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Old 12-03-2009, 05:01 PM
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Sway bar selection

Where do you start in selecting a sway bar or actually anti-sway bar?
I'm looking at using a splined tube from Speedway Engineering or ??? which would have some amount of adjustability.

Rates from 125 pounds to 400 or even higher are available. I have not been able to find any info on how to figure out how stiff the bar should be.

I believe that for the rear, weight and roll center height above ground must be the most important factors, but are there others? I have a solid rear with a triangulated, 4 link with 500 pounds per wheel. I can't remember what the roll center calculations were.

in the front, suspension geometry probably plays a role, also. I have double A arm with 550 pounds per wheel.

Can anyone provide any insight to what I need to know and how to determine what I need?

Thanks

Dan

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Old 12-03-2009, 09:27 PM
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Dan, excellent questions. Unfortunately there is no easy answers, too many variables. You are right that roll center height is a big determining factor, on the front as well as the rear. Wheel rate, track width, tire size etc all play a role. What kind of car are you working on?

I am a believer in "softer" springs and stiffer anti-roll bars, some folks think just the opposite. It is really up to you. If it is an unusual application, an adjustable bar would be desirable. If you could give us some more information on your car we should be able to steer you in the right direction.

Andy
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Old 12-03-2009, 10:09 PM
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Andy,

It is a bit hard to explain so I have attached (I hope) a couple photos to give you the idea.
It is a round tube frame (most of the triangulation is missing in the photo) with a center structural tunnel (also missing). 302 Ford, T5, and 8.8 posi with 4.11 gears. Modified, Mustang II front suspension with Coil-overs. Coil-overs, and triangulated 4 link in the rear. I checked my notes for roll center heights, but I didn't find them yet.
The cab is a '49 Ford, sectioned 4 inches, chopped 2.5 inches, and channeled to the bottom of the lower tube which is 6 inches off the ground. Wheelbase is 108". Total weight will be just under 2200.
While it is somewhat unique, for the purpose of the sway bars it is probably similar to a '30's lowboy roadster with Mustang II front end.

Thanks for responding
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Old 12-04-2009, 12:50 AM
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I'm going thru the same issues with my Willys. There are at least 3 stock type front anti roll bars for the Must II. Unfortunately they are designed for the real Must II which is about 3400 pounds and about 60% on the front end. I got one and tried it out. I have the lightest springs available for the coil over Must replacement shock/spring. It should just float on these but they are still too stiff. Not only that the single adjustable shock are valved too stiffly. There is so much compression in the shock that the ride is really rough. It handles quite nicely but the ride leaves a lot to be desired. The anti roll bar I have was so stiff that going over even a small bump with one front tire caused the other side to follow almost exactly. Inother works way too stiff. I'm told all front anti roll bars for the Must II are about the same. My nice shiny new anti roll bar is now standing up in the corner by itself.

If you can make some nice mounts for the Speedway sprint car anti roll bars, they should work nicey. I'm thinking along the the same lines. Speedway may work with you on exchange but don't mark the bars up.

For the rear there are a number of setups available. All run $250-$450. These are short and very stiff bars as they are normally used with narrowed rear end.

You will need to make your mounts to give you the roll center you want.

My case I have a very narrow rear end with the shocks only 18 inches apart. This means the springs are pretty ineffective at countering torque from the drive shaft. One wheel wonder. yeah you can install a Detroit Locker and make it so both tires spin but al it really does is drive the left tire hard and the right tire just more or less goes along for the ride. The car will drive hard to the right (rear goes to the right). I've already proved this. so I'm also working on a very stiff rear anti roll bar.

I too have the calculation somewhere but SolidWorks gives a very close answer and is much faster and easier.

Be aware that small changes in diameter of short bars changes the rate very significantly. It is also possible to overstress these stiff bars and cause them to stay twisted.

Finally it's not really necessary to depend entirely on the splines for adjustment. Design an adjustable link. You can even preload the system with these.
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Old 12-04-2009, 08:21 AM
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Bentwings,
Thanks for the info. I may have the same problem with my front shocks. We weighed all four wheels when most of the parts were on and I think the front is 500 - 700 pounds lighter than the Mustang II. We also lowered the front roll center, so I think that would require an even softer sway bar. Your experience confirms my thought that Mustang II bar would be way too stiff.

My goal is to have a canyon carver that could navigate a road seam without launching it into the next lane.

I'm planning to make the mounts and build links that can lengthen or shorten the arms. However, there are so many choices in sway bar spring rate, I'd like to get close to begin with.
And I'd like to understand the mechanics well enough to be able to make some sort of educated guess.

Thanks
Dan
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Old 12-04-2009, 11:33 AM
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Sway Bar Rate Calculator- shows you the spring rate of a sway bar:
http://buildafastercar.com/tech/Sway...ate-Calculator

Posted in another forum by Billy Shope:
"If you're using a torsion bar and arms from a place like Speedway Engineering, the calculation for rate, at the end of the arm, is very straightforward:

rate = 1.139E6*D^4/(A^2*L)

where the "E6" indicates the true decimal point is located 6 spaces to the right, the "^" indicates exponentiation, "*" is multiplication, "D" is the diameter of the bar, "A" is the length of the arm, and "L" is the length of the bar.

Unfortunately, with an OEM bar, you simply have to make a reasonable estimate for some of the dimensions. In addition, some OEM bars have additional "jogs" in them. There are ways to get around these problems with an acceptable error, but they're beyond the scope of a short post like this."

A LOT of chassis info:
http://www.auto-ware.com/techref/lib_index.htm
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Old 12-04-2009, 12:34 PM
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Cobalt 327,

Great information on both sites, that's really what I was looking for. Now I'm no longer confused - I know I can't determine what I need.

I didn't realize that what the sway bars are really doing is moving the load between the front wheels and rear wheels, so front sway bar stiffness is dependant upon rear bar stiffness and vise versa.

Also, as Bentwings pointed out, the coil-over spring rate and shock valving are a large factor in roll-resistance and ride. I'm not sure how suited my coil-overs are to my project, I may be in the same boat.
At any rate, I guess I should probably wait on the sway bars until I see how the coil-overs are working.

Thanks for all the help!

Dan
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Old 12-04-2009, 02:48 PM
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Quote:
Originally Posted by DJ3100
Now I'm no longer confused - I know I can't determine what I need.
Well, not yet at least. The answers are out there, it's a matter of picking the right brains and possibly some trial and error along w/all the *math* !lol

But it sounds like you are grasping the intricacies of it all and that is saying a lot, IMHO.

My extent of "handling" cars was a Porsche 928 on extended loan from my overseas nephew and a pair of Datsun Z's, so I'm basically clueless. I did find out that you can get REAL loose by bolting a T/A W41 front sway bar onto a base Camaro with HP but w/o a rear bar! Also it will plow like a John Deere by doing the opposite.
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