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The turning radius or turning circle of a vehicle
Vehicle
A vehicle is a device that is designed or used to transport people or cargo. Most often vehicles are manufactured, such as bicycles, cars, motorcycles, trains, ships, boats, and aircraft....

is the size of the smallest circular
Circle
A circle is a simple shape of Euclidean geometry consisting of those points in a plane that are a given distance from a given point, the centre. The distance between any of the points and the centre is called the radius....

turn (i.e. U-turn
U-turn (maneuver)
A U-turn in driving refers to performing a 180 degree rotation to reverse the direction of travel. It is called a "U-turn" because the maneuver looks like the letter U. In some areas, the maneuver is illegal, while in others it is treated as a more ordinary turn, merely extended...

) that the vehicle is capable of making. The term turning radius is actually a misnomer, since the size of a circle is actually its diameter
Diameter
In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...

In classical geometry, a radius of a circle or sphere is any line segment from its center to its perimeter. By extension, the radius of a circle or sphere is the length of any such segment, which is half the diameter. If the object does not have an obvious center, the term may refer to its...

. The less ambiguous term turning circle is preferred. As an example, Motor Trend refers to a curb-to-curb turning circle of a 2008 Cadillac CTS as 35.5 feet. By contrast, theAutoChannel.com refers to turning radius of the same car as 35.5 feet.
It is often used as a generalized
Generalization
A generalization of a concept is an extension of the concept to less-specific criteria. It is a foundational element of logic and human reasoning. Generalizations posit the existence of a domain or set of elements, as well as one or more common characteristics shared by those elements. As such, it...

term rather than a numerical figure
Number
A number is a mathematical object used to count and measure. In mathematics, the definition of number has been extended over the years to include such numbers as zero, negative numbers, rational numbers, irrational numbers, and complex numbers....

. For example, a vehicle with a very small turning radius may be described as having a "tight turning radius".

Two different measurements can be quoted for a vehicle. A curb or curb-to-curb turning circle will show the distance traveled by the wheels. The wall or wall-to-wall turning circle will include an allowance for the width of the whole car, including the overhang of the bodywork. For example, a van may have been quoted as having a turning circle (in meters) of 12.1(C)/12.4(W).

It may be easier to imagine that on a road with low curb
A curb, or kerb , is the edge where a raised pavement/sidewalk/footpath, road median, or road shoulder meets an unraised street or other roadway.-Function:...

s, you don't need to consider anything other than the tire
Tire
A tire or tyre is a ring-shaped covering that fits around a wheel rim to protect it and enable better vehicle performance by providing a flexible cushion that absorbs shock while keeping the wheel in close contact with the ground...

s (12.1 meters), that is the curb turning radius, but if you were moving the vehicle inside a building, the corners of the vehicle might hit the walls so you need to consider the wall-to-wall turning radius (12.4 metres).

A notable exception in this description is of vehicles that are capable of spinning
Rotation
A rotation is a circular movement of an object around a center of rotation. A three-dimensional object rotates always around an imaginary line called a rotation axis. If the axis is within the body, and passes through its center of mass the body is said to rotate upon itself, or spin. A rotation...

around their central axis, such as a tank
Tank
A tank is a tracked, armoured fighting vehicle designed for front-line combat which combines operational mobility, tactical offensive, and defensive capabilities...

or certain lawnmowers and wheelchair
Motorized wheelchair
A motorized wheelchair, powerchair, electric wheelchair or electric-powered wheelchair is a wheelchair that is propelled by means of an electric motor rather than manual power...

s as they do not follow a circular path as they turn. In this case the vehicle is referred to as a "zero
0 (number)
0 is both a numberand the numerical digit used to represent that number in numerals.It fulfills a central role in mathematics as the additive identity of the integers, real numbers, and many other algebraic structures. As a digit, 0 is used as a placeholder in place value systems...

turning radius" vehicle, although whether or not the turning radius is actually nonexistent is unclear.

Some camera dollies
Camera dolly
A camera dolly is a specialized piece of filmmaking and television production equipment designed to create smooth camera movements . The camera is mounted to the dolly and the camera operator and focus puller or camera assistant, usually ride on the dolly to operate the camera...

used in the film industry
Film industry
The film industry consists of the technological and commercial institutions of filmmaking: i.e. film production companies, film studios, cinematography, film production, screenwriting, pre-production, post production, film festivals, distribution; and actors, film directors and other film crew...

have a "round" mode which allows them to spin around their z axis by allowing synchronized
Synchronization
Synchronization is timekeeping which requires the coordination of events to operate a system in unison. The familiar conductor of an orchestra serves to keep the orchestra in time....

inverse rotation of their front and rear wheel sets, effectively giving them "zero" turning radius.

Turning circle is sometimes measured
Measurement
Measurement is the process or the result of determining the ratio of a physical quantity, such as a length, time, temperature etc., to a unit of measurement, such as the metre, second or degree Celsius...

as the diameter
Diameter
In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...

of the minimum turn rather than the radius.