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Rack and pinion steering is exactly what it sounds like — a rack and a pinion. The rack is a flat, grooved bar that extends horizontally across the bottom of the engine compartment, connecting the left and right front wheels to one another. The rack is connected to the wheels via inner and outer tie rod ends.
The pinion consists of a gear at the end of the steering shaft, which connects to the rack via vertical grooves, or teeth, on the rack. When the pinion rotates, it moves the rack from side to side, controlling the direction of the wheels. Essentially, when you turn the steering wheel to the left, the pinion also turns to the left, which in turn pushes the rack to the right. This pushes the back of the right tire out, and pulls the back of the left tire in, so that the wheels point to the right. The opposite takes place when you turn the steering wheel to the left.
Power rack and pinion, which is a very common type of power steering used in vehicles today, works in much the same way. Technically, it is not fully power steering, but power assisted steering, as the rack and pinion is simply aided by the power steering pump. Essentially, pressurized hydraulic fluid assists the pinion in moving the rack back and forth, making it easier on the driver to turn the wheel.
Another type of power steering is called recirculating ball steering. This is a much more complicated system with a worm gear instead of a pinion, and a pitman arm and idler arm to do the job of the rack. However, since the recirculating ball system has more parts and linkages, it also has more that can go wrong.
Rack and pinion steering is the most common steering system used in vehicles today. One of the advantages of this type of system is that it is simple, meaning there are fewer parts to fail or need repair. Another advantage is that the principles used in this type of system are easily modified for use in a power rack and pinion system.
Rack and pinion steering is possible whether or not it is power assisted, because it allows for a high steering ratio. The higher the steering ratio, the farther you have to turn the steering wheel in order to get the wheels to turn; this decreases the amount of force required to turn the wheels. As a result, by utilizing a higher steering ratio, car manufacturers can make it easy to turn the steering wheel, even without power steering.