Car Parts and Accessories

Auto Wheels move up and down Independently

One-piece front axles, supporting the body on leaf springs, were superseded years ago on cars by independent suspension for each front wheel. This provides better steering and is a great improvement on the old beam-axle arrangement in terms of a comfortable ride.

In an independent front-suspension system, each front wheel is connected to the car’s body by its own linkage and springing, so that its movement does not affect that of the other wheel. However, modern suspensions are joined by an anti- roll bar which is hinged across the monoframe, bent, and joined to the lower wishbone on each side to resist body roll when the car is cornering.

A vital feature of front suspension is that the wheels have to be steered as well as moving up and down. When a car corners or goes over bumps, it is subjected through the wheels to a variety of forces.

The suspension must not allow these forces to deflect the car from the course chosen by the driver. Nor must they cause the wheels to wobble, move any significant distance backwards, forwards or sideways, or alter their angle of tilt to any serious degree. Any such effects would interfere with the handling of the car.

Action of double Auto Wishbones

One common form of independent front suspension is the double-wishbone system. In this, two wishbone links, so called because of their shape, are hinged at their broad ends to the body or sub-frame of the car, and at their narrow ends to the swivel members which have stub axles to carry the wheels. Between each wishbone assembly and the car structure are a spring and a hydraulic damper to intercept road shocks.

As well as positioning the wheels and transmitting loads to the spring, the wishbones must also resist acceleration, braking and cornering forces.

Since the first two of these forces act along the car, a simple sideways link would yield to them. It follows that a form of triangle is necessary to give a broad base, which is why links of triangulated or wishbone shape are used.

The geometry of the wishbone layout— the lengths, positions and angles of the links—governs the path of the wheels when the car rides over bumps ; this path in turn affects steering, road-holding and tyre wear.

If the upper and lower wishbones are parallel and of equal length, the wheels will not tilt as they move up and down over bumps. However, the track (distance between the wheels, across the car) does change somewhat, with bad effect on tyre life. On corners, the wheels lean outwards with the body, which reduces their cornering ability.

In most modern designs, the wishbones are neither of equal length nor parallel, the upper one being the shorter. Hence the wheel no longer remains upright as it moves up or down over bumps, but leans slightly inwards.

This gives better cornering characteristics than the other layout: when the body rolls outwards, the outside wheel—which carries the greater load and therefore exerts the higher cornering power of the two—remains more or less at right angles to the road surface.

MacPherson Strut Auto Assembly

Instead of using double wishbones, some suspension systems have a single arm, a diagonal stay and a telescopic strut for each front wheel; this is known as a MacPherson layout.

The stub axle carrying the wheel is part of the lower half of the strut.

The whole strut is turned by the steering linkage and is connected to the car’s body structure at the top by a flexible mounting and connected at the bottom by a joint to the lower arm.

To absorb road shocks, a coil spring surrounds the upper part of the strut, which contains a hydraulic damper.

This system has gained wide popularity for several reasons; it is mechanically simple; its light moving parts help the wheels to follow road irregularities; and it does not cause the camber (angle of wheel tilt) to vary much as the wheel moves up and down.

On the other hand, the body structure has to be really strong above the wheel arches, where the struts are attached, to absorb the full suspension loads.

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