I am sorry to day that a similar symptom (which may be linked to the same problem, or a different one) has been bothering me since I bought the car (second-hand and out of warranty). It is about a slight steering wheel wobble at speeds within 70-95 mph range.
In order to avoid any confusion, by "wobble" I mean a short amplitude but with quite high frequency clockwise / anti-clockwise rotation of the steering wheel. There's no horizontal nor vertical shaking.
It is strongest around the middle of the indicated speed range. It is not violent: by simply holding the steering wheel I can actually cancel the movement, but I can still feel the tendency to wobble. It is best noticed when I leave the steering wheel free on a straight road. Braking does not influence it. It shows only at steady speeds or very slight acceleration, as stronger acceleration seems to tame it down.
I've taken the following actions (over a period of 1 year and a half), trying to solve the wobble issue:
1. Replaced the front tires.
2. Balanced the wheels over and over again, tires pressure checked very often, rim bolts torqued correctly.
3. Tested extensively (about 700 miles) with another set of wheels (=different rims and tires). The rims were stock ones (style 224) and the tires were half-worn Michelin Pilot Cup, albeit with a different size than stock: 225/40x18 front (instead of 225/45x18 stock) and 265/35x18 rear (instead of 255/40x18 stock). I can say that the amplitude of the wobble was slightly reduced, but probably it may be linked to the smaller overall diameter of the wheels. Unfortunately, due to the rarity of E86 Z4M in my area, I don't have a friend that I could barrow the wheels from for a short test.
4. Replaced the front bearings (for a different reason than steering wheel wobble: the left bearing was worn and noisy at only 30.000 miles).
5. Checked the steering rack for play - tested OK.
6. Checked the front and rear suspension for worn components - looked OK.
7. Replaced the CABs (original BMW) on original front control arms.
8. Replaced the steering tie rods + tie rods ends.
9. Replaced the front control arms (Lemforder) and again the CABs (original BMW).
10. Adjusted the front and rear geometry twice, at two different BMW dealers.
11. Replaced the front brake rotors and pads (but I still suspect that this set of rotors, although new, was not properly balanced from factory).
12. Cleaned the contact area between rims and brake rotors.
And then, after spending a small fortune, I stopped. After all the effort, I still have the same problem.
The BMW dealer is stunned. They said that my best shot would be to replace again the front tires with a top brand (e.g. Michelin). Could be, but considering the points 1 and 3 above, I have reasons to doubt it would solve it.
I suspected flat spots on tires, because the car sits for 5-6 days in a row between trips, as I only drive the car during weekends. However, the wobble never goes away, even after 200 miles long trips.
A few notes:
- The amplitude of wobble seems to vary a bit, from one trip to another. But it's always present.
- Once I managed to make the wobble disappear almost completely. After step 7 and step 8, the amplitude was reduced to almost negligible, and only around 80-85 mph (the middle of the speed interval mentioned above). But that was short lived, as step 8 was not followed by a geometry alignment, and I believe that I dialed too much toe-in when fitting the tie rods. Step 9 plus a geometry alignment brought the wobble back to its usual form.
- The wobble seems to be reduced in very hot weather. Also, by setting the front toe towards the toe-out limit of the accepted range, the wobble amplitude is a bit lower.
I am extremely interested in hearing other owners' experiences on this issue.