I recently did my first track day of the season at The Ridge Motorsports Park in Shelton, Washington. When climbing through speed on the straight the car would start shuffling violently from the rear, starting at around 125 to 130 mph. I couldn't power through it much, with 146 being my highest speed of the day. I'd have to get out of it because it was getting dicey and I didn't want to end up off the track in a pile of rubble. Last year I could easily hit 151 or 152, although I do recall a small shuffle last year, but not like this. I got the car home and started inspecting the car and found that the Continental ECF rear tires are very slightly out of round when run in the air (like 1/4") and the After Dark Speed rear bump steer kit has very slight play (like .001") on both sides. Everything else seems sound and correct. I will also add that over the winter the motor, torque tube, transmission etc, all came out of the car as I blew the motor last year. I have torqued, retorqued, torque checked, etc, and the install checks out.

I had the car set up for this track day with:
- 3.0 camber on both fronts and 1/8th" toe out.
- 2.5 camber on both rears and 1/8th" toe in.
Viking 2-ways with 10 comp/10 rebound on the fronts, rears had 8 comp/8 rebound. I added 2 clicks to the rears between sessions with no change.
Car is corner balanced at 49.6% and weighs 3150lbs. No real aero, only a zr1 splitter/spoiler with a small wicker-bill.

Before I start throwing money at this problem, does anyone have any insight that may be helpful? Thanks!

This is my normal setup. The car rips, other than this issue.

 

Have you check the tie rods? Or maybe the transmission/diff mounts?

 

Yes, that is the bump steer kit. Very minimal play (.001"). The rear transmission mounts are brand new AMT solid mounts.

 

What about rear toe? The only time I've felt what you're describing was in a 2019 ZL1 where the rear end felt loose and disconnected from the rest of the car. Hopefully it's something simple, can't think of anything else that it could be.

 

I'd consider if the drive line is out of balance if you didn't find the problem. Did you have it balanced while you were in there? It should be smooth as buttah.

 

That is what I've been thinking as well. I could give it some more rear toe in before my next track day and see what happens.

 

I would expect more of a vibration from that, not a rear oscillation. I could be wrong. Thanks!

 

put the car one blocks (loaded suspension) and run it up to speed. If you have a driveline vibration you will feel it.

I would be suspicious of the bump steer kit only because it's the unknown.....I'd try loading the rear of the car up and verify the toe is staying in the range you want throughout its movement.

Verify your concentrics also are locked solid (probably camber plates). 1/4" out-of-round tire also seems like it could be pretty serious >140

 

I'd also be suspicious about a 1/4" out of round tire...
Not so much that the1/4" would cause erratic behavior, but whatever is causing that out of round might be a broken belt?

 

I ran it at idle in the air. I will put some more speed to it and see what happens.

I don't like that damn bump steer kit anyways! So hard to adjust!

I do have camber plates... Thanks for the suggestions! I will take a look.

 

Good point! I'm tire shopping right now!

 

i'm no suspension guru, but I run 1/16" toe IN in front and 3/16" toe IN in the back.
When I first got this car, I tried 0 toe front and back and car was scary above 130mph

 

I will definitely try those numbers the next track day! Thanks!