sway bar sizing

[pav8427]

I'm at a point on my project where I need to start thinking of what size sway bars to run. I know that once assembled, that I may need to change them for the proper tune. I have run some numbers through Herb Adams formulas. And although I don't know the front/rear weights or the corner weights exactly(or other numbers for that matter),I have made a good guesstimate.Using these numbers I came up with something like the front suspension being able to do 1.05 G's and the rear able to do .85 G's. without sway bar rates figured in.
Figuring sway bar rate and applying them to these formulas is fairly straight forward.
What I would like to know is should a person shoot for equal numbers front and rear or should there be some kind of a pecentage involved between front/rear?
Would it be better to use sizes from a similar package the aftermarket offers and tune from there?
What I would like to use is a splined sway bar w/custom arms that are adjustable for length. Then I would find a 'good ballpark' rate, and set my arms up so that rate would be in the middle of adjustment.
Any input would be greatly appreciated.

Thank you, Doug

[High Plains Mopars]

I am unfamiliar with the Herb Adams formulas, but I would expect if he has you calculating G force numbers, then somewhere in there is a calculation for the wheel rate necessary for achieving the lateral acceleration number. If you can find out the required wheel rate, then you can back into the sway bar rating via the spring rate and motion ratios for your particular application. Once the spring rate and motion ratio come out, you can then caclulate the sway bar motion ratio and arm lengths and fill in the missing hole with the appropriate sway bar rate.

Front to rear sway bar rates are a percentage and not equal numbers. This is because the roll rate of a vehicle requires a greater percentage of control on the end that has more weight. In your case, the nose of the car. The roll couple percentage is a combination of both the spring rates and the sway bar rates. So being in a percentage game, you can maintain the same roll couple percentage with varying rates of springs and sway bars without changing the overall balance of the car.

Or you can just search out similarly suspended cars, talk to the owners about the spring and sway bar rates they use, and copy them. Ultimatley driver feel is a big part of the equation and you amy end up using varying sway bars to get a percentage and feel that works best for you.

[Gary Wells]

Depending upon what car your question and / or scenario applies to, you are liable to have to give more consideration towards what is currently available on the factory / aftermarketmarket scene than what is theoreticallly best on paper. Case in point: I own a GM "G" body '87 Turbo Buick, and the front factory sway bar seems to be about ideal, but the largest solid Dia rear sway bar that I can find is a 1" which I am currently using. Up from that, next smallest diameter available is the 1 3/8" replica ATR bar, which seems just a little too stiff for optimum handling purposes. Ideally, I would like to find someone manufacturing a 1 1/8" solid or a 1 1/4" solid rear sway bar. HTH, & hopefully you are prepping a car that has more available sizes.

[pav8427]

I left Herbs book up at the shop, so some of this is off memory, and at 40+ that may slip a little.
I don't think there was any mention of motion ratio's, and I don't recall reading anything about a wheel rate necessary for achieving a specific lateral acceleration number.
My spring rate ratio is 51%. Which gives me a 180# wheel rate w/350# springs. I think that is on the small side for spring rate.
Based on a total weight of 3200#'s with a 55/45 front to rear%, heres what I have so far.
Vertical load v.s. Cornering efficiecy- front 930/880=1.05%
rear 830/720=1.15%
Total cornering force- 3520/3200=1.1g's
Lateral weight transfer(factored @ 1g)
-front cornering force-1770/1760=1.005 g's
-rear cornering force1620/1440=1.125g's
both w/200# sway bar rate figured in.
I ran these numbers quite a while ago so I am sure that I have missed something. From there I would have to re-read the book to pick out anything I may still need info wise.

As far as comparing to rigs that are out there, this is a tube chassis under a 65 Chevelle that I will really be fighting to get down to 3200# with a 55/45%. I know the frame is lighter and better balanced front to rear than the factory one, and I hope to get some set back to help. So these numbers may be thrown out the window.

Another option I have is to find a particular factory style sway bar that will physically fit that has a wide range of sizes available and use that for tuning.

