Application specific suspension.....

[SN65]

OK... I typically keep to my own posts, my own topics, my own threads. Something I have been reprimanded for.

That being the case, I have been following other threads and trying to add my two cents when I think I can be of help.

In doing so, I have noticed one thing. People don't seem to ask many questions about the proposed vehicles application before they comment on a thread.

Maybe I am missing something here. Maybe it is generally understood that if they are posting on this site that the car is intended for road racing or a road racing type application.

Is this true?

Also, even if this is true, I would think that there should be more questions asked than comments made.

For example. there is another thread in this section asking about the best suspension, but no one asked any questions about the fellows car. Maybe everyone knows this guy and his car and his intended use, but I had no clue.

Is a watts link always better than a panhard bar? Maybe yes, maybe no. Depends on the application.

Shouldn't we find out at least some basic information before we make any recommendations?

[parsonsj]

Bob,

You're right. Intended application, budget, fab skills, toolset, etc. are all relevant questions that apply to the basic (and oft-repeated) "What suspension should I use" question.

In general, though, most assume that members who post here are looking for insight and help for road-racing, autoX, and street performance. This is Pro-Touring.com, so it's a reasonable assumption. It's an exception when someone asks about drag racing suspensions, other than to ask how their ladder-bars will work on the street or some such.

jp

[SN65]

As I look through more and more posts I am finding that the more experianced person posts at least some data about his intent. It is the newcommer that askes questions witout at least some prelem explanation.

OBTW, congrats on SEMA. Is this going to be your first time displaying there? Your first time with MagnaFlow?

[Marcus SC&C]

Bob, John pretty much hit it on the head. The primary focus of this forums is to bring the performance of older muscle cars up to par with modern performance cars. Your SN65 project fits right in. The hardcore cars take things another step and are basically a cross between a modern road race car and a vintage musclecar. There are a lot of questions to be asked and it`s hard to cover all of the countless details in a discussions forum "Post It Note" format. You may notice I often tell folks to just call us if they want to so we can cover things in much more detail. That said forums like this one are still an invaluable resource and a great sounding board for ideas and combinations. Another useful tool in the box. Mark SC&C

[mpozzi]

I'll always ask about the intended use of the car in question unless it's specified in the thread . . .

Makes answering questions that much easier.

Cheers,
Mary Pozzi

[parsonsj]

As I look through more and more posts I am finding that the more experianced person posts at least some data about his intent. It is the newcommer that askes questions witout at least some prelem explanation.That's a good insight. And it goes right along with maturity levels as people grow up.
OBTW, congrats on SEMA. Is this going to be your first time displaying there? Your first time with MagnaFlow?Thanks! Yes, it will be my first time with SEMA. The folks at MagnaFlow are great to work with, and I'm really looking forward to the show.

jp

[ss dave]

Bob and jp I really appreciate your thoughts and comments in my thread. Bob was the first to question my set-up and goals and made some insightful suggestions: Take it slow, and change things one at a time, amoung other more specific suggestions. In the future when I post I'll be more specific about my app and goals, probably save alot of time! However I found that posting a general thread gets alot of different ideas and comments and allows me to pick up on things from many directions and philosphies. I've learned alot and its all good! Others will be asking these same questions in the furture and will benefit from these posts. I did a search and could not find exactly what I wanted, until the last couple days. It sure beats bugging people on the phone; and they reply because they're interested, have a good comment and the time. Thanks again for the insight, I'm still weighing options and the thread was PRICELESS!

[SN65]

Bob, John pretty much hit it on the head. The primary focus of this forums is to bring the performance of older muscle cars up to par with modern performance cars. Your SN65 project fits right in. The hardcore cars take things another step and are basically a cross between a modern road race car and a vintage musclecar. There are a lot of questions to be asked and it`s hard to cover all of the countless details in a discussions forum "Post It Note" format. You may notice I often tell folks to just call us if they want to so we can cover things in much more detail. That said forums like this one are still an invaluable resource and a great sounding board for ideas and combinations. Another useful tool in the box. Mark SC&C
Hi Mark,

Thanks for the kind words. The SN65 project is still rolling along, but, as many on this forum have pointed out, is old news. It is a shame that most people could not look past the factory apperance of the car and see the unique things we did to the unibody structure.

But that is going to be the drum I beat for a long time.

:pat: :spank2: :box2: :bsjerk:

In order for all the cool suspension and engine packages to work properly, you need to look at the chassis or in most cases with 60/70 muscle cars, the unibody structure. If you do not invest some time and effort towards improving the overall structure first, all the fancy drivetrain and suspension components will not function to the level that they were designed.

[High Plains Mopars]

Very true, to make accurate recommendations, the specific intended purpose is very helpful. However, I also agree that the intent of the forums here are already fairly clear and we can make the assumption that improved performance is the goal of most questions.

I also see a vast majority of cars built this way are never intended to be a "maximum output" application, nor will they ever be driven in a manner where that level of performance is required. So long as they look better, perform better, and/or use the latest go fast tricks, the vast majority are satisfied. Maximized performance within the confines of a rule set usually isn't the goal here.

Honestly now, there is no road racing equivilent to bracket racing and scca rules are so restrictive that almost any pro-touring style car would be put in a class so high that it would be uncompetitive in most cases. So most "pro-touring" style mods are inteded to be primarily for driver pleasure. As such, fully maximizing the performance of most peieces is not always necessary for most drivers. They just need to be better than what they replace.

