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.
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.
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.
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.