Suspension is the term given to the system of springs, shock absorbers and linkages that connects a vehicle to its wheels. Suspension systems serve a dual purpose - contributing to the car's handling and braking for good active safety and driving pleasure, and keeping vehicle occupants comfortable and reasonably well isolated from road noise, bumps, and vibrations. These goals are generally at odds, so the tuning of suspensions involves finding the right compromise. The suspension also protects the vehicle itself and any cargo or luggage from damage and wear. The design of front and rear suspension of a car may be different.
Important properties
Spring rate
This may vary with deflection. For active suspensions, it may depend on other things. The softer the springs, the more important the other requirements are. Spring rate is often a compromise between comfort and handling, but when other things are compromised instead, as in the 1960s Lotus Elan, both may be achieved.
Spring rates typically have units of lbf/in. or N/mm. An example of a linear spring rate is 500 lbf/in. For every inch the spring is compressed, it exerts 500 lbf. A non-linear spring rate (typically increasing) is one that the force exerted increasess exponentially. For example, the first inch exerts 500 lbf, the second inch exerts an additional 550 lbf, the third inch exerts another 600 lbf.
Travel
Bottoming or lifting a wheel can cause serious control problems or directly cause damage. "Bottoming" can be either the suspension, tires, fenders, etc. running out of space to move or the body or other components of the car hitting the road. The control problems caused by lifting a wheel are less severe if the wheel lifts when the spring reaches its unloaded shape than they are if travel is limited by contact of suspension members. (See Triumph TR3B.)
Damping
This may also vary, intentionally or unintentionally. Like spring rate, the optimal damping for comfort may be less than for control.
Damping controls the body movement of the car. An undamped car will oscillate up and down. With proper damping levels, the car will settle back to a normal state in a minimal amount of time.
Camber control
Camber changes with wheel travel and with body roll.
A tire wears and brakes best perpendicular to the road. Depending on the tire, it may hold the road best at a slightly different angle. Small changes in camber, front and rear, are used to tune handling.
Roll Center Height
This is important to body roll and to relative weight transfer, front and rear. It may affect tendency to rollover. All other things being equal the end of the car with the higher roll center will have more weight transfer and therefore more slip in a turn. However, the roll stiffness in most cars is set more by the antiroll bars than the RCH.
Space occupied
Designs differ as to how much space they take up and where it is located.
Force distribution
The suspension attachment must match the frame design in geometry, strength and rigidity.
Air resistance
Currently this is signficant only on racing cars (e.g. Formula One), but may become important on production cars in order to improve aerodynamics and thus fuel efficiency.
Cost
Production methods improve, but cost is always a factor. The continued use of the solid rear axle, with unsprung differential, especially on heavy vehicles
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