The sixth post in this series is about Type Three suspensions – club-sport suspension kits – and the M3.
Club-sport kit designers have no excuses for not producing a good product. They can select from a broad range of springs and dampers, and they don’t have to be daunted by the travel issues that plague other designs.
That said, it’s not a cake-walk. Essentially, you have to allow for at least 100% weight transfer (ie – when your inside front tire is in the air in a corner), preferably more, within the available travel of the suspension. While the stock suspension travel is limited by the damper design, it’s not because BMW couldn’t have made it longer. The tire will hit the inside of the wheel-well at some point, and that’s the true limit of suspension compression. Of course, the clientele that buys club-sports is going to run wider tires than stock, so just letting the tire ride up into the wheel well is just not going to work.
What the Type Three designer can do then is pick a spring rate that keeps the tire under control and allows for 100% weight transfer. That’s about a 500 pound per inch spring at the front. If you’ve got a droop problem, a helper spring will take up the space. Aren’t racing springs wonderful? It’s just that easy, at least at the front.
The rear’s not so easy. Remember the point about the rear spring rate limits because of the suspension design? You know, where a high-rate spring will try to rip the rear subframe right off the bottom of the car? Yeah, that one.
Your Type Three designer is on Easy Street with the front, but the rear is going to be a compromise. The stock rate is 600#/inch, yielding 200#/inch at the tire. There’s a safety margin, but it’s probably not much more than 50%. Let’s say that rear rates over 900# per inch are out of reach for mechanical reasons. That means you can raise the front rate by a factor of 3 from 167#/” to 500#/” but you can’t raise the rear rate from 600# to 1800#. The subframe would be in serious jeopardy.
So you have to go with what works. KW has decided, after a lot of testing, that’s 800#/”. In my survey of suspension kits, I never found one that was higher than that. So, you can triple the front wheel rate, but you can only increase the rear by 33%.
As for what’s available in Club Sport, there are only two: KW and Moton. The Moton kit is the more expensive of the two, and it’s not clear how they’ve dealt with the rear suspension issue. The limited documentation available says “coil over”, which is the only real solution. More on that in the Type Four post.
Last edited by JAJ; 01-10-2010 at 02:15 AM.