PDA

View Full Version : When the Results Don't Match The Principals or Formulas



Ron Sutton
07-02-2013, 07:13 PM
This is a thread I'm starting for Jason Rhoades, myself & others who are interested in figuring out what is the cause ... when the track or test results don't match established principles or formulas.

Post #2 through #9 ... are moved from another thread ... so we could continue discussing this, without completely hijacking that thread.

Posts #10 & later, are the continued discussion.

j-rho
07-02-2013, 07:57 PM
Here's my first post from the old anti rollbars thread-


Ron you've written a lot of good stuff, and pretty much all these folks have the same basic 2 problems:
1. Poor front:rear tire size ratios considering the cars' front:rear weight biases
2. Not enough resulting negative camber on the outside front tire at terminal roll.

The solutions for everybody are pretty much the same:
1. More front wheel/tire
2. More negative camber - if you aren't replacing enough components to get great geometry (whether for budget or ruleset), that means it'll need more static caster and negative camber

Another common element is how the motion ratio of the stock geometry cars is awful, most are running very low ride frequencies with far-too-low damping rates for performance applications. The Corvette geometry front subframes like Matt's DSE stuff allow for reasonable spring and damper rates to achieve a performing setup, but those still on the stock 60's geometries require 4-digit spring rates and absurd damping forces (which basically nobody is doing) before they actually start to work.

I think some of your statements are a little misleading - like sway bars "engaging the inside tires more" - adding, or stiffening a swaybar at one end of car, will increase that end's share of the lateral load transfer distribution in a corner, decreasing dynamic loading on that end's inside tire, potentially to the point of lift - which sounds like the opposite of what you said. Sway bars do not decrease total lateral load transfer (ignoring minor second-order dynamic CG height changes) so they can't add net weight to the inside tires at a given lateral acceleration ("get them more engaged") - only a wider track or lower CG can do that.

I think you probably already know this and are trying to simplify it for the folks here, so I offer this only for those that are maybe "starting to get it" but are having a hard time with those particular elements of your posts.

Ron Sutton
07-02-2013, 08:01 PM
Hi Jason, thanks for chiming in.

Ron you've written a lot of good stuff, and pretty much all these folks have the same basic 2 problems:
1. Poor front:rear tire size ratios considering the cars' front:rear weight biases
2. Not enough resulting negative camber on the outside front tire at terminal roll.

The solutions for everybody are pretty much the same:
1. More front wheel/tire
2. More negative camber - if you aren't replacing enough components to get great geometry (whether for budget or ruleset), that means it'll need more static caster and negative camber.
You’re correct. But, I hate to add any more Camber than needed, because as much as it helps the outside front tire achieve an optimum contact patch, it hurts the inside tire angle, preventing it from achieving an optimum contact patch. All of this is caused by the KPI/Caster Split favoring the KPI too much.

Of course some cars can’t achieve enough caster for an optimum KPI/Caster Split favoring the caster … for a variety of reasons ranging from rules & owner limitations to parts availability & packaging issues. But it makes sense to start with the best KPI/Caster Split we can … then achieve the rest with camber gain & static camber.

Another common element is how the motion ratio of the stock geometry cars is awful, most are running very low ride frequencies with far-too-low damping rates for performance applications.
Agreed. If people … including the shock engineers at most pro touring shock companies ... saw the shock dyno sheets & saw how much dampening rate we’re running on the nose (0 -1/2 IPS) & total rebound at 1-3 IPS … they would faint or be in total disbelief.

The Corvette geometry front subframes like Matt's DSE stuff allow for reasonable spring and damper rates to achieve a performing setup, but those still on the stock 60's geometries require 4-digit spring rates and absurd damping forces (which basically nobody is doing) before they actually start to work.
I agree & even then … on the stock geometry cars … everything they do with springs, bars, shocks, etc … are crutches for the horrible geometry.

I think some of your statements are a little misleading - like sway bars "engaging the inside tires more" - adding, or stiffening a swaybar at one end of car, will increase that end's share of the lateral load transfer distribution in a corner, decreasing dynamic loading on that end's inside tire, potentially to the point of lift - which sounds like the opposite of what you said. Sway bars do not decrease total lateral load transfer (ignoring minor second-order dynamic CG height changes) so they can't add net weight to the inside tires at a given lateral acceleration ("get them more engaged") - only a wider track or lower CG can do that.

I think you probably already know this and are trying to simplify it for the folks here, so I offer this only for those that are maybe "starting to get it" but are having a hard time with those particular elements of your posts.

I respectfully disagree, but Total Roll Stiffness, Roll Couple & the net Roll Angle all play a role in this.

“Adding, or stiffening a swaybar at one end of car, will increase that end's share of the lateral load transfer distribution in a corner, decreasing dynamic loading on that end's inside tire, potentially to the point of lift” … happens only when the car has excessive Roll Angle.

In that case, you are correct, because the car is rolling over with so much force, that any attempts to reduce Roll Angle with ARB stiffness, simply unloads the inside tires. It’s funny, because the stiffer ARB DOES make the suspension run flatter, but the car is rolling with such force it just brings the inside tire, on that end, with it. If it’s big enough, you can see the tire(s) lift. But long before it gets that bad, you can really measure it on a skid pad, because that inside tire starts running cooler.

In your AutoX racing you may have seen, or experienced, stiffer ARB’s leading to lifting the inside tire to a degree, reducing the inside tire’s grip, but the cause is excessive Roll Angle, either evenly or unevenly. This excessive Roll Angle can be caused by three things …. excessive chassis flex, too soft of spring rate and/or too long of a CG lever arm over the roll center.* (Addition: Actually 4 things, counting track width, but most folks on here are not narrowing their track width as a tuning tool.)

*For those following along, that means the car’s CG (Center of Gravity) is too high and/or the Roll Center is too low. The CG acts as a lever on the Roll Center. The further they are apart, the more the car rolls.

I have seen chassis flex cause both even … and uneven … excessive Roll Angle … depending on where & how the chassis was flexing. The spring package can cause “even” excessive Roll Angle when all the springs are equally too soft … and uneven excessive Roll Angle when just one end of the car’s spring are too soft … or they’re all too soft and one end is more so. The excessive distance (leverage) between the CG & roll center can cause “even” excessive Roll Angle if the condition exists at both ends of the car … and uneven excessive Roll Angle when the condition exists only at one end of the car.

Here is a USAC Midget hiking the LF wheel on corner ENTRY … because the crew chief put too soft of a rear spring on the car in an attempt to gain more traction on exit. :hand:

You can see the LF tire is 3”+ off the ground, while the LR tire is on the ground.

78432

Here is an entertaining photo of a Mustang Autocrossing with excessive Roll Angle. :seizure:
You can see the LF tire is 4”+ off the ground, while the LR tire is on the ground.
(Hope I am not embarrassing the car’s owner. Just needed an example.)

78433

But, as long as the car we’re tuning doesn’t have excessive roll angle, stiffening the ARB's do keep the car flatter, spreading the load more evenly over all 4 tires, giving the car more grip. In my experience with all forms of race cars, the race team engineers & myself found increasing ARB rate always made the suspension run flatter … and always worked the inside tires more & the outside tires less … but we were not working with excessive Roll Angle situations.

When testing on 7-post rigs, and even more simple pull-down rigs, you can see the increased loading of the inside tires with stiffer ARB rates. It shows up right on the screen with increased wheel load cell readings. Of course, the track is the best test, and more fun.

In professional oval track & road racing, when we're in the "happy window," a slightly stiffer ARB, or a little pre-load, helps us to works the inside front tire … making the car turn better. In oval track racing this is most critical, because the spring packages in front are very soft.

If we stiffen the ARB too much, it will actually start engaging the LR tire more ... which is normal ... tightening the car up, especially in the middle & exit of the corner. It’s obvious to see with our on-board data acquisition systems, but less equipped racers simply look at the tire temps. When a temp goes up, you’re working that tire more. When a temp goes down, you’re working it less. The only thing that tricks some racers here is when they are sliding, pushing or spinning a tire … will also get hotter.

We do run into excessive Roll Angle situations … which is easy for us to see … because the car doesn’t gain load on any of the inside tires with stiffer ARB rates. How we fix this, depends on the class of racing. If we’re on a coil-bind set-up, we have to install a spring with a taller bind height. If we’re on a bump stop set-up, we simply add a shim. If we’re on a “soft spring/big bar” set-up, we increase the spring rate 1 step. We could adjust on the Roll Center, but only if we run out of options or find it was lower than optimum.

Occasionally, with less experienced Racers, we find they have the CG too high, or the Roll Center way too low (under ground) … and these require work I consider above & beyond normal track tuning … more like their baseline chassis design or set-up wasn’t optimum.

And of course you really see excessive Roll Angles in a lot of street cars with big power, better suspensions, grippy tires & no roll cage to add chassis torsion rigidity. Kinda like that Mustang above.

Best wishes … :cheers:

j-rho
07-02-2013, 08:30 PM
Next post from that thread

Hi Jason, thanks for chiming in.


I think some of your statements are a little misleading - like sway bars "engaging the inside tires more" - adding, or stiffening a swaybar at one end of car, will increase that end's share of the lateral load transfer distribution in a corner, decreasing dynamic loading on that end's inside tire, potentially to the point of lift - which sounds like the opposite of what you said. Sway bars do not decrease total lateral load transfer (ignoring minor second-order dynamic CG height changes) so they can't add net weight to the inside tires at a given lateral acceleration ("get them more engaged") - only a wider track or lower CG can do that.

I think you probably already know this and are trying to simplify it for the folks here, so I offer this only for those that are maybe "starting to get it" but are having a hard time with those particular elements of your posts.

I respectfully disagree, but Total Roll Stiffness, Roll Couple & the net Roll Angle all play a role in this.

Actually, they don't at all. Whether a car rolls dix degrees at one g or three degrees, the amount of weight transferred from the inside tires to the outside remains the same. The only way reducing roll angle could have an effect on total weight transfer, would be if the car gained CG height as it rolled - some cars might but that is not universal and dependent on the geometry. CG might actually sink as the car rolls. In any case the contribution of any CG height change as the result in a change of roll stiffness at one axle is likely orders of magnitude smaller than the handling balance change brought by the same change.



“Adding, or stiffening a swaybar at one end of car, will increase that end's share of the lateral load transfer distribution in a corner, decreasing dynamic loading on that end's inside tire, potentially to the point of lift” … happens only when the car has excessive Roll Angle.

