BC Racing BR type coilovers

[ricky-pinky]

Hi All.

I don't know whether anyone is interested in this thread any more, but I may as well finish it off.

When I installed these shocks the car didn't settle as far as I expected under static load. Just out of interest I checked the spring rate with a Rate Calculator I found on-line and the front was 9.6kg/mm and the rear 7.4kg/mm. However, these can only be ball-park figures since spring rates are very dependant on steel properties and the heat treatment process. I back calculated the spring rates from the distance the car settled under static load and the front came out to be 9.1kg/mm and the rear 7.6kg/mm. Maybe the torsion of the bushes and the pressure in the gas filled shock also contribute. But if this is the case then the original shocks would have had 6mm clearance between the bumpstop and shock when set up with the 5mm of preload as recommended by BC Racing.

I am currently running the front of the car 25mm lower than stock. Since my rear damper body is 30mm longer than the usual damper supplied with these shocks, I can only get the rear 20mm lower than stock using mimimal preload and the height adjuster on maximum low. I am in the process of getting a couple of high-hats fabricated for the rear to allow the shock to sit 30mm higher in the chassis.

I was pleasantly suprised at the ride with these springs - firm but not excessively so. In fact, with the damper rate set to full soft, you would barely be able to tell that the car has non-standard shocks. There are a couple of speed humps around my way which used to cause the suspension to crash against the bumpstops at anything over 50kph. I can now hit them at 70kph and the car just goes up and over and down the other side with the minimum of fuss, although this is probably more to do with the higher spring rate than the longer damper travel.

I am happy with these shocks, but whether the modification was neccessary I accept is a moot point. But I do like having plenty of travel.

And just for interest, while I had the springs out of the car, I checked the motion ratios and found the front to be 1.5 (i.e. when the damper piston moves 10mm the wheel moves 15mm) and the rear to be 1.4 around normal ride height. These figures do change toward the extreme of travel, but not by much.

Also, I when I examined one of the rear shocks, I noted that the bump rubber was pretty long. Also, it was quite worn on the end, I guess from regular contact with the shock. So I accept that MX5's do use their bump stops as part of their normal suspension movement. Mine won't, though!