Zero-offset steering

Zero-offset steering describes a front-end geometry in which the steering axis, when extended downwards, intersects the road surface exactly at the centre of the tyre's contact patch. The horizontal distance between that intersection point and the centre of the contact patch is known as the scrub radius, and in a zero-offset arrangement that distance is nil. The configuration exists to keep the forces generated at the tyre from feeding back into the steering as disturbing moments, which is particularly valuable on front-wheel-drive cars where the same wheels both drive and steer.

The geometry is set by the inclination and position of the steering axis relative to the wheel. On a MacPherson strut the axis runs from the upper strut mounting down to the lower ball joint, and on a double-wishbone layout it runs through the upper and lower ball joints. By combining a suitable kingpin inclination (the inward lean of the axis) with the wheel offset and rim width, designers arrange for the axis to pierce the ground at the contact-patch centre. Because longitudinal forces from braking or traction act through that very point, they produce no turning moment about the steering axis, so they cannot tug at the wheel.

For the driver, the practical reward is calm, predictable steering. In a powerful front-wheel-drive car, unequal traction at the two front wheels would otherwise create torque steer, where the wheel pulls under hard acceleration; with a near-zero scrub radius that pull is largely cancelled. The same applies under braking: if one front wheel grips more than the other, perhaps because one side is on ice or has a soft tyre, the imbalance does not wrench the car off line, which materially improves straight-line stability in an emergency stop.

Designers can choose positive, negative or zero scrub radius, and each has its uses. Positive scrub radius gives more feedback and self-centring but transmits more disturbance; negative scrub radius is sometimes engineered deliberately so that a braking imbalance actually steers the car back towards straight ahead, a useful safety property exploited on split-circuit braking systems. True zero offset sits between these as a neutral compromise that minimises feedback without actively counter-steering.

In practice the exact figure is a compromise, and a genuinely perfect zero is rarely held across the whole range of suspension travel and steering lock, since the geometry changes as the wheel rises, falls and turns. Fitting wheels with the wrong offset, wider tyres or aftermarket spacers will move the contact patch outboard and reintroduce scrub radius, which is why such modifications can bring back torque steer and braking pull that the manufacturer had designed out.

The concept underpins the behaviour of modern rack-and-pinion steering and is closely tied to how electric power steering is tuned, since assistance software must account for the small residual moments the geometry leaves behind. It is a foundational part of suspension geometry on strut-based front ends and works alongside castor and camber to determine how a car tracks, centres and responds at the wheel.