A Suspension Primer
From the early days of the automobile—and even before, in the time of horse-drawn carts—it was already well known that the body of the car, housing both the passengers and the load, has to be decoupled from the unevenness of the road surface.
This isolation is much more than a question of comfort. The vertical force of the jolts caused by the repeating bumps and holes of the road surface are proportional to the square of the vehicle speed. With the high speeds we drive at today, this would result in unbearable shock to both people and the mechanical parts of the car. Jolts in the body also make it more difficult to control the vehicle.
Consequently, there has to be an elastic medium between the body and the wheels, however, the elasticity and other features of this suspension medium are governed by many, mostly contradicting factors.
The softer, more elastic the spring, the less the suspended body will be shaken by various jolts. For the sake of comfort, we would thus need the softest spring possible. Unfortunately, too soft a spring will collapse under a given weight, losing all its elasticity. The elasticity of the spring would need to be determined as a function of the weight carried but the weight is never constant: there is a wide range of possible load requirements for any car. On one hand, a hard suspension will not be sensitive to load variations but being hard, will not fulfill its designated purpose, either. A soft suspension, on the other hand, is comfortable but its behavior will change significantly on any load variation. To cope with this contradicting requirements, an elastic medium of decreasing flexibility would be required: such a spring will become harder as the weight to be carried increases.
When the spring is compressed under the weight of the load, it’s not only its flexibility that changes. The spring deflects, causing the clearance between the car and the road surface decrease, although a constant clearance would be a prerequisite of stable handling and roadholding. At first sight, this pushes us towards harder springs: soft springs would result in excessive variations of vertical position — unless, of course, we can use some other mechanism to ensure a constant ground clearance.
In addition to the static change caused by load variations, the deflection of the spring is changing constantly and dynamically when the wheels roll on the road surface. The body of the vehicle dives, squats, rolls to left and right as the car goes over slopes, holes and bumps in the road, corners, accelerates or decelerates.
When a deflected spring is released again, the energy stored in it will be released but as there is no actual load for this energy, the elastic element, the mass of the suspension and the vehicle form an oscillatory system, causing a series of oscillations to occur instead of the spring simply returning to its neutral position.
Any vertical jolt would thus cause such oscillations: the upward ones are transmitted to the car body while the downward ones make the wheels bounce, losing contact with and adhesion to the road surface. The first is only discomforting, but the second is plainly dangerous. In addition, it’s not only the spring that oscillates; the tires contain air which is a highly elastic spring medium. Oscillation in itself causes unwanted motion but when the corrugation of the road surface happens to coincide with the period of the suspension oscillations, it might lead to synchronous resonance, a detrimental situation leading to serious damages in the suspension elements.
Mass in motion can also be viewed as a source for kinetic energy; because of this, moving parts of the suspension are often reduced in weight to decrease this portion of the stored energy, and this in turn eases the requirements on the dampers as they have to dissipate less unwanted energy as heat. This solution, however, often shifts the frequency of the self-oscillation of the suspension upwards. Unfortunately, occupants are more sensitive to higher frequencies reducing comfort (mostly adding noise), so this is an area where compromise is needed.
Conventional suspension systems use a second element, a shock absorber to dampen these oscillations. The absorber uses friction to drain some of the energy stored in the spring in order to decrease the oscillations. Being an additional element presents new challenges: the characteristics of both the spring and the absorber have to be matched carefully to obtain any acceptable results. The absorber ought to be both soft and hard at the same time: a soft absorber suppresses the bumps of the road but does not decrease the oscillations satisfactorily while a hard absorber reduces the oscillations but lets the passengers feel the unevenness of the road too much. Due to this contradiction, conventionally suspended cars have no alternative but to find a compromise between the two, according to the intended purpose of the car: sport versions are harder but offer better roadholding, luxurious models sacrifice roadholding for increased comfort. This contradiction clearly calls for a unified component serving both as a spring and an absorber, harmonizing the requirements.