Spring rate for a production sedan is designed to give a comfortable ride, primarily. As I remember the front bounce frequency is set to around 1.2-1.5 Hz, and the rear a little higher (say 0.4 Hz higher). These do get tuned a lot further down the track, but these are a good starting point. The higher rear rate is to give a more level ride when you go over a step. These frequencies ar ecalculated using the SDOF equation, f=1/(2*pi)*sqrt(k/m)
The other considerations are to ensure that the spring can handle the loads (particulalry as it approaches full compression), and that it doesn't drop out of the suspension on full droop. Both of these can involve a lot of messing about with variable rate ends, bump stops, and other things. There is a surprising degree of black magic in road spring design.
I would have thought that the effect on handling is not all that great, directly. Many sports suspensions use high spring rates to reduce wheel travel because the tyres are oversize, and it keeps the suspension geometry in a more linear part of its motion.
Cheers
Greg Locock
