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Simplifying Tire Scrub - Kopycinski's Brain

Posted in How To on June 1, 2011
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When truck and 4WD designers develop a front suspension and axle system, they do so to provide proper track width for the chassis and steering geometry to best accommodate the suspension. This total package also includes tire size and wheel offset to optimize stability and handling characteristics. Once we start modifying things (typically a lift and larger tires) steering and handling performance can start to change. Sometimes these changes can bring on some unwanted road manners and added stress to suspension and driveline components.

One characteristic that we should be concerned with is scrub radius. Say we were to draw a line through the king pin axis, or other axis of steering on a front axle assembly and mark its location where it touches the ground (neutral location). We would then measure horizontally from this point to the center of the tire contact patch, and this dimension would be the scrub radius.

Here you can see a typical positive scrub radius. The tire contact patch lies to the outside of the point where the King Pin steering axis meets the road surface.

4WDs are typically designed with some positive scrub radius (point of tire contact is moved outboard from the neutral location). An advantage to this is that it provides good road feel and steering feedback while cornering. However, the greater the positive scrub radius, the more steering effort that is needed and tire kickback is greater when bumps are encountered.

Everything is usually fine with the vehicle rolling forward down a smooth road so there is equal loading and friction on the front tires. However, if one tire hits a bump and the tire contact is momentarily lighter on the one side of the vehicle, the opposing tire with greater traction will tend to want to pull the steering to that side. As the vehicle encounters alternating and varying bumps, this may result in annoying road wander. The greater the scrub radius, the greater this effect.

Wheel backspacing (in inches) determines where your tires sit in relation to the king pin or steering axis. Reduced backspacing pushes the tire further out the fender and increases scrub radius. Increased backspacing pulls the tire back under the fender and often helps reduce the effects of tire scrub.

IFS rigs typically come stock with large amounts of backspacing to best match the A-arm geometry up front. Some owners will swap to wheels with much less backspacing that push out tires much further. This sometimes looks cool and certainly adds to off-camber stability, but can lead to sketchy highway behavior. Steering and wheel bearing components will be subject to more stress, and impacts at the tire edge can transmit more harmful force back into the front suspension and steering through the longer lever created at the wheel. Scrub radius and its effects are characteristics to consider when choosing a new wheel and tire setup for any rig. OR

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