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to put that arc on the car. 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. The distance between the center of the tire's contact patch and the point where that line touches the road is the scrub radius. 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 above.) That's the technical explana- tion, but here's an easier way to remember it. 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 (nega- tive 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. Predictions 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. First we'll look at a front- drive car with negative scrub radius because that's more common. When scrub radius is negative (center of the contact patch is inboard of the pivot point), then under accelera- tion 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 gener- ated. It's just like pushing on a lever: The longer the lever, the more force is generated. A front-drive car with a 100 horse- power engine won't generate much toe-out force, but imagine what happens if scrub radius is increased. If one front tire has a bit more traction than the other, it will feel like the steering wheel is being pulled out of the driver's hands. The same thing happens under braking, except the front wheels try to toe in. In that case, it's not horsepower but the car's weight that pushes on the lever. 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, accelera- tion 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 suspen- sion are more likely to have a n e g a t i v e s c r u b r a d i u s , and cars with double A-arm Compensating for wheel offset If you have a service information system or an align- ment machine with a built-in database, look up "steer- ing angle inclination," sometimes called "steering axis inclination" or "king pin angle." On a vehicle where it's adjustable, changing the angle of that line will adjust scrub radius to compensate for changes in wheel offset. Some OEMs use this adjust- ment to set scrub radius on vehicles that are sold with different wheel size options. Figure 2 8 Feature