[JAJ]

The fifth post in this series is about Type Two suspensions – Street-sport suspension kits – and the M3.

Street-sport kit designers have more tools in their toolbox to attack suspension travel problems. They can use radically wound springs (ie. Bilstein PSS10) to get a high rate in the “normal travel” range of motion, while keeping the ride height at rational levels. They can also change the damper body height and piston stroke to control static height and manage free travel. Either or both might be required to get the springs to deliver the correct ride height and rates.

The challenge that Type Two designers can’t overcome easily is the problem of managing the last two inches. Bilstein solves it by saying there’s a limit on lowering. Moton, in the first version of their kit, didn’t solve it, and their increase in travel (that improved grip) left the tires free to grind holes in the fender liners. Moton is very competent, they’ll figure it out. The key point is not that they messed up; it’s that when someone that good doesn’t get it right, then it’s a really hard problem.

If you look at pictures of the springs, you see one interesting difference between Bilstein and the others. Notably, the Bilsteins have a two-stage progressive wind. The rate is low at low compression, high at high compression and the transition will be quite sharp. It’s much like a racing spring/helper spring combination – two rates that become one high rate for all practical purposes when driving. Other springs tend to look more linear, with one pitch that is closer to stock.

So, what’s available?

- Moton Streetsport kit (double adjustable dampers, height adjustable springs)
- Bilstein PSS10 (single adjustable dampers, height-adjustable springs)
- KW Variant 3 (double adjustable, height adjustable springs)
- AC Schnitzer / Sachs (Single adjustable, height adjustable springs)
- (Future) Ohlins Street Sport (Single Adjustable, height adjustable springs)

One observation I’d like to make on the Type Two kits is that it’s a hot market. Everybody wants in!

Both Ohlins and Moton, damper-makers to the stars, have introduced kits. For Moton, it’s the only twin-tube shock they make, and the initial results have been challenging, shall we say. My opinion on the market (not the products) is that the damper companies who’ve made a good living for years selling high-end racing dampers saw an opportunity to design to a price-point and deliver kits to a waiting enthusiast market. Springs and mounts are new territory, as are low-priced dampers. Teething problems will be rampant. Bilstein, who’ve been doing this for years is in a good position, KW not far behind. The new guys will struggle to find their way.

Overall, the Type Two street-sport products will outperform lowering springs, in that the designers can get the best of the travel design variables and good streetability. For track work, they are limited because of the lack of camber adjustments. Go out a few times a year, they're going to be great. On a steady diet, they might show some limitations. Overall, they'll make a great-handling car handle a bit better without the downside of lowering springs.

They're good value.