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The steering system converts the rotation of the steering wheel into a swivelling movement of the road wheels in such a way that the steering-wheel rim turns a long way to move the road wheels a short way.
The system allows a driver to use only light forces to steer a heavy car. The rim of a 15 in. (380 mm) diameter steering wheel moving four turns from full left lock to full right lock travels nearly 16 ft (5 m), while the edge of a road wheel moves a distance of only slightly more than 12 in. (300 mm). If the driver swivelled the road wheel directly, he or she would have to push nearly 16 times as hard.
The effort passes to the wheels through a system of pivoted joints. These are designed to allow the wheels to move up and down with the suspension without changing the angle. They also ensure that when cornering, the inner front wheel – which has to travel round a tighter curve than the outer one – becomes more sharply angled. The joints must be adjusted very precisely, and even a little looseness in them makes the steering dangerously sloppy and inaccurate.
There are two systems in common use – the rack and pinion and the steering box.
The Rack and Pinion System
At the base of the steering column there is a small pinion ( gear wheel) inside a housing. Its teeth mesh with a straight row of teeth on a rack – a long transverse bar. Turning the pinion makes the rack move from side to side. The ends of the rack are coupled to the road wheels by track rods.
This system is simple, with few moving parts to become worn or displaced, so its action is precise. A universal joint in the steering column allows it to connect with the rack without angling the steering wheel awkwardly sideways.
The Steering Box System
At the base of the steering column there is a worm gear inside a box. A worm is a threaded cylinder like a short bolt. Imagine turning a bolt which holding a nut on it; the nut would move along the bolt. In the same way, turning the worm moves anything fitted into its thread.
Depending on the design, the moving part may be a sector (like a slice of a gear wheel), a peg or a roller connected to a fork, or a large nut.