I'm not sure where you draw the line at "excessive" but this happens all the time - especially in the rear of FWD cars, and in the front of mid or rear-engine RWD cars. For 2WD platforms there are plenty of places where you want to saturate load transfer at the non-driven end to maximize potential tractive force at the driven end.



But, as long as the car we’re tuning doesn’t have excessive roll angle, stiffening the ARB does load the inside tire more, giving it more grip. In my experience with all forms of race cars, the race team engineers & myself found increasing ARB rate always made the suspension run flatter … and always worked the inside tires more & the outside tires less … but we were not working with excessive Roll Angle situations.
This is the part where I get a bit worried...the most basic tenet of tuning automotive handling is tire load sensitivity. A pair of tires (Front pair or Rear pair) will generate the most grip when they are most evenly loaded (all else being equal). When a car achieves a given lateral acceleration, CG height and track width alone determine how much load is going to go from inside to outside - the suspension tuner gets to choose how much of this weight is borne by the front and rear axles via their relative roll stiffnesses (contributed both by springs/bars but also link geometries). For a car that started neutral, increasing front swaybar stiffness relative to the rear, will increase the differential in load between the two front tires (UNloading the inside front tire MORE), which decreases the amount of potential grip that axle has to offer at that lateral acceleration - which means shift towards understeer, all else equal. While this happens, the rear axle is enjoying a reduction in lateral load transfer, with the inside rear now more heavily loaded at that lateral acceleration than it had been with the softer front bar.

Total weight on the pair of inside tires (front + rear) does not change directly from a stiffening of swaybars; it leads to decreased load on the inside of the stiffened axle, and increased load on the inside at the opposite axle, for a given lateral acceleration. Same thing could be achieved with stiffer springs.


When testing on 7-post rigs, and even more simple pull-down rigs, you can see the increased loading of the inside tires with stiffer ARB rates. It shows up right on the screen with increased wheel load cell readings. Of course, the track is the best test, and more fun. The only part of this I agree with is the last part - real world testing trumps just about anything you can do on paper.


In professional oval track & road racing, the stiffer we make the front ARB, through size or pre-load, the more it works the inside front tire … This should be counterintuititve to you - you could put together a front bar so stiff, the inside front tire is almost always in the air - how can you work a tire that never touches the ground?


If we stiffen the ARB too much, it will actually start engaging the LR tire more, tightening the car up, especially in the middle & exit of the corner.This makes perfect sense - increase in load transfer across the front axle means reduced load transfer across the rear axle, which means more load/"engagement" from the inside (always Left in your case?) tire.

j-rho
07-02-2013, 08:35 PM
My next-

Hi Ron,
Appreciate the perspective here. I wonder if maybe our apparent disagreement isn't really a difference over the underlying principles, but maybe just the way we talk about them.

Using your scales approach from above, let's take a hypothetical "totally symmetrical car" - everything is the same about it any way you cut it in half. 50/50 weight distribution with its CG at vehicle centerline along the longitudinal or lateral axes.

Let's say at rest this car looks like this on the scales:
{At rest baseline}
LF 500# RF 500#
LR 500# RR 500#
Left side total: 1000#
Right side total: 1000#
Front total: 1000#
Rear total: 1000#


Now let's say we did something to simulate a cornering situation - maybe we attach a cord to the CG, and pull directly horizontal with some force F. Our tires stay stuck to the scales, but that force rolls the car just as it would in a corner, and based on this car's setup, now our scales read:
{Cornering force F baseline}
LF 750# RF 250#
LR 750# RR 250#
Left side total: 1500#
Right side total: 500#
Front total: 1000#
Rear total: 1000#

Now, we do something to stiffen the front swaybar some amount. But we did so without adding weight or changing anything else about the car. At rest, it still looks the same:
{At rest stiffer front bar}
LF 500# RF 500#
LR 500# RR 500#
Left side total: 1000#
Right side total: 1000#
Front total: 1000#
Rear total: 1000#

The difference, is now what happens when we apply the exact same lateral force F, now that we have increased the front roll stiffness?

I contend we will see something like this:
{Cornering force F stiffer front bar}
LF 750+X# RF 250-X#
LR 750-X# RR 250+X#
Left side total: 1500#
Right side total: 500#
Front total: 1000#
Rear total: 1000#

I contend we haven't changed the total resulting total load on either the left side or right side tires from baseline, even though the car might visually be leaned less now in response to the same lateral force F (e.g. we haven't reduced the total amount of weight transfer). Whatever additional weight transfer X occurred on the front axle, will be the amount of reduced weight transfer we see on the rear axle.

When I read "Stiffer bars reduce roll angle, engaging the inside tires more." What I hear is one of the classic misconceptions in vehicle handling (which seems out of place amongst your otherwise correct information) - that quantity of roll is directly related to quantity of weight transfer, and the more roll, the more weight transfer independent of lateral acceleration. A corollary misconception is that which says the car that squats more, has more rearward weight transfer. I think the basis of this misconception is that when you see two cars with identical setups, and one is rolling more, then there is more weight transfer - but only because that car is cornering harder (more torque about the CG).

If stiffening the front bar resulted in more weight on the inside wheels at the same lateral force F, where is that load coming from? Since our car can't magically gain weight, it has to come from reduced load on the outside tires. This all means a reduction in front-view torque about the CG, and there is no other force available to counteract the torque presented by the cord tugging at the CG. It doesn't add up.

What does make sense is "Stiffer bars reduce roll angle, engaging the inside tire on the other axle more (but not the "inside tires" as a whole)" - if you can live with that interpretation, then maybe we can find some agreement. :)

Ron Sutton
07-02-2013, 08:35 PM
I don’t really like to discuss the new tuning concepts I've learned in the last decade or two, where it conflicts with established engineering principals, because I find it hard to properly explain. But I wouldn’t want anyone to dismiss what we’ve been talking about here, because they think I’ve lost my mind.
Just so you know … I’m not saying I haven’t lost my mind. It happens often … but fortunately for me … I keep finding it again. :jump:

On my regional 9-car race team we had 3 Engineers … and an impressive data acquisition program in-house, including our own satellite based DAQ system … when combined, were critical to our race team’s progress & winning successes (we won 45 races the last 3 years … 2009-2012). The engineers did everything from analyze data on race cars, measure chassis flex, g-forces, tire temps, track friction & slip angles to engineer cutting edge suspensions, aerodynamic aids & torsionally stronger, lighter chassis.

In my profession, we have to be careful not to take 100.00% of engineering training as gospel. It was, after all, developed by human beings based on their understanding of how things work … at the time they developed the principal … with the technology of the day. We occasionally … VERY RARELY … but occasionally run get results that conflicts with what was written by engineers.

This discussion of ARB loading tires is one of those areas.

-------------------------------------------------------------------------------------------------------------------------------

Another is Track Bar/Panhard Bar angle. A few years back, a team was testing a Dodge NASCAR Craftsman Race Truck on a 7-post rig … and the Crew Chief was testing the Track Bar at different angles for an upcoming road race, but with the center always at the same height, so the roll Center stayed the same. He started with a baseline with the left side at 10” & the right side at 10” and ran the test, producing a baseline set of load numbers that were even on the left rear tire & right rear tire.

When he put the left side at 8” & the right side at 12” & tested it … the 7-post rig computer showed more load on the left rear tire & less load on the right rear tire … dynamically … and the Truck had a decreased Roll Angle turning left & increased Roll Angle turning right. When he put the left side at 12” & the right side at 8” … the 7-post rig computer showed less load on the left rear tire & more load on the right rear tire … dynamically … and the Truck had an increased Roll Angle turning left & decreased Roll Angle turning right.

We have known this for years, before doing 7-post testing. This NASCAR Crew Chief wasn’t testing to see IF it worked … he was just testing to quantify the tire load numbers. The Dodge Engineers said there must be something wrong with the 7-post rig, because that is not possible, quoting various engineering experts & engineering principals. All of their engineering training says the CG Height, Track Width & Roll Center location alone affect tire loads … and the angle of the track bar doesn’t matter.

I do not know why the results conflict with the engineering principal. On the track … as on the 7-post rig … Track Bar angle affects tire load. Maybe it's time to update the engineering formula to take that into account.

-------------------------------------------------------------------------------------------------------------------------------

That example doesn’t mean we don’t value engineering. Just the opposite. Engineering has led to almost all of the advances we see today in modern racing suspensions. But there are a few, VERY RARE situations where the test results we see today … conflicts with what was written years back. I surely don't know why they conflict. When we see something like that, we talk about it a bit & move on.

We are all clear you can not outrun the laws of phsyics & fundamental engineering principals will always apply.

Sometimes the formulas are not 100% accurate. I contribute that to we are all human, even the guys that worked out formulas years ago. The professional engineering community continues to learn new stuff & advance as well as the professional racing community. That is what makes progress in engines, suspensions, cars & just about everything in our lives. Otherwise our cars would perform at the same levels they always did.

There are a few rare exceptions where the formula is not accurate or complete, and having experienced them firsthand, I have learned to keep an open mind.

-------------------------------------------------------------------------------------------------------------------------------

An example of a formula that was considered gospel for years, was the quarter mile drag racing power/weight/speed calculation formula.
It is (.00426 x mph)cubed** x weight = HP
**(I couldn't figure out to put the small 3 where it goes.)

If you enter a car with 1000# & 100HP & compare it to car weighing 4000# & 400Hp … both having a 10-1 W/P ratio … the formula provides the same quarter mile top speed mph (106.76 MPH) for both cars, which is then turned into an estimated ET. In the real world, we know that is not accurate. Even though both cars have the exact same weight to power ratio, the lighter car will produce a substantially quicker ET & slower MPH … and conversely the heavier car will produce a substantially slower ET & faster MPH. Said another way … to achieve 106.76 mph … the lighter car will need more than a 100 HP & the heavier car will need less than 400hp.

The formula was created ions ago … and the creators are dead … so we can’t have a beer with them & discuss this. But all indications are they created the formula based on “averages” … and a simple weight-to-power ratio. They did not build into their formula a correct method … or any method … for adjusting for lighter & heavier cars with the same W/P ratio. So the formula is not an absolute. That’s ok. It is still helpful. It’s just not gospel.

