Basic Know How
Basic suspension know-how made easy
Ever more powerful engines and faster cars mean that suspension systems have become a vital development aspect in the automotive industry. And these systems will play an increasingly important role in the future, because the chassis - particularly the springs and shock absorbers - provides the link between the wheel suspension and the car body, ensuring optimum contact with the road. Read on for a simple explanation of how it all works.
Suspension: road grip - traction - comfort
The suspension is the link between the road and the car body. The job of the suspension is to reduce - as far as possible in a controlled fashion - the vibrations from the wheel and the car body caused by an uneven road or changes of direction. Every attempt must be made to prevent these vibrations from being transmitted to the car body in order to reduce rocking, pitching and diving, avoid swaying, and ensure optimum road contact and traction with minimum slip. The suspension system comprises a range of components, including strut support bearings, springs, shock absorbers, connecting rods (coupling rods), stabilisers, axle supports/wheel supports, wheel bearings, suspension arms (control arms and pull rods), wheel brakes, rims, tyres, final drives and steering.
Springs and how they work
A spring cushions the effects of road unevenness and impacts from the road, turning these into vibrations. The spring forms an important link between the individual suspension components, connecting up the sprung and unsprung masses in the vehicle. The unsprung masses include the vehicle components situated between the road and the spring, i.e. the wheels, brake and parts of the wheel suspension and steering. All other vehicle components are classed as sprung masses and include the car body, drive train and the remaining parts of the wheel suspension and steering. In terms of a car's suspension comfort, the basic rule is that the smaller the ratio of unsprung to sprung mass, the greater the comfort. The spring works in conjunction with the stabiliser, the tyres and the seats.
Shock absorbers and how they work
The shock absorbers reduce and slow down the vibrations from the springs, which is why technically they are correctly referred to as vibration dampers. Vibration dampers convert kinetic energy into thermal energy through fluid friction. This involves the flow of oil being slowed down by the valve passages inside the damper. The valve passages in the shock absorber are specifically designed to ensure that the vibrations transmitted by the spring are reduced right from the start. The shock absorbers can heat up to between 100 and 120oC in the process.
Interaction between spring and damper
When a car passes over an obstacle, this first has an impact on the spring, which must not be hampered by excessive damping performance on the part of the shock absorber. When a car passes over a bump in the road, for example, the obstacle forces the wheel up into the wheel housing. In the process, the spring is compressed. The shock absorber is now in its compression stage. Once the spring has levelled out the obstacle, the shock absorber has to slow down the movement of the spring as it releases its tension with great force. The shock absorber is now in its rebound stage. Compression stage (compressing of the springs and the damper, e.g. when driving over bumps in the road) = usually approx. 25% of the damping force. Rebound stage (when the spring pulls the damper apart) = usually approx. 75% of the damping force.
Conclusion: A spring with a higher spring rate (sport or lowered spring) will only work at its best in conjunction with the appropriate high-performance or sports shock absorber.