THE only time we think about our cars’ suspension is when it starts to perform badly. Squeaking, groaning, shaking and swaying are not lewd terms but what happens when we encounter bad suspension. The only remedy is replacement of worn parts but it is interesting to find out about the why and what of car suspension systems.
Suspension would not be necessary if roads were perfectly flat but even the best built highway has undulations and imperfections. It almost certainly has curves and turns as well. The suspension of any car has to deal with forces from bumps as well as forces from cornering. The study of these forces is called vehicle dynamics and is mostly centred on how to maintain the tyres contact and traction with the road surface.
The suspension of a car is actually a part of the chassis and consists of three fundamental components: springs, dampers and anti-sway bars.
Springs
There are four basic designs of springs. The most common is the coil spring which is basically a coiled torsion bar. The second type is the leaf spring which is several layers of metal bound together in different strengths and lengths. Thirdly, there is the torsion bar, a length of steel anchored on one end which works as a spring when twisted by a lever attached to the wheel. Finally, there is air springing, which uses the compressive qualities of air inside a cylinder.
Springs are simple devices designed to absorb the movement of the cars’ wheel but springs alone cannot provide a perfectly smooth ride. That’s because springs do not dissipate the force acting upon it. This is the job of the damper or shock absorber.
Shock Absorbers
Springs have a natural frequency and will tend to bounce until all the energy is dissipated. This would not be very usable in a car and would probably induce car sickness. Shock absorbers slow down and reduce it by turning the kinetic energy of suspension movement into heat energy that can be dissipated through hydraulic fluid.
A shock absorber is basically an oil filled pump where the upper mount is connected to a piston rod connected to a piston, which sits in a tube filled with hydraulic fluid. The inner tube is known as the pressure tube, and the outer tube is known as the reserve tube. The reserve tube stores excess hydraulic fluid.
Orifices in the piston allow fluid to leak through as the piston moves up and down in the pressure tube. Because the orifices are tiny, only a small amount of fluid passes through and this slows down the piston which in turn slows down the spring.
All modern shock absorbers are velocity-sensitive which means the faster the suspension moves, the more resistance the shock absorber provides. This enables shocks to adjust to road conditions and to control all of the unwanted motions that it encounters. However, even the addition of dampers will not allow your car to handle properly. You will also need to corner your vehicle without running out of road and for this you need anti-roll bars.
Anti-Roll Bars
When you encounter a corner and turn into it, the car will experience forces that will try to roll the car one way or the other. This will cause swaying and a general unpleasant feeling in the stomach. The need to stabilise the swaying motion gave rise to the design of the anti-roll bar.
Anti-roll bars, also known as anti-sway bars, are used to give a moving automobile additional stability in corners. It is basically a metal rod that spans the entire axle and effectively joins each side of the suspension together. How the anti-roll bar works is by transferring the movement of one wheel to the opposite side. This reduces the roll of a car on its suspension as it negotiates a turn.
Of course, in actual fact suspension is actually much more complicated than this. There are many factors that affect suspension such as the ratio of sprung to unsprung weight, tyre types and choices and even the ride height of the car. Off-road, street and racing cars have different suspension designs according to their different needs. We’ll explore these different designs in the next article.