The space between the coils...
The space between the coils is greatly diminished when we have the full preload on the spring. This view is of the spring in a relaxed state with no preload. Note the space between the coils.
We can have several springs that are rated the same in pounds per inch, but have a different number of coils and diameter of wire. This difference in construction means they will be different in the size of the spacing between coils and therefore have a difference in how much a particular spring rubber will affect the spring rate.
This all means that you might need an assortment of spring rubbers that are dedicated to one corner of the car, and only for that corner. The likely corners are the left rear and the right rear. The rubbers should be labeled as to the corner and spring where they are to be used.
Uses For Spring Rubbers
We might need to use spring rubbers for compensating for changes in the racetrack surface grip, as in dirt racing where throughout the day and night the moisture level changes to affect changes in handling for entry and exit to and from the corners.
This view shows the spring...
This view shows the spring with full preload. The space between the coils is much less now. The space difference affects the rate and effectiveness of the spring rubber. Springs with the same overall rate, but different diameter wire and number of coils will be affected differently by the same spring rubber.
Most dirt tracks start out wet and tight and usually transition to a more slick condition as the event goes through the stages of practice, qualifying, heat races, and the main events. The spring rates must change as the track loses grip and we can utilize the spring rubbers to adjust to the conditions. Coincidentally, we usually will need to soften the right rear spring under those conditions, so removing a spring rubber is the quickest way to do that.
We would install the best spring combination for the slick conditions as our original setup and then install spring rubbers at one or more corners, usually the right rear, to get the car set up for the tighter track condition early on in the event. Then as the track dries out, we can remove one or more spring rubbers from the right rear to provide more bite.
We can install several rubbers of a softer compound 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 we could add two 25-pound spring rubbers to that spring and start out the day practicing and maybe qualifying with the equivalent of a 200-pound RR spring.
When the track has begun to dry out somewhat, but is not yet dry slick, we can run our heat races with only one spring rubber for a combined 175-pound rate. Then, if the track becomes dryer and more slick, we can yank the remaining rubber out and be set for the main event.
We can see by this chart how...
We can see by this chart how the spring rate increases with the use of different hardness of spring rubbers. We should always rate the spring at or near its installed height to get accurate results. Then we will know exactly how much our spring rates are with the use of each spring rubber.
We can even install different rated spring rubbers to effect smaller changes in spring rate to fine tune the setup. If a more pronounced change in track conditions occurs, then remove the stiffer rubber. If the track changes less, remove the softer rubber, and so on.
For asphalt, we might want to experiment with various amounts of spring split in the front and rear. Spring split up front usually doesn't greatly affect the middle turn handling, but can help our entry into the corner on some types of tracks. If we run an equal rate of springs across the front, we 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 we need to be careful when making changes to the rear springs in order to affect corner exit performance. An example is if we are running a conventional setup and using a 25-pound spring split (RR softer than the LR) in the rear to help provide bite off the corner, we can install a 10-pound spring rubber in the RR 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.