WHEEL BACK-SPACING & OFFSET: Explained and illustrated - Toyota FJ Cruiser Forum
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post #1 of 1 (permalink) Old 02-03-2008, 09:27 PM Thread Starter
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WHEEL BACK-SPACING & OFFSET: Explained and illustrated

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Many questions have come about regarding wheel back-spacing and offset. Hopefully, this will provide a somewhat detailed explanation of those two wheel measurement values.

First off, offset and backspace values are interchangeable. Offset measures the distance from the wheel's centerline to the bolting plane while backspace measures from the bolting plane to the inside edge of the rim. If you use the rim width, you can calculate either one, given the other.

Secondly, knowing the rim diameter, width and backspace (or offset) is NOT enough information to know if a given rim will fit on the FJ Cruiser.

The outside diameter of the brake rotors and calipers is sufficiently small that SOME 16" rims will fit around them.... NOT ALL. This is because the 16" diameter designation is around the OUTSIDE of the rim (inside the tire) and thicker cast alloy rims will have a smaller INSIDE DIAMETER. If the rim is thick enough or shaped with a steep taper, then it will intefere with the rotors and hubs.

Two rims might have the exact same diameter, width and backspace yet one will fit AROUND the brakes and the other will not... based only on material thickness.

Steel rims are thinner walled than alloy because steel is stronger.

Next, not all rims that fit around the OUTSIDE of the rotors/calipers will bolt up correctly. This is because the front brake calipers stick out farther than the wheel mounting surface. The rim design needs to wrap out around these calipers. Alternately, a wheel spacer can be used, but depending on the rim design, it could require a SUBSTANTIAL spacer. If a spacer is used, this has to be taken into account in regard to the backspacing. If the front wheel is mounted out away from the hub far enough, then it will rub the tire into the back edge of the front wheel well during turns.

Here are some illustrations to show what I mean.

The front hub, with brake caliper sticking out FARTHER than the wheel mounting surface:

The rear hub does not have this same configuration. The brake calipers on the rear hubs do not stick out farther than the wheel mounting surface.

Here is a diagram of a generic rim:

Here are a couple of items that are not specified as part of the sizing designations:

Note that the above rim diagram would NOT fit on the hub illustrated above. This is because the back of this rim is FLAT and it would crash into the brake caliper before it seats flatly against the wheel mounting surface.

In order to mate aginst the wheel mounting surface without hitting the caliper, a rim has to have a shape that will reach around the calipers. This shape is not included as part of the standard diameter/width/backspace designation. This is sometimes referred to as "X-factor".

Here is what a rim has to do, in order to get around the front brake caliper:

Note that this diagram has EXACTLY the same diameter, width and backspace as the one drawn previously. It would be designated the same.

ANY RIM... whether it be 16", 17", or even larger diameter, needs to be assessed for fitment in terms of this X-factor. SOME 16" rims will fit around the outside diameter of the rotors, but others will not.
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Last edited by LandCruiserSteve; 02-04-2008 at 03:28 PM.
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