Rack and pinion is by far the most common type of steering system. Picture a flat rack with teeth cut in, connected to the road wheels. This is moved left or right by a pinion gear to which the steering shaft is connected. This turns the rotation of the steering wheel into linear road wheel input.
Recirculating ball car steering is less common nowadays but is still seen on heavier vehicles such as buses. It is a block with a threaded hole within, and gear teeth cut into the outside. The steering rod connects to the block – turning the steering wheel turns the entire ‘block’, moving the gear and turning the road wheels.
Power steering lightens the load on the steering wheel by adding an extra set of muscles. Traditionally, a hydraulic pump was used, driven by the engine via a belt. A piston is connected to the rack, with pressurised fluid either side. The pump supplies higher-pressure fluid to one side of the piston, which forces it to the other side.
When turning a corner, the ‘inside’ wheel to the curve has to turn a smaller radius than the outside. If designers didn’t allow for this, one of the wheels would ‘scrub’ on the road surface as the difference in distance was absorbed. Ackermann steering geometry is a way of setting up systems to avoid this.
Ackermann geometry is created by moving steering pivot points inwards, which means the front wheels turn at increasing differences of angle as the steering is rotated. It means all wheels turn about the same ‘turning circle’ if lines were drawn from their centres. If there were no difference in front wheel angles, there would be different turning circles for each wheel.