Choosing sway bar wheel rate? Sway bar rate as a ratio of spring wheel rate?

[Uhcoog1]

How does one choose the appropriate sway bar wheel rate for a given application?

Car - 73 plymouth duster, 5.7 hemi, tko600, currently in assembly. Weight distribution should be ~54-56% front bias, ~3300 with driver. According to the mopar theoretical handling line, I should aim for a 74-78% front roll bias to stay slightly on the oversteer side of neutral (75-79% for neutral).
Here's what I've done so far math-wise:
1.04 torsion bars - 250 lb/in wheel rate - ~1.65 hz
leaf springs - 130 lb/in spring rate, 73 lb/in wheel rate
front sway bar - open (looking at 1.5", 700 lb/in, 175 lb/in wheel rate (Motion Ratio of .5))
Rear Sway bar - open (looking at 230 lb/in - 86 lb/in wheel rate, or 100 lb/in - 38 lb/in wheel rate)

I'd love to set up the front, and then tune with the rear. I could get the front/rear roll bias close, then tune it for feel once running. Also, I know the frequencies are backwards of advised, what will result from that? And for calculating the leaf spring frequency - does one use the spring rate or the wheel rate?

I've read there were a few good spreadsheets running around - Norm Peterson or David Pozzi - do either of ya'll have anything (read an old thread where ya'll did)?

Once again, the main question is how does one pick a sway bar wheel rate for a given application? Do I need to do calculations for a 1G turn, and aim for a certain body roll? Or what? Is finding the correct suspension combination a reverse engineering exercise?

Roll center should be 5-7" in the front, depending on suspension travel needed and therefore how low I'm able to set the ride height (any calculations for determining suspension travel with wheel rate? more specifically what load I should prepare for in turns and bumps)

Thanks fellas,
Wade

[j-rho]

You can take a stab at initial sway bar rates, but odds are it won't be a perfect choice, regardless of how much math you do in advance - and I'm somebody who likes to do a lot of math. There's also a decades-old debate amongst tuners, about what portion of roll stiffness should be made up of bars vs. springs. There's never one right answer.

Many other variables should be taken into consideration - wheel/tire sizes front to rear, alignment, types of events you'll be doing, even driver preference.

The frequencies you're working to calculate are ride frequencies - for a live axle, you don't need to halve the spring rate to get wheel rate, it's 1:1.

My suggestion is to build your swaybars with modular components, like those from Speedway (http://1speedway.com/Online_Catalog.htm). That way you build arms of adjustable length for fine tuning, and can replace the center section (which is much cheaper than buying an entirely new swaybar) if you'd like to shift your range of adjustment up or down. I'd further suggest basing your front bar on 1.25" components, the rear on 1", as that will allow you a range of stiffnesses usually found at the ends of cars like this.

Good luck and have fun!

[exwestracer]

Another bit (2 bits actually) that you'll need to know to make an informed guess :) are the roll center height and front roll moment arm length.

Keep in mind that the sway bar is only a spring in roll (well, and one wheel bump if you really want to get technical), so wheel rate % doesn't really have as much to do with it as CofG leverage about the roll center.

I'm kind of a fan of soft springs with a lot of stored energy (compression). If the car is super low, you obviously don't have a lot of travel to play around with, so the spring rate will have to be fairly stiff. Super low typically also drives the roll center down, sometimes faster than the chassis height, so the roll moment arm actually gets longer and the car wants to roll MORE, despite not having any suspension travel available for it...

You seem to have a good grasp of levers, so with some measurements and scale work, you can actually calculate how many lb/ft of torque the C of G has about the roll center. Subtract the force holding the car up from the bars, and you can plot how many lb/ft of resistance will let the car roll X degrees.

[David Pozzi]

Use 50% as a minimum bar rate that you will never want to go below.
Your example bar rates look very good as a starting point. I wouldn't go softer, my hunch is you may need a little more bar rate but you have a good place to start. I wouldn't worry about front to rear frequency, the shocks used for handling will dampen the cars reaction anyway.
David

[JRouche]

You really have more knowledge than me. I never thought of frequencies or most of the variables that you look at..

