Please confirm you agree to the use of tracking cookies as outlined in the Cookies Policy. Sign in or register. Matt Kimberley 3 years ago. Remind me later. Share Tweet Email Whatsapp. Note the suspension's greater angle relative to the wheel. Scrub radius relates to the relative angle between the suspension and wheels.
Different scrub radius configurations suit different cars. Steering axis inclination SAI , on the other hand, is the angle between the centerline of the wheel and the steering axis. When the steering axis intersection point is inside the contact patch, the scrub radius is positive.
With the MacPherson strut suspension, the steering axis is the straight line between the top strut bearing the lower ball joint. The angle between the steering axis and the centerline of the wheel is the SAI.
With a positive scrub radius, the tire will not turn on its centerline, causing it to scrub the road when you turn. For you, this means exerting more effort. If you have an RWD car with a positive scrub radius, the friction between the tire and the road will move the front wheels back and will keep the vehicle going straight. 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.
Alternately you can increase the scrub radius by fitting a smaller-size front tire on an RWD car. Your other option is to fit a wider wheel, which will move the tire centerline away from the vehicle. A scrub radius is negative if the steering axis intersection point is outside the center of the contact patch. Looking at the car head-on, imagine a line drawn from the upper strut mount or upper ball joint and down through the lower ball joint, and then on down to the road. If the line touches the road inboard of the contact patch, scrub radius is positive.
If the line touches the road outboard of the contact patch, scrub radius is negative see Figure 2. The point where that line meets the road is the steering pivot point, same as where you held one end of the pen. As the wheel is steered, the point at the center of the contact patch moves through an arc around that pivot point.
The arc can be inboard negative scrub radius or outboard positive scrub radius of the pivot point, and the distance between the arc and the point might be just a few millimeters, but that small distance makes a big difference in what the driver feels in the steering wheel.
So now we know that changing the wheel offset will move the tire contact patch inboard or outboard, and doing that will also change the scrub radius.
The question is, how will that affect the way the car handles? The answer depends partly on whether the car is front- or rear-wheel-drive. When scrub radius is negative center of the contact patch is inboard of the pivot point , then under acceleration the front wheels will try to toe out as they pull the car forward.
The length of the scrub radius influences how much toe-out force is generated. The same thing happens under braking, except the front wheels try to toe in. 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. If scrub radius is positive contact patch arc outboard of the pivot point , the wheels will try to toe out. Cars with strut suspension are more likely to have a negative scrub radius, and cars with double A-arm suspension are more likely to have a positive scrub radius. Chassis engineers use scrub radius to create the feedback the driver feels through the steering wheel.
Negative scrub An advantage of a negative scrub radius is that the geometry naturally compensates for split braking, or failure in one of the brake circuits. Vehicles with a diagonal-split brake system have negative scrub radius built into the steering geometry.
If one half of the brake system fails, then the vehicle will tend to pull up in a straight line. Negative scrub radius also provides centre point steering in the event of a tyre deflation, which provides greater stability and steering control in this emergency situation. If you hit standing water at speed on one side of the car, with negative scrub radius the torque on the steering will steer you away from the puddle. Which balances the effect of drag on one side of the car.
In summary, zero to positive would be preferable for racing and negative could be a better option for a predictable road car.
Tags kingpin inclination , scrub radius , steering axis , toe in , toe out.
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