[Marcus SC&C]

Bob,a really well thought out and executed performance car is never old news,not in my book anyway.
You`re right the chassis itself is often overlooked. There has been some good discussion here on the topic though. Subframe connectors,firmer/solid body bushings/subframe bushings and cages are fairly frequent topics. Not too many people beefing up the unibody itself although that certainly has a lot of merit as well. I`ve done some of that on Mustangs,box Novas and Ramblers in the past myself with good results. Frankly most folks just don`t think about it much. Mark SC&C

[SN65]

Bob,a really well thought out and executed performance car is never old news,not in my book anyway.
You`re right the chassis itself is often overlooked. There has been some good discussion here on the topic though. Subframe connectors,firmer/solid body bushings/subframe bushings and cages are fairly frequent topics. Not too many people beefing up the unibody itself although that certainly has a lot of merit as well. I`ve done some of that on Mustangs,box Novas and Ramblers in the past myself with good results. Frankly most folks just don`t think about it much. Mark SC&C
Hi Mark,

Thanks for the kind words.

It is kind of sad that, as you say, "...most folks just don`t think about it much". Maybe it is because they don't realize just how weak the original unibody designs were/are. Maybe they think that updating the basic mechanical assemblies will be enough. Maybe they are scared to attempt any structural modifications. Maybe they just don't care....

I think that people are blinded by the fancy engine, brake and other mechanical assemblies that they are bolting onto their project car. It is fun shopping for the pretty wheels, etc. There is no glammer in chassis modifications. You can't see a chassis modification. You can't show it off to your freinds at the local cruise night.

The unibody / chassis is where the whole "pro-touring" proccess should start. Unfortunatly, those who don't invest any time or effort into the basic structure of these old muscle cars will never know what they are missing.

[parsonsj]

The unibody / chassis is where the whole "pro-touring" proccess should start. Unfortunatly, those who don't invest any time or effort into the basic structure of these old muscle cars will never know what they are missing.That's true. I remember going through turn 1 at Road Atlanta with David Sloan (my passenger) shouting "My next car has to have a full frame!".

Torsional stability and reduced body flex makes the suspension engineering a lot easier. When you know that any movement of the suspension is just due to the suspension (and not the body moving) it allows better tuning.

Now ... that being said ... I think it's doubtful that handling differences are measurable without the resources of a full-on race team behind you. Us engineering geeks are intrigued and interested, but there aren't any hard criteria or feedback. We're talking ass-o-meter differences, with all of its confirmation bias problems, which makes these exchanges hard to have in a meaningful dialogue.

jp

[High Plains Mopars]

Now ... that being said ... I think it's doubtful that handling differences are measurable without the resources of a full-on race team behind you. Us engineering geeks are intrigued and interested, but there aren't any hard criteria or feedback. We're talking ass-o-meter differences, with all of its confirmation bias problems, which makes these exchanges hard to have in a meaningful dialogue.

jp

I disagree. A parking lot, a permission slip, a stop watch and a friend are all thats needed. I also think any track any were would be willing to rent you an afternoon for verification, and many have skid pads available as well.

However, even with that effort, there is still a lot of "paper engineering" going on by the masses that isn't necessarily bad. If Detroit Speed tests and verifys their products, it is reasonable to assume that even if you don't exactly duplicate their application, simply using their parts will generate at least a percentage of their results as well. This means someone is at least starting to look for, and understand the importance of engineering.

That is what I was alluding to earlier. Most enthusiasts aren't looking for optimization, they simply want it to be better and easier, and better looking, than it was originally. Optimizing the stock stuff is a butt load of work. Buying a new subframe and bolting it on is easy. As an example, lets look at the trans am cars from 1970. These cars were the pinnacle of stock class racing handling then, and are still pretty hot stuff even today and can turn laps that are still sizziling fast. They all used optimized stock suspensions. Many pro-touring cars of today would still be hard pressed to keep pace with a 37 year old race car, even with a hydro-formed subframes, three links, coil overs, 14" brakes and 17" rubber.

Similarly, the tubular suspension, welded seams and braced sub-frames of most pro-touring cars would put them in an outclassed position in any scca meet, which is probably the closest thing to regular guy racing most of us can come for a handling application.

But I agree with Marcus, those of us who know the time, energy , and effort that goes into optimization will never overlook a well thought out and engineered car, even if it is less flashy than the many other sittnig around it. However, I suspect we are the vast minority of the pro-touring subset of the whole automotive hobby. But come on, is that really any different than society as a whole? In general, society is a lot about looks ahead of function. Just watch any episode of a reality tv show to confirm that.

[gt1guy]

Just thinking about the flood of subframes for a moment. How many companies do you think actually test and verify the product? I've seen more than a few that look like it would actually make the car handle worse. Ridiculously short control arms come to mind. It seems today the end user really needs to be on his game, just to keep from getting burned.

Kevin

[parsonsj]

I disagree. A parking lot, a permission slip, a stop watch and a friend are all thats needed. I also think any track any were would be willing to rent you an afternoon for verification, and many have skid pads available as well. I really doubt that you can measure the difference bracing your unibody makes with a skidpad or parking lot autoX. So you go out there on Saturday morning, run your stuff, then weld in some flex reducers and go back out the next day and see if your time is lowered? I'm skeptical the average guy will be able to see or measure any difference beyond normal sampling error ... I could be wrong though.

jp

[High Plains Mopars]

Okay then. How about XV's four post test rig results? While mopar specific, it does demonstrate the differences in effective unibody reinforcement that just a few pieces can have compared to stock. For a mopar, this is only 5 pieces; 2 subframe connectors, 2 shock tower braces, and one radiator support yoke. They also translate how this hard evidence stacks up against lap times and skid pad testing, in this case, that equals a significant seat of the pants as well as stop watch difference.
http://www.xvmotorsports.com/engineering/suspension_process.cfm

[parsonsj]

That's what I'm talking about. Genuine data with a chassis jig, load cells, and a disciplined approach. XV seems to be taking the sort of engineering approach that will result in verifiable and repeatable results.

