How springs affect handling balance:
The handling balance of a car is determined by the front to rear roll-stiffness bias of the suspension (along with alignment and tires). Roll stiffness is a function of springs and anti-roll bars. It basically describes how much the suspension resists the tendency of the car’s body to roll when cornering. How this is calculated is complicated and not really important if you are not designing a car from scratch. Whichever end of the car is made to be stiffer in roll will tend to loose traction first. Increasing the roll stiffness of the front will increase the tendency to understeer; increasing the roll stiffness of the rear will reduce understeer/add oversteer. Most cars come from the factory with quite a bit of understeer since it is considered safer. The handling balance can be made more neutral or even switched to oversteer with the addition of a stiffer rear sway bar or stiffer rear springs. Similarly, understeer can be increased by using a larger front anti-roll bar or stiffer front springs. Softening the opposite end has the same effect (softening the front instead of stiffening the rear to reduce understeer), but allows more body roll.
Progressive v. linear springs
A visual inspection can easily identify most progressive springs. The coil spacing will change—several coils will be spaced farther apart from each other than the others. These springs essentially have two spring rates: one when all the coils are active and another much higher rate with the closely spaced coils stack up. This allows a more comfortable ride since the rate is lower over small bumps while maintaining good performance since the close coils stack up as the body rolls when corning, limiting body roll. The disadvantage of progressive springs is that the transition to the higher rate can alter the handling balance depending on braking and throttle inputs since this will affect when the front and rear springs make the transition to the higher rate. A more expensive (less common) way to make progressive springs is to alter the wire diameter of some of the coils. Even if they are spaced the same, the coils with the smaller wire diameter will be softer and give a lower spring rate until they stack up. If the wire diameter is changed slowly over several coils, the coils will not stack at once since each of them has a slightly different stiffness, eliminating the abrupt transition from one rate to the other. Some springs, once supporting the weight of the car, will already have the closely spaced coils stacked. Since the stacked coils are never active in this case, the spring will act like a linear spring since it never transitions back to the softer rate.
Calculating spring rate from physical measurements:
k = (d^4*G)/(8*D^3*Na)
k = spring rate (lb/in or N/m)
d = wire diameter (meters or inches)
G = modulus of rigidity (80.8*10^9 Pa, 11.7*10^6 psi)
D = coil diameter (average inside and outside diameter), (meters or inches)
Na = number of active coils
Note that the spring rate is VERY sensitive to the wire diameter. If you’re unsure of any measurement you’ve taken pick a high estimate and low estimate and find the rate for both. You may be surprised to find how tiny differences in geometry translate into big differences in spring rate.
This equation is useful even if you’re not calculating spring rate since it shows the effect of changes in spring geometry. Increasing wire diameter increases the spring’s rate. Decreasing coil diameter or the number of active coils raises the spring rate.
Cutting stock springs
Cutting stock springs isn’t always bad if done correctly. Cut them with a saw or something similar, not with a torch. Never heat the whole spring to lower the car, this will destroy the heat treatment that keeps springs from sagging. Cut springs will have a higher rate since there are fewer active coils. Removing one or maybe two coils and upgrading the shocks to account for the additional stiffness should be fine. If you intend to drop the car more than 1-1.5” (conservative estimate) you should plan on getting aftermarket springs. Even though cut springs are stiffer, they are also shorter--there is less spring to absorb energy so your suspension is more likely to bottom out. Aftermarket springs make up for their small loss of length with a greater increase in stiffness so they can absorb additional energy before the suspension runs out of travel.
If you have anything to add or any questions post them below or PM me and I'll make changes as required.