How To Determine Offset

The wheel or wheel rim on an automobile is the circular metal part on which a tire is mounted. The flanges are the two outer edges of the wheel which hold the tire between them. The hub is a smaller circular metal piece which attaches the wheel to the axle with lug nuts. Wheel offset is the distance from the center of the hub to the halfway point between the two flanges.

Things You’ll Need:

• Flat surface capable of supporting wheel weight
• Tape measure
• Tape, such as duct tape
• Paper
• Pen or pencil

Preparation

1. If a tire is mounted on the wheel, remove the tire. If the wheel is still on the car, remove it.
2. Put the wheel on a flat surface with the inboard (car side) flange facing up and the outboard (street side) flange down. Measurements are easier if the wheel is on a work bench, but any sturdy flat surface, such as a garage floor, will do.
3. Stretch a strip of tape from one edge of the inboard flange to the other edge. You need to be able to see the hub center directly underneath one edge of the tape. If the wheel is dirty, first clean the spots where the tape will be attached. The tape must be taut enough not to sag.

Measurement

1. Measure the distance from the outside of the outboard wheel flange, which is resting on the flat surface, to the outside of the inboard wheel flange, which is facing up. Write this measurement down and label it “Wheel Width.”
2. Divide wheel width by 2. Write this down and label it “Center Line.”
3. Measure from the center of the hub to the tape stretched across the rim. Take care to measure the true vertical distance, which will be the shortest distance. Measuring on a slant will give a longer, incorrect distance. Write this measurement down and mark it “Backspace.”
4. Subtract center line from backspace. Write this down and label it “Offset.” If backspace is larger than center line, offset is positive and the hub is closer to the outside of the wheel. If center line is larger, offset is negative and the hub is closer to the inside of the wheel. If they are the same, offset is zero and the hub is on the center line.
5. Offset can be given in either inches or millimeters. Multiply inches by 25.4 to get millimeters. Divide millimeters by 25.4 to get inches.

Tips & Warnings

• You can use a yardstick or straight narrow board instead of tape for your reference line. If you do, measure to the bottom of the yardstick or board.
• Measure to the hub center, not to one of the lug nuts, which may extend beyond the hub center.
• When lifting wheels, use proper technique to avoid back injury. Bend your knees to pick up the weight and keep your back straight when lifting it.

Cheryl Ess. How to measure an Auto Wheel Offset.
Available http://www.ehow.com/how_7914616_measure-auto-wheel-offset.html
February 9, 2011

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