Torque Arm geometry in custom application: multiple questions

[SonomaZ]

Background and pertinent info:

'92 GMC Jimmy (S-series truck) 2 door, 2WD, 101" WB
Axle is a Strange 12 bolt originally for a 3rd/4th gen F-body. (Has t/a mount)

I built my own t/a from 1-1/8" seamless tubing and 3/4" stainless, poly-bushed rod ends.
Since my truck and the F-body share almost identical wheelbases and curb weights, I built the t/a to the same length thinking that would be a good starting point.
The front will be frame mounted as opposed to trans. tailshaft mounted how GM did it.

I think I have a pretty good understanding of Anti-Squat, but my Instant Center definition needs help.
In regular car-guy, non-engineer terms: What is it and why is it important?

Next question:
Since the t/a is fixed at the axle, the pivot (swing) point at the rear is the c/l of the axle, do I include that as part of the overall length dimension?



My ultimate question for the collective hive:
How does t/a length affect the vehicle?

And a follow up question:
Better to be longer than optimal or shorter assuming angle relationships are correct for a/s?

Due to my engine setback and packaging constraints for exhaust, I may need to shorten the t/a from where it is currently.

Thanks all.

[jetmech442]

Hey, since there are no replies yet, I’ll take a stab at kicking this off…
There are much better explanations on Antisquat on this site, but here’s my best in car guy talk: There is a line called the 100% antisquat line. If you google it, you can find how to determine where it is, but it requires you know the center of gravity(for most applications I hear camshaft centerline height is just right).
• If your suspension is set up for 100% antisquat, then the rear of the car neither lifts nor squats during acceleration.
• If it is above 100%, then the suspension actually picks up the rear of the body(forcing the tires down) and increasing traction-but only while the lifting action takes place.
• If it is below 100%, then the suspension actually pulls the body down(squats), and unloads the rear tires during acceleration.

The Instant Center of a Torque arm is the physical attachment point of the T/A to the frame(siting: How to make your muscle car handle by Mark Savistke). The difference in an Fbody and the Jimmy is that the center of gravity is higher. So for the same instant center location, you will have even less antisquat than the Fbody.

Since the Jimmy has a higher Center of Gravity than an Fbody, but the same IC location, you will have less antisquat percentage. The pic below shows my lame attempt at what the setup looks like for an Fbody on the left and Jimmy on the right. What I’m trying to show is that the 100% line is steeper for the Jimmy(since both are 101’’ wheelbase, and the CG is higher on the Jimmy). If the Ic location is the same, then the 100% line is higher, than in the Fbody.



So to answer your questions directly, the TA length affects the instant center location which directly determines how much antisquat you have. A shorter TA length will bring it back and closer to the 100% line, and in your case, you would probably need an extremely short one to get above that line(if that’s your goal, I dunno). –Now there are limits to how short, and optimal ranges here on this site, I think Ron Sutton, Norm Peterson, and some of the other greats have discussed that with a much deeper understanding and more real world experience than I have.

As far as longer or shorter than optimal, I guess depends on what you want to do with the truck. If your trying to drag race I find for my car the 130-150% antisquat really digs the tires in on hard launches, but if your roadcourse/autoX , then I’ve heard that people use much less than 100%-but I don’t have any experience in that yet.

I hope you find this helpful and others chime in with their thoughts(or corrections to anything I’ve said).

[SonomaZ]

That is extremely helpful, thanks much jetmech!

Since I originally posted this I've been able to wade through the majority of Ron Sutton's postings on this topic and my understanding level has increased exponentially.

Granted, my little truck has a higher roofline and carries some of its body weight higher than an F-body, my engine and trans. are sitting a fair amount lower in the frame than stock.
My point is, I'm hoping my CoG is not THAT much higher.... further research and running of the numbers required.

As far as intended use... Street, auto-x, road course (maybe) and drag, in that order.

It's looking like shortening it up some and raising the front mount up (both within reason) would likely be beneficial. Again, I realize I need to measure everything out.

Thanks again and any more input gladly accepted!