Scrub radius

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The scrub radius of a vehicle suspension is the distance between the steer axis of the suspension where it strikes the ground, and the centre of the contact patch, in front view.

By definition it is negative if the steer axis intersection point is outboard of the centre of the contact patch.

Large positive values of scrub radius, 4 inches or 100 mm or so, were used in cars for many years. The advantage of this is that the tire rolls as the wheel is steered, which reduces the effort when parking.

If the scrub radius is small then the contact patch is spun in place when parking, which takes a lot more effort.

The advantage of a small scrub radius is that the steering becomes less sensitive to braking inputs, in particular.

The advantage of a negative scrub radius is that the geometry naturally compensates for split mu braking, or failure in one of the brake circuits.

[edit] References

• Reimpell, Jornsen; Helmut Stoll, Jurgen W. Betzler. The Automotive Chassis Engineering Principles. SAE International. ISBN 978-0-7680-0657-5.