For me it (sway bar) was a final tuning aid, I dont look at the sway bars more than a tuning aid once I have my car set up.

Im not an engineer so I cant think that far ahead.

I did make up the primary suspension to support the car and have some wiggle room on the side of a loose car or a tight car. Its right in the middle ground without the sway bar. And because Im not an engineer that can say YES!!! This is what I want and my math says this is what I need, I shoot from the hip. It was a crap shoot for me picking the sway bar force. Not just the rear but the front also.

Not even the spring rate but what would fit in the package I already made up. Luckily the speedway bar selection is wide and varied. You can pick from hollow bars, solid bars and a variety of arm lengths and materials.

And again it was a crap shoot for me. I knew I had a low rear roll center, not really low yet cause its adjustable, but mid way for adjustments, I made the watts link adjustable so I could play with the rear roll center height.

So in choosing a rear bar rate I wanted a lil more bar spring rate to overcome the low rear roll center. But I have to say. A really stiff rear or front is NOT the best suspension.

Controlling body roll is a decent objective to a point. If you make the suspension too tight while trying to overcome the weight of the car you might overshoot the prime objective which is to plant the tires on the road during ALL conditions.

Compliance in the suspension with control is NUMBER ONE IMO. Compliance means the tires will stay on the road. If the suspension is too tight the tires will lift and any tire off the ground means its not doing its work.

I like soft shocks and moderate springs that includes the sway bar, its a spring. Soft shocks keep the cyclic issues in check and stiff springs keep the body of the car in check while keeping the tires planted..

I have some three piece sway bars on the front and the rear, so far so good. The playing time with the adjustability hasn't come up yet. But as time passes it might :) JR

[Uhcoog1]

Thanks Fellas!

I've been sitting around pulling formula's from Puhn's book today. Looking at only the front suspension, I was able to get some suspension travel figures for braking and turning. But, my results are less than ideal given I was not able to figure the weight transfer percentages for the front vs the rear (though I'm guessing using the roll couple percentage might be close?).

I'll definitely step up to the 3 piece bar. Well worth it, compared to my other options.

As far as calculating braking and cornering forces, what is the max G force one can expect from my car with 275 35 18 R comp tires? My excel work spits out data for .8, 1.0, 1.2, 1.5, and 2.0 G's. I'm guessing I won't see more than 1.0 continuous, although I'm sure spikes are possible (without transferring the full load to the springs because of the shocks, yes?).

The braking formula is simple, and resulted in these figures with the 250lb/in wheel rates:
1G - 2.4" compression
1.2G - 2.9" compression

As far as roll rate, the numbers I've read (Puhn or Martin) state 2 degrees for race, 4 degrees for street. My numbers do not include the full formulas.
Assuming 100% of transferred weight goes to front outside tire:
1.0G = ~2.6", 5 degrees roll
1.2G = ~3.1", 6 degrees roll
Assuming 80% of transferred weight goes to front outside tire:
1.0G = ~2.1", 4 degrees roll
1.2G = ~2.5", 5 degrees roll
Assuming 50% of transferred weight goes to front outside tire:
1.0G = ~1.3", 2.5 degrees roll
1.2G = ~1.6", 3 degrees roll

I'm guessing the goal is 50/50 in the turn, yes? Given the declining increase in traction with vertical load of tires.

So does anyone have a chart of grip vs vertical load for any of the R comp tires out there? If I knew the max adhesion of a set of tires, I could calculate the theoretical limits of cornering and braking, and ensure I have the appropriate suspension travel and wheel rates for braking and cornering.

[j-rho]

No such chart exists, there are too many other variables.
The usual "goal" in suspension tuning is to achieve the right balance between civility and performance for the car owner and their use patterns. There are no hard rules in how to get there, the numbers will be different for every car in every situation.
When you get the balance and alignment and everything else to their best, you should be good for 1-1.2g steady-state in the dry. That's assuming by R compound you mean something like a Hoosier A6. Surface plays a huge role, both in grip, but also in surface irregularities, which drives spring rate, ride height, and shock tuning choices.
There is no one right answer. Best you can do is put togeher the car in a way that gives you a wide range of adjustment, across all facets of the suspension. This will enable you to tune to an optimum in the widest range of scenarios.

