10-11-2012, 12:57 PM
INTRODUCTION TO SUSPENSION
INTRODUCTION TO SUSPENSION.doc (Size: 288 KB / Downloads: 246)
Why do cars have suspension?
The object of a suspension system is to isolate the body and its occupants from the irregularities of the road surface. Ideally the body should ride level and without vertical motion’ however bumpy the road surface. Another important feature of suspension is that it should keep the tires on the ground all the time. If there were no suspension the tires would tend to lift off the ground every time they passed over a bump at the same time, the shock as the wheels left the ground’ and then came down again, would be transmitted directly to the passengers.
What does a suspension?
Ideally the suspension should allow the wheels to move up and down so that they follow the undulations in the road’ while the body rides level. The first requirement therefore is that the wheels should be able to move vertically relative to the body. Every suspension has this wheel travel, which must be accommodated by some means.
How is the wheel movement accommodated?
Springs are normally used (Figure 1.1). As the tire strikes a bump in the road, so a vertical force is applied to the spring’ which is compressed or deflected. Therefore the wheel moves vertically relative to the body, and the tire maintains contact with the road surface. However, some of this force is transmitted through the spring to the body, which also tends to rise. If the springs are very ‘soft’ (i.e. have relatively low spring rates) the body rises little, but if the springs are very stiff the body rises quite a bit, depending on the severity of the bump. For a good ride, therefore, the springs should be soft.
Do soft springs have any disadvantages?
Although soft springs give a good ride in most circumstances, they allow the body to roll a lot during cornering. In practice, spring rates are a compromise between the requirements of ride and handling.
Why does a car roll?
When a car turns a corner, centrifugal force acts on the body and tends to push it outwards. However, this force is resisted by the tires, which grip the road; as a result, the body rolls about its suspension (Figure 1.2).
What is the disadvantage of roll?
When the car rolls excessively it is more difficult for the driver to control its direction. Also, when the car emerges from the corner and the body rolls back to the upright position, it will roll past the upright position if roll is excessive, and will take some time to right itself. Excessive roll can also make the wheels adopt unfavorable angles, depending on the type of suspension fitted.
Are any other components needed?
In its simplest form the suspension consists purely of the springs and their anchorages. However, when a spring is deflected it attempts to return to its normal position immediately, and if left to itself it goes past the normal position and then back again. It would take a number of these oscillations absorb the stored energy, and only then would the spring return to the normal position.