Rack-and-pinion steering translates the rotational motion of the steering wheel into linear motion by using a round pinion gear at the end of the steering column and a toothed rack that connects to the tie rods to move the wheels to the left or right. A metal case encloses the rack-and-pinion mechanism.
The pinion is much smaller than the steering wheel, providing gear reduction to give the driver a mechanical advantage as he steers the vehicle. As the pinion turns, its teeth grasp the teeth on the rack and cause it to move. It generally takes three to four complete turns of the steering wheel to turn the wheels from the maximum right turn to the maximum left one.
The steering ratio determines the amount of force required to turn the wheel. Decreasing the ratio decreases the mechanical advantage but makes the vehicle more responsive to turns of the wheel. Manufacturers often use lower ratios on smaller cars that require less power to steer. However, on larger vehicles, they increase the ratio to make it easier for a driver to turn the wheel, although he must turn the steering wheel farther to make the same turn. Some vehicles use a variable-ratio system. The teeth on the rack are closer toward the center, giving a greater response when starting to turn, but farther apart toward the edges, increasing the driver's ability to turn the wheel as he reaches the edges of the turn.