Springs in your Race Car
What is the purpose of a spring?
The purpose of a spring is quite simply to hold the chassis off the ground during corners and over bumps. Different spring rates in the car help to hold up the car in each corner, and because of suspension geometry designs, different classes of stock cars will have heavier springs in different corners.
A street stock style car will typically have the stiffest spring in the RF.
A late model car will typically have the stiffest spring in the RR. Until we start talking about bump stops and bump springs, then things change, but first where are the springs in your car.
What are the springs in your race car?
Coil springs respond from vertical pressure measured in pound feet
Leaf springs, they are harder to rate, due to the wrap up, flex, materials, thickness etc. We will not be focusing on leaf springs as that isn’t our area of knowledge.
Bump Springs, Bump Stops and Spring Rubbers - Your car doesn’t care what the material is made of that is holding it up. Bump stops made of polyurethane, rubber or bump springs are all the same in that they have a certain amount of elasticity that work to provide resistance. The difficulties in working with rubber materials is that the rate progression often times isn’t linear, so the rate can spike quickly and when it does then you can lose grip.
Sway Bar - torsion spring that increases as the preload is increased
Tires - Springs that increase in rate as the pressure increases. Too much pressure not only decreases the contact patch for grip but also increases the spring rate which also contributes to losing grip.
Chassis Flex (not wanted) - The torsional rigidity of the steel in your race car contributes to how your car responds to adjustments. If the chassis is as soft as a wet noodle then any spring change or adjustment will just be absorbed by the flex in the chassis. In a race car we don’t want chassis flex because we can’t measure it consistently and adjust it. Other forms of motorsports like karting and on-road R/C Touring car racing rely on it heavily.
Springs and Chassis Height
We know that springs in all their varieties help to hold up the car and that is their function. We also know that a low center of gravity is best for speed and the way to lower the car’s center of gravity is to simply lower the car. If there were no ride height rules we could just spring our cars to their maximum dynamic load numbers and set them up 1/2” off the ground so the car didn’t bottom out and it would be fine. That however isn’t the case.
Every class has rules and has ride height rules, so springs can get complicated when we need to figure out how much spring we need in order to get the car down as far as possible without hitting the ground. In addition to getting the car down, we need to monitor the chassis roll. The amount one side is leaning vs the other. THIS IS VERY IMPORTANT!!!!
The chassis needs to be dynamically flat, or the left side a touch lower under full dive or dynamic compression. If the chassis is rolling on the RF more than the LF, then their isn’t enough weight on the LF and the LF can’t do it’s job to push the tire into the ground and help the car turn and the LF needs to have pressure on it in order to make the car turn. One way we can use bump stops/springs and travel to work together is to use a higher static ride height on the right side so that under full dive the RF matches the LF in full travel. This brings us to using the springs together.
How to use Springs Together
Some people assume you use a stiffer RF spring vs the LF spring, but you can use a stiffer LF vs RF if under full travel the chassis is flat in dynamic compression or full travel. To expand on that, when looking at a street stock the LF will have a stiffer spring and a lower static ride height than the RF. Under cornering loads the RF compresses and matches the LF chassis height. This helps the LF to gain heat and traction, helping the car to turn because the spring is helping to push pressure into the LF lower control arm and thus the tire.
In a late model, the LF uses a stiffer bump stop or bump spring together with soft coilover springs. As the car dives into the corner the car travels onto the LF bump stop, then rolls onto the RF and ideally the chassis is flat. If the RF is lower under dynamic load, then the LF tire will lose temperature and grip.
Using engagement points of your swaybar and or bump springs to engage at a similar rate to your coil springs as they travel. Avoid spiking loads when your bump stops engage or too heavy a sway bar that makes the RF spike and not travel smoothly and don’t forget to check for coil bind. Most late models need ultra high travel coilover springs to avoid coil bind under full compression. Travel indicators are a must, so you can check for binds, and measure your dynamic load numbers.
How to use your shocks with your springs
Once you know how much spring pressure you have at full dive or dynamic compression in your car, you can get shocks to match the amount of compression and rebound. Your shock builder will be able to help you match what you need to accomplish if you are looking for a tie down shock etc. If you are racing a street stock style car, with off the shelf shocks then you can reference the shock manufacturers rating system to get a shock with enough compression and rebound to suit your needs based on your spring pressure.