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.