Torque Arm Length

[Joest]

Has anyone had experience with rear suspension systems that use a torque arm to control axle rotation? I plan to use one on my vehicle which as a wheelbase of 108" and a weight distribution of about 55/45 front to rear. My thoughts are that I will make the t-arm so that it extends very close to the front-to-back CG of the car. The reasoning behind this is that the t-arm will come closer to loading the 4 tires equally during braking and biasing the loading even further to the rear tires under acceleration. Currently my t-arm is 51" forward of the rear axle centerline.

 

[bhart]

Joest,
I have a race car w/ a torque tube setup on the rear axle... I think that this is basically the same idea as your application.
Here's what I know:
1. The torque arm is responsible for reacting all of the rear axle torque by a force applied at the pivot/connection of the torque arm w/ the chassis.
2. Therefore the longer the torque arm, the smaller the reaction force required to react rear axle torque.
This can be stated by: Force_torque_arm=Axle_torque/Length_torque_arm
3. The reaction force at the torque arm tries to lift the chassis under acceleration (torque reaction force upward on chassis) and lowers the chassis under braking.
4. The torque reaction force is always a postive "anti" force at the rearend... anti-squat under acceleration, and anti-lift under braking. So a shorter torque arm corresponds to more anti-squat and anti-lift at the rear suspension.
I'm not sure what you're getting at by your first post, that is why I just presented a brief explanation of what I know about a torque tube/arm suspension, w.r.t. the forces exerted on the chassis.
What I mainly don't understand is what is meant by the loading the four tires more equally under braking part. It is true that a longer torque arm will reduce the instantaneous unloading (reduce anti-lift at the rear) of the rear tires, and so in that way may prolong the load transfer from rear to front a little, but it is also true that if you are trying to slow down w/ X amount of G's of deceleration, that you will have Y amount of rear to front load transfer... and that is primarily dependant on the ratio of cg height to the wheelbase of the vehicle. Same thing for the rearward load transfer under acceleration, a longer torque arm will prolong the load transfer, but, in general, will not affect the total amount of load that is transfered.
In case you aren't sure how to calculate the front to rear location of your CG, it should be 59.4 inches ahead of the rear axle for the weight distribution that you gave (CG_from_rear_axle = front_weight%*wheel_base/100%)
Hope this helps,
bhart

 

[Joest]

bhart,
Thanks for your response. I have run some of the numbers in the past and have a good idea of the dynamics of a T-arm, however I was hoping to have a few people tell me what lengths they have used, under what conditions (car types and racing applications), and their impressions of the performance. Are you happy with the performance of the T-arm on your racecar? What kind of vehicle is it? Wheelbase? CG? Any information would be greatly appreciated. Thanks

 

[bhart]

My racecar is called a midget (4-cylinder version of a sprint car, also called speedcars in New Zealand/Australia). We race only asphalt circle tracks in the US. Here is some of the info, but it comes w/ a disclaimer, it is difficult for us to adjust torque arm length... so basically we don't tune w/ the torque arm but I have had different cars w/ different torque arm lengths, and the car w/ the shorter torque arm definitely gets better forward traction (the shorter torque arm is accomplished by moving the rear axle closer to the motor plate, so the short torque arm cars also have more rearward weight bias).
Here are my specs anyway, hope they are somewhat useful.
Weight: 1100 lb.
HP: ~300
Weight Dist.: F/R 40/60
Wheelbase: 73"
Rear axle to CG: 29.2"
Torque Arm Length: ~35"

This is the shortest torque arm I have ever run on a car, but it has the best traction. Also, it is worth noting that the midgets that run on dirt have even shorter torque arms (~33&quot for the purpose of increasing rearward weight bias and anti-squat to help get better traction on the dirt.
One final thing, if your setup is anything like ours, a longer torque arm has the potential to significantly decrease the friction in torque tube/torque ball joints b/c of the lower forces transmitted by longer torque arm.
Hope you are able to find something useful from this,
bhart

 

[EKEngineering]

Here is a link to a thread that may be of some help.

http://corner-carvers.com/forums/showthread.php?s=&threadid=795

 

[Joest]

EK,
Thanks for the post. The tread was interesting to read...a good mix of knowledge and ignorance.