It's just not something the average guy is going to be able to do. The message there is that Mopar guys have a real resource with XV, and one should talk with them about chassis stiffening.

The rest of us need somebody to do the same thing with GM and Ford bodies and chassis.

jp

[gt1guy]

You don't measure tortional rigidity on a 4,5, or 7 post rig. It's done a completely different way. Anyone, at home with just a few tools, can measure the rigidity of thier chassis, and the changes that modifications do. As far as the "shaker" rig goes. It was the topic of this months Chassis Newsletter from Mark Ortiz. Hopefully I can paste it here. It's long. I think it worked.

Kevin




October 2007
Reproduction for free use permitted and encouraged.



Reproduction for sale subject to restrictions. Please inquire for details.




WELCOME


Mark Ortiz Automotive is a chassis consulting service primarily serving oval track and road racers. This newsletter is a free service intended to benefit racers and enthusiasts by offering useful insights into chassis engineering and answers to questions. Readers may mail questions to: 155 Wankel Dr., Kannapolis, NC 28083-8200; submit questions by phone at 704-933-8876; or submit questions by e-mail to: [email protected] ([email protected]). Readers are invited to subscribe to this newsletter by e-mail. Just e-mail me and request to be added to the list.


WHAT EXACTLY ARE SHAKER RIGS FOR?

Over the past few years I have seen and heard about all of the developments with 4-post, 5-post, and 7-post shaker rigs. However, it seems that most articles focus on the technology employed by the latest and greatest rig rather than what type of results the rig is going to yield. My question is this, when teams test on a shaker rig, what exactly are they going to get in return in terms of results (i.e. lateral and longitudinal G forces or something else)? Also, maybe even a better question, what is it they are testing for (i.e. is there a way these rigs measure “grip” in particular) and what type of testing regiment is going to be employed? For instance if you are a team that is on a limited budget and can only afford to put your car on the rig once per year, do you put the car through a generic set of accelerations and simulated corners or do you try and tune for something more specific or maybe even a particular track? And lastly, at the end of the day how do you know what was the better setup (if there isn’t a way to measure “grip” specifically)?

Shaker rigs are an example of street driving improving the breed for racing. They originated in passenger car development, and were later applied to race cars. The original objective was to explore the car's response to excitation at the wheels, in a more controlled and observable situation than could be achieved on a test track. Improving handling wasn't the main goal. Engineers were more concerned with isolating frequency-sensitive effects that would impact durability and noise, vibration, and harshness (NVH). Does a portion of the roof or the floor pan resonate at a certain frequency? Are there brackets or ducts that buzz or might break? Does anything rattle? At what speeds and frequencies do the wheels "dance", and does this agree with calculated natural frequencies?

Early shaker rigs had posts only under the wheels, and the sprung mass was allowed to do whatever it wanted to do in response to excitation at the contact patch. It soon became apparent that it would be more desirable to allow the contact patches to float both laterally and longitudinally, so that the track and wheelbase could vary with suspension movement, without having tire scrub fight the movements. A fifth post attached to the sprung mass allowed the car to be anchored horizontally


while all four contact patches could float. The car still needed either an anti-rotation feature in the fifth post, or some other mechanism, to constrain the car in yaw.

This was good for replicating highway driving, but it was impossible to explore suspension behavior in conditions of sprung mass displacement caused by aerodynamic loads, banked turns, or pitch and roll due to longitudinal and lateral acceleration. To reproduce these displacements, three posts were attached to the sprung mass. Conveniently, three posts also constrain the car in yaw, without any additional devices. With this addition, it is possible to reproduce any combination of suspension displacements that the data acquisition system may have recorded on-track, or that an engineer might imagine.

Importantly, this does not mean that the rig reproduces all the forces acting on the suspension. The posts only move vertically. The rig cannot apply or reproduce any horizontal forces. It also cannot measure any horizontal forces. Therefore, we cannot measure grip at all. We also cannot reproduce loads and frictional influences in the suspension that result from horizontal forces at the contact patches. We do not even know vertical wheel loads with any accuracy, because horizontal forces at the contact patches affect these.

This last phenomenon is particularly significant in stock car rear suspensions, and in any suspension with large geometric "anti" effects. (Stock car rear suspensions, and most beam axle suspensions, have ample geometric anti-roll.) With formula cars, where there is modest anti-roll and anti-pitch, the wheel load values on the 7-post are closer to reality, but still not highly accurate.

I am referring here to individual wheel loads. On the rig, we do get reasonably accurate total loads for all four wheels, and for the left, right, front, and rear wheel pairs. It's the loads for individual wheels and for diagonally opposite pairs that are inaccurate.

Even though we do not read accurate individual wheel loads on the shaker rig, we can get a reasonable comparative evaluation of how much these loads vary dynamically in the excitation conditions that we test. What we are after is minimum load variation. We cannot measure grip itself, but we do know that variation of normal load is bad for grip, and minimizing such variation will improve grip.

This is true for two reasons. First, if the tires unload for any significant length of time, the car is limited by whatever grip the tires have at that load. If the grip limit at that load is exceeded, the car will break traction, and once traction is broken it is hard to regain. A slide, once initiated, tends to persist.

Second, even if we are dealing with load variations over small time spans, so that vehicle inertia masks the intervals of low load and grip, we do not fully recover during the highly loaded periods what we lose in the minimally loaded periods. Why? It's our old friend, load sensitivity of the coefficient of friction. Adding 100 pounds of load doesn't help as much as losing 100 pounds of load hurts.

So we try different combinations of shock valving and springing, and we try to minimize load variation at the wheels.

I was asked recently how you valve shocks based on suspension displacement traces provided by data acquisition. I had to reply that I don't know of any way to do that, and I don't know anybody who claims to be able to do that. However, with the use of a 7-post rig, we do at least have a means to make gains by trial and error.

