Learner Zone
Motor Vehicle HubSteering
The steering system converts the rotation of the steering wheel into a swivelling movement of the road wheels. The steering wheel rim turns a long way to move the road wheels a short way.
This system allows a driver to use only light forces to steer a heavy car. The rim of a 15 inch diameter steering wheel moving four turns from full left lock to full right lock travels nearly 5 metres, while the edge of a road wheel moves a distance of only 12 inches.
The steering effort falls 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 steering angle. They also ensure that when cornering, the inner front wheel is more sharply angled. The joints must be adjusted very precisely, and even a little looseness in them makes the steering dangerously inaccurate.
There are two steering systems in common use; the rack and pinion and the steering box.
Watch this short video to introduce you to steering systems.
rack and pinion
The typical rack-and-pinion steering diagram above shows how the rack acts directly on the road wheel steering arms.
At the base of the steering column there is a small pinion inside a housing. Its teeth mesh with a straight row of teeth on 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. A universal joint in the steering column allows it to connect with the rack without angling the steering wheel sideways.
This system is simple with few moving parts, so its action is precise.
steering box
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.
The nut system has hardened balls running inside the thread between the worm and the nut. As the nut moves, the balls roll out into a tube that takes them back to the start. This is a recirculating-ball system. The worm moves a drop arm linked by a track rod to a steering arm that moves the nearest front wheel. A central track rod reaches to the other side of the car, where it is linked to the other front wheel by another track rod and steering arm. A pivoted idler arm holds the far end of the central track rod level.
The steering-box system has many moving parts, so is less precise than the rack system.
power-assisted
On a heavy car, either the steering is heavy or it is low geared, i.e. the steering wheel requires many turns from lock to lock. Heavy gearing can be troublesome when parking in confined spaces.
Power-assisted steering overcomes this. The engine drives a pump that supplies oil under high pressure to the rack or the steering box. Valves in the steering rack or box open whenever the driver turns the wheel, allowing oil into the cylinder. The oil works a piston that helps to push the steering in the appropriate direction. As soon as the driver stops turning the wheel, the valve shuts and the pushing action of the piston stops. The power only assists the steering – the steering wheel is still linked to the road wheels in the usual way.