A good turning radius may be important to you when considering a new car or truck. Generally speaking, it refers to the tightest turn a vehicle can make and is dependent on several factors. One obvious one is the width of the wheel base — that is, how far apart the front tires are. All other things being equal, a smaller wheel base can offer a tighter radius, while a large wheel base, such as those on large trucks, will have a larger radius, regardless of other factors.
Another factor in a car’s turning radius is the steering gear box, which translates the turning of the steering wheel into the pivoting of the front wheels of the car. Not all gear boxes are created equal; moving the steering wheel clockwise a quarter turn will have a different result in a big pickup truck than it does in a tiny sports car. Typically, the steering gear box is designed in accordance to the purpose of the vehicle: that big truck probably won’t ever need to turn on a dime, whereas the driver of a sports car will want solid cornering abilities.
There are several different types of steering setups, each with different advantages. The oldest and most basic type of steering is called rack-and-pinion. The rack spans the width of the car between the front wheels; the pinion gear, which is on the end of the steering wheel shaft, attaches to the top of the rack. As the steering wheel is turned, the spinning motion of the pinion gear moves the rack from side to side via the gear teeth on the top of the rack. In the days before power steering, the driver moved this rack from side to side with only the force of the steering wheel, which explains why cars without power steering can be hard to steer, especially at slow speeds when the road offers more resistance.
To relieve the driver of some of the work of turning the wheels, power steering was invented. This type of system has become quite popular in modern times. One type is called recirculating-ball, which is a complex system that basically takes all of the work out of turning the steering wheel.
The gearbox is attached to the end of the steering wheel shaft. As the wheel is turned, the ball bearings in the gearbox circulate around the shaft in spiraled grooves, in a manner rather like the direction a stripe takes around a candy cane. This moves the gears, which in turn pivot an arm called the pitman arm. The pitman arm is attached to the gearbox on one end and the “track rod” on the other, so that when the pitman arm pivots, it moves the arms that control the direction of the wheels.
Another type of power steering is power-assisted rack-and-pinion. This means that the steering pump uses pressurized fluid to help the driver move the pinion with the steering wheel. Although this is a type of hybrid system, somewhere between manual and power steering, it is preferred by some drivers, as it is a more responsive type of steering.
The type of steering system a car has might have an effect on the vehicle’s turning radius. In a classic rack-and-pinion car, it is harder to turn the steering wheel far enough to achieve a tight turn, particularly because the car has to be moving slowly — or not at all — to make a U-turn. In a car with power steering, it is easy to turn the wheel all the way in one direction or the other, but the size of the gears used in the steering system will determine how tight the turning radius is.