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      08-26-2012, 04:45 AM   #1
mlhj83
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Rear Toe Keeps Changing

Need some help from geometry gurus.

I had my E92 ZCP aligned at a reputable shop with the Hunter Hawkeye 811 Pro Wheel Aligner and got everything adjusted to factory specs. However, upon leaving the shop, my steering wheel was slightly down on the left side, so I figured that they didn't centre it perfectly prior to adjustment. Took it back and they found that the rear toe had gone out again, and they repeated the adjustment 3 consecutive times - driving the car round the car park between each attempt. They managed to adjust everything to spec each time, but the rear toe seemed to go out of spec by a few minutes every time they put the car back on the rig.

The bolts were all torqued up to spec, and the car has only covered 21800, with no previous damage. Tyre pressures are correct and the car is completely OEM. What is going on?
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      08-26-2012, 10:16 PM   #2
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could the adjustments be loading the rear tires laterally and when the load is removed by driving, the alignment is changing slightly?
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      08-27-2012, 12:06 AM   #3
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Edit: A few minutes is not that bad at all. Factory front toe spec (non-ZCP) is 10 minutes +/-8 minutes, rear 10 minutes +/-5 minutes.

So, if your toe is only changing by a few mintues, that should not significantly affect the steering wheel centering.

Did you guys check your tire wear? How about swapping left/right tires to see if the steering wheel off-centers to the other direction?
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      08-27-2012, 01:24 AM   #4
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Quote:
Originally Posted by RLDZHAO
Edit: A few minutes is not that bad at all. Factory front toe spec (non-ZCP) is 10 minutes +/-8 minutes, rear 10 minutes +/-5 minutes.

So, if your toe is only changing by a few mintues, that should not significantly affect the steering wheel centering.

Did you guys check your tire wear? How about swapping left/right tires to see if the steering wheel off-centers to the other direction?
Thanks for your reply. My steering wheel centring was actually fine after the 2nd attempt at adjustment, the rear toe going out of factory specs was an incidental finding. Tyres are all relatively new and all of the same make (PSS), and are in great condition. Just to be clear, the rear toe was going outside of target range by a couple of mins. The shop said that there might be excessive play in the rear steering arm, which controls rear toe, although I find it hard to believe given the condition of the car and the pristine appearance of the bushes and rear suspension arms. I have done 4 track days but I highly doubt the bushes would wear that quickly. Will swap my rear tyres round and see what changes. Any other ideas? Could it be that the mere fact that the car is never going to be in the same exact position on the rig between each attempt, and that the steering wheel is not going to be pointing in the exact direction between each attempt, that is causing the change in rear toe read outs?
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      08-28-2012, 03:51 PM   #5
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If the rear toe is not the same from side the side it will create an un-zero'd thrust angle.

BMW clearly states in their TIS, that if a thrust angle of anything besides 0.00 is achieved, the car will pull to one side with an un-centered steering wheel.

In regards to the rear alignment going out repeatedly. It must be something they are doing during their procedure. They must find a way to relax the suspension (rock the car back and forth, settle the suspension, etc).

What I have seen from time to time, is that the adjustment hardware needs to be replaced. BMW also states this in TIS. If the fasteners have been loosened and torqued multiple times, they may begin to lose their torque holding capability.
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      08-29-2012, 10:54 PM   #6
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Their machine is screwed up.
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