There is another kind of test rig that does let us look at the effects of horizontal forces at the contact patches. It's called a kinematics and compliance tester, or K&C rig. Until recently, only passenger car manufacturers had these, and nobody was offering testing by the day or hour to racers. There is now a K&C facility available to the motorsports community, just up the road from me in Salisbury, NC. It's called Morse Measurements. The people who run this facility are friends of mine, so I'll give them a plug. They are at www.morsemeasurements.com (http://www.morsemeasurements.com/) or 704-638-6515.

What the K&C rig does is grab hold of the frame of the car, and then apply horizontal loads at the tire contact patches. The sprung mass can be kept from moving, and the changes in wheel loads measured that way. Alternatively, the sprung mass motion can be controlled to keep the total normal force at the contact patches constant and we can measure the displacements and load changes that result. Roll and pitch moments are applied to the frame, based on an input sprung mass c.g. height. Individual wheel horizontal forces are applied based on estimates, which in turn are based on tested or estimated tire properties and calculated wheel loads.

We then see from the test results whether our estimated wheel loads approximate those produced in the test, and we may want to adjust our contact patch forces and do another iteration on the rig.

There are other tests on the K&C rig as well, including simply cycling the car slowly in heave through its suspension travel and measuring the camber changes, contact patch scrubs, and wheel loads, with the contact patches allowed to float horizontally so that all horizontal forces at the contact patches are eliminated. This test can be done on either a K&C rig or a 7-post, so it can be used to compare the agreement of the two types of rig.

What the K&C rig does not do is move the chassis fast. The 7-post can load and move the chassis at racing speed, but only up and down. The K&C can load and move the chassis in all directions, but only slowly. At this writing, there is no test rig that moves and loads and measures a car in all directions at once, at realistic on-track speeds. And even the low-speed testing done on the K&C rig uses estimated or assumed grip levels at the contact patches.

We do have a test that does measure grip: the skid pad. It measures average grip over a lap, by stop watch, or over smaller intervals, using accelerometers or other sensors. Of course, we have difficulty observing the car during testing, because the car is in motion rather than on a test rig. And the grip measured is only on a particular surface, on a particular radius.


We thus have three useful methods of testing, all of which provide useful comparative measurements, but none of which allows us to fully replicate track events in a controlled environment. I could even say we have four methods, if we include wind tunnels, or a much larger number if we include component testing devices such as tire testing machines, shock dynos, and engine dynos. But so far we have no device that really puts the car through its paces in a manner that replicates in full what happens on the track, on a stationary platform where we can safely watch and measure it. The best we can do is look through a number of windows, each of which affords a view from a different angle, and use our mind to try to put these glimpses of reality together.

[SN65]

You don't measure tortional rigidity on a 4,5, or 7 post rig. It's done a completely different way....

....SNIP
Testing for differences in torsional ridgity is similar to testing engine modifications on a dyno. Dosn't really matter what dyno you use (as long as it is the same dyno for every test) and what the nunbers are, it is the changes in the numbers, as modifications are made) that tell the story.

Yes, I know that the test bed we use on our mustang is not the same that is used for testing torsional ridgidity on a NASCAR chassis. But if we keep the setup the same on every test, we can measure the change and determine if any improvements have been made.

On our primative test bed, we have measured a reduction in torsional deflection of more than 1". We went from 1.3" of deflection on a stock unmodified chassis to less than .25".

Tell me that this type of change, regardless of the suspension components used, will not be noticable in both the seat of the pants and also at the track.

[parsonsj]

Tell me that this type of change, regardless of the suspension components used, will not be noticable in both the seat of the pants and also at the track.Bob, who are you asking? Or is the comment rhetorical?

jp

[SN65]

Bob, who are you asking? Or is the comment rhetorical?

jp
Hi JP,

No one in particular. it was more of a rhetorical comment. That is, unless someone is in disageement.

:poke:

[parsonsj]

OK. I don't disagree, but I must (sorry ... it's this engineering background thing again) ask for some measure of quantity. In other words, how much? You've cut torsional deflection to 25% (or so) of the factory value. In and of itself, it is meaningless.

Any idea how it will affect spring rates? (less body movement means more suspension movement for a given input load) Shock valving? (shocks have to handle more direct motion)

Any calculation on a resultant improvement on lateral acceleration?

Don't get me wrong, I'd make those same improvements in a heartbeat ... I'm just wondering about the measurable effects on a car's performance.

I remember somebody once telling me that their sub-frame connectors made the car "feel a foot shorter". That's great, but what does that really mean?

Thoughts?

jp

[JMarsa]

I'll jump in...I'd like to be able to quanitfy the results of my "engineering" changes but like many, unsure exactly how to do that with the tools I have at home.

For example, I've seen the "tortional" test of attaching a lever to a chassis (width-wise) and measuring the results with a dial indicator from a fixed base.

Should that test start with a stock subframe?
Should it then be attatched to the car?
If so, what body mounts ?

If there is an improvment what does it prove?

What I'm getting at is what's already been brought up here, what standards used and what do the results really say?

--JMarsa

[David Pozzi]

A stock 73 Camaro I tested in running condition had (from memory 4000 ft lbs/deg). We added the Herb Adams style braces from firewall to upper A arm bolt on each side, and upped torsional rigidity to 6000. It was an improvement that was felt by the driver and the feeling was described as quicker response to steering inputs.

I've heard 10,000 ft lbs/deg mentioned as a good goal for a tube chassis race car. I also read a blurb about the new Aston Martin model having 12,000 ft lb/deg stiffness. I'd like to know what a new Z06 is, the only figures I've seen are in percent of improvement over the past models.
David

[SN65]

OK....

