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In Reply to: M5 compared to 540i and other stuff posted by John K on July 21, 2001 at 19:15:19:
The E34 M5 is stuck with the suspension design of its more sedate siblings. Struts front and rear, enormous understeer and modest room for the wheel/tire package.
My M5 was treated to a Dinan chip, brake and Stage III suspension upgrade early in its life. I don't have a clue about the spring rates, but the lowered the car about 3/4 inch and have probably sagged some too. The front bar is huge compared to the rear. In my opinion, Dinan did not size them correctly, I run full soft in front, full hard in rear and still wish the rear bar were stiffer. The camber plates result in about 2 degrees negative front camber. The last owner added 750 thrust arm bushings and replaced the Throwing Stars with 8.5 x 17 ET15 wheels. This, combined with the camber plates, allowed 255/40R17's to be fitted. The ride is pretty stiff and the handling is fabulous. The car is stable at speed.
However, violent tram-lining (darting from side to side) occurs on uneven roads, even at 30 mph. I concluded that this was due to the camber plates, wider tires and incorrect offset (should be ET20). My plan was to ditch the camber plates, install a stock wheel/tire combination and reduce rear camber. This last item deserves discussion.
All E34's are blessed with severe understeer. Front camber is often increased to reduce understeer and promote good turn-in. The same thing can be achieved by reducing rear camber, and I reasoned this would be a better way to achieve it, resulting in less tram-lining and more even tire wear. So I had the alignment shop do a baseline, camber was -2.0F, -1.9R caster and toe were great, except for a minor rear toe bias that was within tolerance (going be memory on all this).
Then I got to reading my handling and race car design books and found the kicker. Most suspensions are designed so that as the spring compresses, the top of the wheel leans slightly in towards the center of the car (camber curve). This is done so that as the body rolls when cornering, the outside tire is kept flat on the ground. For a MacPherson strut, camber remains nearly constant. THERE IS NO CAMBER CURVE. The factory put a lot of rear camber in so that as the body leans, grip increases and you don't spin out. I canned the idea of reducing rear camber.
Talking with a guy that road-races an M5, I have come to appreciate that we are limited by tire size. He is running relatively hard compound 275's front and rear (they rub). I have decided to keep the ET15's for now, and hope that some of the tram-lining will go away when I replace the Sumitomo HTRZ II's that came with the car (known problem with these tires).
If I were to do it from scratch, I would run factory springs, an adjustible front camber plate and look for a kit with the biggest rear sway bar I could find. I think there's a small Australian company...
Now to your car. I vaguely remember reading about fitting E30 camber plates. There was an issue with having to cut down the nuts that retain them (???) The person was willing to live with it, but not willing to endorse the idea. I'd keep them if I were you.
Stiffer springs, lowering and stiffer sway bars all reduce body roll. This is critical for making a MacPherson strut equipped car handle better, remember, we have no camber curve to counteract body roll. You will have to decide how you like the ride (spring stiffness) and compare the size of your sway bars with others before changing anything.
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