Suspension Issue - Car Crab Walking On Turns

[r0nd3L]

Hi All,

I may have a potential problem with my car that I'm not able to solve. Actually, I'm not sure if it is a problem or me being crazy.

Problem: On higher speed sweeping turns, toggling throttle between 0% and 20-30%, the car feels like it changes directions from the rear suspension. On throttle, it feels like rear end changes directions and starts "crab walking" with rear end swinging towards inside of the turn which forces me to provide more steering input to maintain same trajectory. Lifting off the throttle, tightens the car and I have to remove steering input. Remember, this is at light partial throttle, not at the limits of grip. When attempting to drive more aggressively, this also presents a problem on the sweeping turns as it's hard to find confidence.

I was hoping this had something to do with the soft bushings in the rear, so everything has been updated and the issue still persists. What is interesting, I recall similar feeling on my M235i as well as my friend's 997TT RWD which I recently drove. On another hand, my F80 M3CS and E70 X5 (xDrive Delete), do not behave in this way. Basically, can somebody tell me I'm crazy or there is a legitimate problem?

Current Setup:

• 2009 135i 6MT
• 500WHP
• Ground Control Koni Coilovers 300/700 (Before: 400/650)
• Front M3 Control Arms
• Rear M3 Camber arms w/OE bushings and bearings
• Rear M3 Upper Links
• Solid Aluminum RSFB
• Ground Control Toe Arms
• BimmerWorld Trailing Arm Bearings
• MFactory LSD
• New M3 Front Diff Bushings, New OE Rear Diff Bushing
• Front Camber: -2.4 / -2.4
• Front Toe: 0.05 / 0.05
• Rear Camber: -2.0 / -1.9
• Rear Toe: 0.17 / 0.19

[fe1rx]

I suggest putting a camera on the car looking at the rear tires. See if you can see any toe change under the conditions you mention. Check both sides.

To some extent the characteristic you describe is expected, but due to shifting grip due to weight transfer and rear tire thrust/drag. If you see a toe change, presumably the remaining rubber bushings in the rear are worn.

[r0nd3L]

Originally Posted by fe1rx

I suggest putting a camera on the car looking at the rear tires. See if you can see any toe change under the conditions you mention. Check both sides.

To some extent the characteristic you describe is expected, but due to shifting grip due to weight transfer and rear tire thrust/drag. If you see a toe change, presumably the remaining rubber bushings in the rear are worn.

Thanks for the response!

I actually did film the rear suspension few weeks ago, but didn't see anything out of ordinary besides the differential moving quite a bit, which I assume is normal due to soft rear diff bushing. I did try diff lock-down brace, but that did not change the behavior and was removed. I did try more or less rear toe-in, but also did not make a difference.

The only remaining original bushings are the lower camber arm ball joint in the wheel carrier and the inner rubber bushing in the sub-frame. Both of them looked in great shape, but perhaps I should change them anyway. Everything else is new.

Technically, I agree, what I describe could sound normal based on the conditions. However, it just feels wrong in the driver's seat, if that makes sense.

[hoki06]

Sounds normal to me and you're experiencing throttle lift oversteer. Lifting off throttle mid-turn shifts weight back to the front of the car which increases grip and will help pull the nose in towards the apex. Reapplying throttle will have the opposite effect as it shift weight back to the rear and understeer. Based on the vehicle dynamics I would expect different steering inputs for each situation.

I used to use this technique around the carousel at Summit Point to help tuck the nose in between turns 6 and 7. Only takes a credit card thickness of throttle lift to tuck the nose in and help set up for the next turn.

Your LSD lockup characteristics will also play a role in how the car behaves in this situation.

[r0nd3L]

Originally Posted by hoki06

Sounds normal to me and you're experiencing throttle lift oversteer. Lifting off throttle mid-turn shifts weight back to the front of the car which increases grip and will help pull the nose in towards the apex. Reapplying throttle will have the opposite effect as it shift weight back to the rear and understeer. Based on the vehicle dynamics I would expect different steering inputs for each situation.

I used to use this technique around the carousel at Summit Point to help tuck the nose in between turns 6 and 7. Only takes a credit card thickness of throttle lift to tuck the nose in and help set up for the next turn.

Your LSD lockup characteristics will also play a role in how the car behaves in this situation.

Generally, I would agree, but this is casual driving and can be replicated at less than 10-20% throttle slowly rounding a sweeping curve. This cannot be replicated on F80 and E70 I drove.

I understand the dynamics on the race track and performance driving as I was racing in NASA GTS2 for a while with my E36.

[r0nd3L]

Well, ya'll will laugh at this. After tons of troubleshooting things such as checking alignment, tire pressures, diff brace, I found the problem. The upper camber link to carrier bolts came lose at some point after install and first alignment was done. Good lesson to double check the torque of all the fasteners after major overhaul. Note that installation and alignment was done by yours truly , so nobody else to blame but myself!

fe1rx , your video suggestion helped as I noticed the wheel was moving forwards quite a bit under slight acceleration, which prompted me to check everything for play. Thanks for that

That said, it seems my issue is mostly resolved, but I will need some more seat time. Currently waiting for E85 tune to be sorted, so haven't driven it much.