Doug

[High Plains Mopars]

I would think the manufacturer of your chassis would have a variety of sway bars available. If not, then check Speedway Engineering. They have a very large assortment of splined sway bars in solid and tubular design as well as a variety of arms.

For a baseline set up, you numbers don't make a lot of sense to me. Not knowing how Herb gets to all his numbers, I'd think you would be around 800# front spring rate, 150-200# rear spring rate, and then dial in the specific handling with variations in sway bar sizing from 1-1.25 front and .5-1 in the rear.

Back in my oval track days, we ran on a fairly flat bank tracked using 800# front springs, 1.125-1.25 front sway bar and 150# rear springs. On a higher banked track we stepped up to 1000# front and 200# rear. We never used a rear sway bar. This was on a stock style suspensison under 2nd gen camaro/nova bodies running at 3200# competition weight with around 52-55% front weight bias. While you are a street vehicles and wouldn't use as high a spring rate, you still don't want to ride in a marshmallow and a set up for a flat type oval track may be pretty close to a high performance street appplication these days.

[pav8427]

Back to the math class black board. But for clarification, heres what Herb writes.
For those who know more, this will probably be old news.
Total cornering force=traction available(from tire performance curve) divided by vertical weight. This will give you, in theory, how
many g's a car will be able to do.
Lateral weight transfer(factored at 1g)=overall weight x center of gravity height/track width. This will give you how much weight is transfered in a turn of 1g. Resulting in how much weight will be added to your static,at rest,corner weights.

Total cornering force is an average of the front and rear suspensions. If you apply the same formula to both the front and rear suspension separatly, you will see how balanced the car is.Using the numbers I posted earlier,(1.005g's front,1.125g's rear),says that the front will not take as hard of a turn as the rear,therefor on paper it will push. If these were
bass ackwards there would be oversteer. If they were equal or closer, it would turn better and probably faster.

I will be upping the spring poundage to around 550# and set up for the old reliable 1.25 front/.875 rear to start until I get more accurate corner weight and some tuning time.

Does anyone happen to know where I can find corner weights for an early Chevelle w/ BB or SB? Total,front and rear weights would be useful, for future reference also.

Thanks, Doug

[Norm Peterson]

. . . For a baseline set up, . . . Not knowing how Herb gets to all his numbers, . . .

The only way you can get to the lateral g potential of each end of the car mathematically is to know something about the lateral force vs vertical load and lateral force vs camber tire curves. And the suspension camber curves.

As a rather rough approximation, you can use some sort of "upside-down parabola" for the camber curve effect and an 0.7 exponent of vertical tire load for the effect on lateral grip.

Norm

[Norm Peterson]

Depending upon what car your question and / or scenario applies to, you are liable to have to give more consideration towards what is currently available on the factory / aftermarketmarket scene than what is theoreticallly best on paper. Case in point: I own a GM "G" body '87 Turbo Buick, and the front factory sway bar seems to be about ideal, but the largest solid Dia rear sway bar that I can find is a 1" which I am currently using. Up from that, next smallest diameter available is the 1 3/8" replica ATR bar, which seems just a little too stiff for optimum handling purposes. Ideally, I would like to find someone manufacturing a 1 1/8" solid or a 1 1/4" solid rear sway bar. HTH, & hopefully you are prepping a car that has more available sizes.

If you're willing to scrap the LCA mounting and go to an endlinked arrangement you can build up something out of circle track/stock car pieces with nearly any rate you want. 3rd/4th gen F-body bars are available in several diameters and can be made adjustable as well with a little fabrication. But keep in mind that due to the different arrangement any comparisons made between the A-/G-body bars and endlink-style bars based only on their diameters will not be correct.

If you actually need more than a 1" rear bar and are still running the OE front bar, that suggests that you've swapped front springs. That, in turn, suggests that a better overall solution to adding rear roll stiffness might be by stiffening the springs instead of the bar.


Norm