Spring Rate Balance

[Equinox]

This has been on my mind for a while and I've been meaning to ask someone knowledgeable about it...

I have an S14, and was wondering what should the spring balance front to rear be? All aftermarket coilover combos seem to have a stronger front rate to rear (eg. 10/8, 8/6, etc) whereas aftermarket springs seem to either be neutral or stiffer in the rear! (eg. Tein 3.1/3.1, Whiteline 2.8/3.1, Nismo 4.6/5.5)

Is there a reason for this? For neutral handling which ratio is preferred?

Thanks.

[Sydneykid]

This has been on my mind for a while and I've been meaning to ask someone knowledgeable about it...

I have an S14, and was wondering what should the spring balance front to rear be? All aftermarket coilover combos seem to have a stronger front rate to rear (eg. 10/8, 8/6, etc) whereas aftermarket springs seem to either be neutral or stiffer in the rear! (eg. Tein 3.1/3.1, Whiteline 2.8/3.1, Nismo 4.6/5.5)

Is there a reason for this? For neutral handling which ratio is preferred?

Thanks.

Spring rate on its own is meaningless, S13/14/15's have movement and leverage ratios, being MacPherson strut front suspension, of around 1.1 to 1. The rear (double wishbone) on the other hand is 1 to 1.

So a 100 lbs/inch spring in the front actually gives an effecrtive rate at the wheel of 83 lbs/inch, whereas 100 lbs/inch spring in the rear actually gives an effective rate at the wheel of 100 lbs/inch. This is important because a tyre doesn't care what the actual spring rate is, it is only cares about the effective spring rate that acts on it.

BTW, whoever told you that Whiteline springs were 2.8/3.1 was wrong as the rears are progressive, hence they have more than one rate.

To further complicate the issue, the effective spring rate is also added to by the anti roll rates (stabiliser bars), the anti dive and anti squat geometry and most complexly by the shocks damper rates. Then of course you have the question of what effective spring rate do the tyres like, for example some tyres like softer rates than others. Lastly you have the contact patch of the tyres, for example how much camber, caster and toe the car is running.

So, what do I do to determine a starting point for the spring rates?

1. I ask the tyre technicians what effective spring rate the tyres like. For road and circuit use that is usually the front tyre, for drag use that is the rear tyre.

2. Once I know that, I use the movement and leverage ratios to determin the actual spring rate for that end of the car.

3. I know the front to rear weight distribution (via the corner weight scales) so I use the spring rates at that end to work out the spring rates at the other end based on the weight distribution.

4. Armed with those rates I then go testing with 3 or 4 alternative springs rates (front and rear) and tune down to a reasonable traction balance.

For fine tuning the handling balance stabiliser bars are by far the best, that's why almost all circuit race cars (where the regs allow) have driver adjustable bars.

Simple isn't it?

cheers