Suspension  - Axles and wheel suspension

The axles and wheel suspension determine the amount of the wheels in relation to the body of the vehicle. Various wheel movements are to be distinguished:

(a) The wheel moves only in the vertical direction in its plane in relation to the body;
(b) The wheel undergoes sideways displacement;
(c) The angle of the wheel in relation to the body is changed;
(d) Sideways displacement occurs in combination with change of angle.

Various forms of construction for the axles and wheel suspension are employed for eliminating these wheel movements as far as possible.

With a rigid axle the two wheels share the same suspension and springing. This type of axle is used chiefly for heavy vehicles, and more particularly for the rear axles. Such an axle often consists of a welded split sheet-steel casing. If one of the wheels encounters an irregularity on the road, this will cause the whole axle to tilt, with the result that the two springs are unequally stressed.

The wheels undergo no parallel displacement when the axle tilts, but their angle does change. A floating axle is a rigid axle provided with a transverse spring above it.

If the spring stop is located at the level of the centre of gravity of the vehicle, any tilting over of the body when the vehicle is negotiating a bend can be obviated. The floating axle therefore provides particularly good road-hugging ability during cornering (side-tilt stability).

The swing axle provides independent wheel suspension and springing, so that only the wheel that encounters a ‘bump’ alters its position. For normal rear axle drive the independent suspension is usually provided in the form of the single-pivot swing axle.

The drawback of this arrangement is that the road contact surface of the wheel undergoes a certain amount of sideways displacement when the springs are compressed, for which reason it is not used for front axles (steering axles).
If it is desired to use the swing axle principle for the steering axle, the so-called wishbone suspension can be applied.

 This embodies a parallelogram system which ensures that the wheels are kept accurately parallel. However, here too, sideways displacement of the wheels occurs when the springs are compressed; in addition, angular displacement occurs when cornering. Another form of construction for steering axles is based on the use of double transverse springs (two leaf springs mounted one above the other).

The greater the deflections of these springs are, the larger will be the amounts of sideways and angular displacement of the wheels. The wheels may be mounted on arms, which are connected at one end to the frame and at the other end to the wheel axle. These arms may be single or double. In this case, the springing takes the form of a torsion bar. With suspension systems of this kind there is no sideways nor angular displacement when the springs deflect.

The same is true when the coil spring is enclosed in a tubular casing - except when the vehicle is cornering.