Steering

The function of the steering system is to enable the vehicle to change its direction of travel and negotiate corners. The main requirement applicable to a good steering system is to ensure geometrically precise rolling of the wheels (without slip) when travelling in bends.

In the case of motor cycles and three-wheelers with single-wheel steering this condition is automatically satisfied, because with such vehicles the centre of the circular motion is always located at the intersection of the extensions of the steered and the non-steered axle.

The so-called bogie or fifth-wheel steering system is geometrically satisfactory for four-wheeled vehicles. In this system the whole steering control shaft forms a rigid system which is rotated about its centre; it is usually employed on horse-drawn vehicles and trailers. The drawback of this steering system is that the stability of the vehicle decreases as the front axle and wheels swing farther round, and a further disadvantage is that the swing-round of the wheels takes up a good deal of space under the frame.

These draw-backs do not occur in the king-pin steering system, since each steered wheel has its own pivot point; on the other hand, it only approximately satisfies the main condition as to slip-free geometrically accurate rolling of all the wheels on the road surface. The extension of the rear axle coincides at one point with the extensions of the two front stub axles only if the wheel on the inside of the curve has a larger angle of lock than the wheel on the outside of the curve, because only then will all the wheels be rolling on concentric circles around the centre of the circular motion. The track arms can, however, be installed, not parallel to the longitudinal axis of the vehicle, but at an angle to it (so that the front axle, track arms and track rod form a trapezium instead of a parallelogram). Then all the wheels will run approximately at right angles to the lines of connection to the common centre of the curved path.

In the king-pin steering system each wheel has its own pivot point, namely, the king pin, to which is attached the stub axle on which the wheel rotates. Each stub axle is connected to a short lever called the track arm.

The two track arms are interconnected by the track rod. Thanks to this arrangement, the force applied by the steering control arm need act only at one king pin; the movements of the other king pin will be automatically controlled by the trapezium linkage system. When the driver turns the steering wheel, the steering gear transmits the rotation of the wheel to the drop arm and drag link which in turn actuates the above-mentioned steering control arm.

The latter then rotates the stub axle about the king pin.

There are various forms of steering gear construction: In the worm-and-sector system the bottom part of the steering column is provided with a worm which engages with a toothed worm-gear sector. In the case of worm-and-nut steering a nut moves along the worm when the latter rotates. When the steering column is rotated, the worm-gear sector or the nut moves up or down and transmits its movement to the drop arm.

A variant of the worm-and-nut system is the cam steering system. In this arrangement a tapered projection (follower) engages with the worm-shaped cam and moves the drop arm.