How it Works
A rack-and-pinion gearset is comprised of two interlocking, notched gears: a circular pinion and a straight rack. In a rack-and-pinion steering system, the rack-and-pinion gearset is set inside a metal tube, from which each end of the rack protrudes.
The pinion is attached to the vehicle's steering shaft. A tie rod attached to each end of the rack connects the rack to the steering arm on each wheel.
When the steering wheel is turned, the pinion spins, moving the rack to the right or to the left, depending on which way you turn the wheel.
For the majority of cars, three or four revolutions of the steering wheel are required to make the car’s wheels turn from the far left to the far right.
 Steering Ratio
The steering ratio is the ratio of the turn of the steering wheel (in degrees) and the turn of the wheel (in degrees). If a 360-degree revolution of a steering wheel makes the car's wheels turn 20 degrees, the steering ratio would be 360 divided by 20, or 18:1. The lower the steering ratio, the faster the wheels change direction in response to a turn of the steering wheel.
 Power Steering
Power steering involves a slightly different rack-and-pinion configuration. In this case, part of the rack includes a cylinder with a piston in the middle, which is connected to the rack. A fluid line on either side of the piston supplies high-pressure fluid to the piston and forces it to move. The movement of the piston moves the rack, which provides the power assist element.