Turning radius
The turning diameter of a vehicle is the minimum diameter (or "width") of available space required for that vehicle to make a circular turn (i.e. U-turn). The term thus refers to a theoretical minimal circle in which for example an aeroplane, a ground vehicle or a watercraft can be turned around. The terms turning radius and turning circle are sometimes used, but can have different meanings (see the section on Alternative nomenclature below). The Oxford English Dictionary describes turning circle as "the smallest circle within which a ship, motor vehicle, etc., can be turned round completely".[1]
On wheeled vehicles with the common type of front wheel steering (i.e. one, two or even four wheels at the front capable of steering), the vehicle's turning diameter is a measure of the space needed to turn the vehicle around while the steering is set to its maximum displacement from the central 'straight ahead' position - i.e. either extreme left or right. If a theoretical marker pen was placed on the point of the vehicle furthest from the center of the turn, it would draw a circle and the diameter of that circle would give the value of that vehicle's turning diameter.
Theoretically speaking, the tightest turning circle possible for a vehicle is the one where the vehicle does not move either forwards or backwards while turning and effectively simply rotates on its own axis. Taking a rectangular vehicle capable of doing this, its turning circle would in fact be equal to the diagonal length of the vehicle. As an example, some boats can be turned in this way.
Turning diameter is sometimes used in everyday speak as a generalized term rather than with numerical figures.[citation needed] For example, a vehicle with a very small turning circle may be described as having a "tight turning radius" (aka. being easier to turn around very tight corners).
Other terms are sometimes used synonymously for turning diameter includes, which can lead to confusion.
The term turning radius is sometimes used as equal and interchangeable to the turning diameter. However, strictly mathematically speaking, the turning radius (r) will always be defined as half of the turning diameter (d). The turning diameter thus will always give a higher number for a given vehicle, and the turning diameter measurement is usually preferred by automotive manufacturers.[citation needed] For example, the 2017 Audi A4 is specified by the manufacturer as having a turning diameter (curb-to-curb) of 11.6 m (38.1 ft).[2] However, another page refers to the turning radius of the same vehicle as also being 11.6 m,[3] while the correct turning radius in this example would equate to 11.6 m/2 = 5.8 m. Such mixing of terms can lead to confusion among consumers. The term turning radius has become somewhat popular automotive jargon in the mathematically erroneous sense to mean the full diameter of the smallest circle,[citation needed] but, as mentioned, in mathematically correct usage the turning radius is still used to denote the radius. In practice, the numbers for turning diameter tend to be used more,[citation needed] and the term turning diameter will therefore be more correct in most cases.
