Porsche 944 suspension
 

I have a 1987 944 and I have blown struts. I have decided on going with the the koni cup one coilover kit with coil sleeves for the front and full coilovers in replacement of the struts in the rear. I have done a fair bit of research and I dont want to delete the torsion bars. I've become very confused on how to calculate spring rate for the rear and how the coils will interfere with the torsion bars. Has anyone done this swap or something similar? I also want to lower my car a little to remove some of the wheel gap and I want it to be some what stiff but not ridiculous. I know I can adjust the spring plates in the rear and lower it that way but how will that change the stiffness of the torsion bars or coilovers? What's my best option for stiffening it up? I really dont want to reindex the torsion bars

There are RL postings on this subject. Do an advance search. One specific posting name to circle the search is Van for how to make coilover work with rear torsion bars.


Also, call Jason at Paragon Porsche. Paragon website has the equivalent spring rate for rear torsion bars. Adding coilovers, the equivalent rear spring rate is 0.56 the coilover spring rate because as the wheel travels one unit, the coilover spring moves 0.56 units

The rear spring rate is calculated by adding the T-bar rate to the coil-over rate adjusted for motion ratio.

1) the torsion bar rate is a 1:1 wheel rate. So, if you have 175lb/in T-bars you have 175lb/in wheel rate as one component. This is the rate for 25.5mm M030 T-bars for my '89.
2) the coil-over motion ratio was tested by Van to be 0.63 on his lowered race car. Take your coil spring rate and multiply it by .63^2 or .63 x .63 = .397 (You don't get much rate bang for the buck.)

So if you install 400lb/in coil springs you would add 400 x .397 = 159lb/in to 175lb/in = 338lb/in total rate

edit: other questions you asked
-coil-overs will not "interfere" with the T-bars. Their rate will add to the T-bar rate as shown above
-adjusting ride height using the spring plates will not significantly affect the spring rate of either T-bars or coil-overs. However, adding coil-overs can prevent the car from compressing (twisting) the T-bars as much, which means the ride height could/would increase with their addition. The static load is now being shared by two springs. So, you may not (probably not?) be able to avoid re-indexing the T-bars. This is probably why so many either 1) remove the T-bars and go with coil-overs only, or 2) go with higher rate T-bars and re-index once. (I just re-indexed bigger T-bars and was able to get exactly what I wanted in one shot given the procedure I found.) With 944 suspension there is no free lunch.

Good discussion.

Interesting...have typically used R(m) = 0.65 for stock ride height. Ergo...R(w) = 0.65^2 = 0.4225. I suppose R(m) could be effected by ride height.

Nice explanation!
Cheers,
Mike