Torsion Bar Rates Vs Coil Spring Rates
09-01-2003, 07:39 AM
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Torsion Bar Rates Vs Coil Spring Rates
The article by Will Ferch in the Rennlist Technical site to establish the spring rate of torsion bars is quite comprehensive and well worth a visit. However, should a car be sprung with coil-overs, the calculations change. I quote the formulas for the two kinds of springs for the sake of clarity:
TORSION BARS
K= 1 178 000 (D)*4 /(L)(A)*2
K = is to the fourth power and A is to the second power
D = is diameter of torsion bar;
L = torsion bar length;
A = is the lever arm length
COIL SPRINGS
K=W4G/8ND3.
K= Spring stiffness in lb/inch;
W= Wire diameter in inches to the fourth power;
G= Constant 12 000 000; N= ACTIVE COILS + 1/2;
D= Diameter of coil (measured from centre to centre)
The problem is that when you convert from a full torsion bar suspension to a full coil-over the above calculations to not take into account the mechanical advantage of the suspension. How is the ‘wheel rate’ of the coil spring suspension calculated?
As an example running 22 mm (210 lb) torsion bars in front and 31 mm (332 lb) rear. Which coil springs would give the same suspension stiffness?
Should you want to complicate matters even more, how about running torsion bars and coils simultaneously?
TORSION BARS
K= 1 178 000 (D)*4 /(L)(A)*2
K = is to the fourth power and A is to the second power
D = is diameter of torsion bar;
L = torsion bar length;
A = is the lever arm length
COIL SPRINGS
K=W4G/8ND3.
K= Spring stiffness in lb/inch;
W= Wire diameter in inches to the fourth power;
G= Constant 12 000 000; N= ACTIVE COILS + 1/2;
D= Diameter of coil (measured from centre to centre)
The problem is that when you convert from a full torsion bar suspension to a full coil-over the above calculations to not take into account the mechanical advantage of the suspension. How is the ‘wheel rate’ of the coil spring suspension calculated?
As an example running 22 mm (210 lb) torsion bars in front and 31 mm (332 lb) rear. Which coil springs would give the same suspension stiffness?
Should you want to complicate matters even more, how about running torsion bars and coils simultaneously?
09-01-2003, 02:57 PM
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Here's some examples of running both simultaneously. Unfortunately, these are for a different platform than yours, but maybe it will be useful.944 spring rates
09-25-2003, 08:33 PM
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Coil overs and torsion bars together is a really good set up on 911s, especially where you are using a lot of travel.
Torsion bars are a really nice type of spring so I usually set cars up with the torsion bar as a base rate, and use a coil over to fine ture the rate for the event or track. Coils are cheap, so you can use different lengths and rates to see what works on your car.
As far as working out the different rates for coils vs torsion bars - just convert everything back to the wheel rate (the ratio of coil to wheel movement). The formula for working out coil rates is not the most accurate thing in the real world due to different steel quality and age - Try working it out for a German spring vs a Australian spring and then put is on a spring tester!
The other thing that makes a big difference to the different rates is friction. A torsion bar setup with a rate of 300lb may feel very different to a coil only setup with the same rate, because of things like friction changes in the spring plate bushes vs monoballs or needle rollers and even the friction in the shock seals if you ave changed brand shocks.
Torsion bars are a really nice type of spring so I usually set cars up with the torsion bar as a base rate, and use a coil over to fine ture the rate for the event or track. Coils are cheap, so you can use different lengths and rates to see what works on your car.
As far as working out the different rates for coils vs torsion bars - just convert everything back to the wheel rate (the ratio of coil to wheel movement). The formula for working out coil rates is not the most accurate thing in the real world due to different steel quality and age - Try working it out for a German spring vs a Australian spring and then put is on a spring tester!
The other thing that makes a big difference to the different rates is friction. A torsion bar setup with a rate of 300lb may feel very different to a coil only setup with the same rate, because of things like friction changes in the spring plate bushes vs monoballs or needle rollers and even the friction in the shock seals if you ave changed brand shocks.