For working in engineering for almost 30 years now, my basic math skills suck the big one. :-(

I am applying 750 lbs of preasure 2.5 feet off axis. (750 lbs up on the drivers side and 750 lbs down on the passengers side).

That comes to 1,875 foot lbs???

I am getting .3 degrees of deflection.

That same setup netted me 2.5 degrees of deflection before I started. If all this is right I would end up with 750 foot lbs to move the unmodified chassis 1 degree, and that just does not sound right.

So, I don't think I can assume that increasing the force to 5625 foot lbs will get me 1 degree of deflection.

With that said, I think that I am going to have to increase the presure to whatever it takes to get 1 degree of deflection and see what the reading is at that point. I have noticed that the preasure required to twist the chassis is not a "linear" curve. It takes more and more pressure to move the chassis less and less.

Does this sound correct or am I off somewhere????

[High Plains Mopars]

OK. I don't disagree, but I must (sorry ... it's this engineering background thing again) ask for some measure of quantity. In other words, how much? You've cut torsional deflection to 25% (or so) of the factory value. In and of itself, it is meaningless.

Any idea how it will affect spring rates? (less body movement means more suspension movement for a given input load) Shock valving? (shocks have to handle more direct motion)

Any calculation on a resultant improvement on lateral acceleration?

Don't get me wrong, I'd make those same improvements in a heartbeat ... I'm just wondering about the measurable effects on a car's performance.

I remember somebody once telling me that their sub-frame connectors made the car "feel a foot shorter". That's great, but what does that really mean?

Thoughts?

jp

So what do you expect to see?

A 25% increase in torsional rigidity resulted in a 10% decrease in needed spring and shock rates?

or

A 25% increase in torsional rigidity netted a 1/2 second faster slalom time and .25 increase in lateral acceleration?

I think we would all agree that a more rigid chassis allows better control of the suspension components. Are you looking for what does this additional control benefit you in skid pad testing or some other measure?

[parsonsj]

So what do you expect to see?

A 25% increase in torsional rigidity resulted in a 10% decrease in needed spring and shock rates?

or

A 25% increase in torsional rigidity netted a 1/2 second faster slalom time and .25 increase in lateral acceleration?

I think we would all agree that a more rigid chassis allows better control of the suspension components. Are you looking for what does this additional control benefit you in skid pad testing or some other measure?Any of the above, though the 2nd would be better since it is directly tied to performance. We have all been taught that a stiffer body is a better body, but that's the end of the discussion (as far as I know anyway). I'm wondering if there is anyone with direct knowledge of how making the body stiffer makes the car faster with quantifiable results free from confirmation bias ...

As I said, I'm a believer ... I'm just wondering if how much of my belief is mythology and how much is quantifiable.

jp

[SN65]

So what do you expect to see?

A 25% increase in torsional rigidity resulted in a 10% decrease in needed spring and shock rates?

or

A 25% increase in torsional rigidity netted a 1/2 second faster slalom time and .25 increase in lateral acceleration?

I think we would all agree that a more rigid chassis allows better control of the suspension components. Are you looking for what does this additional control benefit you in skid pad testing or some other measure?
I expect to see an improvment in three areas.

1... Ride quality
2... Overall responsivness
3... Better accelleration all around (straight line, lateral and brakeing)

The only way to quantify any of this would be to make all the mechanical changes without any structural changes what-so-ever. Run the car and then make all the chassis modifications.

Unfortunatly, this is not the proper order of things when you build a car. The chassis is always first.

[parsonsj]

this is not the proper order of things when you build a car. The chassis is always first.In our world, sure. We don't have the resources to do before/after/before testing. But if we're buying chassis/body stiffening products, shouldn't the manufacturers of such products provide such data? Or should we just buy the mythology? Stiffen thy body, and thou shalt be blessed. :)

jp

[High Plains Mopars]

In our world, sure. We don't have the resources to do before/after/before testing. But if we're buying chassis/body stiffening products, shouldn't the manufacturers of such products provide such data? Or should we just buy the mythology? Stiffen thy body, and thou shalt be blessed. :)

jp

IMO, most manufacturers don't have the resources to perform such tests nor do their customer base demanded it, so why bother with it since it is expensive and time consuming. XV is the first aftermarket rganization I've heard of conducting some pretty exhaustive testing...which, ironically, a lot of mopar guys I've seen have then questioned the value of. Go figure.

The way I see it, there is a lot of "associative acceptance" going on with this. I.E. the designer is an engineer, their other product line is race specific so they should know what they are doing, feedback on their products is that they work great. Any or all of these three attitudes prevail when contemplating the purchase.

In addition to that is then going to be either A)cost or B) looks. For better or worse, I see the majority of pro-touring cars as looks first, performance second. It is an appearance driven hobby as there is no universal rule book of what consitutes pro-touring or that regulates what constitutes goodness. Now I'm not saying that it isn't performance based, just, like I said earlier in this thread, so long as the result is better than stock, it is accepted as good with little to no proof behind it. Since the vast majority of PT cars are never judged against a stop watch, the majority of owners don't use quantified results as a primary decision driver.

Is that bad? Well, in this case, not really. They do get improved performance. Is it application specific? Well, since the application is improvement in looks and performance and not necessarily against a stop watch within the confines of the rule book, then yes.

[parsonsj]

so long as the result is better than stock, it is accepted as good with little to no proof behind it. I agree. I would even assert there are a lot of things in the aftermarket that results in performance worse than stock that is accepted as good with little or no proof.

This has been an interesting thread ...

jp

[wiedemab]

My take on chassis stiffening has been that it allows you to be in control of the items that have more predictable behavior - ie: springs, shocks, sway bars, and the basic geometry of the suspension.