Years later, a formula was created as a correction factor to be used with the original formula. It goes …

a. For cars weighing 2300# use the original formula.

b. For cars lighter than 2300# … Car weight/2300 = X … 1.00-X = Y … Y times .3875 = Z … Z+1.00 = B … B times the result of the original HP in the formula … provides the corrected, real HP to achieve that mph.

c. For cars heavier than 2300# … … 2300/Car weight = X … 1.00-X = Y … Y times .3500 = Z … 1.00-Z = B … B times the result of the original HP in the formula … provides the corrected, real HP to achieve that mph.

Now let’s re-run the calc’s …
The 1000# car actually needs 121.9HP to go 106.76 MPH in the Ľ mile.
The 4000# car actually needs 340.5HP to go 106.76 MPH in the Ľ mile.

This new formula is correct, because some one learned & advanced it. But I still see the original formula being used & relied upon as “fact.” :hammer:

-------------------------------------------------------------------------------------------------------------------------------

Anti-Roll Bars

In an oval track race car, if we preload the front ARB … it adds “wedge” or “cross weight” (pick your term) to the car statically & dynamically. Meaning it loads two opposing tires. Statically, we can see it on a set of scales. In fact, as a part of most race team’s set-up, they adjust their “Wedge” or “Cross Weight” to a number lower than their final desired % … then add in bar preload to achieve the final target “Wedge” or “Cross Weight” % they are targeting.

Follow me close here … when we do this on the scales … it looks like this:
This car weighs a total of 2570# ... with driver & 175# Lead
Left side weight: 58.2%
Front weight: 48.77%
Rear weight: 51.23 %
*These #’s & %’s do not change with cross weight.

LF 636# RF 618#
LR 860# RR 457#
Cross Weight: 57.49% before preloading the front ARB

LF 616# RF 638#
LR 880# RR 437#
Cross Weight: 59.03% with the front ARB preloaded

The scale under the LF tire gets lighter. This would indicate the LF tire gets worked LESS with a preloaded or bigger sway bar. But dynamically, that is not what we find.

Some people think of testing race cars on pull down rigs and/or on 7-post rigs as “dynamical” testing. I do not. I think of it as “semi-dynamic” testing … because the tires are not rolling, nor are they twisting their contact patches into the slip angle they experience on track. It is helpful … VERY helpful ... to see the loads on the on each corner. In the case of the 7-post “shaker rig” as some call them … the forces of braking, cornering & accelerating are recreated exactly from the data pulled from track testing. It is so accurate the track surface is recreated down to the pebble in turn 3. Teams can test any suspension part to see how well the tire & wheel follow the track surface (contact %) & how much it is loaded in pounds or weight %.

On the 7-post … when the crew installs a bigger, stiffer sway bar, or preloads the existing bar … and the 4 load cells under the tires are monitored on corner entry & middle … the load goes up on the inside front tire & down on the outside front tire … as compared to a smaller, softer bar or a bar with less or no preload.

I think the 7-post teaches us HOW the inside front tires gets loaded in the first place and what keeps it there. When the car is braking hard setting up for the corner, most of the Force is transferring load from the rear tires to the front tires … compressing both front suspension corners & loading both front tires. Because the car is in a slight roll condition ... not driving perfectly straight ... the inside front wheel compresses about 3” & the outside wheel 3-˝”. Obviously the outside tire is loaded more than the inside tire. In hard threshold braking, there is little load on the rear tires & a lot of load on the front tires.

Testing shows the bigger the sway bar, the more the inside front tire gets loaded on this braking transition, which is good, because the more we load the inside front tire now, while we have braking force, the better grip it will have on corner turn in.

The question was, if the inside front tire gets loaded on, more or less, straight line braking, what keeps the inside front tire loaded through the corner entry & the mid-corner roll through zone?
The answer is the stiff rebound valving & small bleeds in the shocks … keep the whole front end “tied down.” Otherwise, when the driver lifts off the brakes … the inside front corner would “pop up” pretty quick 7 roll over in the process. In oval track racing the LF is “meaner” on rebound valving. In Road Racing we run both shocks the same, which is why you see a tick more Roll Angle in a Trans Am car than A NASCAR Car. (Look at the splitters).

How do you REALLY KNOW when you’re DYNAMICALLY working a tire more or less?
On the track, where tires are rolling, and they’re twisting their contact patches into the slip angle, the best method of measuring tire load … is tire heat … with on-board infrared tire temp sensors pointed at each wheel. Some set-ups use 3 per tire to measure how well the set-up is working the whole contact patch. Other set-ups just use 1 per tire, pointed at the center. Regardless, the results are the same here.

When the crew installs a stiffer ARB, or preloads the existing bar … and the 4 tires are monitored on corner entry & middle … we see the temps go up on the inside front tire & down on the outside front tire … as compared to a smaller, softer bar or a bar with less or no preload. So fully dynamic ... you can see it in tire temps.

In my experience most all factors in the race car’s geometry, suspension & chassis tend to have an effect on other factors. Some more than others. Some statically … some dynamically … some both. What I shared with you is what we see on highly developed race cars utilizing big soft springs, big bars & stiff rebound shocks … ending up with Roll Angles of 0.75-1.5 degrees. I believe Roll Angle to be a factor. If we let the race car roll too much, the ARB is not capable of keeping load on the inside tires as well, or at all if it rolls too much.

I didn’t want to take the time to get into this conversation, because I knew it would be complex, lengthy ... and is always controversial. I am not trying to be a know it all, because I do not know it all by a long shot. I’m learning every time we test or race. I encourage you to think for yourself & not rely on 100.00% of what you read … my stuff included.

Best wishes !!!

j-rho
07-02-2013, 08:36 PM
One more-


Hi Jason & Cobra,

...
In my profession, we have to be careful not to take 100.00% of engineering training as gospel. It was, after all, developed by human beings based on their understanding of how things work … at the time they developed the principal … with the technology of the day. We occasionally … VERY RARELY … but occasionally run across something that conflicts with what was written by Engineers … even top Engineers.

This discussion of ARB loading tires is one of those areas.

-------------------------------------------------------------------------------------------------------------------------------

Another is Track Bar/Panhard Bar angle. A few years back, a team was testing a Dodge NASCAR Craftsman Race Truck on a 7-post rig … and the Crew Chief was testing the Track Bar at different angles for an upcoming road race, but with the center always at the same height, so the roll Center stayed the same. He started with a baseline with the left side at 10” & the right side at 10” and ran the test, producing a baseline set of load numbers that were even on the left rear tire & right rear tire.

When he put the left side at 8” & the right side at 12” & tested it … the 7-post rig computer showed more load on the left rear tire & less load on the right rear tire … dynamically … and the Truck had a decreased Roll Angle turning left & increased Roll Angle turning right. When he put the left side at 12” & the right side at 8” … the 7-post rig computer showed less load on the left rear tire & more load on the right rear tire … dynamically … and the Truck had an increased Roll Angle turning left & decreased Roll Angle turning right.

We have known this for years, before doing 7-post testing. This NASCAR Crew Chief wasn’t testing to see IF it worked … he was just testing to quantify the tire load numbers. The Dodge Engineers said there must be something wrong with the 7-post rig, because that is not possible, quoting various engineering experts & engineering principals. All of their engineering training says the CG Height, Track Width & Roll Center location alone affect tire loads … and the angle of the track bar doesn’t matter.

On the track … as on the 7-post rig … Track Bar angle affects tire load. It’s time to re-write the engineering formula to take that into account.

-------------------------------------------------------------------------------------------------------------------------------
Hi Ron,
When the test was done, what happened to loads on the front tires? Wasn't there an equal and opposite change in load transfer across the front axle?



When the crew installs a bigger, stiffer sway bar, or preloads the existing bar … and the 4 tires are monitored on corner entry & middle … the temp goes up on the inside front tire & down on the outside front tire … as compared to a smaller, softer bar or a bar with less or no preload.

So fully dynamic ... we see it in tire temps.

Now … in my experience I have learned that most all factors in the race car’s geometry, suspension & chassis tend to have an effect on other factors. Some more than others. Some statically … some dynamically … some both. What I shared with you is what we see on full blown, highly developed race cars utilizing cutting edge suspensions … ending up with Roll Angles of 0.75-1.5 degrees. I find Roll Angle to be a factor.

In cars with much higher Roll Angles above 2.5+ degrees … I think the original engineering principal may apply. But I don’t know … because I‘m not involved with high tech testing of those types of cars.
I bolded that last bit just to call it out - those are just the sorts of cars Matt is asking about, and that the people here build and drive.

What I suspect is happening, are a number of other contributing factors come into play in your situation (could be aero, or a change in the bump stop engagement) that combine to trump the fundamental effects of the original change, producing an end result (hotter inside front tires) opposite of what a pure and simple "textbook scenario" would say. It's not that either is necessarily wrong, or even in contradiction; it's that in real life we never really get to just change one variable at a time in an experiment, and the other things that change with one variable, have an effect on the outcome. However, your observed cause-and-effect relationships are likely somewhat specific to your situation, not necessarily the same as would be seen in all (or most of the time in typical street) situations, and perhaps not the best representation of the basic mechanics of these sorts of changes, to a beginning audience like the Pro-Touring forum members. I bet some Pro-Touring cars roll 2.5 degrees just when the driver sits down! :)

j-rho
07-02-2013, 08:37 PM
Last one from the rollbar thread-


Hi Jason,


Hi Ron,
When the test was done, what happened to loads on the front tires? Wasn't there an equal and opposite change in load transfer across the front axle?
Yes. For example, when the track Bar was 8" on the left & 12" on the right, and the RR was loaded less, the LF was loaded more.
So if there was an equal and opposite reaction on the other axle, wasn't the total amount of weight transferred to the outside (weight transferred to outside front + weight transferred to outside rear) the same between the different scenarios? Or at least, close enough that a discrepancy could be explained by a second-order effect like a change in resulting CG height?

Ron Sutton
07-02-2013, 08:37 PM
Hi Jason,


Hi Ron,
When the test was done, what happened to loads on the front tires? Wasn't there an equal and opposite change in load transfer across the front axle?
Yes. For example, when the track Bar was 8" on the left & 12" on the right, and the RR was loaded less, the LF was loaded more.


I bolded that last bit just to call it out - those are just the sorts of cars Matt is asking about, and that the people here build and drive.
True.