[JRouche]

So does anyone have a chart of grip vs vertical load for any of the R comp tires out there?


Ummm... Try not to get too carried away. We are talking about road surfaces that are NOT detailed like where you are driving.

KISS.. The old age saying. Sounds like you want to govern every input that the car will see. Not gonna happen.

Suspensions are meant for variations in the road. Pick a number and drive the car. If it doesn't suit you then make sure to have your specs on hand and change the specs for the suspension. I think you are over thinking the entire process.

There really is a point where you need to say ok, Ill start here.

Even the massive money groups like say Formula One set a working point and work off of that. Sure, they might have 15 different bar settings but they have to pick a starting point, and they do.

There is the time to sit on the porcelain and work things out. But eventually you have to get off the seat and make a decision. In other words get off the pot, your time is up.. You cant control EVERYTHING. Some things are out of our control. JR

[Norm Peterson]

Thanks Fellas!

I've been sitting around pulling formula's from Puhn's book today.
Then you have enough information to develop the total lateral load distribution, which considers the effects of roll center height, roll stiffnesses, and unsprung mass. The chassis can be considered to be a rigid body at this stage. Once you've got a handle on how the lateral load transfer is distributed, then you can estimate the roll and the suspension deflections, without running endless guesses at how much of the transferred load is going to happen up front and never knowing which one(s) are even close.

5" - 7" sounds a bit high for a reasonable front geometric roll center.



The braking formula is simple, and resulted in these figures with the 250lb/in wheel rates:
1G - 2.4" compression
1.2G - 2.9" compression
Have you accounted for the anti-dive (front) and anti-lift (rear) effects?



I'm guessing the goal is 50/50 in the turn, yes? Given the declining increase in traction with vertical load of tires.
A better target is to get the front TLLTD to be around 5% higher than the front weight %.


If you're looking at the amount of roll in terms of camber settings, don't forget that there is about a degree per g of "apparent roll" due to vertical tire deflection effects. This still affects the camber that the wheels will actually run at.


Norm

[Uhcoog1]

Norm-

Looks like I'm going to have to put some more effort into Puhn's book and formulas. I calculated weight transfer when braking using his formula, but only guessed my center of gravity height of 20".
1G braking
3300 lb car
20" COG
108" wheelbase
Weight Transfer = 1G x 3300lbs x 20" / 108"WB = 611 lb

So, 611 lb/in would cause 2.4" of dive in the front. Is anti-dive something else I need to calculate?

I used the same formula for weight transfer in a turn. Is that too oversimplified? Do I need to go read again the more complex weight transfer formulas? I recently realized that I didn't account for the sway bar wheel rate being double in a roll, given that in a turn the roll that causes 1" of compression will also cause 1" of rise on the other side.

1G turn
3300 lb car
20" COG
57.125" track width
Weight Transfer = 1G x 3300lbs x 20" / 57.125" = 1155 lb

Torsion bar wheel rate: 250 lb/in
Sway bar wheel rate: 175 lb/in (700 lb/in bar, .5 MR)
Total wheel rate in turn = 250 + (175x2) = 600 lb/in wheel rate in turn (last time I used 425)
Total compression = (1155*.74) / 600 = 1.4" compression

Obviously I didn't account for roll center. I haven't figured out how to do that yet. And the 5-7" roll center is what the mopar's have. Bill Reilly did the measurements and published most of the data quite a while ago.

My original plan was to model after a certain company's suspension set-up. But, it turns out the development was done with a different K frame than what I have, and the front sway bar rates are different by 200 lb/in (bar rate), or 50 lb/in wheel rate. Their weight bias was 56% front, with a front roll couple of 74%. That's very close to the mopar theoretical handling line. I figured I would start close to their work, and adjust for my different weight bias.

