General suspension questions

[NoHachi]

Hiya,

Dunno what would be the best setup for you (everyone is different), but here is what I did.

Rates:
I set the springrates to match those of the faster cars on the nordschleiffe. After some calculating that comes to 5.8kg/mm and 4.5kg/mm. Close to the rates you mention, but with some more room to firm things up for ultra-smooth tracks with stiffer swaybars. 6/4.7 should be fine.
You'll have to measure the inner diameter of your springs, depending on what it is you can find your springs cheapest at Faulkner or Rally Design:
http://www.dfaulknersprings.com
http://www.rallydesign.co.uk/
No TRD logo, thus less then a quarter of the price. Good enough for plenty of racers to use them. dFaulkner might be able to do you a rear set as well, otherwise go TRD if you must.
You'll need approx 9" free length at the front.

Height: You can keep your coilovers at the front no probs. With these rates your total stroke will be approx 100mm front, 120mm rear. So 5 to 6cm's either way. When the time comes to change the springs, remove springs from the struts (or car in the rear) and put everything back on the car. Put a jack under your wheel and jack the wheel up until the wheel hits the inner fender. Do some hammering for clearance (if you want to do this), jack wheel up until it hits again. Also make sure its set close to the camber you plan on running. This is max possible compression travel
Now measure the distance from the top of the wheel arch to the hub centre of your wheel and scribble it on a piece of paper (we'll call it M1).
Drop the jack down untill the wheel is hanging in the air freely. Again measure between hub and arch and make a note. This is max rebound travel (M2).
Subtract the first measurement from the last and you have total available travel dictated by your chassis.
max.travel = M2-M1
If your car is anything like mine it'll have close to 100mm of travel (no coincidence that that is the same amount of travel that the springs allow, usually you plan them so they use all available travel at max lat.accel.).

A good starting point for height is to use around 60% of total travel for bump and 40% for rebound. That way, when you are cornering at max lateral acceleration. The suspension is fully compressed, but you still have around 10% of travel left to absorb road irregularities (warning -> this leads to grip..if you want it to slide, go ahead and set things at 50%). So take 60% of total travel and add it to your first measurement.
height = M1+0.6*max.Travel

After that, throw the springs on and drop the car onto the ground. Measure the distance between hub and arch and adjust the spring perches until the car is close to the variable "height" all around.
My guess is that TRD did their home work and the rear springs will be close to the sweet spot.
From that height you can start making small changes so that the corner weights are in the ballpark and the car has a bit of aerodynamic rake (lower front then rear).

End result: Still an utterly stiff ride..nowhere near wallowy or slow reacting. However the car now rides over sharp road irregularities instead of crashing over them. Its not exactly slammed, but not quite back at stock height either. Small price to pay.
Haven't taken it to the track yet, but my guess is it will be much easier to drive. She communicates a lot more (old setup was 9k/7k dorifto). On bumpy tracks like the ring its no contest: the forgiving setup is faster for everyone except the people with thousands of laps under their belt.