By Jim Kerr
To the untrained eye, suspension designs vary little from car to car. Some use struts to hold the wheels upright, while others use control arms to do the same task. Springs may be spring steel coils, flat leaf springs or long torsion bars, but again they all have to do the same task: hold the car off the ground. Then there are the shock absorbers, which all look very similar. Some are bigger or have fancy paint schemes on them, but again they all are supposed to do the same task: keep the tires in contact with the ground.
With all these similarities, why is it that some vehicles handle and ride so much better than others? It all comes down to small details. Change the angles on the control arms a couple degrees and the engineers can build in anti-dive characteristics, or change the roll centre so the car remains flatter on corners. Of all the variables that owners can change on their vehicles to improve handling, tires and shock absorbers create the most difference. Performance tires are easier to recognize, with their lower sidewalls and wide profiles. Shocks, on the other hand, all still look similar. So what makes the difference between a good shock absorber and a poor one?
To understand what makes a good shock absorber, one must understand what the shock needs to do. MacPherson strut suspensions have the shock absorber integrated into the strut but still function the same as externally mounted shocks. When a rolling tire hits a bump in the road, it quickly moves upward. The shock must allow the tire to move up quickly or the whole vehicle will bounce up. The vehicle’s spring will then push the tire back down so it contacts the road, but if this happens too fast, the tire will bounce when it hits the road. Instead, the shock must now allow the tire to move down smoothly and slowly (compared to how fast it moved up) so it won’t bounce. To summarize, the shock must allow the tire to travel up quickly but move down slowly. This will prevent the tire from bouncing and maximize the tire contact on the road.
To accomplish this task, the shock absorber has a piston inside that slides in a cylinder. The piston is connected to a shaft that bolts to the body of the car, while the cylinder is bolted to the moving suspension. As the tire goes up and down, so does the shock piston in the cylinder. Oil in the cylinder is forced through orifices in the piston to control the speed of movement of the piston. If there were just holes in the piston, the jounce (upward movement) and rebound (downward movement) rates would be the same, so small spring-loaded valves block some of the orifices on the rebound stroke. This allows the shock to slow the tire movement so it doesn’t bounce.