This will calculate the amount cornering force your car can exert, or how well it sticks to the road during turns. This is measured in lateral G's, 1.0 G being equal to the force of earth's gravity. The numbers you need to provide are obtained by driving the car around a circle of known size and timing a few laps. My suggestion would be to find an empty parking lot, but make sure it's a smooth and level surface. Attach some chalk to a measured string and draw a circle (a 150 ft radius is used by Car & Driver). Then drive the car around it as fast as possible without letting it break out of the circle. Do this a few times, timing each lap with a stopwatch, and use the average time.
Stock IROC-Z's had skidpad numbers in the upper .8 G's, a very respectable figure for a street car. Some all-out road racing cars can pull in excess of 1.6 G's.
Spring Rate:
This will calculate spring rate for steel coil springs. Note that this will not work with variable rate springs. Spring rate will never change over the life of the spring. When selecting spring rate, stiffer is not always better. If the springs are overly stiff the wheels won't be able to follow road irregularities as well. Click the blue text for more info on the variables.