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Old 12-26-2003, 04:24 PM   #43
Join Date: Feb 2002
Location: Stance
Posts: 13,107
Okay, here are some quotes from reputable members of the E30 community:

I've used Bilsteins with all of my various spring setups. They work best with stock springs and are ok with sport/lowering springs such as H&R Sports or Eibach Prokits because they have good compression damping, but they don't have nearly enough rebound damping to contro the rebound of stiff race springs, especially on the rear. BTDT.

Koni Single Adjustables are a *huge* improvement over Bilsteins when you're using stiff racing springs. My car now settles immediately over bumps and is especially noticeable on the highways and freeways at moderate to high speeds.

You can have Bilsteins revalved with a lot more rebound damping (i did this once) but for the money you can get Konis which still do a better job, with the added benefit of being adjustable. I adjust my shocks a lot and i can never again do without this ability.
Konis have a lot of rebound damping so they somewhat mimic the effect of a larger swaybar (controls shock extension so the inside of the car doesn't lift as much in a corner). Bilstein Sports/HDs don't have nearly enough rebound damping to control stiff springs and they also don't inhibit body lean like the Konis do on the stiffer settings.
I had Bilstein Sports with H&R Sports and the shocks were barely able to control the rebound. When i went to the Ground Control setup the Bilsteins severe lack of rebound damping made it impossible for them to control the stored energy of the stiff GC springs (450 fr & 600 rear) especially at the rear. Bilsteins Sports and HDs are good with stock springs or other soft springs because such springs don't have a lot of stored energy after compression, but they don't work well with stiffer springs unless you have them revalved with a lot more rebound damping.

I recommend Koni adjustables for stiff springs because they have WAY more rebound damping than Bilsteins and that rebound is adjustable so you can fine tune them to a wide range of spring rates.
Bilsteins act much differently than Konis. Bilsteins have a lot of compression damping but have hardly any rebound damping. They're hard to compress but very easy to pull apart - actually if you compress them and lay em on a table they'll expand all on their own until they reach full length. This action works well with stock springs - the compression damping helps add to the car's stock spring rate so the car doesn't dip as much when it hits a shallow dip (as an example) so the springs don't compress as much as they did with stock shocks. Also, the low spring rate of stock springs means they don't rebound as hard so they don't put much demand on rebound action of the shock. For this reason the Bilstein is well suited to the stock springs.

Now with Stiffer lowering springs things change, these springs have a much higher spring rate so they don't need the help of the Bilsteins' stiff compression damping - the stiff springs are doing more to keep the car from dipping on that same shallow dip, not the shocks. But these stiffer springs bounce the car upward a lot harder after they've compressed (like a pogo stick effect) and since the Bilsteins have almost no rebound damping they can't harness the stored energy of the rebounding springs so the car bounces upward with nothing to stop it's motion. Then when the springs have fully extended the weight of the car pushes down again and the stiff springs bounce the car upward again. This is really bad on the rear of an E30 with stiff springs so you need shocks with a lot of rebound damping, and the Koni has lots of rebound damping.

So, Bilsteins work well with stock springs, but aren't enough for stiff springs. I tried them with M3 springs, H&R Sports, H&R OE Sports, then finally with stiff Ground Control race springs. As i went stiffer and stiff with the spring rates, the Bilsteins became more and more useless. Once i installed shocks with a lot of adjustable rebound damping the suspension is much more controlled and settles immediately both on the highway and autocross course.

I went through the same development with my former kick-ass 85 Golf GTi canyon/track/autocross car and once again the Koni adjustables workedway better than the Bilsteins. I'd only use Bilsteins with stock springs, but with anything stiffer than stock i'd do with the Konis.
This is where I get my info from.
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