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Thread: Effect of Wheel Offset Change on Scrub Radius

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    Village Idiot goldstar's Avatar
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    Effect of Wheel Offset Change on Scrub Radius

    One of the mods sometimes considered when switching to aftermarket wheels involves using a rim with less positive offset than OEM for the purposes of increasing the vehicle's track and, for those who think it enhances appearance, filling out the wheelwell. Assuming that either tire/fender interference is not a problem, or has been resolved, changing the offset of a wheel will change the scrub radius of the tire/wheel combination.

    Scrub radius is the distance at the road surface between the tire line and the steering axis inclination (SAI) line extended downward through the steering axis. SAI is an imaginary line, that on strut suspensions runs through the lower ball joints and the upper end of the strut. 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, positive scrub radius results. When the lines intersect above the road, negative scrub radius occurs. The point where the SAI line contacts the road is the fulcrum pivot point on which the tire is turned. One of the main effects of SAI is to promote directional stability, the tendency of a wheel to straighten from a turned position and remain straight. SAI is not adjustable on most vehicles and is usually only affected by loose, worn or damaged suspension parts or by frame damage.

    Characteristics of Different Types of Scrub Radius
    At zero scrub radius, the car steers easily and will have little or no kickback from bumps. At the same time there will be virtually no road feel or feedback and there will be a feeling of directional instability while cornering due to the tendency of the tires to squirm. This condition is also known as center-point steering.

    A positive scrub radius will increase steering effort, torque steer and kickback on bumps to a considerable degree. At the same time, a blowout or a failure of one front brake could yank the wheel hard enough to pull it out of your hands. The advantage is that there is much greater road feel and feedback so that you can feel when the front tires start to break loose in a corner. Consequently, this is often the set-up of choice on race cars.

    Negative scrub radius will also increase steering effort, torque steer and kickback but to a noticeably lesser degree than the positive kind. Additionally, front tire blowouts and single brake failures will act with less force on the steering wheel. Finally, there will be less road feel and feedback and less ability to feel when the front tires are about to break loose as compared with the positive state. In general, front-wheel drive cars are set-up with negative scrub radius.

    Incidently, 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.

    A significant reduction in positive offset of the wheels will EFFECTIVELY change the steering geometry's scrub radius, possibly increasing the steering effort and making the car harder to control during turning and cornering. As already stated, SAI is not adjustable on most cars so alignment changes may not be adequate to compensate for changes in scrub radius. That being said, I have no idea how much positive offset would have to be reduced for the change to be considered significant. It's just something to think about, particularly for those of you who consider your cars to be primarily daily drivers.

    Reference:
    http://www.ingallseng.com/alignment-terms.html?&id=10

    Happy Motoring!

    Below are illustrations of SAI and Scrub Radius:
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    suspension mods intheworks FE3-323's Avatar
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    excellent read, thanks for posting this.
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