In the example of 2.5deg of deflection, if you assume that it is rotating about the centerline of the car and the suspension pickup points are approx. 20" from the centerline the vertical deflection for each side is .87" (total of 1.75"). I am very far from an expert, so someone else chime in, but if this chassis flex wasn't included in the original calculations when designing the suspension geometry - I would think that this much deflection could cause some undesired results.

If you lessen this to .3deg for the same torque applied you end up with .1" per side (.2" total).

Could you model the suspension with the pickup points moved for deflection and see how much it affects the design characteristics?????

OK - Hopefully that made some sense, but again - I'm just thinking outloud and that's scary for me!

I agree that this in an interesting topic. I'm sure I'll learn something!

Later,

Brandon

[David Pozzi]

OK....

For working in engineering for almost 30 years now, my basic math skills suck the big one. :-(

I am applying 750 lbs of preasure 2.5 feet off axis. (750 lbs up on the drivers side and 750 lbs down on the passengers side).

That comes to 1,875 foot lbs???

I am getting .3 degrees of deflection.

That same setup netted me 2.5 degrees of deflection before I started. If all this is right I would end up with 750 foot lbs to move the unmodified chassis 1 degree, and that just does not sound right.

So, I don't think I can assume that increasing the force to 5625 foot lbs will get me 1 degree of deflection.

With that said, I think that I am going to have to increase the presure to whatever it takes to get 1 degree of deflection and see what the reading is at that point. I have noticed that the preasure required to twist the chassis is not a "linear" curve. It takes more and more pressure to move the chassis less and less.

Does this sound correct or am I off somewhere????

Only count the 750 lbs once, 750 X 2.5 = 1875 foot lbs applied. Chassis twists .3 deg, 1875/.3 = 6,250 foot lbs per degree. Pretty close to the Camaro I measured after the brace install. I'd say this is in the "good" range for an old muscle car.

I'm assuming the end being twisted is supported in the center and not constrained. I use either a small block of wood on a floor jack, or a piece of angle iron open end down on the jack to act as a pivot.

Measure from where you apply weight, to the chassis centerline in feet, multiply weight applied by lever length to get foot lbs applied. Divde that by degrees or tenths of degrees the chassis twisted = ft lbs/degree.

The stiffness may not be linear but applying the same test torque will at least give you some consistency. I don't think you really need to twist it 1 full degree, but enough to get some resolution in your readings would be good.

My original level only had .1 deg increments, not enough when my first test yielded .3 deg.

I bought a new digital level with .01 deg resolution, it's ten times more accurate. A dial indicator can be used to measure twist very accurately but you need to convert inches of movement into degrees.
David

[Mean 69]

My take on chassis stiffening has been that it allows you to be in control of the items that have more predictable behavior - ie: springs, shocks, sway bars, and the basic geometry of the suspension.


Exactly. The real story behind any suspension system is how well it keeps the tires in contact with the road (track) under all of the circumstances it faces: braking, linear acceleration, vertical bumps, turning forces, and all combinations. It's a heck of a thing to model and solve for, the goal of the suspension is to maximize the tire's performance. Because the chassis is such a complex structure to model dynamically, and once built, there's no tuning to it, the goal is to eliminate, or at least, greatly reduce, it's influence on the suspension characteristics. In other words, if the chassis is flexing, it is effectively PART of the overall suspension system.

I'd imagine that there are examples where a car with a noodle chassis structure has been characterized really well for a given circuit, or whatever the case. It's all about real world tuning, imagine a car that some guy has been running for years and has it setup perfectly. Take that same car, in that same trim with no other changes, and increase the chassis rigidity, and I'd bet that in many cases the car would be slower. Of course, once the new state is sorted, the car would probably overall be faster if the increase was significant. Further, changes in the tune of the car (spring rates, etc) would probably (will) be a lot easier to characterize and feel with a stiffer structure because there is less coupling between the two.

How much is enough? Like everything, it depends. I am certain that you will get to a point of diminishing returns on a street driven car when working exhaustively to increase rigidity. At some point, the other aspects of the performance package will become the weak link, driver, suspension, tires, etc. Clearly all cars are different, what's sufficient for one won't be for others.

I have a lot more opinions regarding other points expressed here, but not enough time to write at this moment.

Mark

[Damn True]

The question that often occurs to me is the following:

According to the guy I bought it from ^that guy^ the rear suspension I have selected (and damn its gonna be soooo slick) will work well with the chassis as I intend to build it.

According to the guy I am buying my front suspension from those products too will work well with the chassis as I intend to build it.

But.....will they work together? I don't know enough about it to evaluate it, but things like roll couple and such are sure to be a factor. But to what degree? And, would one have to be named Andretti to notice the difference?

[David Pozzi]

True,
Our 73 Camaro has a front weight bias of aprox 56% but the front roll couple is 74% give or take.
As you can see, the front suspension handles more cornering load than the percent of weight it carries. This tells me the rear is trying to twist the front subframe when cornering. If the front to rear weight bias is not close to 50/50, then I'd expect the torsional load the chassis "sees" to be greater.

A flexable chassis acts like a un-dampened spring, this would result in a time lag to steering input, and disconnected feeling to the steering. Lots of people say they feel subframe connectors after install, so a stiffer front sub connection should be just as noticeable. I don't think Mario is the only one who would feel it.

David

[Norm Peterson]

Very recently posted in response to a similar topic elsewhere by a guy in the automotive business (OE level, I believe),


The shock absorber is the most sensitive to the torsional stiffness. The shock absorber 'sees' the point impedance at which it is mounted. This is measured in units of mm/s/N, typically, and is a function of frequency.

For handling purposes we are probably interested in the frequency range 0-10 Hz.

