Rack and pinion steering lines consist of a pinion gear attached to a steering shaft that presses against and moves a rack to ultimately make steering possible. The rack connects to the steering arm on the spindle of the tire, so turning the steering wheel begins a line of communication that travels from the wheel to the steering shaft, through the pinion gear to the rack, then to the tie rod, and finally to the steering arm on the spindle.
Rack and pinion gearsets accomplish two primary functions: they convert the steering wheel's rotational motion into the linear motion that turns the wheels, and they make it easier to turn the wheels by requiring one gear fewer than some other steering methods. Many cars require between three to four full revolutions of the steering wheel to turn the wheels from the far right position to the far left; some vehicles use rack and pinion gearsets that alter the number of teeth per inch on the rack in order to reduce the necessary revolutions and improve a car's responsiveness when beginning a turn. This function, along with the benefits provided by using rack and pinion gears in power-steering systems, is responsible for making the rack and pinion steering type more and more common on cars, small trucks and SUVs.