What I suspect is happening, are a number of other contributing factors come into play in your situation (could be aero, or a change in the bump stop engagement) that combine to trump the fundamental effects of the original change, producing an end result (hotter inside front tires) opposite of what a pure and simple "textbook scenario" would say.
It's not aero or bump stop engagement, because we see it without bumps stops & on the 7-post, where there is no aero. It is connected to low roll angles, because when we get too much roll angle, it stops working.

It's not that either is necessarily wrong, or even in contradiction; it's that in real life we never really get to just change one variable at a time in an experiment, and the other things that change with one variable, have an effect on the outcome.
Too true. Everything affects everything else, which is what makes this challenging & fun.

However, your observed cause-and-effect relationships are likely somewhat specific to your situation, not necessarily the same as would be seen in all (or most of the time in typical street) situations, and perhaps not the best representation of the basic mechanics of these sorts of changes, to a beginning audience like the Pro-Touring forum members.
I agree for almost all passenger cars & most PT cars.

But Matt's situation with his car was he made his suspension too stiff and started fighting a pushing problem. It's a Roll Angle vs Pitch angle equation. A performance or race car meant to handle ... can’t run flat … it needs the suspension to travel. So it’s either got to Roll more and lessen the grip on the inside tires ... or Pitch more and lessen the grip on the rear tires … to turn well. Matt's was neither rolling much or pitching much.

That’s the primary difference between Conventional vs High Travel set-ups. Conventional set-ups pitch less & roll more. High travel set-ups roll less & pitch more. Matt's spring & sway bar combination was simply not traveling enough either way. It is less a problem on big tracks with fast, sweeping corners & more amplified on small AutoX tracks.

I bet some Pro-Touring cars roll 2.5 degrees just when the driver sits down! :)
That's funny ... because it's true.

Ron Sutton
07-03-2013, 09:29 AM
Jason,

We had two topics going on in our previous discussion. One about track bar angle effects & the other about front sway size affecting loads on the inside front & inside rear tires. Which conversation do you want to start with?

j-rho
07-03-2013, 10:49 AM
Lets start with #1: I contend total lateral load transfer at a given lateral acceleration is governed by two things: CG height and track width. You believe that roll angle plays a part - that a car that rolls less, transfers less weight, even if CG height and track width are kept constant.

#2 will be the contribution of sway bars. I contend stiffening a swaybar at one axle, increases the amount of weight transferred across that axle at a given lateral acceleration, while decreasing the amount of weight transfer across the opposite axle. But as above, total weight transferred to the outside does not change. My post #5 in this thread is an example of this question.

j-rho
07-03-2013, 11:52 AM
If we're addressing common fallacies and misconceptions, another one I see:
When one stiffens their shocks, there is no question that the visible transient motions of the car are slowed down. But does that shock stiffening decrease the actual rate of load transfer between tires, or increase it?

The reality is that stiffening the shocks increases the rate (the speed) of weight transfer. It doesn't reduce quantity of weight transfer. However it is very commonly believed (even by a few of the shock vendors selling stuff on this site) that stiffening shocks slows weight transfer, and some appear to believe it reduces the total amount. I believe this misconception goes hand-in-hand with the ones we're talking about above -that reduced roll (and its side view buddies squat/pitch) are indicators of reduced weight transfer. It's based upon a flawed but fundamental belief that the chassis' displacement of each relative corner must be tied, at that moment in time, to the load present upon that corner's tire. Once people get over that, their depth of understanding can rapidly advance.

Ron Sutton
07-03-2013, 01:38 PM
Hi Jason,

First, for us to converse well, I need us to speak more "car guy" and less engineer. I'm not an engineer. I'm a car guy/racer that happens to design, build & tune winning race cars. In my racing world, the Engineers & I respect each other backgrounds because we both contribute to this overall goal, so they tolerate my simple language & at times, I find myself trying to understand what they're saying. I use some engineering terms ... correctly ... and sometimes I use them wrong. For Pete's sake, let's not discuss polar moments. I understand it, use it, live & breathe it ... but don't explain it correctly. This drove my lead engineer nuts.

One of my other engineering friends said, Ron is a race car designer that did not go to engineering school, so he speaks "car guy". I think it was a compliment, but I'm not 100% sure. Engineering speak is like French to me. I understand some of it, but if I have to go look it up, I'm not having fun.


Lets start with #1: I contend total lateral load transfer at a given lateral acceleration is governed by two things: CG height and track width. You believe that roll angle plays a part - that a car that rolls less, transfers less weight, even if CG height and track width are kept constant.

Let's use terms like force, load, weight, weight transfer (not literally), working the tire, suspension travel, roll, pitch, dive, lift, rise, rate, preload, etc.

Roll & pitch play a role in loading & working the tires ... and at the end of the day ... all I really care about is working the tires in an optimum fashion, for optimum performance.

When we brake hard, in a soft sprung front end race car, and the front end suspension travels far & fast ... and the rear end rises similarly ... creating a higher degree of pitch angle ... the front tires are loaded & working more ... while the rear tires are loaded less & working less ... than if we have a stiffly sprung front end race car, and the front end suspension travels little & slowly ... and the rear end rises similarly ... creating a smaller degree of pitch. The same concept works with roll angle.

What part of that do we not see eye to eye?

djfxall
07-03-2013, 01:50 PM
Awsome thread getting into shocks is like opening a can of worms along with sprung weight I know on a bike you can load the suspension into a corner and use that for exiting the corner. I am new to the 4 wheel race world but valving controls the speed of the compression allong with decompression. Is there dual speed shocks for cars which have different rates for high speed compression?

Ron Sutton
07-03-2013, 02:01 PM
Awsome thread getting into shocks is like opening a can of worms along with sprung weight I know on a bike you can load the suspension into a corner and use that for exiting the corner. I am new to the 4 wheel race world but valving controls the speed of the compression allong with decompression. Is there dual speed shocks for cars which have different rates for high speed compression?

Yes. We can tune shocks internally with parts ... or externally with knobs ... in ways that control the low speed valving (where handling is) and high speed valving (where bumps are controlled). Of course 2, 3 & 4 way shocks are spendy.

If you'd like to discuss it, let's start another thread. We're created this one to work on reasons "why" track & test results conflict with proven principles occasionally.

j-rho
07-03-2013, 03:00 PM
Let's use terms like force, load, weight, weight transfer (not literally), working the tire, suspension travel, roll, pitch, dive, lift, rise, rate, preload, etc.

Roll & pitch play a role in loading & working the tires ... and at the end of the day ... all I really care about is working the tires in an optimum fashion, for optimum performance.

When we brake hard, in a soft sprung front end race car, and the front end suspension travels far & fast ... and the rear end rises similarly ... creating a higher degree of pitch angle ... the front tires are loaded & working more ... while the rear tires are loaded less & working less ... than if we have a stiffly sprung front end race car, and the front end suspension travels little & slowly ... and the rear end rises similarly ... creating a smaller degree of pitch. The same concept works with roll angle.

What part of that do we not see eye to eye?

Let's say we have two cars almost exactly the same - both have the same CG height. One is softly sprung, but when it brakes, the sinking front and rising rear combine to keep the CG height unchanged. The second car is different only in that it has a totally solid suspension that can't move at all.

Both cars brake at 1g. The soft car's nose dives and its rear end rises. The stiff car doesn't move at all. Which car transferred more weight to the front tires?

You'd say of the soft car, "the front tires are loaded & working more". I say, the front tires of both cars are equally loaded, because they have the same CG height, wheelbase, and are both braking at 1g. This is just basic physics and nothing discovered in the last 10 or 20 years makes this basic principle any less valid under any circumstance.

This isn't to say both cars would work equally well on a racetrack. A simple example like this is only used to explain the fundamentals of what's happening with weight transfer. In reality there are many variables, and sometimes when you make a change it may have a net effect (after all other affected variables are considered) opposite of what a simple textbook says happens when just that one variable is changed in isolation.

A classic example is stiffening a front swaybar on a crummy car - the textbook says doing so will increase relative weight transfer at the front tires, loading the outside front more and inside front less, which will reduce total grip of the front axle, which will shift the car's balance towards understeer. But in real life, your crummy car might have really awful front geometry, where a stiffer front swaybar does the opposite - shifts balance towards oversteer - because the additional roll stiffness of that bar, kept the resulting alignment of the outside front tire at a "happier" place, resulting in more grip from the front axle than before. It's not that the swaybar somehow magically wasn't having its fundamental textbook-defined load-transfer-altering effect; it's that a second-order effect (resulting alignment change) altered the equation in a way that trumped the change's primary effect (load transfer change).

In my experience there is no such thing as track tests conflicting with proven principles. There is only failure to observe and properly account for all the variables that go into the equation. Technology has improved our ability to measure, record, and model the contribution of these variables to the end result, and as development progresses under ever-changing rulesets, we often see the importance of certain variables overtaking others in the process.

Ron, you aren't the first person I've come across to have a successful career in something, without having a very thorough understanding of what was going on under the covers. While that probably sounds disrespectful it isn't meant to be, and speaks to the importance of a combination of other great qualities like your kind personality, hard work, dedication, and wisdom to choose the right employees to compliment your own abilities. I thought it might be interesting to hear about more cases like the swaybar example I gave above, but it is clear now we aren't even starting with a compass of solid fundamentals, which will make progress in exploration of apparent real world vs. theoretical contradicions, a wild goose chase.

Last tangential note not to Ron but to those reading - I don't normally post much on message boards these days, but one thing that gets me fired up is people selling you junk, or the wrong parts. It's part of why I put so much effort into my blog - I want to help everyone learn, so they can get better, and be better competitors, and maybe turn into people I can learn something from someday. The Pro-Touring movement as a whole I see as a big community, hungry to learn about vehicle dynamics and handling and driving well. Not all vendors, but many, don't really know what they're talking about, but won't let that stop them from taking your money. Educate yourself on the basic principles of tuning your car's handling. Make your own choices, think for yourself. And practice driving! You can't figure out what car changes to make if you don't know how to drive and evaluate present state.
http://www.rhoadescamaro.com/build/?p=338

Ron Sutton
07-03-2013, 08:06 PM
Zing! Duck! Ouch !

Help! I’ve been insulted & can’t get up. :lol:

Jason I know the formulas for calculating g-forces & weight transfers too.

Let’s take the formula for Acceleration Weight Transfer, which is:
(G x W x CGH)/WB = T

Let’s take a real car, like the one in the photo below, that has a 109” wheelbase, weighs 3000#, with 54% on the front (1620#) & a CG height at 16”, launching with 2.0 G’s.

