Battle Magazine — March 1997

What matters is front-to-rear balance. When aftermarket springs are developed, their rates are usually based on the stock front/rear distribution. If the front rate is increased by 20%, the rear is often increased by a similar amount. But real-world tuning and aftermarket specifications are not always that simple—that is part of the difficulty. Stock springs are designed primarily for ride comfort as well as handling, so retaining the same balance does not necessarily produce the best handling. Consider this carefully. Spring-rate selection also depends greatly on driver preference: do you want to reduce understeer and make the rear step out more, or reduce excessive oversteer and add understeer? The goal is a setup that lets you drive fastest.

Along with the shock absorber, the spring supports the vehicle’s weight, maintains its posture, and plays a major role in determining handling. Spring stiffness is expressed as force per unit deflection and is called the spring constant or spring rate. In Japan it is often called “bane rate,” or simply “K.” The rate indicates how much force is needed to compress the spring by one unit.

For example, a 4.0K spring requires a force of 4 to compress it by one unit. A car usually has four springs, one at each wheel. Left and right rates are normally the same, while front and rear rates differ on most vehicles. The overall rate also varies with vehicle weight. Stock springs generally have a higher front rate because the engine and transmission are at the front. Manufacturers usually select spring rates according to the load on each axle: heavier loads need stiffer springs, lighter loads softer ones.

Too stiff is bad, but too soft is also bad; you need a compromise that allows an appropriate amount of roll. Spring stiffness is closely related to tire grip. If the tires have little grip, they may slide before the suspension rolls, even with stiff springs. Match the spring rate to tire performance. Springs also determine ride height, which strongly affects handling. A higher end tends to break away sooner: a higher rear (slight nose-down rake) makes the rear slide sooner; a higher front tends toward understeer. Consider these factors when choosing springs, but avoid extreme changes. If you use springs intended for your model, they are unlikely to be wildly inappropriate; experiment within the intended range to find a setup you like.