Suspension Handbook
Introduction to Vehicle Dynamics
An FSAE car will experience three movement types: pitch, roll and yaw. Pitch refers to the front vs back vertical movement of the car, for example if the car is pitched forward the front will be lower to the ground than the rear. Roll refers to the side vs side vertical movement of the car, for example a car with soft suspension turning right the body will noticeably lean to the left when looking at the vehicle face on. Yaw is the vertical rotation of the vehicle around the yaw axis, for example a car losing traction on its rear wheels and starts to swing around is experiencing yaw.
Suspension controls the dynamics of these movements. This is done using dampers and a form of spring. A spring provides resistance and compliance for sprung mass of the car relative to the unsprung mass. A damper controls the oscillation of the spring. Unsprung mass is as it sounds; everything on the vehicle that is not supported by the spring. This includes: the tire, the wheel, the hub, the upright/knuckle assembly, as well as a portion of the spring assembly and control arms. Sprung mass is all of the mass on the vehicle that is supported (or sprung) by the spring.
Our Goal
Our goal is to build and set up a suspension system that provides positive vehicle dynamics. Positive vehicle dynamics is a quite subjective term, but for us it largely making the car drive as fast as possible by maintaining large amounts of grip through the tires. Contrary to road vehicles, comfort is not a priority. Another complication is added as the suspension also must work with the aero of the car in order to provide downforce and less drag. For example, with the F25 it was discovered that stiffer springs improved the aerodynamic consistency of the vehicle, thus allowing it the maintain more downforce through pitch, roll, and yaw.
Springs and Dampers
Every spring has a rate which determines the force needed to compress or extend it. This is the k-constant or spring constant. Changing a springs rate can change the amount of load, or force, a given tire receives at a given time. Because springs ossiculate and in a perfect system would ossiculate forever, a damper is needed to control its osculation. A damper is a hydraulic device placed within the spring most often that will convert the kinetic energy of a force into another form, often heat. This is done by forcing a hydraulic fluid through small passageways, thus reducing the springs kinetic energy. It is important to note that dampers do not change tire load levels but rather control the speed at which the load changes. The dampers we use are 4-way adjustable. This mean we can adjust the rebound and compression (called bump by people who are wrong) dynamics of the damper separately as well as adjust the "fast" rebound and compression separately. Fast rebound and compression are described as a relatively (usually significantly) quicker compression or expansion of the damper. This most often occurs when attacking something like a racing kerb, where it is beneficial for the damper and spring to compress quickly to absorb the impact rather than send the car flying. Compression controls the kinetic energy the damper absorbs when being compressed. Rebound controls the kinetic energy the damper absorbs when being extended. The terminology concerning dampers is often extremely confusing and contradictory, so in this page we will be using the following terminology: Lowering the damping force means lowering the resistance the damper provides to the spring. This makes the spring and suspension move faster.
Basic Theory
A tires grip level is determined through the friction coefficient between it and the ground, the force the tire is putting into the ground, and contact patch (the area of which the tire is in contact with the ground. A higher grip level allows the car to take corners faster. Suspension controls the dynamics of pitch, roll, and yaw which in hand controls the force each tire is putting into the ground at a moment. This force changes through a "weight shift" of the vehicle. This is because the center of mass of the vehicle is higher than the tires, thus it pushes a set of tires into the ground with a greater force than the other. With changes to the suspension, we can change the speed and extent of which a weight shift occurs. We want to design and tune our suspension in such a way that the car maintains high levels of grip while also promoting rotation through corner and driver control.
In Pitch
Pitch of a car is caused by either acceleration or braking in a straight line. When braking, due to the vehicles center of mass being higher than the unsprung and driving mass, the weight shifts to the front tires. Suspension can be used to change the dynamics of braking. If a driver prefers a car with a oversteer bias when braking, a greater spring rate can be used for the rear wheels or a softer spring can be used for the fronts, thus increasing the fronts load and decreasing the rear wheels load and allowing the car to rotate upon entry to the corner. The damper forces can also be changed to influence pitch under braking. A lower compression damping force on the fronts will cause the front to pitch rapidly forward while a high compression damping force will cause the front to ease slowly into its pitch. Too low of a compression damping force on the fronts can cause oversteer as a rapid pitch forward and extreme change in weight transfer to the front causes a sudden loss of rear grip. Too high of a compression damping force on the front can cause understeer as the front becomes "lazy" and weight does not transfer to the front quickly enough to provide the grip needed to the front tires thus causing understeer. Lowering the rear rebound force can also reduce oversteer as it allows the rear tires to quickly move to be in constant contact with the ground as the cars weight shifts forward, thus providing more consistent rear grip.
When accelerating, due to the vehicles center of mass being higher than the unsprung and driving mass, the weight shifts toward the rear tires. In most cases this is inherently beneficial as the rear wheels drive a FSAE car and more force leads to more grip which leads to better acceleration. When accelerating out of a corner, the front dampers are rebounding, and the rears are compressing. In extreme cases, lack of traction and therefore oversteer can be caused by too low of a rear compression damping force as the initial weight shift may cause the rear spring to oscillate and thus skate the rear tires across the surface. Understeer can be caused under acceleration by too low of a rear compression damping force, as a quick pitch rearward will cause the front wheels to become unloaded and lose grip.
In Roll
Roll of a car occurs during a turn where the centripetal force will push the body of the car outwards. The dynamics of roll include a weight transfer to the outside wheels and the inside wheels traveling less distance in a circle.
Accounting for Aero
In general, the more a fluctuation aero package has when driving, the less consistent downforce will be produced. Because suspension basically controls the fluctuation and location of the whole cars mass, the suspensions effect on aero needs to be considered. Generally, the stiffer the car is, the more consistent the downforce is produced. Additionally, when the car is lower to the ground, more downforce is produced (until aero stall) (think 2022 F1). Driving the car with stiffer suspension allows it to be closer to the ground as the cars body will not bounce as much into the ground. Recently, Aero has become responsible for setting the ride height.