The formula says (2.0g x 3000# x 16”cgh)/109”wb = 881# of weight transferred from the front end … to the rear tires. But the front end weighs 1620# … and it’s in the air … and all of the weight of the 3000# car is on the rear tires. Will you be my huckleberry and educate us all on how that computes?

Feel free to change the numbers around, but keep them realistic.


78466


---------------------------------------------------------------------------------------------


While we’re having fun, let’s take this cool PT car at the AutoX track. (LOVE this car.) :)


78467


The formula for Cornering Weight Transfer is:
(G x W x CGH)/TW = T

Let’s say it weighs 3200#, has a track width of 58”, a CG of 16” & pulls a g-force of 1.2.

The formula says (1.2g x 3200# x 16”cgh)/58”tw = 1059# of weight transferred from the left side … to the right side tires. But the left side of the car … with the driver, weighs at least 1600# … and it’s in the air … and all of the weight of the 3200# car is on the right side tires.

How do we use the formula to make that compute?

CA B4C
07-03-2013, 08:39 PM
Gentlemen, thank you very much for starting this thread and discussing vehicle dynamics in a professional manner. I truly appreciate each of your points of view and it kind of reminds me of the ol Guldstrand vs Adams debates from the 80's.

Ron, in the photo of the SpeedTech Nova, I think you would have to use a launch equation and not a lateral weight transfer equation. It appears from the photo there is an elevation change in the course. Granted photos are only 2D and freeze a moment in time.

Again, thank you very much for the highly intelligent discussion.

Ron Sutton
07-03-2013, 08:49 PM
Gentlemen, thank you very much for starting this thread and discussing vehicle dynamics in a professional manner. I truly appreciate each of your points of view and it kind of reminds me of the ol Guldstrand vs Adams debates from the 80's.

Ron, in the photo of the SpeedTech Nova, I think you would have to use a launch equation and not a lateral weight transfer equation. It appears from the photo there is an elevation change in the course. Granted photos are only 2D and freeze a moment in time.

Again, thank you very much for the highly intelligent discussion.

No worries. And no on the elevation change ... it's flat at Irvine.

CA B4C
07-03-2013, 09:05 PM
78474

This was at El Toro also, pretty good elevation change at speed

Ron Sutton
07-03-2013, 09:18 PM
That's hairy! I'll find another example to use. I just picked that one because it is a PT car.

Ron Sutton
07-03-2013, 09:34 PM
Jason,

Let's use this Mitsu Evo instead

78475

Weighs 3550#, has a track width of 60.8”, a CG of 17” & pulls a g-force of 1.2.

The formula says (1.2g x 3550# x 17”cgh)/60.8”tw = 1191# of weight transferred from the right side … to the left side tires. But the right side of the car weighs 1775# … and it’s in the air … and all of the weight of the 3550# car is on the left side tires.

How do we use the formula to make that compute?

j-rho
07-03-2013, 09:41 PM
Ron, in my very first reply to you I talked about second-order effects like dynamic CG height changes in roll:

Sway bars do not decrease total lateral load transfer (ignoring minor second-order dynamic CG height changes)

If you really need me to explain it (?) - your pictured drag car may have had a 16" CG height at rest but the CG height was raised by launch reaction; such an effect is a positive feedback mechanism for further weight transfer to the rear, more launch grip, and a higher CG, to the point of front tire lift. The Nova is clearly cresting a berm.

j-rho
07-03-2013, 09:47 PM
In the real world there are bumps and spikes in lateral grip above what the car might average over an entire skidpad lap. Production cars might have their CG move upwards in roll. Doesn't change the underlying principles.

Why don't we just cut to the chase and talk about with this one? :)

78476

Ron Sutton
07-03-2013, 10:29 PM
Haha, Yeah, I know about the changing CG. I was yanking your chain ... but only a little ... but to make a point.

Before the tires left the ground ... on both the wheelie & the Evo in post #22 ... the load on them was decreased progressively ... rapidly ... but not instantly.

On the drag car, the formula says 881# of weight will be transferred from the front end … to the rear tires ... regardless of what we do with the suspension. To lift the front end completely off the ground, which weighs 1620#, requires us to transfer another 739# of weight ... before the front tires can come .001" off the ground. If we fully extend the front suspension 4" & lift the rear end 2" the CG will raise pretty close to 3-1/4" ... making the dynamic CG 19 1/4". That helps us move another 165# ... but we're still short 560# of weight transfer.

The challenge with the formula, is it implies if the front end were fully extended, but with the tires barely "just" touching ... say .002" from "wheelie" ... then there should be 560# of load on the front tires ... and .002" later ... the formula would still say there is about the same 560# load ... but the front wheels are in the air ... clearly with no load on them.

The car did not go from 560# of load to 0# load in .002" of travel. Yes, as the car continues to rotate on its pitch axis, if not prevented from doing so with wheelie bars, the CG would continue to climb. But to get the formula to read correct ... with zero load on the front tires ... the CG would need to rise to 29.5" ... another 10" at the CG and another 18.5" at the front wheels. Yet, common sense tells us once the tires broke "daylight" there was no load on the front tires & all the load was on the rear tires.

What do you feel describes this incongruity between formula & results?

j-rho
07-03-2013, 11:37 PM
You are describing a dynamic state (drag launch) where longitudinal acceleration (not to mention vertical acceleration) of the Cg are changing. This differs from the steady state cornering situation we have been using to describe the contribution on sway bars. If you are legitimately interested in understanding the differences I can take you through them.

Ron Sutton
07-04-2013, 07:25 AM
Hey Jason,

I detect you’re taking a superior, dismissive tone with me, which is amusing as you asked me to discuss this topic. I moved the forum thread only so we wouldn’t hijack Matt’s thread anymore than we already did. As in any debate, each person takes one side of the position and makes their points using the only the facts that support their side of the position.



When we brake hard, in a soft sprung front end race car, and the front end suspension travels far & fast ... and the rear end rises similarly ... creating a higher degree of pitch angle ... the front tires are loaded & working more ... while the rear tires are loaded less & working less ... than if we have a stiffly sprung front end race car, and the front end suspension travels little & slowly ... and the rear end rises similarly ... creating a smaller degree of pitch. The same concept works with roll angle.

What part of that do we not see eye to eye?

This rhetorical question I used in Post #13 was designed for you to solve this riddle with the correct engineering conclusion. An example of the correct answer would have been anything like … “This can not be true if the g forces are the same.”

Bingo. The statement in post #13 is true … as well as the answer. The reality is, with a properly tuned high travel suspension, we can make the tires grip better & the car brake & turn better, carrying more speed than the low travel car … which creates higher g-forces. My goal in taking that position was to move you out of static thinking to help foster a better conversation. But you just dismissed it as wrong.

I think the challenge for many racers … non-engineers & engineers alike … is they look at the car and its components statically or as a constant. It is this point of view they approach problems with that causes them to not truly understand race cars. I often hear racers & engineers mention a number … be it a suspension travel number, g-force, mph, roll angle, etc … like it is a constant. When in reality, on track, it was only at that number for a fraction of a second.

In my experience I have found the best racers … non-engineers & engineers … always look at the car and its components dynamically, and therefore in constant transition, which it is. The race car is rarely in a steady state. We’re accelerating, braking, turning & accelerating again. Even in the roll through zone of a corner, where the driver is not on the brakes, nor the throttle, and the steering wheel may be steady … the car is still settling from being on the brakes (pitch change) and experiencing slight roll angle change as the shocks work.

I was offering similar, but slightly different, challenges to you with posts #17 & #22. The math didn’t make sense. This was intentional, because we know the math must make sense. My goal by taking this position in the debate was to change your point of view from static to dynamic … leading you to find the answers … and the correct answers that make the weight transfer formulas work … are in the car’s transitional states.

There are lots of cars that can do 11 seconds in the quarter mile that can’t “wheelie.” When you think out how this racer achieves big wheelies, the answer not only makes the math work … which I’m going to leave you to figure out … but more importantly leads us as suspension tuners to three valuable conclusions.

1. We should not look at any of the numbers as a constant … but a mere point in time of a race car in constant transition.
2. We can utilize the suspension & tires to create more g-force, if only temporarily in a transitional state.
3. We can proactively improve our cars performance, with suspension tuning, as long as we’re not stuck in the reactive mode.

“Reactive mode” in #3 reminds me of accountants that try to manage a business from the Profit & Loss statement. They see an annual marketing expense of 10% and say, “if we reduce or eliminate this expense, our profit will go up.” Savvy business owners & managers say, “if we didn’t market, we wouldn’t have the sales number we have, which the marketing budget is simply a percentage of.” It’s a chicken & egg thing.

I like to look at it as … we don’t have an X.xx g-force number to work from … we MADE the X.xx g-force number happen in the first place by what we did with the car & suspension. And we can improve the g-force number more, with suspension tuning to better work the 4 tires.

My goal, from my side of this debate, was to present only the facts that made 1 side of the discussion … and to challenge your thinking and lead you solve these problems. But there is no reason for me to carry both sides of the discussion.

Where you say things like …



Ron, you aren't the first person I've come across to have a successful career in something, without having a very thorough understanding of what was going on under the covers. While that probably sounds disrespectful it isn't meant to be, and speaks to the importance of a combination of other great qualities like your kind personality, hard work, dedication, and wisdom to choose the right employees to compliment your own abilities. I thought it might be interesting to hear about more cases like the swaybar example I gave above, but it is clear now we aren't even starting with a compass of solid fundamentals, which will make progress in exploration of apparent real world vs. theoretical contradicions, a wild goose chase.

Last tangential note not to Ron but to those reading - I don't normally post much on message boards these days, but one thing that gets me fired up is people selling you junk, or the wrong parts. It's part of why I put so much effort into my blog - I want to help everyone learn, so they can get better, and be better competitors, and maybe turn into people I can learn something from someday. The Pro-Touring movement as a whole I see as a big community, hungry to learn about vehicle dynamics and handling and driving well. Not all vendors, but many, don't really know what they're talking about, but won't let that stop them from taking your money.



Ron, in my very first reply to you I talked about second-order effects like dynamic CG height changes in roll: Sway bars do not decrease total lateral load transfer (ignoring minor second-order dynamic CG height changes) If you really need me to explain it (?) - your pictured drag car may have had a 16" CG height at rest but the CG height was raised by launch reaction; such an effect is a positive feedback mechanism for further weight transfer to the rear, more launch grip, and a higher CG, to the point of front tire lift.



