This chart indicates how the spring rate increases with the use of different hardness of s
Several rubbers of a softer compound can be installed at one corner so that we can make two changes as conditions deteriorate. An example would be to install a 150-pound spring in the right-rear corner that would be great for a dry-slick track. Then, two 25-pound spring rubbers could be added to that spring to start out the day's practicing, and maybe qualifying with the equivalent of a 200-pound right-rear spring.
When the track has begun to dry out somewhat, but is not yet dry-slick, the heat races can be run with only one spring rubber for a combined 175-pound rate. If the track becomes drier and slicker, the remaining rubber can be yanked out for the main event.
For asphalt, you might want to experiment with various amounts of spring split in the front and rear. Spring split in the front usually doesn't greatly affect the middle handling, but can help entry into the corner on some types of tracks. If there is an equal rate of springs across the front, you can experiment with a reverse spring split up front by installing a 50-pound pre-rated spring rubber in the left-front to see if having a 50-pound split helps corner entry. Many times, it will help the transition into the corners on the flatter racetracks.
Changing the spring split in the rear greatly affects the mid-turn handling, so care must be taken when making changes to the rear springs in order to affect corner exit performance. For example, when we are using a 25-pound spring split (right-rear softer than the left-rear) in the rear to help bite off the corner, we can install a 10-pound rated spring rubber in the right-rear spring to see if only a 15-pound split will suffice. The Panhard bar height will need to be lowered along with this spring change in order to remain neutral through the middle of the corner.
The advantage of using spring rubbers to change our spring rate on each corner of the car is the speed and ease of making those changes. Do not guess the resulting rate change when putting in the rubbers; use a spring tester and rate your spring rubbers for each corner and each spring that will be used in that corner. The change in rate will be predictable, and you will be able to see the true results.
Changing springs during testing is all part of the process of finding the best combination that will make the car faster. To make the test session go better, use spring rubbers to simulate different spring rates. Rubber inserts are much faster to install and will display the same results if done correctly.
The space between the coils is greatly diminished when there is full preload on the spring. The view on top is an unloaded spring, and on the bottom is a spring with average preload. The space difference affects the rate and effectiveness of the spring rubber.
Note the difference in space between the coils outside the spring rubber and the space inside the rubber. At full preload, the rubber effectively takes almost 100 percent of one coil out of the spring equation.