The distribution of stifnnesses, and masses, and spring-mounted masses, in the vehicle will alter the shape of the impedance graph, but may not have much effect below 10 Hz.
Having units of time involved in addition to length, force, and mass suggests to me that transient behavior is affected much more than ultimate grip.


Norm

[SN65]

And it all brings us back to the basic chassis / unibody. The foundation upon which we build.

Really, all things being equal, the major difference between one pro-touring car and another is the chassis. How much it weighs. Where the weight is distributed. How it is constructed. Etc...

Everyone uses the finest mechanical components they can find, or at least they believe so. In fact, most in the hobby will spend tens of thousands of dollars on high performance components. And then again most will never give thier chassis a second thought, other than the manditory sub-frame connectors.

Hell, even Dynacorn has added some of the convertable structural items to the 67 stang chassis they build. They have even altered material thicknesses and composition. They understand the application that their chassis are are going to be used for and have taken steps to make sure that they are up to the challenge. I would have gone a few steps farther, but then again I am a glue, nails, screws kind of guy. :-)

[parsonsj]

Having units of time involved in addition to length, force, and mass suggests to me that transient behavior is affected much more than ultimate grip. That makes sense to me. Steering input (turn-in, accel out) would see the most benefit in terms of "feel" to the driver. I doubt very much you can measure any difference on a skidpad ... and the bigger tracks would show less difference since transient behavior is a lower percentage of the total lap time. Perhaps it is measurable on a tight autoX ... but the best way to measure difference due to chassis flex might be slalom testing. Though as Bob has said, it isn't something the average enthusiast can do since it will require before/after/before testing on a single vehicle, and a statistically significant number of cars if testing a vehicle type.

jp

[Damn True]

True,
Our 73 Camaro has a front weight bias of aprox 56% but the front roll couple is 74% give or take.
As you can see, the front suspension handles more cornering load than the percent of weight it carries. This tells me the rear is trying to twist the front subframe when cornering. If the front to rear weight bias is not close to 50/50, then I'd expect the torsional load the chassis "sees" to be greater.

A flexable chassis acts like a un-dampened spring, this would result in a time lag to steering input, and disconnected feeling to the steering. Lots of people say they feel subframe connectors after install, so a stiffer front sub connection should be just as noticeable. I don't think Mario is the only one who would feel it.

David

So David,

Am I correct in assuming from the above that since my car will be lighter in the front by virtue of an aluminum block and a bunch of fiberglass that my weight distribution is likely to be closer to 50/50 and therefore I will be needing a relatively stiffer chassis?

Good thing I am planning an 8pt (maybe 10) cage.

[Norm Peterson]

74/26 may be the roll couple distribution, but the effects of roll center heights and unsprung mass (at axle height) also enter into this, and those other two are significantly rearward-biased. I'm given to believe that you want the total lateral load distribution, which considers all of those factors, to be slightly more front-biased than the weight distribution. At least for a FE/RWD/stick axle car. Maybe a total lateral load distribution of 58/42 for a car with 53/47 weight distribution, or 55ish/45ish for a 50/50 weight?

Simplified, chassis torsional stiffness is a spring in series with the suspension roll stiffnesses. I'm sure that that 74/26 value is a theoretical one based on a completely rigid chassis and the sprung mass taken as a single lump at the CG. In actuality, the roll moment distribution will vary from that depending on mass distribution, locations of discrete lumped masses, and variations in chassis stiffness over its length. Less simply, I think it's part of an overall parallel-series combination of springs and masses, but that gets really complex in a hurry.

Seems to me that with less front end weight, you wouldn't need to drag quite as much LLT forward, and could perhaps be satisfied with a slightly softer chassis. But that's using a pretty sharp pencil, and I doubt that you could actually end up with a chassis whose stiffness differs by some "precisely calculated" amount from a value that you don't really know in the first place. The better solution still is to build it stiff and work with the springs/shocks/sta-bars for the tuning.


Norm

[David Pozzi]

Norm,
Thanks for pointing that out, I didn't think of it.
This Camaro has 60% rear RLLD, I had to go look it up, but it's software generated (Perf Trends) and I don't have full confidence in it since it predicts mild to heavy oversteer and we have understeer.

Here's our car running with no rear anti roll bar as a test, it had heavy understeer. Notice also the LR tire is very lightly loaded too. Since the rear is not stiff enough to carry it's proper cornering load, the front has to carry it. That was enough extra load to lift the inside wheel Acceleration forces were probably involved too. Anyway, the chassis get's pretty high loads on it when doing things like this, and the stiffer it is the better. If the subframe is flexing, then the fenders are too, pretty hard on the body. We are seeing some cracks at the upper rear side window corners. Which is very common on this model.
https://static1.pt-content.com/images/noimg.gif

[SN65]

So David,

Am I correct in assuming from the above that since my car will be lighter in the front by virtue of an aluminum block and a bunch of fiberglass that my weight distribution is likely to be closer to 50/50 and therefore I will be needing a relatively stiffer chassis?

Good thing I am planning an 8pt (maybe 10) cage.
Be carefull in assumming that installing a cage is going to stiffen the chassis in a way that you want it to.

By design, a cages main purpose is to protect the passenger, not reinforce the chassis.

Most of the torsional loads (in the front) transmit from the front frame rails to the cowl to the "A" pillars to the roof structure.

To stiffen the structure the "cage" needs to transmit the loads from the front frame rails (at the suspension mounting point) to the cowl structure and then to the "A" pillars and the roof structure. If the cage is just attached to the front and rear of the rockers, and then the rear frame rails and spring mounting point it will be floating inside the roof structure. Even if you go through the firewall to the front frame rails, doing so without attaching the cage to the cowl and "A" pillars will not help as much as you would like. Also attaching the halo to the roof above the windshield and the side glass will help considerably, but this is difficult if you are planning on installing the factory head liner.

