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Rear sway bar mounting location?

loose nut dan

Well-Known Member
#1
Some friends and I are having a debate about the mount location of a rear sway bar, and it's effectiveness. Using the scientific "logic" approach, we're having a hard time coming up with an answer we can all agree on.

On solid axle, coil-sprung desert truck, should the torsion bar be mounted to the chassis behind the rear axle, in front of the rear axle, will it even make a difference?? And Why?

Please discuss...

-Dan
 

scottm

Well-Known Member
#2
I dont think it will make any difference wether it is in front or behind the axle. There is a delicate balance in the bar diameter, bar length, arm length, where the arm link attaches, and how much travel the suspension has. Say you have a 4 link axle with 28" travel, and you want to attach the swaybar links to the axle. In my opinion the swaybar needs to survive a situation where one wheel is in full bump and the other is in full droop. Some people say that will never happen, but if it did and if your bar was thin enough to survive, they say it probably wont have much effect as a swaybar. I think it does happen, and the stories of broken swaybar parts you often hear are good evidence. So you will have a compromise between effectiveness and wether or not it survives the race.

Anyway, with the example of 28" travel, the bar will see a lot of angular twist, perhaps 45° depending on arm length. That is far more twist than a sway bar will see on a road or circle track car. So you will need long arms or a relatively small diameter bar to survive the max twist situation. You could mount the arm links to the trailing arms to reduce the amount of travel and twist the bar sees, but then the bar will lose leverage over the axle, like trying to push a door open from the middle rather than from the door handle. So to answer your question, the best place to mount it is wherever you can that will allow you to attach the links where you want, and to fit the arms you need to make it work. I show the deisgn of my front swaybar in pretty good detail in the shop thread about my class 8 truck, here, post 106:

http://www.race-dezert.com/forum/showthread.php/62194-My-D-I-Y-Class-8-Chevy/page6
 

scottm

Well-Known Member
#4
I picked 24" entirely because it was the longest length I could get without redoing the cage in front of the radiator. It was really a lucky guess. The arms could have been shorter which would have made it work better as a swaybar, but that would require the bar be located closer to the a-arms. The current location is ideal for structural and convienence reasons. Keep in mind that the bar links are about 2/3 of the length of the a-arms, so the sway arms move about 2/3 of the 21 inch front wheel travel, or 14 inches per side. To use this same bar in the back with twice as much sway arm motion, the arms would have to be twice as long for the same amount of stress in the bar. Assuming the stress could actually be 50 percent higher, or around 120,000 ps, you could use 36" arms with 28 inches of travel and be ok. Or you could assume your axle will never see a full bump/full droop condition, and use a sway arm shorter than 36 inches.
 
#5
Most triangulation is done in front of bump stops. The swaybar puts a huge amount of leverage and torsional load on cage. It just makes sense to put that load where most of your triangulation in the cage is, when possible. I notice on most trucks that run them behind rearend either have alot of tubing, that wouldnt normally be needed, or fatigue cracks developing. Just my thought.
 
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