924/944/968 Frequently Asked Questions

Torsion Bar Spring Rates

The following note was created by Jim Pasha.

I have a 1984 944. Someone suggested some larger torsion bars for the rear. What do the diameters correspond to for conventional spring rate?

I did some calculations some years ago and the following factors apply to the rate. The lengths and diameter of the bar are the largest contributors. The material is the next largest contributor.

Considering the material used, diameter and length, this is what the diameters represent:

Length = 675 mm (26 5/16 in.)

diameter (mm) raw rate (lb/in) effective rate (lb/in) original application
22464292Stock 924
23554344Stock 944
23.5628379VW Transporter
25773486Stock 944 turbo, 944S2 and 968
26904568 
271062661 
281212765 
291400886 
3016031008 

This is only part of the equation. The figures are only for twist of the bar itself. If you add the spring plate, you have to factor in the total distance of the rotation, center of the torsion bar to canter of the rear wheel. That is the effective lever length. As the lever arm gets longer, the spring rate decreases. The lever, spring plate in this case, will factor into the equation and reduce effective rate.

One size torsion I did mention was the 23.5 mm VW transporter torsion bar which I found to work very well at the rear of some early 924s I've owned. This was something we did in early 1977 to greatly improve rear suspension spring rate. It was very cheap to do though took some time to make the changes. As the 924 family rear torsion bars are based on VW units, any of those will work at the rear of these cars.

Why do you think Porsche changed from torsion bars to springs with the C2/C4?

I think the big thing was the use coil springs allowed for easier changes and progressive rate springs. A major disadvantage is that torsion bars, unlike coil springs, usually cannot provide a progressive spring rate, forcing a compromise between ride quality and handling ability - progressive torsion bars are available, but at the expense of durability since they have a tendency to crack where the diameter of the bar changes. The major objective was improved ride quality with the expected Porsche handling.

back