[JRouche]

Ok really? You are still going all engineering on us. Do you have any part of the car assembled yet? Pics!!

It really sounds like a "bench" racing issue so far. I havent heard of what you did. Only what you would like to do. Only tossing some ideas around. Ideas are good. But unless you get to the point where you get the car to the ground its all "bench" racing or paper car building.

A sway bar is one of the last things that you should be thinking about. Get the wheels on the ground and make the car a car. Otherwise its just a paper car that wont do much.

Once on the ground and a drivable car it will be easier to detirmine the sway bar rate needed.

Yer killing me with ALL the tuning issues for the sway bar and its not even a car at this point. Sway bars are NOT a suspension design issue.. They are a tuning aid.

Get the car on its feet and place it one the scales. You dont even know what it scales out at. Why would you focus so much on the "white paper" work without any real numbers? Bench racing....

Errr. Put the car on its tires and THEN think about dialing it in the the minute specs that you think matter. Then there will still be a car that drives circles around you. There is NO winning. Its about driving and having fun. NOT about figuring the smallest details to get it within a gnats rear before it hits the road. Drop the wheels...... JR

[Uhcoog1]

3gduster.com

You can see a couple pics before I took it apart. Currently I'm waiting on a timing chain set to assemble the short block. The heads are ready minus assembly. The tranny install is half done, the wiring is in progress, the front k member is out for welding, and I have a pile of suspension rebuild parts, new torsion bars, brakes, wheels, tires, etc. And a sway bar that I'm not sure I want to keep.

Whats wrong with running the numbers? I'm dealing with tire clearance issues, and Im trying to establish the suspension travel needed. Ive got this sway bar that is almost the price of a 3 piece set up, and I'm trying to decide if I should return it and go to a 3 piece, or if I should run it and figure it out later. Further, most Mopar guys recommend larger torsion bars, with a wheel rate upwards of 350-450 lb/ft, with small sway bars. I'd like to go with the smaller torsion bars I have (250 lb/ft) and a larger sway bar. Given the lack of information in this direction for Mopar guys, it makes sense to run the numbers for roll angle, and get it close to right on paper. Further, if that's close, and I get my roll resistance close, the I can tune the car with rear suspension changes to dial it in. Maybe I get it close enough out the gate, and maybe I don't, but running the numbers and understanding the formulas is part of the fun and part of the hobby for me.

[Norm Peterson]

Weight Transfer = 1G x 3300lbs x 20" / 108"WB = 611 lb

So, 611 lb [not lb/in - ed] would cause 2.4" of dive in the front. Is anti-dive something else I need to calculate?
You might want to. 17% anti-dive would reduce that 2.4" down to 2" . . . and since 17% is just a number I pulled out of thin air, you'd want to know what yours actually is.



I used the same formula for weight transfer in a turn.
That approach works as far as determining the total lateral load transfer is concerned. But it won't work for determining the amount of roll or the suspension displacements or how the load transfer divides, front vs rear. The effect of roll center height is much like the effect of anti-dive and anti-lift (and the other anti's), where some of the load transfer occurs without getting any roll.


I'd measure up the suspension for roll center and anti-effect determination, with the car sitting as close to the final ride height(s) as you can estimate. If you're at a much different height from what the Mopar guidelines assume, the guidelines themselves may not be worth as much as you'd like. If you want to do some more math, once you've got the suspension measurements at static ride height, you can estimate what it becomes as the suspension moves and get camber curves and a few other things of interest.


Norm

[MIKE67]

Uhcoog1, I agree with you. Running the numbers and understanding the math is part of the fun, may save you some $'s and will help with the tuning later. You seem to be getting a good handle on the situation and are thinking it through. I also like all the input from the members.

[Bjkadron]

Roll center height is right at 4 inches on the stock set-up. Assuming the lower pivot points (LCA pin and ball joint pivot) are level.

Puhn talks about it from page 32-44 with the actual weight transfer equations on page 42 if you have "how to make your car handle."

