Severe rear end wobble on hard accel

[Rich Kruger]

Thanks Howard. I’ll be busy this weekend. It is interesting that a inch or so could affect the bump steer so much. But I’m confident there is sound engineering applied with the design based upon the excellent overall package the SLC is as well as the other products RCR produces.

 

[Howard Jones]

Tom, I am working on a fix for that right now as a matter of fact. But the thing to remember is the standard setup (parts) will tune in just fine on a street car and at street car/tire performance levels. Just set it up correctly the first time and you should be good to go for the foreseeable future. But trying to extract all the possible grip out of a track car being driven in a completely different performance envelope is.............well completely different. You would never see the thing I am currently trying to tune for on the street. In fact, I didn't see it until I had taken more than 10 seconds out of a 21/2 minute lap while still trying to incrementally further reduce laps times a second or two at a time.

Rich, the washers in the picture above are 1/16" thick. I can dial mine in +/- one washer and see measurable differences. Having said all that I am trying to dial in a track car on slicks, learning how to drive it, and getting old simultaneously. If I remember correctly the toe change thing will not really be an issue in the middle 2/3s of the rear suspension travel. So if the spring rates and shock setting are pretty close, it's not really anything I would go to extreme measures to correct. These cars, given the correct setup and on modern street high-performance tires are frigin fast. Cornering grip limits probably should not be explored often and without runoff room factored in.

 

[Neil]

Initial setup. The toe link arm should be parallel to the upper a-arm as seen from the rear. Then measure toe change and make minor changes in the stack of the washers until you get the least possible change to toe as the suspension travels through its range up and down, I then install spacers to achieve this alinement angle for best toe stability.

Make measurements with the spring out and the car sitting on a block under the chassis to achieve normal ride height. Support corner suspension assembly weight with a jack under the lower a-arm, jack up and down with jack through the suspension travel range as you measure toe at the full bump, full droop, and at normal ride height. Set final toe to achieve no toe out at any point and about 1/16inch per side toe in at the point of max toe in. You can refine this to as close to no rear toe as possible but do not allow ANY toe out no matter what you do.

Here is an early on picture of my car when I finished my first setup. Note that the toe link arm is sorta in the middle of the upper and lower a-arm angles. Your car will be different but the concept is the same.

I agree with Howard. You NEVER want to have your rear suspension assume a toe-out condition; that generates a horrible instability. As Howard explained, checking your bump steer will tell the tale.

 

[Joel K]

This is an interesting thread.

Here is a pic of my rear upright. I put a thin AN washer on each side of the toe rod-end and a single washer 1/8” thick that is placed on the bottom. The thin washers take up all the slack in the stack…

I took some quick measurements and at ride height of 4.5” I get 2 degrees less angle on the toe rod than the upper control arm. So moving that washer up would further reduce the angle of the toe arm.

Rich, it will be interesting to see what measurements you come up with.

 

[Rich Kruger]

I swapped the links today in different combinations.I have 5 sixteenth washers stacked on the bottom of the long link. Ended up with about 0.2 degrees less angle on the tow arm than the upper control arm with zero toe at 4.5 inched rear tire height. I was limited on how much I was able to measure bump steer but It appears to not vary much if any through the range I measured, never toe out. Put 32 psi in the tires ( the roads in northeast PA are terrible, scared to bend a rim).Took a ride and its a different car. This LS3 becomes a new animal over 5000rpm.