If you are legitimately interested in understanding the differences I can take you through them.

Maybe I’m interpreting these paragraphs from different posts incorrectly, but they seem to come off as superior, egotistical & insulting … which I find slightly humorous. When I need to learn something engineering wise, I reach out to the one of the many veteran race engineers I have worked with. You wanted to have this discussion and I’m ok doing so as long as it’s civil & productive. But if not, I’m quite happy to let it go & get back to fun stuff. I’m temporarily stuck at 498 personal race wins, so I have a car to build and people to help.


P.S. There is a good story about veteran Racers & Engineers struggling to work together initially, but overcoming it to produce winning results ... here (http://www.nascar.com/content/nascar/en_us/news-media/articles/2013/02/06/NASCAR-engineering-changes-2013-season.html).

j-rho
07-04-2013, 07:58 AM
I apologize if I've insulted you. I've tried to simplify the discussion as much as possible to help the beginning audience form a grasp of the basics. Statements like "Stiffer bars load the inside tires More" are counterproductive to their education in my opinion. They have to get the fundamentals before being in a position to understand the higher level concepts you want to jump right into.

My continued participation was only in an effort to help the less initiated interpret some of what you were saying. But I'm done now, only have so much charity in me :)

Again, my apologies. I promise in person I'm actually pretty nice, the bay area AutoX guys (Andy McKee, Vic Sias, Scott Fraser, Chris Cox) can vouch for that...I come across much less well on the Internet. :)

Ron Sutton
07-04-2013, 08:14 AM
No worries Jason.

I often come off as brusque or rude, when I am "results focused." My wife sees it at the track & makes me apologize.

I look forward to meeting you at the track & having a beer sometime. Best wishes. :cheers:

ace_xp2
07-04-2013, 10:58 AM
Okay, well now I've got some questions about the feasibility of using those dynamic changes to actually gain grip. For instance, I've read that Anti-Squat can indeed increase load on the rear contact patch. This doesn't occur during acceleration though, but during a change in rate of acceleration. This is great from a standstill when the only force that matters is positive forward acceleration.

However, once you get into adding negative acceleration and turning forces things start to get pretty muddled. That anti-squat turns into pro lift under brakes and adding cornering forces tends to wedge and de-wedge the car in unpleasantly non linear ways. Or better put, while the changes will be linear, they'll lead to a car that has additional factors leading to understeer or oversteer while cornering that may not feel linear to the driver.

I guess what I'm asking is, if you can take advantage of the dynamic forces during a turn, can you do it in a way that won't be too application specific? IE a one corner one loading situation, which is basically what a drag launch is. And what will that feel like once the dynamic situation is over? IE when you aren't at 10/10, will the car give the sensation of increased initial grip followed by breakaway once the dynamic situation can no longer contribute to tire loading?

Norm Peterson
07-10-2013, 05:06 PM
I just found the thread that this discussion was separated out from. FWIW, I'm an engineer by education and profession with a side interest in chassis dynamics, a little bit blunt or stubborn at times (but I've got a fairly thick internet skin). Anyhow, I couldn't read through this thread without commenting. I do have some idea what's going on here, and finding out that my early guess of circle track involvement was on target clears things up a bit. I agree that the technical side there is underestimated - the Saturday night/"aw-shucks" stereotype on the one hand and failure to recognize that there can be advantage to asymmetric car setup (and how to understand it mathematically) on the other.

I'll be posting other comments and questions on this and the original topic, just not sure where yet.



. . .
Sorry, but I've got to agree with Jason on this point. I think there's a little confusion between cause and effect going on - any reduction in roll due to increasing the suspension roll stiffness at one end does not mean that the lateral load transfer off the inside wheel at that end is reduced. IOW, just because the roll looks like there's less LLT going on does not mean that that's what is really happening. It is analogous to squat and rearward load transfer at the drag strip - the "weight transfer" still happens even when the car does not squat at all (in fact, it happens even faster when squat = 0). It's best to think of roll, squat, and all of the other chassis motions as being no more than the generally visible evidence that some sort of load transfer is happening at that instant. I say "generally visible" because it's not guaranteed to actually be visible (as in cases where ~100% anti- geometry effects are present).


I think some of your statements are a little misleading - like sway bars "engaging the inside tires more" - adding, or stiffening a swaybar at one end of car, will increase that end's share of the lateral load transfer distribution in a corner, decreasing dynamic loading on that end's inside tire, potentially to the point of lift - which sounds like the opposite of what you said. Sway bars do not decrease total lateral load transfer (ignoring minor second-order dynamic CG height changes) so they can't add net weight to the inside tires at a given lateral acceleration ("get them more engaged") - only a wider track or lower CG can do that.


“Adding, or stiffening a swaybar at one end of car, will increase that end's share of the lateral load transfer distribution in a corner, decreasing dynamic loading on that end's inside tire, potentially to the point of lift”[/I][COLOR="#0000FF"] … happens only when the car has excessive Roll Angle.
What you're attempting to describe involves a roll angle large enough to displace the sprung mass CG laterally far enough to be significant. While this might be approached by an old Citroen, it is not a reasonable situation for a car that you're attempting to improve the cornering limits and handling behavior for.

There's way too much to deal with all in one bite here, so let's concentrate on the concept that a sta-bar is nothing more than a rotational spring and ignore the rest of post #1 for a moment.
What it is, is a roll moment-resisting spring connected to the chassis (for roll moment input) and to the suspensions (where the roll moment picked up by the bar is reacted by roughly equal and opposite couple forces). In structural-ese, this is a text version of a "free body diagram", and unless I've left something out nothing else matters. Anyhoo, the outboard suspension is pressed down by the downward force and the inboard suspension is lifted up by that side's upward force. There is no other possible answer.

Whether the inboard wheel maintains contact with the ground does not change the fact that a sta-bar always tries to lift its inboard wheel as it plants the outer wheel more firmly.


Norm

Norm Peterson
07-11-2013, 04:24 AM
[B][COLOR="#0000FF"]But, as long as the car we’re tuning doesn’t have excessive roll angle, stiffening the ARB's do keep the car flatter,
Nobody is arguing this.



spreading the load more evenly over all 4 tires, giving the car more grip. In my experience with all forms of race cars, the race team engineers & myself found increasing ARB rate always made the suspension run flatter … and always worked the inside tires more & the outside tires less … but we were not working with excessive Roll Angle situations.

When testing on 7-post rigs, and even more simple pull-down rigs, you can see the increased loading of the inside tires with stiffer ARB rates. It shows up right on the screen with increased wheel load cell readings. Of course, the track is the best test, and more fun.

In professional oval track & road racing, when we're in the "happy window," a slightly stiffer ARB, or a little pre-load, helps us to works the inside front tire … making the car turn better. In oval track racing this is most critical, because the spring packages in front are very soft.
This may be the key to understanding the difference of opinion. I haven't gone through all 30 or so replies yet, but the cornering situation with oval track cars is not one of pure roll. There is also 'heave' loading induced by cornering speed and the angle of the banked turn, which does add load to the inside tires. Yes, this applies to some road course corners and maybe on rare occasions at autocross, but it's outside that which is necessary to understand what a sta-bar does. (Out of curiosity, what do you suspect that the influence of stiffening a sta-bar would be on a road course turn that is either off-camber or crests a hill at the apex instead of being banked into a bowl? One of the road courses closest to me has two cresting turns and one bowl.).


Norm

Norm Peterson
07-11-2013, 04:58 AM
In my profession, we have to be careful not to take 100.00% of engineering training as gospel. It was, after all, developed by human beings based on their understanding of how things work … at the time they developed the principal … with the technology of the day. We occasionally … VERY RARELY … but occasionally run get results that conflicts with what was written by engineers.

This discussion of ARB loading tires is one of those areas.

-------------------------------------------------------------------------------------------------------------------------------

Another is Track Bar/Panhard Bar angle. <snip>
I do not know why the results conflict with the engineering principal. On the track … as on the 7-post rig … Track Bar angle affects tire load. Maybe it's time to update the engineering formula to take that into account.
The engineering formula is not wrong, but it is apparently incomplete as far as modeling what was tested. Track bars (aka Panhard bars) are inherently asymmetric in their behavior. For road course/autoX/performance street cornering purposes the PHB is normally set to be horizontal with the car at rest at normal weight (driver + some fuel load). 4" of pivot height difference is huge, and outside those driving environments. But dynamically it has to have effect simply because the track bar/PHB is no longer strictly a lateral locating device (as is commonly assumed).


Sometimes the formulas are not 100% accurate.
Make that "not 100% inclusive" and I think you'll get more understanding.



An example of a formula that was considered gospel for years, was the quarter mile drag racing power/weight/speed calculation formula.
It is (.00426 x mph)cubed** x weight = HP
**(I couldn't figure out to put the small 3 where it goes.)

If you enter a car with 1000# & 100HP & compare it to car weighing 4000# & 400Hp … both having a 10-1 W/P ratio … the formula provides the same quarter mile top speed mph (106.76 MPH) for both cars, which is then turned into an estimated ET. In the real world, we know that is not accurate. Even though both cars have the exact same weight to power ratio, the lighter car will produce a substantially quicker ET & slower MPH … and conversely the heavier car will produce a substantially slower ET & faster MPH. Said another way … to achieve 106.76 mph … the lighter car will need more than a 100 HP & the heavier car will need less than 400hp.

The formula was created ions ago … and the creators are dead … so we can’t have a beer with them & discuss this. But all indications are they created the formula based on “averages” … and a simple weight-to-power ratio. They did not build into their formula a correct method … or any method … for adjusting for lighter & heavier cars with the same W/P ratio. So the formula is not an absolute. That’s ok. It is still helpful. It’s just not gospel.
As I understand it, the original formula was the result of some curve fitting to actual race or test results. I think it was done at Chrysler, and if so, probably by the Ramchargers drag team back in the early 1960's, possibly as early as the late 1950's. You might try getting in touch with Billy Shope, who is a member here and still alive last I knew, as he'd know more about this than I.

At best, quarter mile predictions based only on HP and weight are approximations. In no way are they truly "engineering formulas".

It is entirely possible to simulate acceleration, which can get you past any "scale factor" difficulties that crude approximations cannot, though they need a whole lot more data. But even those sims need to make a few rather gross assumptions as far as the launch is concerned if you're trying to get a good match with measured short times and ETs.


