The rack and pinion steering system in cars converts the rotational motion of a steering wheel into linear motion that turns the wheels through the use of gears. Pinion refers to the circular gear turned by the steering wheel, while rack refers to the straight bar on which the pinion acts. The gears interlock along toothed edges. As the pinion turns, it rolls along the rack, causing the rack, and the tires, to shift either to the left or right.
The rack connects to the wheels through tie rods connected to the wheel's spindle. The pinion connects to the steering wheel via a steering rod that includes a universal joint to maintain an ideal steering wheel angle for the driver.
The rack and pinion connection takes place inside a metal tube, which in power steering systems fills with power steering fluid. Systems with power steering include a pressurized tube that includes a piston, valves and fluid lines that connect to the fluid pump, and other systems using the fluid. When supplied to either side of the piston, the high-pressure fluid assists the movement of the rack, thus requiring less effort by the driver to turn the wheels.
Some vehicles utilize a variable-ratio rack and pinion system with variable tooth spacing in the rack to provide greater steering accuracy when the pinion is near the center of the rack.