Scrub radius
Encyclopedia
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.
The kingpin axis is the line between the upper and lower ball joint
s of the hub. On a MacPherson strut
, the top pivot point is the strut bearing, and the bottom point is the lower ball joint. The inclination of the steering axis is measured as the angle between the steering axis and the centerline of the wheel. This means that if the camber angle
is adjustable within the pivot points the scrub radius can be changed, this alters the width and offset of the tires on a vehicle.
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 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. This also allows greater width in the engine bay, which is very important in some compact sports cars.
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.
An advantage of a negative scrub radius is that the geometry naturally compensates for split 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 situation.
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. A vehicle provides stable handling without any of the drawbacks of high positive caster because of SAI.
The line through the steering axis creates a pivot point around which the tire turns. If these lines intersect at the road surface, a zero scrub radius would be present. When the intersection is below the surface of the road, this is positive scrub radius. Conversely, when the lines intersect above the road, negative scrub radius is present. The point where the steering axis line contacts the road is the fulcrum pivot point on which the tire is turned. Scrub radius is changed whenever there is a change in wheel o/s as on the t/l where the wheels are pushed outboard causing scrub radius to become more (+). Older cars tended to have very close to zero scrub radius but often on the (+) side, newer cars with ABS all have negative scrub radius(that's why all the newer cars have the wheels o/s more inboard)
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 and the more road shock and pivot binding that takes place. 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.
The kingpin axis is the line between the upper and lower ball joint
Ball joint
In an automobile, ball joints are spherical bearings that connect the control arms to the steering knuckles.More specifically, a ball joint is a steel bearing stud and socket enclosed in a steel casing. The bearing stud is tapered and threaded. It fits into a tapered hole in the steering knuckle. A...
s of the hub. On a MacPherson strut
MacPherson strut
The MacPherson strut is a type of car suspension system which uses the axis of a telescopic damper as the upper steering pivot. It is widely used in modern vehicles and named after Earle S. MacPherson, who developed the design.-History:...
, the top pivot point is the strut bearing, and the bottom point is the lower ball joint. The inclination of the steering axis is measured as the angle between the steering axis and the centerline of the wheel. This means that if the camber angle
Camber angle
thumb|100px|From the front of the car, a right wheel with a negative camber angleCamber angle is the angle made by the wheels of a vehicle; specifically, it is the angle between the vertical axis of the wheels used for steering and the vertical axis of the vehicle when viewed from the front or...
is adjustable within the pivot points the scrub radius can be changed, this alters the width and offset of the tires on a vehicle.
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 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. This also allows greater width in the engine bay, which is very important in some compact sports cars.
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.
An advantage of a negative scrub radius is that the geometry naturally compensates for split mu braking, or failure in one of the brake
Brake
A brake is a mechanical device which inhibits motion. Its opposite component is a clutch. The rest of this article is dedicated to various types of vehicular brakes....
circuits. It also provides center point steering in the event of a tire deflation, which provides greater stability and steering control in this emergency situation.
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, 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 suspensionSuspension (vehicle)
Suspension is the term given to the system of springs, shock absorbers and linkages that connects a vehicle to its wheels. Suspension systems serve a dual purpose — contributing to the car's roadholding/handling and braking for good active safety and driving pleasure, and keeping vehicle occupants...
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. A vehicle provides stable handling without any of the drawbacks of high positive caster because of SAI.
Scrub radius
The scrub radius is the distance at the road surface between the tire line and the SAI line extended downward through the steering axis.The line through the steering axis creates a pivot point around which the tire turns. If these lines intersect at the road surface, a zero scrub radius would be present. When the intersection is below the surface of the road, this is positive scrub radius. Conversely, when the lines intersect above the road, negative scrub radius is present. The point where the steering axis line contacts the road is the fulcrum pivot point on which the tire is turned. Scrub radius is changed whenever there is a change in wheel o/s as on the t/l where the wheels are pushed outboard causing scrub radius to become more (+). Older cars tended to have very close to zero scrub radius but often on the (+) side, newer cars with ABS all have negative scrub radius(that's why all the newer cars have the wheels o/s more inboard)
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 turned. 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 and the more road shock and pivot binding that takes place. 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.