The terms backspacing and offset as applied to automotive wheels suggest the tire placement on the wheel relative to the centerline of the wheel. It is important primarily when considering changing the size of the tire or its relative position within the vehicle’s wheel well.
The term offset is used to describe the position of the back face of the center flange in relation to the centerline of the wheel. If the center of the wheel is farther to the rear of the centerline of the wheel, it is considered a negative offset. Conversely, when the center of the wheel is forward of the centerline, it is considered positive offset, and centered would be neutral.
Front-wheel drive cars often use positive-offset wheels. The face of the wheel with the lug nut holes for mounting is close to the outside of the wheel, with most of the tire behind the face of the wheel. A deep-dish wheel where the majority of the tire is forward of the wheel’s face has a negative offset.
The effects are, for the most part, similar to backspacing in that it directly affects the distance between the wheels. In most cases, the offset can’t be changed without modification to the body or the vehicle’s suspension. This is due to the space provided by the body contours for aerodynamic effect as well as other considerations. If the offset is changed, the tire will be moved either outward, where it will contact the fenderwell opening, or inward, where it will contact the body, shocks or some other suspension component. For example, increasing the positive offset increases backspacing, whereas increasing negative offset decreases backspacing.
Backspacing is a term that defines the location of the tire under the fenderwell, similar to offset with the exception that the face or center of the wheel is not bent to create the backspacing as it is in offset. The entire inside face of the wheel is moved to a different point within the wheel. Essentially, offset measures from the wheel centerline, while backspacing measures from the back of the rim.
The current wheel must be mounted on the vehicle to determine backspacing. Measure the distance from the backside of the outer rim of the wheel to the nearest obstruction. Allow sufficient room for obstacle avoidance and the result is the amount of additional positive backspacing that can be used. When the current wheel backspacing is measured, the additional can be added. This will allow a larger-width rim to be used while still keeping the tire under the wheel well.
The reverse is used to increase wheel width for a much larger tire and move it out beyond the wheel well. In this scenario, just subtract the additional movement of the rim outwards from the current backspacing, which results in negative backspacing. It is important to consider the additional load negative backspacing will place on the wheel bearings and studs.
The tire must be dismounted from the wheel. Lay the wheel on the floor, face down. Lay a straight edge across the wheel and measure from the straight edge down to the center-mounting surface of the wheel. This is the current backspacing. Add the additional backspacing for positive or subtract for negative backspacing to the measurement.
Always keep in mind that there isn’t anything that can be done to a vehicle’s suspension that doesn’t affect something else. The further out the wheel is mounted, the more it affects handling. If the backspacing is decreased by increasing the negative offset, the wheel and tire assembly will be moved outward, widening the space between the tires on the same axle. If the wheel is widened and a larger-width tire is used, the distance between the tires will remain, however, the tire will extend out even farther. If the offset is changed on the rear only, the vehicle will increase traction and resistance to roll, however, it will create an understeer situation that decreases the handling response. On the other hand, if the front and rear experience the same changes, yaw and roll will be decreased due to the change in the center of roll, thus increasing the handling characteristics. Of coarse, the degree to which this effect is felt is reliant on the quality and type of tire. Decreasing negative offset will place the tire and wheel farther under the car, making the tires closer together. This results in poor handling with an oversteer situation and increased roll and yaw.
