Is negative scrub radius bad?
For high-speed stability in the presence of road disturbances, zero or slight negative scrub radius is desirable (negative is outside the wheel center plane) Braking forces introduce steer torques proportional to the scrub radius.
How does scrub radius affect handling?
Negative scrub radius decreases torque steer and improves stability in the event of brake failure. 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.
Does offset affect scrub radius?
Changing the offset of a wheel will change the scrub radius of the tire/wheel combination. SAI (Steering Axis Inclination) is not adjustable on most vehicles and is usually only affected by loose, worn or damaged suspension parts or by frame damage.
What is the function of scrub radius?
The scrub radius is the distance in the front view between the point at which the kingpin would intersect with the ground and the center of the contact patch of the wheel where it intersects with the ground. The main effect of this is to introduce a turning moment.
Is a positive scrub radius bad?
When you brake and your effort is greater on one side of the car than the other, a positive scrub radius will make the car turn to the side where effort is greater. You’ll find that a positive scrub radius is useful with certain racing setups. It’s also a standard setting with many double-wishbone suspension designs.
How do you measure a scrub radius?
From a front view of the car, draw a line between the ball joints and make it intersect the ground. The distance between the centerline of the tire and where this line meets the ground is your scrub radius.
Does scrub radius matter?
A larger scrub radius amplifies the toe-in affect, which might actually reduce stopping distance just a bit, but only if he driver can hold the steering wheel straight. On a rear-drive car, acceleration and braking forces both have the same effect on the front wheels.
Do wheel spacers affect turning radius?
Spacer increase the track of the car which increases the turning radius a little but it also adds stability. Using wheel spacers also increases the load on the bearing because you are pushing the wheel out of the car whereas offset is a little easier.
When is the scrub radius positive or negative?
If the intersection point is inside the contact patch, the scrub radius is positive (B in the illustration). If the steering axis intersection point is outside the center of the contact patch, the scrub radius is negative (C in the illustration). In either the positive or negative scrub radius mode, the tire does not turn on its centerline.
What causes a tire to have a negative scrub radius?
If the steering axis intersection point is outside the center of the contact patch, the scrub radius is negative (C in the illustration). In either the positive or negative scrub radius mode, the tire does not turn on its centerline. This causes it to scrub the road in a turn. Due to the increased friction, more effort is needed to turn the wheel.
What does zero scrub radius do to a car?
However, zero scrub radius, under hard braking, causes the suspension to be skittish because varying road conditions created varying amounts of torque (both positive and negative) around the steering axis. Therefore, some amount of scrub radius, positive or negative, is preferred.
Why do you need a positive scrub radius on front suspension?
On the front suspension, a positive scrub radius gives stability because it causes toe out when braking and toe in when accelerating. A negative scrub radius is suitable for vehicles with high power engine and a positive scrub radius offset for small power engines.