[JRouche]

Whats wrong with running the numbers? I'm dealing with tire clearance issues, and Im trying to establish the suspension travel needed. Ive got this sway bar that is almost the price of a 3 piece set up, and I'm trying to decide if I should return it and go to a 3 piece, or if I should run it and figure it out later. Further, most Mopar guys recommend larger torsion bars, with a wheel rate upwards of 350-450 lb/ft, with small sway bars. I'd like to go with the smaller torsion bars I have (250 lb/ft) and a larger sway bar. Given the lack of information in this direction for Mopar guys, it makes sense to run the numbers for roll angle, and get it close to right on paper. Further, if that's close, and I get my roll resistance close, the I can tune the car with rear suspension changes to dial it in. Maybe I get it close enough out the gate, and maybe I don't, but running the numbers and understanding the formulas is part of the fun and part of the hobby for me.


Oh no, dont get me wrong. Its GOOD to run the numbers. I F'IN LOVE numbers!!! But... Re-running them over and over you end up with the same conclusion. You are stuck with what one piece bars are available. If its a solid or hollow bar. There really is not much available for various spring rates when you are considering a one piece sway bar.

And IMO the wheel travel is not determined by the sway bar. Your wheel travel should be determined by the spring, shock, control arms and body limitations, never the sway bar.

A sway bar is a tuning device that should be worked into the existing suspension geometry. The suspension geometry should not be designed around or even considered as far as the sway bar is considered.

And thats why a three piece bar is GREAT for its purpose. It has the most adjustablity as far as spring rates available (the range of spring rates cant be beat).

Problem with a three piece bar? Fitment. Yep, I have a three piece bar at the front and rear and believe me.. It took some creative thinking to incorporate them into the suspension I made up.

The benny? I can change the spring rate to some extent with the arm length (where the vertical link attaches to the arm). I am limited on the front, its really tight, prolly wont be able to do that. The rear arms do have some wiggle room. But the thing is for the front or rear a diff torsion bar rate only costs 90 bucks. So if I really wanted to change the rate its just a matter of ordering the torsion bar and swapping it out.

And really, my intention wasnt meant to say you are wasting your time trying to figure out sway bar rates. My bad...

Cause really, I was in the same bucket. Sway bar rates are some of the hardest numbers to get with research. Mainly cause most manufactures and dealers only mention size VS rates. I really did do alot of research to try and figure out what would be a good starting point (spring rate for the sway bar) for my car. I ran into a brick wall. Most folks talk about sway bars in terms of size and not spring rate.

I was lucky enough to get some actual numbers from this forum to get a good starting point. Mainly for cars that were similar in weight and that was good enough for me. Otherwise it would have been a crap shoot. I had no idea what some decent starting numbers would be. David Pozzi, Norm Peterson and others helped me out with a starting point.

And my car is a lil goofy. Its a mix of "parts". Mustang II front end, parallel four link with a watts link on the back. And an air ride tech spring setup all around. Its kinda a spaghetti of sorts. Why all the links?? Adjustability, plain and simple (but also a nightmare)..

YES!!! I love crunching numbers also. But after awhile I found out I really just needed to make a stand and go with SOMETHING.. I did.. Was it good? OH YES!! Can it get better? Yup... Is it fun to drive without any odd handling issues? YUP :)

Problem now.. Which monkey do I want to adjust? Lets see. Have the four link with some major adjustments possible. Have the watts link with a large amount of possibilities. The sway bars have some wiggle room. And shocks (not the air springs, they have to be set to the ride height that is best for the suspension, NO adjustability there as some might think). The double adjustable shocks have a wide rage of valving.

So yes!!! I LOVE to crunch the numbers. But.... I have found its easier for me to make up a very adjustable suspension that suspends the car, then work on making it handle better. Crazy thing is my car will never see a race track. But I still love the ability to mess with the numbers and see (or not see) if it changes the handling of the car. I might not ever see a diff unless I stress the car on a track. But I cant afford that so its a nice 45mph up to the store to get some groceries and back. Maybe a fun drive out to the coast and tool around some.

Umm as far as a stiff spring for the car or a soft suspension spring and a heavier sway bar?? I would be inclined to use a very complient (read soft) torsion bar for the suspension and beef up the roll stiffness that will be induced with a heavier sway bar.

