The seventh post in this series is about Type Four suspensions – DIY Engineering Specials – and the M3.
The Type Three suspensions solve most of the front suspension travel issues. They might be a bit stiff, but you’re in linear mode with reasonable travel most of the time. It’s the rear suspension where the going gets tough. Type Four suspensions essentially pick up at this point and keep going up the development curve.
The bad news is that unless you can afford to buy a Type Three suspension kit and throw away the rear suspension, you have to start over at the front. You have to find dampers and strut bodies that will fit and work in the front of an M3. Moton, KW, Bilstein and Ohlins *appear* to have parts that work but you’re in territory where there are no guarantees. YMMV takes on a whole new dimension. You can also go with front kits from Ground Control and TCKline – I assume they’ll sell you axle sets one at a time. My own dealings with GC were very satisfactory on this point when I was upgrading my E39 M5.
The solution to the Type Three rear suspension compromise is to convert the rear to coil-over. By putting the coil spring around the damper, the motion ratio goes up from 57% to 83%, drastically reducing the leverage on the subframe. It increases the load on the body where the spring mounts, but the leverage is much less than on the subframe mounts. Also, because the action is more “direct”, the spring rate is much lower. If you want the rear rate to be a good match to a new, higher rate 500# front, then you can use a 600# coil-over spring, delivering a rear wheel rate of just over 400# that will work nicely. To do the same using the stock spring location would call for an unhealthy 1200# spring.
At first, the coil-over conversion sounded a little abstract to me, so I spent considerable time looking at photos of the BMW M3 GT4’s racing. I eventually found one with a shot of the rear suspension with the car in the air and the wheel off. Sure enough, it was a coil-over rear spring setup. It’s what the big boys use. Interestingly, the photos of that car also made it clear that the rear dampers and the front dampers were from different suppliers. The rear dampers had an “Ohlins” look to them; the fronts were probably Moton, if the orange color means anything.
So, what are your choices? Well, you can contact Sachs Racing, Bilstein Racing, Moton, Ohlins, KW and probably a few others to get real racing dampers that will support a rear coil-over setup. While nobody is talking about it, I suspect that a call to Turner Motorsports (who make a race damper spherical ball upper mount for the M3) would turn up other choices. Certainly, GC and TCKline have the capability, although perhaps not the motivation, to build something for you using Koni single or double-adjustable dampers.
I wouldn’t be surprised if sometime soon one of the Type Three club sport companies comes out with a coil-over rear setup for the M3. In fact, it’s possible that Moton has one already – my research showed that they refer to a coil-over rear setup on the Moton Clubsport, but I found no drawing or picture that confirmed it.
Last edited by JAJ; 01-10-2010 at 01:44 AM.