A SHORT HISTORY OF SUSPENSION  

A PERPLEXING PHENOMENON: LEVEL VARIATION

We know that the softer the spring (and this is desirable), the more sensitive it is to load variations (this is undesirable). The softer the spring, the more it will give under the load, thus decreasing the space between the car floor and the ground level (road clearance). There will also be a corresponding decrease between the wheels and the body.

When a driver is alone in his car, the only load is the driver's weight, say 170 lbs. But if five people and their luggage are carried, the load will be about 900 lbs. The springs will give under this weight. Thus the clearance will vary according to whether the car is driven empty or with a full load. Now, for good road holding qualities, the car should always have the same position. Conventional suspensions are prevented from being made softer than they are by the excessive variations in position that would result. On the other hand, a suspension of the decreasing flexibility type restricts the clearance variations and guarantees the essential clearance the wheels require.

But it is possible to do better still, by adopting a suspension which corrects the car's position due to static and dynamic variation in height. 

Static variations are due to load increase which, of course, causes a spring deflection. Dynamic variations in height pose another problem for suspension engineers. Let us take an example: a car running on a flat road.  It comes to a steep hill. The front wheels will reach the hill first and start climbing up the slope. But the suspended part of the car, which was so far horizontal, will tend to remain in this position, owing to the force of inertia. The lay of the land has changed, causing a dynamic variation.  The faster the car is going and the softer the springs, the greater it will be. This phenomenon will occur whenever the lay of the land changes significantly: hills, hollows, humpbacks. Also, when going up a hill, the car's centre of gravity is displaced towards the rear, and when going down hill towards the front. These changes in the centre of gravity will also affect the ground clearance. If these ups and downs are close together - if, instead of being hills to drive up and down, they are holes and bumps in the road, or if the road surface is corrugated, constant dynamic variations will occur and give the car an unpleasant pitch. It will behave like a ship on a rough sea, heading into the crest of the waves to lurch down again afterwards. Like a ship, it will pitch.

If the ship sails along the crest of the waves, they will make it sway from side to side. It is said to roll. This occurs when a soft sprung car is driven over a road with holes and bumps staggered on both sides if and when sharp corners to the left and right are taken in rapid succession. Other influences must also be taken into account: side winds, centrifugal force on corners; a highly cambered road, making the car look lopsided; the rearing of the car in a quick getaway; or the way it puts its nose down when the brakes are jammed on. To counteract these phenomena, a certain number of devices have been designed, for example stabilizer bars, to prevent the car from rolling.

Several manufacturers, including Citroën, have also adopted independent wheels, with each wheel reacting individually to the state of the road. Watch a mule on a mountain track; its four legs are independent It can bend its left legs to walk along the hill-side or climb on a stone with a joint bent, without unbalancing the load it is carrying. In the same way, the independent wheels of a car respond separately to the state of the road, each wheel reacting on its own to the bumps of the surface.

A good suspension, then, requires four independent wheels. But, clearly, this would not be sufficient. It would also require an automatic clearance corrector which, as we have seen, would not only reestablish the car's position if the load were altered, but also react immediately to road surface conditions and to any change in the centre of gravity.

We shall see that this rare bird - the automatic height corrector - really does exist. But first let us continue our review of suspension problems. Unfortunately, we have not finished with them. How are shocks to be counteracted?

THE DAMPING PARADOX

The elasticity of a spring works both ways, as we have seen. Thus any stress on the spring will lead to a series of oscillations comparable in theory to those of the pendulum of a clock.

Take your favorite clock and stop the pendulum at a standstill. Then push it gently to the left. It will start moving in this direction, to return through its position of equilibrium and swing over to the right, and so on.  Your action has produced a series of oscillations on either side of the position of equilibrium. This is a pendulum motion.

Similar things happen in an automobile. At the standstill, the spring is in the equilibrium position. In motion, the variations of terrain create series of oscillations similar to the pendulum, i.e. series of movements going from one side to the other of the equilibrium position. This has two tiresome results: first, the repeated, upward oscillations are transmitted to the body and thus to the passengers; secondly, downwards, the same oscillations transmitted to the wheels make them bounce on the ground, thus losing adhesion -- the basic factor of road holding.

This is all the more marked in that the wheel has its own highly elastic suspension, the tyre, which reacts to road surface conditions immediately.  It is vital to dampen these oscillations, both in the suspension and in the wheels. This is all the more necessary when the road surface is corrugated in such a way that the frequency of the bumps coincides with the period of the suspension (time between two complete oscillations on either side of the position of equilibrium). This will produce the phenomenon of synchronous resonance, which will increase the oscillations beyond limits.

It is the same phenomenon that enables children to reach dizzy heights on a swing, merely by coordinating the impulsions of their legs with the period of oscillation of the swing.

As any impulsion on the spring makes it store up energy which is gradually released by oscillation, part of this energy must be removed in order to decrease the oscillations.

This is a problem schoolchildren are familiar with. If a tap fills a bath in a given time, how long will the drain require to empty it before it overflows ? In the motor car, the drain is the shock absorber. The shock absorber has to eliminate part of the energy the spring has stored.  Friction, an excellent energy consumer, is used for this purpose.

There are various types of shock absorbers: the friction or solid type (friction damper) and the hydraulic type, in which the speed of the movement is reduced by the effect of lamination of a fluid. This is also called viscous dampening and is preferable to the solid type because it is more sensitive and responsive, its action is continuous and progressive in all circumstances.

On most cars, the shock absorbers are added to the suspension proper, as an extra. However, it would obviously be preferable for the shock absorbers to be built into the suspension. Anyway, the result is a paradox. We are trying to obtain a suspension as soft as possible and yet we are doing everything in our power to prevent it from being too soft.

If we exaggerate, we may say that a very powerful shock absorber would stop entirely the motion of the spring. This would amount to doing away with the suspension altogether. If the shock absorber is very weak it will have no effect at all. Here again, the difficulty consists in reaching a happy medium between a suspension that is sufficiently soft to absorb the bumps of the road and a shock absorber firm enough to brake the oscillations of the body and wheels quickly. According to the type of vehicle, manufacturers have to choose between two solutions:
 

for a sports car, where road holding is the vital factor, the shock absorbers will be powerful; thus, the springing will be hard.    for a luxury Sedan, on the other hand, the shock absorbers will be weak: the suspension will therefore be very soft, but the road holding will not be so good.

Lieutenant de la Besse's sledge.  1909
 

Westinghouse car.  1908

For years, attempts have been made to combine these two points of view: comfort and road holding. For years, people have put up with a more or less satisfactory compromise.