Turning Circle

The turning circle expresses how tightly a vehicle can be manoeuvred, defined as the diameter of the smallest circle the car can describe when the steering is held at full lock. It is the single most useful figure for judging how a car will cope with multi-storey car parks, narrow country lanes, three-point turns and reversing into tight bays. A smaller number means a more wieldy vehicle, and figures typically range from around 10 metres for a city car to 12 metres or more for a large saloon or SUV.

Two measurements are commonly quoted. The kerb-to-kerb turning circle reflects the path swept by the outer front tyre and is the smaller figure, while the wall-to-wall measurement adds the overhang of the front bodywork beyond the wheels and is therefore larger. The distinction matters in practice, because it is the front corner of the bumper, not the tyre, that strikes an obstacle when squeezing through a gap.

Geometrically, the turning circle is governed chiefly by the wheelbase and by the maximum steering angle the front wheels can achieve. A long wheelbase forces the rear wheels to track a wide arc, enlarging the circle, which is why limousines and long-wheelbase estates feel cumbersome. A greater steering lock shrinks the circle, but the available angle is limited by how far the wheels can turn before fouling the suspension, the wheel arch or the driveshaft joints, a constraint that is especially acute on front-wheel-drive cars where the driveshafts impose a practical ceiling.

Engineers employ several strategies to improve the figure. Careful packaging of the driveline, slimmer tyres and revised arch clearances all help. On larger and more expensive vehicles, rear-wheel steering offers a dramatic solution: at low speeds the rear wheels steer in the opposite direction to the fronts, effectively shortening the working wheelbase and allowing a long luxury car to turn as tightly as a model two sizes smaller.

It is worth noting that the turning circle describes a steady-state manoeuvre at crawling pace and tells you nothing about high-speed agility, which depends on track width, suspension and weight distribution. A car with a small turning circle is not necessarily nimble through a series of bends, and vice versa.

The figure is closely related to the wheelbase, which is its dominant determinant, and to four-wheel steering, the technology most often deployed to overcome the manoeuvrability penalty that comes with a large, long-wheelbase body.