How to Measure Backspacing

 

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The offset (sometimes called the backspace) is the distance from the inside surface of the dished center (B) to the inside rim lip (A). See diagram below.

On a 15x8" wheel this dimension can be anywhere from 1" to 5" for modifieds or 1" to 4" for stock cars.

The same holds true for 15x10" and 15X12" wheels because the only dimension that changes is the width of the 'valley.'

While it may appear that the offset range should increase with the width of the valley, actually the critical dimension is the distance between the inside rim lip (A) and the inside valley edge (C). In the illustration below you will see that it is labeled (X) and has a dimension of 4". Regardless of the width of the wheel, X will always be 4" and Z will always be 1.5" in the case of Allied wheels. (There is an exception to this in our 'Wide Back Bell' shells. In this case X is increased to 5".)

 

MAXIMUM OFFSET

If (I) x2 (times 2) is greater than 12.125" (G), obviously your maximum offset must be just under 4". Otherwise the caliper will hit the valley of the wheel.

 MINIMUM OFFSET

The minimum offset is determined by the 'dish' of the center and the Back Bell Depth (X). While the typical center is 2-1/8" deep, part of that is the weld surface, leaving only a part of the actual center depth to work with. Also, the actual flat weldable part of the valley is shortened by the curve of the valley wall.

Placing the center at the extreme usable edge of the valley gives only a 3" offset.

YOUR BEST BET

When you're ready to buy a new set of wheels, measure the offset of the wheels that you know work - the ones on the car.

 HOW TO MEASURE OFFSET

1. Place the wheel (without tire) on a flat surface with the outside up (hub side down).
2. Put a ruler down through the center hub hole.
3. note where the bottom side of the center plate strikes the ruler - That's the offset.

THERE IS NO SUCH THING AS A 'STANDARD' OFFSET!

 There are too many variations between car and axle manufacturers. 2", 3", and 4" offsets are popular and common, but not standard. We depend upon you to give us accurate offset measurements. If you don't, you may have just purchased a very expensive set of paperweights.


STEEL WHEEL OFFSET - A PRIMER

1. Hub
2. Brake Rotor
3. Brake Caliper
4. Axle
5. Stud

A. Inside rim lip
B. Dished center
C. Inside valley edge
D. Center weld point
E. Outside valley edge
F. Outside rim lip
G. Inside diameter 12.125"
H. Outside diameter 14.5"
I. Center of axle to outside edge of caliper
X. Back bell depth 4"
Z. Front bell depth 1.5"

 

Phone

888-316-2940

Toll Free