What is the turning circle of a car wheel?
The turning circle (also called turning radius or turning diameter) is the smallest circle a car can describe while its front wheels are turned to their maximum angle. In practical terms, it measures how easily a vehicle can maneuver in tight spaces and is commonly listed by manufacturers as a radius or diameter.
Definition and measurement
When a driver steers the front wheels to maximum lock, the vehicle traces a circular path. The turning circle is the size of that circle. Depending on the convention, it may be described as the radius (distance from the vehicle's center to the circle) or as the diameter (twice that radius). In many manufacturer specs, turning circle is reported as the turning diameter—the distance across the circle traced by the outer front tire.
Notes on measurement standards
Different regions and testers use slightly different reference points. Some specs measure from the vehicle’s center line to the turning center, others use the outer wheel path across the front. Always check the standard used, especially when comparing cars for parking or city driving.
Key factors that influence turning circle
Several vehicle characteristics and conditions determine the turning circle. The following list highlights the main influences.
- Wheelbase (distance between front and rear axles)
- Track width (distance between left and right wheels)
- Steering geometry, including how the front wheels axis align to achieve Ackermann steering
- Maximum steering lock angle of the front wheels
- Tire size and suspension geometry
- Vehicle load, passenger weight, and cargo distribution
- Road surface, incline, and tire inflation
In short, longer wheelbase and wider track generally increase the turning circle, while higher steering lock angle can decrease it. Real-world values also shift with load and road conditions.
Measuring and interpreting turning circle in practice
Automakers and regulators use standard methods to report turning circle. The most common approaches are:
- Kerb-to-kerb method: measuring the diameter of the circle described by the outer front wheel as it reaches full lock on a flat surface.
- Center-based radius: measuring the distance from the vehicle’s centerline (often the midpoint of the front axle or the vehicle's geometric center) to the turning center.
- Reference to turning diameter: many manufacturers publish the turning circle as a diameter rather than a radius.
Official figures depend on the testing protocol and the exact reference points used, so it’s wise to read the test standards or ask the dealer for the method used when comparing cars.
Calculating turning radius (basic approach)
A simple first-order approximation links the turning radius to the vehicle’s wheelbase and steering angle. If the front wheels are at the maximum lock angle δ and L is the wheelbase, then R ≈ L / tan(δ). For more accuracy, especially with Ackermann geometry, the inner and outer wheel angles differ and the turning center radius R satisfies tan δ_in = L/(R − t/2) and tan δ_out = L/(R + t/2), where t is track width. In real-world terms, manufacturers often publish the turning circle as a radius or diameter under full-lock steering.
Example: a compact car with a wheelbase of about 2.7 meters and an average full-lock angle around 30 degrees yields an approximate turning radius of about 4.7 meters, implying a turning diameter near 9.4 meters. Actual measured values can differ by several percent depending on load and tire size.
Summary
In essence, the turning circle is a practical gauge of how maneuverable a car is in tight spaces. It depends on the vehicle’s geometry, steering design, and load, and is typically published as either a turning radius or a turning diameter. For urban driving and parking, a smaller turning circle is advantageous, while highway stability and design constraints can influence the final figures.
