Spring Rates for the Rear of the M coupe

After running the M coupe on the track for two years and racing it for 1-1/2 years, I thought I would write up a little information on spring rate setups for the rear springs . I'm no expert on this subject but I believe this information is good food for thought.

I've had problems with the back end of the car coming loose when on the track. The rear spring gets loaded up, coil-binds in the corner and the suspension stops traveling, back end of car starts to twitch and breaks loose. I'm running a coil-over setup in the front and a ride-height adjustable springs in the stock rear location, shocks are Koni double-adjustable front and rear.

FYI, Spring rate comparison between the M roadster and M coupe can not be done, because the weight of the M coupe and the distribution of the weight is different. So be careful on the information you hear and collect, if you know what I'm saying.

I have tried the H&R race springs and springs from different kits they sell in the rear stock location and they all coil-bind, a common cause of the rear end locking up and then breaking loose. Part of the problem with H&R springs in our stock rear position is the size of the wire used to make the spring and how the wire is wrapped in the making the spring. Eibach springs on the other hand use a smaller diameter wire and they wrap the wire differently when making the spring. If you are going to use a "race spring" and adjustable ride-height collars in the rear stock position go with Eibach.

Now, numbers like 450 lbs/in of spring rate in the stock rear location is very light and will coil-bind just driving it on the street. How do you tell if a spring coil-binds, using a cable (wire) tie, you know the little plastic ties, wrap it around the wire of the spring on a few of the coils. Do this on a few of the coils in the center section of the spring. Do this on a few of the coils facing the front of the car and on the rear facing coils, drive the car and then look to see if the cable ties are still on the coils of the spring or have any indentations in them. Little indentations in the ties, some might say that that's just right, but if the ties are gone and the powder coat of the spring (in the center section) starts to show contact marks, you need more spring rate. Testing the spring rate on the street is good for just that the street. Track setup will need to take into account many other variables like; tires DOT or slicks, tire pressures, tire temps, sway bar settings, etc.

I'm not saying that H&R Race Springs are bad, just don't us them in the stock rear location. Now if one were to use them on a true coil-over system for both the front and rear, that's a completely different setup.