With rack and pinion steering, the rotation of the pinion causes linear motion of the rack, which turns the vehicle's wheels left or right. Rack and pinion systems are a common component in railways.
A rack and pinion steering system consists of a pinion (a circular gear) with a rack (a linear gear). The system works by converting a revolving motion into linear motion. Most cars, small trucks, and SUVs come equipped with a rack and pinion system, rather than the recirculating ball steering found in larger trucks, larger SUVs, and other heavy-duty vehicles.
Rack and Pinion
With rack and pinion steering, the rotation of the pinion causes linear motion of the rack, which turns the vehicle's wheels left or right. Rack and pinion systems are a common component in railways. In between train rails are racks that interact with pinions attached to locomotives and train cars to assist trains with moving up steep inclines.
While a rack and pinion system might seem complicated, according to Advance Autoparts, it is simply a gear attached to a toothed bar. The bar attaches to a set of tie rods. A generating rack is a rack outline used in the design of a generating tool, such as a hob or a gear shaper cutter, to indicate the details and dimensions of the teeth. Simple linear actuators often consist of some combination of rack and pinion. The shaft rotation of the pinion is powered by hand or by a motor to create linear movement.
While the rack and pinion steering system has been used by U.S. automotive manufacturers for less than 50 years, the concept is nearly a century old in other countries. Hemmings Motor News reports that in the 1930s, BMW produced the first rack and pinion gearbox. The first American automotive manufacturer to use rack and pinion steering in production was Ford, which used it for the 1974 Mustang II and the 1974 Pinto. While AMC adopted the system soon after for the 1975 Pacer, GM and Chrysler would not manufacture cars with rack and pinion steering until the 1980s.
Although it took some time for U.S. manufacturers to start producing rack and pinion steering systems, they soon realized what European and Asian automotive companies had known for decades. Rack and pinion steering is a more straightforward design compared to the recirculating ball steering system that came before it. That more straightforward design makes rack and pinion steering systems more cost-effective to build.
Hemmings also notes that the rack and pinion steering system weighs less than a recirculating ball gearbox, which helps improve gas mileage. Rack and pinion systems are lighter because they don't require the idler arms, Pitman arms, center links, and tie rod sleeves found in conventional steering systems. The size and weight of a rack and pinion system make it a better fit for front-wheel-drive applications because manufacturers can install it right next to the transverse drivetrain. It is easier for manufacturers to tailor rack and pinion gearboxes to fit specific wheelbases and handling packages.
Rack and Pinion: Applications
While most consumers are familiar with rack and pinion systems for steering cars and small trucks, rack and pinion combinations have several other applications. Not only are rack and pinion systems used to help trains climb steep gradients, but they also provide better brake control, especially in snowy and icy conditions. The Stairlift.com states that rack and pinion systems are standard components in most stair lifts. The rack and pinion mechanism often operates using hydraulic or electrical energy.
In the 1970s, Arthur Ernest Bishop invented the variable rack. Combined with a standard pinion, his variable rack was used to improve vehicle handling.
How Does Rack and Pinion Steering Work?
According to an article by Moog Parts, rack and pinion steering works by using a gear system to translate the steering wheel's circular motion into the linear motion needed to turn the wheels. A metal tube houses the gearset. The tube has openings on each end to allow the rack to attach to an axial rod. The pinion gear connects to the steering shaft so that the gear will spin and move the rack when the steering wheel turns. The axial rods connect to a tie rod end, which attaches to the spindle.
The rack and pinion gear set has two main functions:
- Conversion of the steering wheel's rotational motion into the linear motion needed for the vehicle's wheels to turn
- Reduction of gears, which makes it easier for the steering wheel to turn the wheels
Rack and Pinion Steering Ratio
Moog Parts defines "steering ratio" as the ratio of how far the steering wheel turns to how much the wheels turn. For example, if a 360-degree turn of the steering wheel causes a car's wheels to turn 20 degrees, then that car's steering ratio is 18:1 (360 divided by 20). A higher steering ratio requires more turns of the steering wheel to turn the wheels. A lower steering ratio is desirable because it indicates more responsive steering.
Light sports cars tend to have a lower steering ratio when compared to large cars and trucks. Thanks to power steering, all consumer vehicles have an improved steering ratio.