I'm having a very strange issue with my car.

At full throttle in 3rd gear around 100mph, the rear starts wagging it's tail. It's a very strange sensation. Almost like the car has rear steer and the rear wheels are steering back and forth.

The frequency feels a lot like a tank Slapper for those of you that ride bikes.

I've already checked the air pressure (front and rear) and all the suspension bushings and mounting points. The car never did this before and I haven't performed any work on the car lately. When the car is in the air, there is no play in the rear wheels and the car drives and tracks fine all the rest of the time.

I checked the front suspension as well, but forgot to check the front wheel bearings.

Anyone had a similar issue?

Thanks, Steve

 

I'd check your rear motor mounts. Also the trans mounting points to chassis.

 

https://www.6speedonline.com/forums/...eleration.html

 

My M3 did that, it was the RTAB.

 

This is waaaayyyyy worse than an M3 with worn RTAB's. Been there done that.

 

Thanks, but the drop links seemed fine. A little play, but very little play. I may try removing the drop links to see if it changes.

Later, Steve

 

Tail wag, I my experience has been two things. 1) lsd. 2) toe is moving about. Check toe arms and the lca eccentric to make sure that it is not moving as that will change toe also.

 

My Turbo's rear end acted strange under hard braking from speed and the cause proved to be a low rear tire.

Under acceleration I can't recall anything. Under steady state speed the rear end can feel weird enough I've pulled off the road to check for a low tire. Tire pressures were fine. The problem was road surface and worn tires causing the rear tires to squirm/wander and enough to make me really notice.

If the rear tires are really worn that might account for the behavior. But you need to make sure nothing else is wrong first.

Oh, and do not discount alignment. Around town at low speeds a bad alignment may only eat tires. At higher speeds it could have the car dancing around some. I'd not go right out and align the car. I'd like I said above make sure there is nothing else wrong, as in worn out, broken, bent, or loose.

 

+1 It's like a frequency right? Also check the track arms and bushings

 

ive experienced exactly that, but it wasnt gear or speed dependent, it was road dependent.
it was the eccentric bolts on the rear suspension that adjust the toe. the higher the speed, the harder the "snap".
ive had this exact same problem on my 996.

 

The issue is fixed.


I found two issues.


1. The bolt holding the lower end of my PSS10 coil-overs in the upright, was loose on both sides. It wasn't very loose, as I couldn't feel any play, but it took about 1/2 to one full turn to snug up the bolt. I need to get the correct torque spec now and loosen and re-torque the bolts. Interestingly, it was like this on both sides.


2. The alignment was off. Though everything was a little off, I believe the issue was the rear toe setting.


New rear toe setting: L=0.23 degrees and R=0.23 degrees
Old rear toe setting: L=0.13 degrees and R=0.32 degrees


The total toe was within 0.01 degrees, but the thrust angle was -0.10 versus 0.00.


It tracks straight now with the new alignment.


Later, Steve

 

Just to give an update.

After the alignment, my car didn't have the issue in 3rd gear, so I thought my problem was fixed.

Unfortunately the car began having the same issue, so I took it back and had the alignment rechecked. The rear toe settings had again changed, so we again corrected the alignment and made sure you tighten the bolts on the eccentric bolts very well.

It again seems better, but I finally did a pull up near redline in 4th gear and the cars starts the wobble again. Not as severe, but definitely there.

I beginning to believe that the alignment can make this issue worse, but isn't the root cause. I'm going to check air pressures again, but I believe there is something else going on.

My passenger side rear drop link has a little play (very little). It's not even bad enough to hear a clunk in the car, but I make unhook the swaybar and see if it makes a difference.

Later, Steve

 

I had a similar issue with my car due to the camber bolt not being tightened during alignment and the camber being way off. That was the bolt the technician doing the alignment told me wasn't tight, but I can't be sure that was definitely it. It may have been the toe. However, it was apparent even at lower speeds and was independent of gear, but it was also WAY off.

I can't think of any reason it would be happening only in a certain gear(s). Can you confirm it only happens in fourth gear, and only near redline? Do you notice it at all in fifth or sixth at the same speed? If it only happens in a certain gear near redline, that to me points to the engine and/or tranny mounts being at least part of the problem. That also makes sense to me with the parts in the rear loosening up repeatedly. There has to be some uneven (and significant) force being applied to them to force them to move.

 

It appears to be a false alarm.

I had a nail in my driver side rear tire and the pressure was down to 22psi. I had the tire repaired and raised the pressure to match the other side and had no issue.

I still believe I can feel it slightly, but it may just be me being paranoid.

Scott,

It's a very strange issue, because it only does it near redline in 3rd and 4th gear (never tried 5th. It's not speed dependent, because it stops as soon as I let off the throttle. It may do it in 1st and 2nd, but you accelerate through the last 1,000rpm so quickly, that you may just not feel the wiggle.

It's interesting that it appears load based, but doesn't do it at the peak torque rpm. Keep in mind that even though my car is a mild build (relative to this forum ), it still makes 516 wtq.

Either way, I'm not having the issue at this moment.

Later, Steve

 

A new update.

It appears that my car has a tendency to exhibit this issue. It becomes less frequent when the alignment and air pressure are perfect, but still happens randomly. If anything get off even a little, it goes into "tank slapper" mode every time.

I have replaced the front control arms and tie rod ends, so I know that's not the issue.

Unlike before, I starting to get a noise (similar to a wheel bearing, but with more pulsing), but I have checked the wheel bearings. It is speed dependent. Having the clutch in or out doesn't change the sound, nor does having the brakes on or off. It is not gear dependent. This noise was not apparent when the "tank slapper" issue first started, so I'm not sure it they are related.

I'm starting to wonder if it's the front differential and if one front tire is gripping and then slipping could cause the issue. I guess I need to remove the front axles, differential and drive shaft.

Anyone had a front diff go out? If so, what were the symptoms?

Later, Steve