Changing spring rates during...
Changing spring rates 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 faster, we can use spring rubbers to simulate different spring rates. Installing spacers is much faster than a spring change and tells us the exact same results if we do it correctly.
There are times when we need to make a spring rate change quickly due to lack of 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'll continue to refer to them as rubbers for the sake of consistency. I love traditional terms anyhow. You get the meaning.
We preach having a balanced setup and whether you dial your car in with trial and error or with the use of computer software, 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.
Spring rubbers are not necessarily...
Spring rubbers are not necessarily made of rubber. They come in different degrees of hardness and shapes to fit every application of springs from the coilover design to the big spring stock sizes.
One way to quickly enact a spring rate change in our cars is by installing or removing spring rubbers. 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 coil in the spring. We can even 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's because a 200 lb/in rated spring that holds up 950 pounds will be compressed 43/4 inches, whereas a 200 lb/in spring holds up 500 pounds will only compress 21/2 inches. That means there's 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 that supports 500 pounds.
Spring rubbers come in different...
Spring rubbers come in different "rates," or hardness, and styles. They can be used with the large stock-type coil springs or the coilover sizes. The longer coilover springs have more room to place multiple rubbers.
It's also true that if we change to a 150 lb/in rated spring in place of the 200 spring, the compressed height on the corner holding 950 pounds will be 61/3 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's 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 racetrack. 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.
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.
For the Big Bar, Soft Spring asphalt setups, fine tuning is usually done with the RR 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're sure of the correct RR spring rate, just install that rate of 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.
Note the difference in space...
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. This reduction in the number of coils increases the rate of the spring.
Many designs of modern spring...
Many designs of modern spring rubbers come with plastic handles so the effort required to remove them is less. If you're using these in a race condition, you may need to pull a spring rubber during a pit stop to affect the setup as the track changes conditions. In a long dirt race, it might be wise to give up positions to make these kinds of changes if it will make a significant difference in lap times. Example: a dirt track starts out tight, but quickly turns dry and slick. You need to soften the right rear spring rate, so you pit under a caution and have a crew member pull one or more rubbers out of the right rear without losing a lap. Your lap times get better by a half second and you drive to the front. It could happen.
Spring rubbers are color coded...
Spring rubbers are color coded so you can easily chose the correct stiffness. This assortment demonstrates the various sizes to fit coilover springs and big springs.