Changing Linear Spring Rates On Coiovers

[PT]

Looking at lowering my spring rates both front/rear by 2kg/mm (manufacturer specifies acceptable range +/- 2kg/mm for my model)

Currently have "firm" rates:

Front 10kg/mm with free length of 175mm (Stroke 98mm)

Rear 8kg/mm with free length of 200mm (Stroke 111mm)

Now if I were to change to 8kg front and 6kg rear can I just use the longer rear spring in the front and lower the spring perch to compensate for height difference (final ride height won't matter as have 2 way height adjust). Just wondering if this will cause issues, wouldn't think so as springsare linear in rate.

Seeing if I can just buy x1 pair of springs?

thanks

Edited January 23, 2007 by PT

[salad]

200mm free height spring is fine in the front. If you have preload and base height adjustment, that is more than enough to make the height right. And then a new pair of 200mm free height 6kg/mm in the rear.

So yes, you can just buy 1 pair of springs

[PT]

sweet, thanks salad

[PT]

OK new springs have arrived and I have swapped them over and adjusted preload on them, not fitted to car as it's at the workshop getting other stuff done.

Main concern is the coilovers hitting the bump stops due to lighter spring rates. I have done rough calculations to try to offset this, I know it's not "exaclty" like this in real world as other factors occur but...

FRONT: factory weight=690kg, divide by two=345kg per side (L and R)

10kg/mm spring will compress ~34mm

8kg/mm spring will compress ~43mm

Difference ~9mm, so I have increased preload by 9mm over original set up (I have then taken into account I am using a longer spring too)

REAR: factory weight=550kg, therefore ~275kg per side

8kg/mm spring will compress ~34mm

6kg/mm spring will compress ~45mm

Difference ~11mm, so have increased preload by 11mm compared to before (BUT I hace a helper spring too 1.9kg/mm with 80mm free length=too hard to factor in)

Long winded post yes, plenty of crap figures/numbers yes, but have I gone about this in the right way???

Should or do I need to trim the bump stops, they are 2-3" long

Edited January 25, 2007 by PT

[salad]

How much you trim the bump stops depends on the height you have the car set to. Whether or not it reaches the bumpstops depends on height, shock rates, where you're driving etc.

Increasing preload as you have done will raise the car, this may bother you.

How much the strut compresses also depends on the shock. A high pressure monotube shock will compress less than a low pressure twin tube shock because of the gas pressure inside acting as a spring.

Your rear struts have helper springs so that effectively gives bugger all preload when they are uncrushed. With the weight of the car down, the helper spring completely crushes so they dont have an effect on how the main spring behaves.

Edited January 25, 2007 by salad

[PT]

yeah they have 2 way height adjustment so I can alter final ride height independently from the spring perch, hence preload affecting height can be negated.

they are a twin tube (Tein)

surprisingly despite the helper spring there the main spring has now got 5mm of preload on it by rasing the spring perch 11mm total

I'll wait til fitted before cutting the bumpstops in that case.

[BHDave]

You haven't considered the dynamic weight transfer when cornering/braking/accelerating in your calculation

The strut will now compress further when cornering with the same load even if you increase the pre load to compensate for the difference in static loading.

If you are really concerned you can increase the pre load on the spring to the point where the weight of the car is only compressing it by a small amount (say 10-15mm either end) and adjust the height via the base adjustment in order to maximize travel.

[Sydneykid]

Try this;

This is how much much to trim off a Group Buy front bump stop, shown next to a standard Skyline front bump stop. The orange line is for standard height, the red line is for ~350 mm centre of wheel to guard and the yellow line is for ~330 mm centre of wheel to guard.

cheers