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Rear end issue at the track, need some help diagnosing

Ok track guys I need some help diagnosing an issue. This past weekend at the track I started experiencing an issue with what I believed to be the rear end. I ran my first session at MPH in cool temps (47F) and everything went great. There was a lot of traffic and I only got one clean lap and it was 0.2 seconds faster than my best lap over two days at the Mustang Roundup a month ago. In the second session I started getting an issue with what I thought was the rear end in Turn 12 which is a sharp left-hander that comes right after a sharp right hander. The right rear "felt" "sounded" like a growl/vibration and I seemed to be getting excessive rear tire slippage when I laid the power down at apex. The noise/feel issue was only on left hand turns and mostly just in that turn. However, I felt that I was getting excessive wheel slippage powering out of most turns. My lap times climbed 2-3 seconds, but part of it was probably the fact that I was backing off a bit being concerned about the rear end coming apart. We put the car on jack stands and one of the track mechanics diagnosed it as outer wheel bearings on the rear axle. We took the rear end apart, inspected the gears and axles and there was no apparent wear or damage. There were NO metal filings in the fluid or at the bottom of the diff. We also replaced the wheel bearings to eliminate that issue since there seemed to be excessive play according to the mechanic. The bottom line, the rear end, Torsen and axles all appear to be fine so shouldn't be an issue.

I went back out the following day and experienced similar issues, albeit maybe not as noticable. My lap times were more like 1.5 seconds slower, but so were a couple other guys so could have been conditions. When I got home I looked at my tires from Friday and noticed some very weird wear patterns on the rears. There's a 3" section that's worn more than the rest of the tire but doesn't go the full circumference. The rears also seemed to wear way faster than normal. I don't wear out the rears very fast running R6s or Conti slicks, but I go through fronts due to cording the outer edges. Someone suggested possibly a brake issue and I'll have them inspect for that.

Here's the kicker. When I got home Saturday night and went to unload the car off the trailer, it wouldn't start! There was plenty of voltage and the gauges would sweep and fuel rail charged, but wouldn't turn over at all. We even put a jump pack on it and nothing. I've had a "check charging system" error only on track. The dealer can't diagnose it because it never does it on the street and doesn't leave any codes. I'm hoping whatever is causing it not to start is connected to this issue so we can kill two birds with one stone. I should have this diagnosed today.

Any ideas on what potentially could be going on with the rear end would be appreciated. These wear patterns have me puzzled. I did email the pics to Hoosier to see if they've seen it before. Best case scenario would be the tires were just bad and I don't have an issue with the car.

20121020215517184.jpg


20121020215319768.jpg


20121020215719850.jpg


20121022061335909.jpg
 

pufferfish

Supporting Vendor
1,094
66
Maryland
i can only speculate, of course, but i have had issues with the TCS reducing slip by applying brakes. yes, even with the TCS off-off (the hold the brake and button for 7 seconds procedure). granted, i have a GT, not a boss. but the wear "stripe" looks like a brake lockup on a road paint line? another thought, have you checked your rotors to see if they made contact with the caliper bracket?

good luck. hope you get it figured out
 

Sesshomurai

Bad shock/strut/damper? Just a wild guess. If its damping poorly or the shock/strut is bad the tire could be rubbing? (also check bump stops) Any wear on inside the well?

I think the car starting issue is unrelated.
 
pufferfish said:
i can only speculate, of course, but i have had issues with the TCS reducing slip by applying brakes. yes, even with the TCS off-off (the hold the brake and button for 7 seconds procedure). granted, i have a GT, not a boss. but the wear "stripe" looks like a brake lockup on a road paint line? another thought, have you checked your rotors to see if they made contact with the caliper bracket?

good luck. hope you get it figured out
I agree the stripe would indicate potential brake lockup....possibly on the apex curbing where all the tire isn't fully contacted? The rotors look good....no excessive wear from heat or any visible damage.
 
darreng505 said:
Bad shock/strut/damper? Just a wild guess. If its damping poorly or the shock/strut is bad the tire could be rubbing? (also check bump stops) Any wear on inside the well?

I think the car starting issue is unrelated.
I wasn't suggesting the car starting issue is related to the rear end issue, but hopefully to the "check charging system" warning I've been getting on track ionly the past few months.
 
