Modern Tire Dealer

Handbook 2019

Magazine for the professional tire industry

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P e r f o r m a n c e H a n d b o o k 2 0 1 9 18 W h e e l O f f s e t 3. Measure the distance from the floor to the straight edge and divide by two. at calculates the centerline of the wheel. If the centerline number is smaller than the hub measurement, offset is positive; if it is larger, offset is negative. 4. Measure the distance from the floor to the straight edge and write down that number. 5. Divide that number by two to calculate the centerline of the wheel. 6. Now measure from the hub flange to the straight edge. 7. Subtract the smaller number from the larger number. at gives you the offset of the wheel. If the centerline number is smaller than the hub measurement, offset is positive. On most cars, changing offset by just 5 mm is enough to notice a change in the car's handling because it changes scrub radius by the same amount, and that might already be only a few millimeters. If the OEM wheels have a positive offset, you can safely install wheels with even more positive offset and use spacers to move the contact patch out to its original position, keeping scrub radius the same. Just make sure the new wheels and tires fit inside the fender without rubbing anything. If the new wheels have negative offset, there's nothing you can do to move the contact patch in toward the center of the car. e only way to know for sure how much the new offset will affect scrub radius is to put the car on an alignment rack. A FEW FINAL DETAILS Even if you're confident the wheels have been properly selected for the vehicle, it's up to your technician to make sure everything fits properly. It's a good idea to test-fit the wheel onto the front and rear hubs before mounting the tire. e first thing to check is brake caliper clearance: On OEM wheels it might be as small as 1/8 of an inch (4 mm). is can be hard to see, but it's easy to simply install the bare wheel and turn it by hand. If clearance is that tight, the tech must take extra care when placing the wheel weights. Mounting a tire on an aermarket wheel versus an OEM wheel may be different; it depends on the location of the wheel's drop center. Usually the drop center is toward the outside of the wheel, so the wheel is placed outside up on the tire changer. But some aermarket wheels have the narrowest part of the drop center toward the inside of the wheel. ese must be placed on the tire machine inside up. If not, the machine will try to stretch the bead — which can't stretch — and that can damage the wheel, the tire, the tire machine or the technician. Finally, make sure your techs tighten the lug nuts with a properly calibrated torque wrench every single time! Uneven lug nut torque is almost guaranteed to cause warped brake rotors within a few thousand miles. Finally, remind your customers to come back aer five to 50 miles to get re-torqued. OFFSET VS. BACKSPACING Closely related to offset is the concept of backspacing. Offset and backspacing are often spoken of as if they are the same thing, but they are very different. Backspac- ing is defined as the distance between the mounting plate and the inner flange of the wheel. Backspacing is, therefore, the com- bination of the offset and the wheel width. This comes into play if the new wheels are wider than the old, as the offset may need to change to compensate for the greater width. The distance from the hub flange to the straight edge is the last measurement needed to determine offset. If that number is smaller than your centerline calculation, offset is positive. Changing offset by just 5 mm is enough to change handling. COURTESY OF JACQUES GORDON

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