Norm

Norm Peterson
07-11-2013, 05:12 AM
If we're addressing common fallacies and misconceptions, another one I see:
When one stiffens their shocks, there is no question that the visible transient motions of the car are slowed down. But does that shock stiffening decrease the actual rate of load transfer between tires, or increase it?

The reality is that stiffening the shocks increases the rate (the speed) of weight transfer. It doesn't reduce quantity of weight transfer. However it is very commonly believed (even by a few of the shock vendors selling stuff on this site) that stiffening shocks slows weight transfer, and some appear to believe it reduces the total amount. I believe this misconception goes hand-in-hand with the ones we're talking about above -that reduced roll (and its side view buddies squat/pitch) are indicators of reduced weight transfer. It's based upon a flawed but fundamental belief that the chassis' displacement of each relative corner must be tied, at that moment in time, to the load present upon that corner's tire. Once people get over that, their depth of understanding can rapidly advance.
As soon as you start discussing dampers and load transfer you have to look at what's happening to all of the sources of load transfer over time. LLT through the dampers will peak before LLT through the springs and any sta-bars (and relatively long after LLT through the roll centers), so it is somewhat indeterminate when and at what suspension displacement the total LLT actually peaks. I'm guessing that when the total LLT has essentially peaked is when you might define the car as having taken a set.


Norm

Norm Peterson
07-11-2013, 05:26 AM
Let's use this Mitsu Evo instead

78475

Weighs 3550#, has a track width of 60.8”, a CG of 17” & pulls a g-force of 1.2.

The formula says (1.2g x 3550# x 17”cgh)/60.8”tw = 1191# of weight transferred from the right side … to the left side tires. But the right side of the car weighs 1775# … and it’s in the air … and all of the weight of the 3550# car is on the left side tires.

How do we use the formula to make that compute?
It is not a pure steady-state cornering condition, so a static analysis is not sufficient. The car is also undergoing braking, and the LF contact patch has migrated inward (effectively narrowing the track slightly). It is also being pitched - the chassis rotational moments of inertia in pitch and roll may no longer be negligible. And tires that can sustain 1.2g will briefly spike somewhat higher (1.4g would not surprise me).


Norm

Norm Peterson
07-11-2013, 06:27 AM
Zing! Duck! Ouch !

Help! I’ve been insulted & can’t get up. :lol:

Jason I know the formulas for calculating g-forces & weight transfers too.

Let’s take the formula for Acceleration Weight Transfer, which is:
(G x W x CGH)/WB = T

Let’s take a real car, like the one in the photo below, that has a 109” wheelbase, weighs 3000#, with 54% on the front (1620#) & a CG height at 16”, launching with 2.0 G’s.

The formula says (2.0g x 3000# x 16”cgh)/109”wb = 881# of weight transferred from the front end … to the rear tires. But the front end weighs 1620# … and it’s in the air … and all of the weight of the 3000# car is on the rear tires. Will you be my huckleberry and educate us all on how that computes?

Feel free to change the numbers around, but keep them realistic.


78466


---------------------------------------------------------------------------------------------


While we’re having fun, let’s take this cool PT car at the AutoX track. (LOVE this car.) :)


78467


The formula for Cornering Weight Transfer is:
(G x W x CGH)/TW = T

Let’s say it weighs 3200#, has a track width of 58”, a CG of 16” & pulls a g-force of 1.2.

The formula says (1.2g x 3200# x 16”cgh)/58”tw = 1059# of weight transferred from the left side … to the right side tires. But the left side of the car … with the driver, weighs at least 1600# … and it’s in the air … and all of the weight of the 3200# car is on the right side tires.

How do we use the formula to make that compute?
Right away I'll question the 16" CG heights and the (peak) accelerations. Corvettes and the like excepted, typical road car CG heights run more like 20" and up (there is a NHTSA inertia database that contains this information for many cars - I'll email you a spreadsheet copy of it if you'd like, as it is way, way too big and too cumbersome to paste here).

It is a gross assumption to use 2.0g for a dragstrip launch, as it doesn't explain how you got there (even if it may be a good enough number for "coefficient of friction" purposes - which I don't know either). With respect to the wheelstand, once the car is in nose-up pitch motion, it has acquired some momentum in pitch, which requires time to be decelerated to zero pitch velocity (which occurs at the peak wheelstanding position). Static analysis cannot account for this. Think of this effect as the car overshooting its statically determined equilibrium pitch position.

Don't overlook the effects of anti-squat, which plants the tires faster and much more firmly than the pitch itself happens and can briefly transfer more load than the static-g acceleration predicts, which implies that still greater acceleration is possible (subject to power limitations). Think in terms of over 100% anti-squat here, which can actually make the rear of the chassis rise as well as the nose. Keep also in mind that the pitch angle itself results in a raised CG once it does occur if there is any anti-squat at all.

A small pitch moment exists courtesy of reaction to the rear tires' rotational inertia that is additive here even though it does not contribute to forward acceleration.



I have pictures of my cars at autocross showing very little load on both inboard tires - on true street tires in one case and on really weenie competition tires (non R-compound) in another.


Norm

Norm Peterson
07-12-2013, 10:26 AM
Ron - this is where the questions I mentioned were coming start. Some are from here and some from SSLance's Suspeneion forum thread, which I'd rather not disturb. Hopefully the text colors will copy over from Word.

Anti-Roll Bar, ARB, Sway Bar & Anti-Sway Bar … all mean the samething.
Let me add stabilizer bar and sta-bar to that list, as that is in line with OE mfr and engineering usage. Sta-bar ismy preferred term (saves a few keystrokes).

ARB Rate = Pounds of torsional force to twist the ARB 1”
I'm assuming that this follows the bar tables rated in lbs required for 5°bar twist (and which assumes the arms do not deflect at all).

Rise = can refer to either end of the car rising up.
Normally I'll use the terms 'rise' and 'anti-rise' for the front only and 'lift' and 'anti-lift'for the rear. Saves confusion and a little more typing.


5. Force (weight & weight transfer)applied to a tire adds grip to that tire.
True, with an important qualification. Grip does not increase as fast as load is added. I've seen grip estimated as the 0.7 power of tire load. IOW, if you start with a tire that has 1000 lbs load and develops 1000 lb of grip (1.00g), if you then increase the load to 1100 lbs the grip only goes to about 1069 lbs (0.97g). This has individual tire implications as well as tire pair total grip consequences.



6. Anti-Roll Bars primarily control how far the front or rear suspension (andtherefore chassis) “rolls” under force, and only secondarily influences therate of roll.
By 'rate of roll' do you mean chassis rotational velocity in roll (degrees per second of chassis roll)?


Stiffer bars reduce roll angle, engagingthe inside tires more.
I think this has to either involve heave as well as roll or coil bind on the outside. This is a"nonlinearity" that a simple look at sta-bars in isolation cannot account for. A good engineer recognizes this, though it would likely escape somebody with "just a little book knowledge".


10. The front tires need force, from weighttransfer on corner entry, to provide front tire GRIP. Too little & the carpushes … too much & the car is loose on entry. The rear tires need force,from weight transfer on corner exit, to provide rear tire GRIP. Too little& the car is loose … too much & the car pushes on exit.
Are you referring to forward load transfer on corner entry and rearward oncorner exit?



12. Softer springs allow more compression travel & therefore more forceonto the tire … for MORE GRIP on that tire ... and less grip on the oppositetire. Stiffer springs reduce compression travel & therefore lessen forceonto the tire … for LESS GRIP on that tire ... retaining more grip on theopposite tire.
Here I think we're looking at an effect from very different perspectives. What I see with softer springs is greater overall mechanical grip due to smaller variations in vertical tire loading. It's the same thing as I mentioned above to "5. Force" (weight & weight transfer), just that here it is in response to normal pavement bumps and roughness rather than such intentional changes in tire loading as those coming from driver control inputs or weight physically added or subtracted.




13. Optimum roll angle works & loads both sides of the car’s tires “closer toeven” ... within the optimum tire heat range … providing a consistent"long run set-up" & optimum cornering traction throughout amulti-lap event. Long run set-ups typically take ˝ a lap to a whole lap for thetires to “come in” 100%.

14. Higher roll angles work better in tight corners but suffer in high speedcorners. Lower roll angles work better in high speed corners but suffer intight corners. The goal on a road course with various tight & high speedcorners … is to find the best balance & compromise that produces thequickest lap times.

15. Too much roll angle overworks the outside tires in corners & underworksthe inside tires. Too little roll angle underworks the outside tires in acorner. Excessive roll angle works the outside tires too much … may provide an“ok” short run set-up … but will be “knife edgy” to drive on long runs. Thetire heat up quicker & go away quicker. If it has way too much roll angle …the car loses grip as the inside tires are not being utilized.

16. Too little roll angle produces less than optimum grip. The car feels“skatey” to drive … like it’s “on top of the track.” The outside tires are notgetting enough load from force, therefore not gripping enough. Tires heat upslower & car gets better very slowly over a long run as tires gain heat.
I think I need some help here. Is this purely circle track related? I do understand that too stiff (particularly with too much bump damping) will skate.



Norm

Norm Peterson
07-12-2013, 10:30 AM
19. Goal: To have optimum grip on all tiresand disengage the inside rear tire (to a degree) to turn well … then re-engagethe inside rear tire (to a higher degree) for maximum forward bite on exit.
Isn't this commonly accomplished via bump damping at the inside rear shock?


Let’s expand on #1 …
You have 4” less tread in the front. That’s a 20.4% static tread difference.That’s 20% LESS grip for the front tires, than the rear.
1. We can't make a car optimum byreducing a positive quality (grip/tire traction) in one area to balanceanother weak area. Sometimes you have to … but my first inclination is toINCREASE the positive quality(grip/tire traction in this case) … in the weakarea.
Agreed. Fix the end that needsfixing.


In production based cars, where the majority of the weight mass is on the fronttires, usually around 52-56%. The last thing we want to do here is make thefront tires have less grip than the rear. If it didn’t look completely dorky,we would put wider tires in front than the rear, to help make up for the poorweight balance.
This has actually been done - I think on at least one of GM's V8-poweredtransverse FWD cars. I remember this because of its oddity with respect to "convention".