Once there is a flat spot the tires will cause all sorts of issues with the electronics. So it is possible you streaked them early on and the ABS went nuts after that because of the traction changing as the tire spun. I had a similar result from picking up massive amounts of marbles when the instructors made us do side-by-side off-line laps, my rear ABS went bonkers until I scraped them all off with a scraper. One of your streaks starts at a big marble but probably coincidence.

Did you change tires between days or did you mean that the pic was taken on the first day but you used again second day? If it is a flat spot issue then maybe they wore down enough to even out a bit so it got better the second day? Did you swap tires side to side? That could also move the grip point around so that the flat spots are not affecting as much.

If, and it is a big if, the flat spot caused the issues then you would still need to figure out why you flat spotted them initially. You use take offs? If so did you inspect them well? Maybe they were used on a non-abs car and had some damage already. Or a big marble caused the ABS to go wonky and that made the tires slide. I'm no expert as I run RA1s/888s which are not as prone to these issues as gumball slicks, and I have only used the stock tires on the Boss.



edit: oops puffer's post mixed me up. Your issue was powering out of corners. The flat spots can still cause that issue, even without TCS interfering. If that is the reason, then figuring out why they flat spotted is the next step. Hoosier should be able to tell you if those streaks are enough to cause the issues.
 

ArizonaBOSS

Because racecar.
Moderator
8,545
2,376
Arizona, USA
Gary did you guys look at the VSS magnets on the rear to make sure everything is clean and there are no shorted wires for those sensors?

Might be time for the 302R ABS controller.
 
The tire wear patterns are strange indeed. How did your Saturday tires look? Does the darker section feel higher or lower than the rest of the tire? If the problem had gone away after changing the rear tires, I would say it looks like a defective tire.
 
Your discription sounds like it is the differential. You did state ther was no metal filings in the gear oil, but maybe one of the worm gear shafts in the torsen diff sheared? I for one, do not know alot about the torsen diff other than how it works. When taking off from a standing start, can you spin just one wheel? or are both still spinning? If only one spins then if would be a diff problem for sure. And not that I have a lot of experiance with tires (as I do not), I have never seen wear patterns like that. They sorta look like spin marks?
 
All good suggestions. I stopped by the dealer this morning since I had a meeting in the area. They had it up on the lift and had gone through everything inspecting for rubs, or anything unusual. I looked closely at the rear rotors and they looked perfect. They aren't even heat checking and no visible grooving or hot spotting you'll sometimes see on them when they're getting cooked, so that rules out a brake issue.

The starter was bad and that's why it wouldn't start so that will be here tomorrow. I doubt that's going to solve the "check charging system" issue though, but I won't know until my next track day in April after the long winter.

I like the suggestion of doing a burnout to make sure I don't have a "one wheel peel". Kevin is going to take it for a spin when they're done with it just to get his take since his GT has the Torsen and he knows how it's supposed to feel. I'll have him do a burnout in the mall parking lot to check that.

The tires on the car now don't show the unusual wear from the Friday tires. It's looking like those are just defective, almost like they're delaminating. There isn't a noticeable difference in tread depth like you'd expect if they were peeling apart as some have had with the Corsas, but they definitely look defective. If the burnout shows two stripes, I'm going to have to chalk it up to defective tires.
 
I was putting the car back into street condition yesterday and found my problem. Unfortunately it was a front end issue creating looseness in the rear end on left hand corners which had me looking in the wrong place.

Healthy driver side front sway bar mount:



Uploaded with ImageShack.us

Passenger side, not so much:



Uploaded with ImageShack.us

I'm just glad I figured it out. I was starting to think I was "imagining" things ???
 

PeteInCT

#LS-378 - So many Porsche's, so little time....
Moderator
2,848
13
Connecticut
Hmmm, I think I remember asking you if you were sure it was coming from the rear! ;D (just joking) Funny that no one else saw that...
 

Grant 302

basic and well known psychic
cloud9 said:
I was putting the car back into street condition yesterday and found my problem. Unfortunately it was a front end issue creating looseness in the rear end on left hand corners which had me looking in the wrong place.

Healthy driver side front sway bar mount:



Uploaded with ImageShack.us

Passenger side, not so much:



Uploaded with ImageShack.us

I'm just glad I figured it out. I was starting to think I was "imagining" things ???

Crazy. What broke? The strap or one of the bolts?
 

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