I think it would be useful to go through the different aspects of spring and damper rates, eg compression and rebound. How over stiff handles differently to over soft, and front rear balance.
There are many ins and outs on this subject.....so I will try and keep it brief.
Too soft a spring rate will provide a good ride quality. But, the suspension takes too long to absorb the energy, thus the tyre isn't in full contact with the ground for as long as it could be. So not providing the maximum grip.
Also when cornering, especially with a McPherson strut set up, the tyre rolls to a greater angle and doesn't stay square to the ground when under full load. Again, not providing the full grip.
If the spring rate is too high, then when the tyre hits a bump, the spring will not absorb enough of the energy. That is passed onto the chassis. The chassis is sent in the upward direction thus the tyre is not in full contact with the ground. Again not providing full grip. If this happens whilst cornering, it could spell trouble.
Damper rates work hand in hand with spring rates and can give similar effects if the rates are wrong.
In general, the compression side of damping will influence the cars stability and response. The rebound side should influence comfort and traction.
But, for example if the compression side of damping is too high, then the ride will be harsh and crashy. This is why, setting up and diagnosing problems can be very difficult.
Dampers can provide a very useful tool for tuning handling balance especially on aspects of cornering such as entry and exit. This is because the diagonally opposed dampers are usually working in the opposite direction. Working out exactly which direction each damper is moving, at the different stages whilst cornering, you can learn to change the damping accordingly to alter the handling balance.
Anyone looking to improve handling in general, should spend as much as possible on dampers. Modern dampers have improved so much since Active Suspension was outlawed in F1 in the early '90s.
Racing dampers using Digressive technology have so much more control over suspension movements than either progressive or linear dampers.
Can I just point out......that the different types of damping effect the shaft speed in damping, rather than the position of the shaft within the stroke. I have heard people mention that progressive dampers have a higher rate of damping the shorter the shaft becomes. This is not true........it is the speed of the shaft movements that effect the rate of damping.
This is the reason that Digressive dampers are so effective. These dampers can control slow shaft speed movements such as cornering.....where other types of dampers would have little influence. Then when the shaft speed increases on bumps etc.....digressive dampers are more comfortable......where the other dampers are harsh and crashy.
As far as balance is concerned.........in general the end of the car that has the most roll stiffness (provided by springs and roll bars) will have the most lateral movement and will lose grip first.
This is why on cars like Peugeots (that have struts and coils at the front.....and torsion bars a the rear) it is OK to raise the front spring rate to control roll etc, but if the rear rate isn't increased accordingley......then the car's balance will be very much towards the front.......and understeer.
Another "general rule" is that the opposite end of the car to the drive wheels....will have the higher roll resistance. This is so the drive wheels are kept flat and the suspension is more complaint to enable the tyre to follow any undulations.