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Suspension, 1

23rd April 1983, Page 60
23rd April 1983
Page 60
Page 60, 23rd April 1983 — Suspension, 1
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Which of the following most accurately describes the problem?

WHEN A WHEEL on a motor vehicle meets a sudden bump in the road the wheel has to follow the contour of the bump and must rise rapidly. It is true that small irregularities are smoothed out by the tyre but, nevertheless, the wheel will move upwards, with high acceleration, when meeting humps of a noticeable size. When the wheel drops into a potholes similar shock will be experienced.

If springs were not fitted between the road wheel and the frame of the vehicle the passengers and the load would be subject to all the shocks suffered by the wheel. The suspension system, consisting of springs of some sort or another, allows the wheel to move up and down without the frame of the vehicle moving a similar amount, giving a smooth and comfortable ride to passengers and load alike.

Various forms of springs are used, as detailed below, and in this and subsequent articles each type and their application will be reviewed. The springs in general use are: (1) Steel springs, which are of the leaf, coil or torsion bar type.

(2) Rubber springs.

(3) Pneumatic springs. On heavy commercial vehicles it is the leaf spring (see Figure 1) which is most commonly employed. This type is built up by several layers of leaves, the top leaf, termed the main leaf, is rolled at each end to form an eye (see Figure 2). The other leaves, fitted below the main leaf, gradually reduce in length.

A centre bolt passes through all the leaves at the centre of the spring. The head of this bolt is cylindrical in shape and serves to locate the spring at the axle. . The spring is fastened to the axle by "U" bolts or plates and ordinary bolts.

Clips are located at intervals along the spring to stop the leaves from splaying out. A more important function of these clips, however, is to support the main leaf when the spring is on the rebound. When the road wheel drops into a pot hole or when the chassis goes on rising after the vehicle has gone over a bump the main leaf, without these clips, would be supporting the whole weight of the axle and road wheel placing a very heavy load on the main leaf at the "U" bolt ends.

On heavy vehicles, the eyes at the end of the main leaf are fitted with phosphor bronze bushes and pins, known as shackle pins. The latter are fitted with grease nipples so that they may be lubricated. Rubber bushes are used for the shackle pins on ligher vehicles eliminating the need for lubrication thus cutting down on service time. Swinging shackles are fitted at the rear en of the springs to allow the sprin to lengthen as it straightens out under load, On some heavy vehicles, the rear end of the spring is not fitted with a swinging shackle but to a slipper seat. The end of the spring is left flat and the flat end bears on a flat slipper bracket bolted or rivetted to the chassis frame, as shown in This design can only be used at the rear end of the spring as the front end has to take the braking and driving forces.

It will be remembered from physics lessons at school that: "To every action, load or force there is an equal and opposite reaction," so when a vehicle is being driven, the axle casing wi try and turn in the opposite direction to the road wheels.

In the majority of commercial vehicles this torque reaction is resisted by the springs; the spring will deflect as shown in Figure 3 (b) when the vehicle is accelerating and as in Figure 3(1 when the brakes are applied.

The front end of the rear springs, therefore, must be connected directly to the chassi frame and the slipper type end cannot be used. Incidentally, thi front end of the spring also pushes the vehicle when it is travelling forward, pulls it back when it is reversing and holds ii back when braking.

Certain designs incorporate a special rod to deal with these forces, leaving the spring just tc cater for irregularities in the road.

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