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The term turning radius is a technical term that has become popular automotive jargon. In the jargon sense, it is commonly used to mean the full diameter of the smallest circle, but in technical usage the turning radius is still used to denote the radius. The less ambiguous term turning circle avoids the mistaken jargon use of the word radius. As an example, Motor Trend refers to a curb-to-curb turning circle of a 2008 Cadillac CTS as 35.5 feet (10.82 m), but the terminology is not yet settled. AutoChannel.com refers to the turning radius of the same car as 35.5 feet (10.82 m). It is often used as a generalized term rather than a numerical figure. For example, a vehicle with a very small turning circle may be described as having a "tight turning radius".
Curb to curb
Two different measurements can be quoted for a vehicle. A curb or curb-to-curb turning circle will show the straight-line distance from one side of the circle to the other, through the center. The name "curb-to-curb" indicates that a street would have to be this wide before this car can make a U-turn and not hit a street curb with a wheel. If you took the street curb and built it higher, as high as the car, and tried to make a U-turn in the street, parts of the car (bumper) would hit the wall. The name wall or wall-to-wall turning circle denotes how far apart the two walls would have to be to allow a U-turn without scraping the walls. One can find these two ways of measuring the turning circle used in auto specifications, for example, a van might be listed as having a turning circle (in meters) of 12.1(C)/12.4(W).
A notable exception in this description is of vehicles that are capable of spinning around their central axis, such as certain lawnmowers and wheelchairs as they do not follow a circular path as they turn. In this case the vehicle is referred to as a "zero turning radius" vehicle.
Some camera dollies used in the film industry have a "round" mode which allows them to spin around their z axis by allowing synchronized inverse rotation of their front and rear wheel sets, effectively giving them "zero" turning radius.