In this installment of Suspension 101, the overall function of the suspension of a vehicle will be explored: why do cars have suspension, how is it used to improve the handling of the vehicle, and what are some of the different types of suspensions? This will lay the groundwork for future discussions of design of the the suspension to behave in a way that maximizes handling, and is tailored to the type of vehicle and track in question.
Introduction
Why do cars have suspension systems? Are they even necessary? Using a go-kart as an example, it would appear that under certain conditions it’s not necessary. A kart has no suspension system, but instead has the hubs mounted directly to the chassis, leaving only the tires to absorb bumps. So why is the kart a lonely example of this lack of suspension? It is a machine designed for use on very smooth surfaces, and without bumps to absorb it has been decided that low weight will take priority. The result is a vehicle which has unrivaled agility on a smooth track, but would be extremely uncomfortable and handle very poorly over the slightest bumps. Comfort is obviously a benefit of a suspension system. But why would it handle so poorly? As a wheel passes over a bump it moves upward, but because it is directly connected to the chassis the entire chassis must move with it, and fast too! The result is not only a vehicle that is dramatically upset from the chassis having to follow the single wheel, but the speed at which it follows it means it is likely to be thrown into the air momentarily. An airborne car generally doesn’t handle as well as one that is touching the ground. So a suspension system allows one wheel to travel over a bump while having as small effect as possible on the chassis, and thus the other wheels. Instead of the chassis having to directly follow the motion of the wheel in a bump, the wheel is allowed to move independently relative to the chassis, controlled by the spring. This separates the vehicle into two categories, as illustrated in the diagram below. The first category is unsprung mass, which is those objects between the ground and the spring which move along with the wheel as it moves in bump. The second category is sprung mass, which is the chassis and suspension components of the car that do not move as the wheel travels in bump. Because the weight of the unsprung mass is a very small fraction of the weight of the chassis, it has much lower inertia and is able to respond much faster to bumps. As the wheel moves, the kinetic energy of the upward motion is converted into potential energy in the spring. This energy is stored in the compressed spring, and can be transferred to either the chassis or back to the wheel as kinetic energy through extension of the spring. Due to the greater inertia of the sprung mass, this energy does not cause as quick of an acceleration of the chassis.
Vehicle Suspension Schematic, Courtesy Wikimedia Commons
The suspension allows the wheel to move independently of the chassis, using the spring as a buffer between bumps in the road and bumps in the seat. This helps in ride comfort over bumps, which is a reasonable goal for any vehicle. And a car bounding through the air also sounds undesirable. So what is it about the suspension controlling the wheel motion that actually helps handling? Obviously a tire that isn’t touching the ground can’t help the car at all; conversely the harder a tire presses onto the ground the more work it can do in accelerating, braking and turning the car. So the goal of tuning of the shocks and springs is to keep the tire pressing down as hard as possible for as much time as possible, thus keeping the average force of the tire on the ground at a maximum. The role of the spring has already been covered, but what about the shock? When the energy of the bump is input into the suspension, the spring first absorbs the energy and then releases it into the rest of the system through kinetic energy. But if there were no mechanism to control the transfer of this energy it would continue to transfer back and forth from the suspension to the spring, causing the system to oscillate at its natural frequency. Without a shock to damp this oscillation, the primary way it would dissipate is through friction in the suspension joints, which is usually low compared to the force in the spring. Working in parallel with the spring, the shock (or more properly, damper) converts kinetic energy into heat energy, damping the system and helping to control oscillation. Shock selection depends on the stiffness of the spring and the weight of the sprung and unsprung masses.
Next, we’ll take a look at a few of the different types of suspension systems.
