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Offset
The offset of a wheel is the distance from its hub mounting surface to the
centerline of the wheel. The offset can be one of three types:
The hub mounting surface is even with the centerline of the wheel.
The hub mounting surface is toward the front or wheel side of the wheel.
Positive offset wheels are generally found on front wheel drive cars and
newer rear drive cars.
The hub mounting surface is toward the back or brake side of the wheels
centerline.
"Deep dish" wheels typically have negative offset or a very low positive offset.
Calculating the Offset of a Wheel
Calculating the offset of a wheel is a fairly easy mathematical equation. First,
measure the overall width of the wheel (remember, just because a wheel is
18x7.5, does not mean that the OVERALL width is 7.5”. It means that the
measurement from outboard flange to the
inboard flange is 7.5”). Next, divide that width of the wheel by
two; this will give you the centerline of the wheel.
Overall width/2 = Centerline
After determining the centerline, measure from the mounting pad
to the edge of the inboard flange (if the wheel were laying flat on the ground –
face up – your measurement would be from the ground to the mounting pad). This
is your back spacing.
Centerline – Back Spacing = Offset in Inches
Inches x 25.4 = Offset in mm
All of the wheels in the American Racing catalog indicate the wheel’s offset.
Determining the Right Offset for Your Car
Short of adding fender flares or a body kit to your car or rolling and pulling
your fenderwells, there is no way to significantly change your car’s offset. If
your car has a high offset, you will have to buy a high offset aftermarket
wheel. If you car has a low offset, you will need a low offset aftermarket
wheel. Typically, front wheel drive vehicles have a high offset (+35 mm or
greater), and rear-wheel drive applications will have lower offsets (this is not
true in all cases, as the Honda S2000 is rear wheel drive, and has a very high
offset). Your installer or tire and wheel dealer should be able to tell you what
the offset of your vehicle is.
The common definitions are:
Low – 0-15 mm
Mid – 16-34 mm
High – 35-40 mm
High + - 41+ mm
Other factors in determining what offset is right for your vehicle is whether
not you are making any other changes to your car (lowering or raising,
aftermarket brakes, etc.).
If the offset of the wheel is not correct for the car, the handling can be
adversely affected. When the width of the wheel changes, the offset also changes
numerically. If the offset were to stay the same while you added width, the
additional width would be split evenly between the inside and outside. For most
cars, this won't work correctly.
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