Why? Because on the road, the straight roads you really want the tire to conform to the road. Which means a spring that will allow the tire to follow the road. Too stiff of a suspension spring means the shocks will be over powered. Use a semi soft spring and a soft compression but stiffer rebound shock (if you have some adjustable valve shocks) and the car will ride like its on air but still have the responsiveness needed to keep the tire on the ground at all times. And thats the key really, keeping the tire on the road.

Now for turns thats when the heavy sway bar comes into play. It will keep the inside tire planted while the outside tire is taking all the abuse.

Try not to think about the sway bar as something to control body roll (body roll is not always bad). Think of the sway bar for what it is and thats to link up the inside and outside tire in a turn. NOT to limit body roll as the name (sway bar, anti roll bar) implies. Its sole purpose is to connect the left and right side tires in a turn and thats ALL. And the byproduct might be less body roll over, but its just a byproduct. The need for a sway bar is to plant the inside (weak griping) tire to the road in a turn and thats all.

If you think you have too much car body movement in a turn then its time to slow down, reduce the weight of the car or re-think the purpose of the car.

Race cars dont have much body roll due to the weight and specific purpose for the car. A race car is not a great street car. AND... Some folks manage to blend the two together while riding the fine line.


Ok... So I have exhausted my allowed soap box drivel.

I think you are good to go with all the info you have gathered up. Put the tires on and drive the car.. Ummm, YES I love the 70s Plymouths. Yeah, some folks go for the Cuda. But really, the Duster is a sleeper that has some great body lines. Kinda why I went with a Nova instead of the more popular Camaro. JR

[exwestracer]

JRouche,
I've seen that we tend to think along the same lines in most cases, but sway bar theory is not one of those areas. If you think about what a sway bar does in the suspension, it is not keeping the inside tire planted, rather exactly the opposite! If it did, a bigger bar would reduce understeer, which we all know isn't the case... Keep in mind that NOTHING will stop weight transfer. Just because the car is sitting upright doesn't mean that any less force is going to the outside tire. As the car goes into a corner the outside spring compresses and the inside spring wants to extend a certain amount depending on roll moment, etc. The sway bar limits the amount of extension on the inside spring. It is using a certain amount of load from the outside spring to lift on the inside lower control arm. If that isn't reducing load on the inside tire contact patch, I don't know what is...

As a chassis builder, I'd love to design geometry to eliminate the need for bars altogether (and on some cars I can). As you have stated, they are a tuning aid and a "necessary evil"... Bottom line is, whatever makes the stopwatch happy was the right answer!

[Uhcoog1]

I'll dive back into Puhn's book, see what I can do. Does anyone have a worksheet or program that does some or all of the calculations?

[High Plains Mopars]

IMO, your overthinking it a bit. The sway bar is just an additional spring to be caclulated into the roll couple percentages. Get the s-bar rate, calculate the motion ratios, add it to the t-bars, apply it to roll couple and see what side of the line you fall on. Adjust sway bar diameter up or down from there.

BTW, 700# seems kind of light for a 1.5" diameter sway bar. Is it tubular with attaching locations that differ from stock? According to my calcs, a stock type, solid, A body sway bar that is 1.125" in diameter is 685#. I'm not sure of the exact A body motion ratio though.

[Uhcoog1]

Tony-

Thanks for the insight! I just did some math (from Puhn's book), and I came up with ~1446 lb/in for the sway bar in my garage (not factory, but close). With a motion ratio of .5, that puts me down to 361, which is nearly the 375 I was quoted. I guess he was talking wheel rates, and I was assuming I still needed to accomodate for the Motion Ratio.

That makes things VERY different... recommendations make so much more sense now... haha

Did you use 72 and older measurements or 73+?

[High Plains Mopars]

I've got very limited info on A body stuff and what I do have is all 73-76. I've never had a chance to play around with any of the earlier stuff.

Like j-rho said, there are lots of variables you can throw at this to arrive radically different combos that produce very similar percentages.