Spring
A spring is object that stores energy when compressed or extended. In FSAE, a coil spring is most commonly used in the suspension as part of the shock or coilover. There are other forms of spring besides coil such as leaf springs and torsion bars. All springs have a k-constant which determines the stiffness of the spring. The dynamics of springs will change a car's dynamics in roll, heave and pitch. This k-constant is also called the spring rate. A stiffer spring has a higher spring rate and higher k-constant. Spring stiffness can be measured in N/m (newtons per meter) or lbs/in (pounds per inch).
Choosing the Correct Spring[edit | edit source]
When deciding what spring rate your car should run some factors to consider include the effect on aero, drivability, and mechanical grip. One is not more important than the other (unless you have no aero somehow) and the trick is finding a balance between them.
Effect on Aero[edit | edit source]
Because the spring determine the stiffness of the suspension, they determine how much and often a vehicle's ride height will change while driving. A car with stiffer springs has less ride height variation. This allows the car to be run closer to the ground as there is a reduce risk of scraping. Running closer to the ground without scraping is extremely beneficial to the floor and downforce of an FSAE car.
Springs are also a factor in the roll of a car; stiffer springs increase an axle's roll stiffness. A sensitive aero package may benefit from less roll. This was the case with the F25 which kept getting faster the stiffer the springs were.
Drivability[edit | edit source]
This may surprise you, but someone actually has to be able to control the car for it to be fast. You may find extremely stiff springs will be beneficial for aero, however this may make the car extremely uncomfortable to drive. Extremely stiff suspension is also very susceptible to bumps as instead of the spring absorbing most of the impact, much of that load will be transferred into the chassis, launching that wheel off the ground and unsettling the car.
Extremely soft suspension can be just as difficult to drive. Soft suspension technically provides the most consistent mechanical grip; however, it can make the car feel extremely lazy and unresponsive. This can lead to an unpredictable and downright slow car as it will likely become difficult to rotate. Additionally, extremely soft suspension is more susceptible to bottoming out the car which can lead to a loss of control.
Mechanical Grip[edit | edit source]
In roll, lateral load transfer occurs through a torque moment. The total amount of load transferred laterally across the car cannot be changed by the spring stiffness. However, spring stiffness changes an axle's roll stiffness, which changes the vehicle's load distribution between the front and rear axles. Stiffer springs on an axle will increase the axle's roll stiffness and thus increase that axle's load and decrease the other axle's load. By changing the load distribution between the front and rear axles the vehicle's mid corner over/understeer bias can be changed.
Extremely stiff suspension also lacks consistent mechanical grip due to the wheels not having as much contact with the ground (wheel hopping). This will cause an extreme deficit in any corners that are not smooth.
What-ifs[edit | edit source]
This is a vague push-in-the-right-direction on choosing spring rates. Spring rates are not the only things that can be changed to fix the scenarios, so you may want to look somewhere else first like dampers, ride height, etc.
| Scenario | Spring rate change |
|---|---|
| Floor keeps scraping excessively | stiffer springs all around or on the end of the car that is scraping |
| driver complains of oversteer | softer springs in the rear |
| driver complains of understeer | softer spring in the front |
| driver complains of oversteer under braking | stiffen front springs |
| driver complains of understeer under braking | stiffen rear springs/softer front |
| driver's ass hurts really bad, and they developed scoliosis | softer springs all around |
| the car goes airborne a lot (or excessive wheel hop) | softer springs all around |
Spring Preload[edit | edit source]
Preload is a set compression on a spring. If the spring has a constant k-constant throughout its length, preload does not change stiffness of the spring. If the spring has a variable spring rate (variable k-constant), preload does have an effect as the more preload on the spring, the stiffer it will be for its first distance of compression. As of 2025, we do not use variable k-constant springs.
When setting preload, it should be as close to 0 as possible. 0 preload means that the spring is fully extended, and any suspension movement will compress the spring immediately (i.e. the spring should not be able to move up and down on the shock). This is to maximize the travel of the spring. The spring should be measured to have 0 preload on the car in its lightest condition that it will be racing in (dry car, lightest driver). Weight (fuel) and downforce will force the suspension down, thus compressing the spring and change its "preload" to more than 0. This is expected because you absolutely do not want a car with negative preload because that means for a portion of the suspensions travel there will be effectively no spring.