PS: If your weight distribution is closser to 50/50 I believe that you can get by with a weaker chassis than you could if your distribution were 80/20.

[SN65]

SNIP...

Here's our car running with no rear anti roll bar as a test, it had heavy understeer. Notice also the LR tire is very lightly loaded too. Since the rear is not stiff enough to carry it's proper cornering load, the front has to carry it. That was enough extra load to lift the inside wheel Acceleration forces were probably involved too. Anyway, the chassis get's pretty high loads on it when doing things like this, and the stiffer it is the better. If the subframe is flexing, then the fenders are too, pretty hard on the body. We are seeing some cracks at the upper rear side window corners. Which is very common on this model.
https://static1.pt-content.com/images/noimg.gif
Hi Dave,

Is the gap between the door and fender typically like that (it looks much tighter at the top than at the bottom) or is that a result of chassis twist / flex???

[SN65]

Speeking about application specific rear suspensions, here is the rear suspension we are currently building for our 66 mustang project car.

https://static1.pt-content.com/images/noimg.gif

https://static1.pt-content.com/images/noimg.gif

This is for a street, strip application. The components are designed for relativly high HP (1,100 HP, 1,000 FPT) and relativly high shock loads. Light springs, heavy sway bar, etc...

All of the chassis mods we have made are most likely counter to the typical drag race way of thinking, but we are also concerned with driveability on the street. Also, I believe that the weight we added to the car by reinforcing the chassis, will help transfer the power to the ground better than one that was left stock.

[Damn True]

Be carefull in assumming that installing a cage is going to stiffen the chassis in a way that you want it to.

By design, a cages main purpose is to protect the passenger, not reinforce the chassis.

It's not an assumption. The guy that will build my cage builds SCCA compliant cages every day for Porsche's, BMW's, Miata's, Corvette's, and has built a number of quality endurance racing cars as well. He isn't experimenting, he knows what he is doing.



Most of the torsional loads (in the front) transmit from the front frame rails to the cowl to the "A" pillars to the roof structure.

To stiffen the structure the "cage" needs to transmit the loads from the front frame rails (at the suspension mounting point) to the cowl structure and then to the "A" pillars and the roof structure. If the cage is just attached to the front and rear of the rockers, and then the rear frame rails and spring mounting point it will be floating inside the roof structure. Even if you go through the firewall to the front frame rails, doing so without attaching the cage to the cowl and "A" pillars will not help as much as you would like. Also attaching the halo to the roof above the windshield and the side glass will help considerably, but this is difficult if you are planning on installing the factory head liner.

We have that covered I assure you.



PS: If your weight distribution is closser to 50/50 I believe that you can get by with a weaker chassis than you could if your distribution were 80/20.

That seems 180 degrees out from what David said above. What is your "belief" based on?

[David Pozzi]

Hi Dave,

Is the gap between the door and fender typically like that (it looks much tighter at the top than at the bottom) or is that a result of chassis twist / flex???

It's like that sitting still, and I believe it was that way before we started autocrossing it.


On the subject of a cage helping torsional stiffness. I tested our 69 camaro with a Trans Am spec cage, it has forward reaching bars off the vertical bar near the A pillars through the firewall that attach just behind the steering box on the frame rail kickup.

As far as I can tell, the cage in this car does NOTHING for torsional rigidity. I WAS SHOCKED, IT WAS TERRIBLE. The car had no extra braces to the sub other than the front braces. SCCA did not allow the cage to connect to the subframe except for the forward two braces. There really is no triangulation with this setup.

Full tube chassis cars were not allowed in Trans Am in the time this car was built. The main hoops of the cage were not allowed to connect to the subframe. A proper diagonal brace structure should do a lot to stiffen the chassis.

True,
I do think a Camaro with closer to 50/50 would have less torsion load. As weight bias shifts from front to rear, the roll couple would shift to the rear too. My Lola has almost the reverse of my Camaro in roll couple distribution.
David

[High Plains Mopars]

PS: If your weight distribution is closser to 50/50 I believe that you can get by with a weaker chassis than you could if your distribution were 80/20.

I don't know that I'd say you could get by with a weaker chassis. Rigidity yields benefits regardless of weight distribution. Now your roll couple percentages would be radically different. As a matter of fact, in a front engine, RWD chassis, your front roll couple percentage should be going up as your weight percentage moves back if you want to maintain neutral handling.

David's Gen 2 Camaro with 74% roll couple with 56% front weight bias is smack dab in the middle of the theoretical neutral handling line. A car with 50/50 distribution would be closer to 86% front roll couple.

[SN65]

It's not an assumption. The guy that will build my cage builds SCCA compliant cages every day for Porsche's, BMW's, Miata's, Corvette's, and has built a number of quality endurance racing cars as well. He isn't experimenting, he knows what he is doing.

We have that covered I assure you.

That seems 180 degrees out from what David said above. What is your "belief" based on?
I am sure that your "guy" knows what he is doing. Building cages that are SCCA compliant takes a great deal of knowledge and expertise.

I have not read the SCCA rulebook in regard to cages, but, if it is anything like the NHRA rulebook, it covers all of the safety issued and nothing more.

So, unless the cage is something more than a typical 8 or 10 point safety cage, it will do little to nothing in regard to the torsional stiffness of your particular chassis.

With that said, you mentioned that you are attaching the upper structure of the roof and cage together. Have you decided on the type of gussets and their attachments yet? Instead of the typical gusset, maybe you would consider a 10 or 12 gage plate with the free edge reinforced with a 1/2" diameter tube.

In regard to my belief that a chassis can be less stiff as the weight distribution becomes more and more even. Maybe I am wrong. I tend to look at things from a structural steel point of view. In that world, structures become less complex as the loads become more uniform.