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Date: 20-May-1997 16:46:01
From: Pete Read
Subject: Re: <E24><E28> Worn Diff Mount Causes Negative Camber

Don Mies doesn't see how a worn differential mount, which lowers the differential height, can cause extra rear wheel negative camber on an E24/E28 trailing arm suspension.

Then Dale Phelps explains that the half-shafts are fixed length and push the hubs out slightly when the differential drops below its design height.

Dale is right about a worn differential mount causing negative camber, but not for the reason he mentions. The half-shafts don't push the wheels out. It's actually caused by movement of the subframe which is rotated slightly by the weight of the unsupported differential.

I've seen this worn differential mount causes negative camber mentioned a number of times, and have always agreed with Don's point of view. I couldn't see any way that differential height affects camber.

The motion of the rear wheels is completely controlled by the relationship between the trailing arms and subframe, to which they attach. Because of the orientation of the trailing arms, as the suspension compresses, negative camber and toe-in increase. That is why sagging springs or shorter aftermarket springs cause more static negative camber.

After this latest mention, I decided to renew my search for a reasonable answer. At the Summit Point Vintage Car races this weekend, I bumped into Tom Baruch, a very experienced BMW mechanic and driving school instructor.

Tom explained that the differential hangs off the back side of the subframe. If the rear diff mount doesn't properly support the differential, the weight of the differential rotates the subframe downwards towards the back of the car.

This rotation of the subframe affects negative camber the same as compressing the suspension. It changes the orientation between the subframe and trailing arms.

While I can't take credit for the explanation, at least I was able to recognize a good answer <grin>.

Pete Read
'88 M5
Arlington, VA

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