In fact, not only is the tire’s contactpatch with the road reduced, but you’re on the sidewall. Tire sidewalls are just like a spring .. really ... justlike a spring. I use an Intercomp “Tire Sidewall Spring Rater” to learn thespring rate & characteristics of different tires. And they DO DIFFER.
Sidewalls make lousy treads, and don't even gain you area or width as fastas you lose useful tread area/width. FWIW, I've heard (reliably) that grip is something like an 0.15 powerfunction of tire width (I assume that meant an individual tire's tread width,but it's rough enough that using nominal section width is probably nearly asgood).



The problem is …
When the KPI/Caster Split favors the KPI …

I get what caster is and that it makes thewheels want to center themselves more but what other effects does it have?
Caster ... creates Camber as you know it ... but only when the wheels areturned. As you know, to achieve caster, you're moving the upper BJ back, and/orthe lower BJ forward to "lean" the spindle back.

This specific problem is generally causedby too much KPI/Caster Split … meaning the degree of KPI angle is muchgreater than the degree of positive caster. The greater the KPI/Caster Split …the more the top of both front tires tilt the WRONG direction when turned. Theamount of camber error is minimal with slight amounts of steering input onlarge sweeping corners, but grows exponentially worse with higher ratesof steering input (front wheel steering angle) on tighter corners.
I think this boils down to caster → negative camber on the outside tire and positive camber on the inside tire as you steer, while KPI → positive camber on both tires. It's related to how caster can be computed by sweeping the steering from x° right to the same x° left. Somewhere I may have a spreadsheet I worked up showing camber as a function of caster, KPI, and steer.


When you …
a. Stiffened the front spring rates … from 450# to 550# to 575# to 700# …you were reducing front end suspension travel and therefore reducing weighttransfer & loading force onto the front tires … reducing the traction ofthe front tires in cornering situations.
I'm going to say that more lateral load was transferred up front, reducingoverall the total grip of the two front tires taken together. Ultimately, it's still a loss of frontlateral grip, just looked at from different perspectives.


b. Increased the rate of the rear ARB … combined with stiffer springs ... madethe made the car run flatter … making it harder for the inside rear tire todisengage on corner entry ... making it easier for the big rear tires tooverpower the much narrower front tires.
c. Decreasing the rate of the front ARB would normally allow the car totransfer more weight off the inside rear tire & onto the outside front tirehelping it to turn … unless the car rolls so much it also unloads the insidefront tire too. But the front springs were too stiff, not allowing muchtransfer … and the contact patch got even narrower as you turned the wheelfurther, due to geometry.
I sense a banked turn being visualized here.



Norm

Norm Peterson
07-12-2013, 10:34 AM
Finding the “happy window” for the Roll Angle is key.

Your spring & ARB’s combine to create a term called Total Roll Stiffness,discussed in post #21. The optimum TRS is different for different tracks and/orcorners. Softer TRS work better in tight corners but suffer in high speedcorners. Higher TRS work better in high speed corners but suffer in tightcorners. The goal on a road course with various tight & high speed corners… is to find the best balance & compromise that produces the quickest laptimes.

Your goal with the ARB’s is to achieve a front Roll Angle that keeps both fronttires working optimally … and a rear Roll Angle that allows the IR tire todisengage “slightly” … on corner entry & middle … and re-engages the IRtire more on corner exit.

The term for this is Roll Couple. Although the term sounds different … it issimply breaking the TRS into front & rear %. You want the Front Roll Couple% (front spring & ARB combined stiffness) to be higher than the Rear RollCouple % (rear spring & ARB combined stiffness). Most race engineers willtell you 5% more than the front weight percentage is a good place to start. Inyour car, with 51/49 F/R weight … that means a good baseline would be 56% FrontRoll Couple % and 44% rear … adding up to 100% of the TRS #.
Roll couple is the term I'm much more familiar with. AKA roll stiffness distribution (what the suspensions see), which is not the same thing as total lateral load distribution (what the tires see). I know that Bob Bolles continually mentions the front and rear WANTING to roll differently - is your usage essentially the same as his?



Don’t get too lost in the numbers or terms.They’re just a starting point anyway,
This ↑↑↑ . . . . . eventually you have to strap yourself in and drive the thing.



Basic Tips:
If the car has too much Roll Angle … use the right tool … and increase the ARBsize … keeping the front ARB rate significantly stiffer than the rear ARB rate.If you think you need to increase the rate of the rear ARB to near the rate ofthe front ARB, you have other problems.

If the car is loose on entry … consider the Roll Angle first … and if it’s good… then stiffen the front spring rate. If it is tight/pushy on entry … considerthe Roll Angle first … and if it’s good … then soften the front spring rate.

If the car is loose on corner exit … consider the Roll Angle first … and ifit’s good … then soften the rear spring rate. If it is tight/pushy on exit …consider the Roll Angle first … and if it’s good … then stiffen the rear springrate.
I think what you're saying is to fix looseness with the bars if the roll angle isn't right, but with the springs otherwise.




The key difference between the two ... isadjustability of the front Instant Center, rise leverage & anti-squat.

Torque Arm suspensions are common as a factory style rear suspension in somecars, because they are the simplest of the designs, allow a high degree of rearend articulation & can take high shock loads from hard launches. TheirInstant Center, if designed properly, is automatically in a "good"spot ... and can be made "a little" adjustable ... but offer theleast adjustability of the common PT rear suspension designs, for tuning thefront Instant Center, rise leverage & anti-squat. These make a great allaround suspension for the person that doesn't want to tune much.

3-Links can handle drag racing up to a point, but it wouldn't be my choice ifthe car was planned for high hp, high rpm, clutch dropping, slick running,wheelie pulling launches ... as there are only 2 rod ends "pulling"to lift the whole car.

Centered 3-Links are common in oval track racing & offset 3-Links are verycommon in road racing, especially in full body cars like GT1 & the Trans Amseries, because they allow for a high degree of articulation & are highlyadjustable & tunable for track conditions. You also see them a lot on topAutoX racers. 3-Links can be very adjustable if designed & installed withmultiple or variable mounting points. Many "street kits" are soldwith little or no adjustment to protect non-tuning novices from themselves.

There are pros & cons both ways. If designed right, Torque Arm suspensionsmay not be optimum, but will always be in the ball park, and are difficult, ifnot impossible, to tune yourself out of the happy window. If you know chassisset-ups & tuning ... or plan to learn ... and want a rear suspension with ahigh degree of tuning adjustability, to fine tune & optimize your rearsuspension's performance, the 3-Link is the better choice. If not, it makesmore sense to go with the Torque Arm suspension.

Personally, for my car, the choice is anadjustable 3-link for AutoX, Road Course track days & street driving.
We're right on the same page here, despite all the little differences along the way. The opinions of an individual or two running Cortex torque arms on the current Mustang chassis notwithstanding.



Norm

Norm Peterson
07-12-2013, 10:40 AM
-When you have the outside front tire rolled over & bound up, it is overloaded … and it will break free … and hop… then regain traction … and continue this break free-hop-regain traction activity … although it is lessening each time as the car scrubs off speed …until the speed comes down enough that the front tires can hold.

How do you know when this is happening? What are they symptoms?
If it is minor ... you'll feel & see the car pushing from inside the car... and a trained eye or video camera can catch it from outside the car. If it is significant, you'll also feel it in the steering wheel.
I'm not sure I've ever experienced hop while cornering. I assume that shock bump damping gets involved here. Is it also a function of wheel width relative to tire size?



"Wedge" is a Stock Car term meaning the same thing as "CrossWeight" used by Road Racers & Open Wheel Racers, even in oval racing. Some racers simply shorten it to "Cross" ... as in "put some cross in the car". Then throw in a mix of Engineers and it's a wonder any of them understand each other.
This . . . if the engineer is unwilling to work the translations.


Oval track racing is way more challengingthan outsiders know. I love all forms of motor racing ... but I prefer roadracing the most.
The fact that the only serious turns all go the same way means that suspension asymmetry can reap benefits. Which in turn means that symmetry-based thinking doesn't work well, or even at all.


With about a degree of static negativecamber I was still chewing up the outside of my front tires.
This is partially a Roll Angle issue … to be worked out with springs &sway bar … and partially a KPI/Caster split issue.
I've probably mentioned to SSLance some camber settings that I'd used on my roughly similar 1979 Malibu when I autocrossed it, and I did post a couple of approximate alignment numbers in his current Suspension forum thread. What I didn't mention there was that I'd swap between street (-0.7°-ish) and competition settings (beyond -2.5°) at the lot. Shim-adjusting is about as dead-nuts reliable as it gets. That toe varied outward as camber was pulled further negative was a helpful convenience. The OE camber was some small but silly positive value that was never used after I aligned the car the first time (and there's a little story behind that).



The discussion about springs is quite involved. More than most people would think … with something as seemingly simple as springs.

This is going to be controversial … because there are new* concepts … and there will be people who have run traditional** spring packages for decades … that have worked well for them … they have been successful with traditional** spring packages … and they don’t understand the new concepts & technology.
This has to be headed off in the Big Bar/Soft Spring direction at minimum.




Weight transfer: Speaking literally … weight doesn’t transfer. I mean weight doesn’t unbolt itself and move around the car, re-attaching itself somewhere else just because you jumped on the brakes … I mean other than your coffee cup.

What is really happening is the car’s weight mass at the CG is acting on theroll & pivot axis of the car and applying “Force” when the driver tries toget the car to stop, turn or accelerate.

Calling it “Weight Transfer” is not technically correct, but it is simple & easier for most of us to understand... so we’ll call it Weight Transfer or WT for short.
This ↑↑↑ . It's something that us engineers tend to be fussier about and use the term Load Transfer instead. Though all fluid slosh and loose cargo and/or passengers floating/stretching about the interior really does count as "weight transfer" :naughty:



In most every situation in life &racing there are “exceptions to the rule” … this is one of them. What you read is not a typo. As you accelerate out of the corner, the inside rear spring WOULD allow weight transfer from the outside front corner to the inside rear corner … IF THE FORCE was going that direction. But it’s not. The Force … when accelerating out of a turn, while still turning … is to the outside & rear. So there is NO FORCE pushing the car onto the left rear … yet. BUT … for optimum acceleration you still need to utilize all the potential grip available with the inside rear tire.
Doesn't the inside rear get involved to the point that the resultant force is no longer purely lateral to the car but has picked up a smallish rearward component? IOW, isn't there some force change at the inside rear, just nowhere near what it'd be once you're fully straightened out?



Norm