The scrub radius is the distance in front view between the king pin axis and the center of the contact patch of the wheel, where both would theoretically touch the road. It can be positive, negative or zero.

The kingpin axis also known as steering inclination is the line between the upper and lower pivot points of the steering knuckle.

If the kingpin axis intersection point is outboard of the center of the contact patch, it is negative; if inside the contact patch, it is positive. The term scrub radius derives from the fact that either in the positive or negative mode, the tire does not turn on its centerline (it scrubs the road in a turn) and due to the increased friction, more effort is needed to turn the wheel.

Large positive values of scrub radius, 4 inches/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, provided you're not on the brake.

The advantage of a small scrub radius is that the steering becomes less sensitive to braking inputs. More scrub radius adds to road feel by pushing the inside wheel into the ground.

An advantage of a negative scrub radius is that the geometry naturally compensates for split μ (mu) braking, or failure in one of the brake circuits. It also provides center point steering in the event of a tire deflation, which provides greater stability and steering control in this emergency.

Steering axis inclination

The steering axis inclination (SAI) is the angle between the centerline of the steering axis and vertical line from center contact area of the tire (as viewed from the front).

Effects of SAI

SAI urges the wheels to a straight ahead position after a turn. By inclining the steering axis inward (away from the wheel), it causes the spindle to rise and fall as the wheels are turned in one direction or the other. Because the tire cannot be forced into the ground as the spindle travels in an arc, the tire/wheel assembly raises the suspension and thus causes the tire/wheel assembly to seek the low (center) return point when it is allowed to return. Thus, since it has a tendency to maintain or seek a straight ahead position, less positive caster is needed to maintain directional stability. SAI adds positive camber while turning for both steering tires. A vehicle provides stable handling without any of the drawbacks of high positive caster because of SAI.

The scrub radius is the distance at the road surface between the tire center line and the SAI line extended downward through the steering axis.

Squirm

Squirm occurs when the scrub radius is at zero. When the pivot point is in the exact center of the tire footprint, this causes scrubbing action in opposite directions when the wheels are steered. Tire wear and some instability in corners is the result.

Applications in suspension

MacPherson strut equipped vehicles usually have a negative scrub radius. Even though scrub radius in itself is not directly adjustable, it will be changed if the upper steering axis point or spindle angle is changed when adjusting camber. This is the case on a MacPherson strut which has the camber adjustment at the steering knuckle. Because camber is usually kept within 1/4° side to side, the resulting scrub radius difference is negligible.

Negative scrub radius decreases torque steer and improves stability in the event of brake failure. SLA suspensions usually have a positive scrub radius. With this suspension, the scrub radius is not adjustable. The greater the scrub radius (positive or negative), the greater the steering effort is. When the vehicle has been modified with offset wheels, larger tires, height adjustments and side to side camber differences, the scrub radius will be changed and the handling and stability of the vehicle will be affected.

References

1. ^ Chassis handbook : fundamentals, driving dynamics, components, mechatronics, perspectives / Bernd HeiÇing, Metin Ersoy (ed.). page 25
• Reimpell, Jornsen; Helmut Stoll; Jurgen W. Betzler (2001). The Automotive Chassis Engineering Principles. SAE International. ISBN 978-0-7680-0657-5.