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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".
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. For example, a vehicle with a very small turning circle may be described as having a "tight turning radius" (aka. being more difficult 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. 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). However, another page refers to the turning radius of the same vehicle as also being 11.6 m, 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, 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, and the term turning diameter will therefore be more correct in most cases.
The term turning circle is another term also sometimes used synonymously for the turning diameter. Some argue that the term turning circle is less ambiguous than the term turning radius, but "turning circle" may introduce its own ambiguities since a "circular measurement" mathematically can refer to several measurements, like for example the radius (r), diameter (2r, twice as big) or circumference (2πr, about 6.28 times as big). As an example, Motor Trend refers to a "curb-to-curb turning circle" of a 2008 Cadillac CTS as 10.82 metres (35.5 ft), but the terminology is not yet settled. AutoChannel.com refers to the "turning radius" of the same car as 10.82 metres (35.5 ft).
Different measurement methods
There are two methods for measuring the vehicle turning diameter which will give slightly different results. These two methods are called wall-to-wall and curb-to-curb (US spelling), or alternatively kerb-to-kerb (UK spelling).
The kerb to kerb turning circle is smaller than the turning circle as it refers to only a partial circle (~180°) with the vehicle alongside one kerb to start with. To perform a U turn in a forward direction only, the centre of the turn is not coincident with the centre of the road - thus a complete circle would not be possible (without driving onto the pavement to complete the manoeuvre). It also does not take into account that part of the vehicle that overhangs the wheels where as 'turning circle' does.
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 to the terminology used in this article in 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 left and right wheel sets, effectively giving them "zero" turning radius.
- "turning, n." OED Online, Oxford University Press, December 2020, www.oed.com/view/Entry/207704. Accessed 15 February 2021.
- 2017 Audi A4 - Audi Canada - Product Information Book
- "Used 2017 Audi A4 Specs & Features". Edmunds.com. Retrieved 2017-08-18.