I am abit confused here. this says to measure with a straight edge from the OUTSIDE lip of the rim, is it not from the INSIDe of the lip of the rim? I am test fitting a set of 20x8.5 Foose wheels and need to be precise on the backspacing & offset. Thanks in advance, JPT.
In the example cited in the diagram
, say it's a 7" wide wheel, say out side to outside measurement is 8 inches, center will always be center. It's halfway between the edges. Offset
is a measurement expressed in mm
) and reflects the difference between this imaginary center line, and the mounting face of the wheel that sets against the brake rotor or drum.
means they are the same.
means the center of the rim is offset in-ward from the mounting surface, or that the mounting surface is towards the outside of the wheel. Like a front wheel drive car.
means the center of the rim is offset out-ward from the mounting surface, or that the mounting surface is towards the inside of the wheel. Like the deep dish Cragars I had on my '65 GTO back in the day.
If you know the offset, and whether it's negative or positive, you can measure in / out from the mounting surface of your brake drum / rotor where the wheel sets and that is where the center of the wheel will be. If it's a 15 X 7 or a 16 X 8 wheel, the center will be .... the center of that rim width measurement.
The reason they used the out side to outside lip method is simple, it's easier placing a ruler / straightedge across a wheel's outer lip when it's setting on a flat surface, measure down to the flat surface and splitting that figure (say it's 8 inches, half is 4", thus 4" from the flat surface is the center whether measuring out to out or in to in
). Then while the wheel is still lying on that flat surface, or by flipping it over and using a straight edge, measure from mounting surface to one of the same lips (say it's 3" from mounting surface to inside lip, you have 1" negative offset, or 25.4 mm negative offset when converted to metric
Most wheels have the same size lips on both sides, but it's just easier measuring out side lips when dealing with measuring down through a center.
, that is the way it used to always be done before metrics and all these front wheel drive cars and such. Properly, it does refer to the difference between the tire mounting surface of the inner lip and the wheel's mounting surface, not outer edge of inner lip as depicted. It can easily be 1/2 - 3/4" difference depending on the wheel. To do it correctly, I would measuer as shown, then measure the width of that inner lip back to tire bead surface and subtract that. A 15X 7 wheel with 3 " back space is telling me that 4" of that wheel's tire bead mounting width is outside towards the viewer and away from the brake drum / rotor.
Your T-Bird's wheels will almost always have positive offset, or else stick way outside the fenders.
Also, watch that front upper control arm @ ball joint & and ball joint stud sticking down.