Spring Rate: The amount of weight required to compress a spring one inch. “500lb/in” means it takes 500 lbs to compress the spring 1 in, 1000lb to compress it 2 in.
Load Rate: With a given length, the amount of weight required to compress a spring that length.
Free Length: The uncompressed length of a spring, or, the free standing length of the spring.
Coil Bind: The length of the spring when fully compressed, or, the point at which the coils touch each other.
Total Travel: The difference of the Free Length to the Coil Bind length.
“What springs do I need for my race car?”
To answer that, you need to have three important pieces of information.
That may sound obvious, but think about this, one driver can get in to the race champion’s car and do a miserable lap. And the CAR just won the race at that TRACK. The difference? The DRIVER. Assuming the drivers are of similar build so as not to change the weight balance of the car. The race champion had a different ‘feel’ for the suspension dynamics at that track for that car, compared to the second driver.
There are other basic factors to consider outside of the three just mentioned. They are the "set" load (or preloaded weight) of each wheel, shock type, caster & camber, tire choice (width and compound) and body aerodynamics. You may have a preference for a little more body roll through a corner, therefore requiring a lighter spring rate, and possibly an adjustment to the shock’s dampening rate. If you prefer less body roll through a corner, you would require a stiffer spring for that car in that situation.
But to do any of that, you need to understand the preloaded or “set” weight on the spring then factor the Spring Rate and Total Travel to determine if you are over stressing the spring.
So, a 12” Free Length spring with a Coil Bind of 5.75” gives 6.25” of Total Travel, therefore it can handle 3125 lbs. of weight (6.25” x 500lb/in) before it becomes over stressed. If the Preload Length 1.5” (750 lbs) you are left with 2375 lbs. of available cornering load, but you also will have 4.75” of travel. Quite a considerable distance in road racing, but not necessarily a lot for rally and rally-cross.
So, you can see that the question “What Springs do I need for my race car” requires many variables to be answered before a good answer can be given.
Our friends at Eibach have a good set of diagrams and equations to sort out all the intricate details.
The Eibach Race Spring-System
EIBACH DOUBLE-SPRING SYSTEM
Suspension tuning is a precise balance. On one hand, the spring system should be soft enough to absorb track irregularities assuring maximum adhesion, while on the other hand the spring system must be firm enough to reduce the body roll, squat, and dive of the vehicle during cornering, acceleration and braking. For optimum balance, Eibach has developed the ERS double-spring system. By combining Eibach Main, Tender and/or Helper springs, the suspension engineer or weekend racer can create the perfect combination of spring rates for the ultimate suspension set-up. With our extensive range of motorsport spring sizes and rates—our ERS double-spring system is unsurpassed in performance and flexibility. For Off-Road racing where extreme wheel travel is a must—combining two different rate main springs gives you the softer rate required for small bumps and a stiffer final rate to prevent bottoming on the bigger bumps and jumps.
EIBACH MAIN AND TENDER SYSTEM
The Eibach Main and Tender spring concept provides a softer initial rate when both springs are compressed together, then delivers the desired firmer final rate once the tender spring closes completely. The Main spring has a linear-rate characteristic and determines the final rate of the system. The Tender springs are available in linear or progressive-rate (shown) and determine the initial rate of the spring-system.
EIBACH MAIN AND HELPER SYSTEM
A Helper spring is used to prevent the Main spring from becoming loose in the spring seat when the suspension is unloaded or at full droop. The Helper spring, unlike the Tender spring, has very little spring rate, and therefore has no effect on the suspension characteristics of the vehicle. Up to 2″ (50mm) of spring-to-perch gap can be covered with the use of a Helper spring.
(image courtesy Eibach Australia)