There are times when we need to make a spring rate change quickly due to lacking the time needed to make a full spring change. We can do this with what are commonly referred to as spring rubbers. If we elect to do that, there is some information about spring rubbers we need to know. To give you a better understanding of the effects of these coil spacers, we did some experimenting.

The term spring rubber comes from the original use of rubber as the component in these devices. Modern spring rubbers can be made from different materials, including Polyurethane, but we will continue to refer to them as rubbers for the sake of consistency. I love traditional terms anyhow. You get the meaning.

We preach about having a balanced setup, and changing spring rates is all part of the process. The truth is we can balance the setup in the car through the use of many different combinations of spring rates. The middle of the turns must remain balanced, but what we might need to improve is the entry and exit. Various front and rear spring split combinations will feel different to the driver and may result in a change in performance.

One way to quickly enact a spring rate change in our cars is by installing or removing spring rubbers that will change the rate of the spring. This is useful with asphalt stock cars, and almost essential with dirt cars. The spring rubber causes an increase in the rate of the spring by eliminating the effectiveness of one or more coils in the spring. We can use multiple spring rubbers in a spring if that will provide us with the necessary changes we seek.

How to Rate a Rubber

The success we will have using spring rubbers is only as good as our knowledge of how much the spring rate changes. The rate change for a particular size and hardness of spring rubber is different for each different rate and design of spring and for different amounts of preload on a particular spring.

If I install a “25 pound per inch” spring rubber in a 200-pound spring, the amount of rate change will be different as the magnitude of preload on the spring changes. That is because a spring rated at 200 pounds per inch that holds up to 950 pounds will be compressed 4 3/4 inches whereas a 200-pound spring the holds up 500 pounds will only compress 2 1/2 inches. That means there is a different spacing between the coils for each application and the spring rubber will be compressed more in the spring that supports 950 pounds than the one the supports 500.

It is also true that if we change to a 150-lb/in rated spring in place of the 200-lb/in spring, the compressed height on the corner holding 950 pounds will be 6 1/3 inches and the spring rubber will be compressed even more, yielding a greater overall spring rate change.

The correct way to rate a spring rubber for a particular use is to do it like we rate a spring, in a spring tester. We first install the spring without the rubber in the tester. We need to compress the spring to the same compressed length as when it is installed in the car at ride height and with all of the weight in the car such as driver, fuel, and so on. Then we rate the spring in the next inch or two or however far the spring compresses on the race track. This establishes the actual installed rate of the spring itself.

Next we relieve the pressure on the spring and install the spring rubber. We then repeat the process and compress the spring to its competition height and go the added distance and record the rate. Whatever the increase is over the “spring only” rate is the rate of that spring rubber for that corner of the car and that particular spring under those conditions. That rate will not be the same for other springs and corners of the car.

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 a different in the width of the spacing between coils and therefore 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, and only one, corner of the car. The likely corners are the left and right rears. The rubbers should be labeled as to the corner and spring they are to be used with.

Uses for Spring Rubbers

We might need to use spring rubbers for compensating for changes in the race track surface grip, as in dirt racing, where throughout the day and night the moisture level changes to effect changes in handling for entry and exit to and from the corners. 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. We can utilize the spring rubbers to adjust to the conditions. Coincidentally, we will usually 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 rate of 175 lb/in. 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 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.

For asphalt, we might want to experiment with various amounts of spring split in the front and rear. Spring split up front usually does not greatly affect the middle turn handling, but it can help our entry into the corner on some types of track. 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 prerated 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 race tracks.

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 in a metric stock car 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 spring rubber rated at 10 pounds in the RR spring to see if only a 15-pounds split will suffice.

For the asphalt bump setups, fine tuning is usually done with the right rear spring. If you think you need a 400-pound spring in the RR, you can install a 375 spring and a 25-pound spring rubber. If the setup is off slightly in either direction (tight or loose), you can either add a stiffer or additional rubber or remove the rubber to tune the setup. Then when you are sure of the correct RR spring rate, just install that rate spring for the race.

The advantage in using spring rubbers to change our spring rate on each corner of the car is the speed and ease of making those changes. We just need to be sure not to guess at the resulting rate change when we throw in or remove the rubbers. Use a spring tester and rate your spring rubbers for each corner and each spring that will be used in that corner. Then the change in rate will be predictable and we will be able to see the true results.