Spring Rates

When I first ordered my shocks and springs I went with the springs rates suggested by NTS:

400lbs/inch front
250lbs/inch rear

Initially these felt ok, but the more I drove, and the more confident I got, the more clearer it became that something wasn't right with the handling...
I found that the ride was pretty poor on bumpy roads (and we have a lot of bumpy roads round here...), and I was getting jolted about all over the place.
More importantly though, the car had disasterous understeer when pushed to the limits of grip.
It was very difficult to get the tail out, and when I did it'd very quickly switch between under and oversteer resulting in some funky handling.
Another indication that something wasn't right was that the tyres squealed like a pig going round tight corners, even when I wasn't pushing it hard.

After spending a while playing with ride heights, damper settings, camber, and toe, and not getting anywhere I decided to do a bit of research on spring rates.

A good starting point seemed to be to calulcate the wheel frequency (using the Allan Staniforth method in his book Competition Car Suspension - great book by the way!).
My calculations were full of measurement errors, estimates, and assumptions, but I figured they'd still give me a reasonably good idea of what was going on.

The calculations suggested my wheel frequencies were around 130cpm on the front and 100cpm on the rear.
Staniforth suggests 80-100 cpm for a sportscar and 100-125 for a race car.  He also suggests that the rear should be slightly higher than the front to enable it to catch up with the front over bumpy surfaces.
So that might explain a few things then!

At that point I decided to work through the calculations the other way to see what sort of spring rates I might actually want to start with.
After playing around I came to the conclusion that the rear needed to be around 100lbs higher than the front and that a good starting point would be 300lbs on the front and 400lbs on the rear, giving 115cpm front, and 130cpm rear.

As a quick and dirty test I decided to swap the front and rear springs giving me 250 on the front and 400 on the rear.
I've had it set like this for a few days and it's much much better!  The fronts does feel a tad soft, but the tyre squeal and understeer have been eliminated, she feels far more composed on bumpy ground, and as an added bonus the front seems far less prone to locking under heavy breaking.
It's probably gone a little far the other way and is slightly tail happy now, but it's more fun like that :)

I still think 300 front / 400 rear will give a good balance and started looking for a new set of springs for the front.
I managed to find a pair of 325lb springs secondhand (but unused) for less than half the price of a pair of new 300lb springs and will hopefully get time to fit them next week.
That should still keep it slightly biassed towards the rear assuming my calculations aren't too far out!
I'll let you know how I get on.

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UPDATE 30/09/2013:

I've been out a few times with the current spring rates (325lbs front / 400lbs rear), and I'm reasonably happy with them now.  I'm certainly in no rush to change them again!
The car feels fairly well balanced front to rear now and goes into a nice controllable oversteer when provoked, which is what I was aiming for :)

The only thing I would say is that the ride is possibly a little too hard for some of the dodgy roads I drive on, and I *might* think about fitting softer springs next year.