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Old February 8th 2010, 08:29
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evilC evilC is offline
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Location: UK Where Leics is more
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Right! I have used Humbles corner weight and calculated the theoretical spring rates for the following spec:

Super Beetle Macpherson strut front and IRS rear with a sprung weight of 401.5lb at each front corner and 564lb at each rear corner. The lever arm at the front is 1:1 and at the rear is 1:1.277
For a quick road car the spring frequency would normally be between 80 and 100 CPM and for an unaerodynamically assisted race car would be 100 - 120 CPM
This gives the following results:
Road car:
Front
80CPM = 72lb/in spring rate
100CPM = 113lb/in spring rate
Rear
80CPM = 131lb/in coilover spring rate (no torsion bar)
100CPM = 204lb/in coilover spring rate (no torsion bar)

Comp Car
100CPM rates as above
120CPM front = 163lb/in
120CPM rear = 294lb/in (no torsion bar)

The above rates should be amended so that there is a 10 - 15% difference between the front and the rear spring frequency to prevent uncomfortable pitching that will occur if the spring frequencies are too close together (sympathetic resonance) with the fronts being softer. Please note that these rates are the theoretical ones that will only be a starting point for selecting the final settings, much of which will be governed by the feel and handling as well as the grip.

For comparison a standard 1303 super has the following equivalents:
Front spring rate + 80lb/in (83CPM)
Rear notional spring rate of the TB at the damper position (coil over position) = 186lb/in (95CPM)

Intuitively, the rears on the calculation are less than I would have expected although the normal recognised torsion bar upgrade to 23.5mm dia gives an equivalent spring rate of 216lb/in and a CPM of 102.

Hope these figures are of interest.

Clive
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