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
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
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>.