Backspacing vs offset - calculations don't match up

[EricsXJ]

OK you wheel tech guys heres a question for you -

What is the offset of a 15x8 wheel that has 4.0" backspacing?


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If you said the offset is "zero" like I did, you are WRONG! At least according to this chart you are wrong - http://www.rsracing.com/tech-wheel.html#backspace

The chart in the above page says the offset is -12mm. How can this be since 12mm is almost 1/2 inch???

 

[satan]

The BS measurement includes the flange (where weights would normally clamp); by including that value from only "one-side" the data is scewed by that flange and the thickness of the rim material.

Normally rim-width is measured as bead-seat to bead seat, essentially an "ID" value where the material thicknessesand "flanges" are not considered...


Just like it's always been --- I noticed the shift from spec'ing a rim from offset to Backspacing came about the time tat Front-wheel-drive got really popular -- either as a function of the dramatic off-sets or the tighter fits (knowing EXACTLY where the metal stops with a BS measurement seems much better)...

 

[EricsXJ]

I thought BS was to be measured from the bead, and not the outermost edge (flange). When I measured my stock wheels this way I got 5.25". I hear people say stock is 5.5" and I'm guessing they are including the flange which is about 1/4".

Anyways thanks for the explanation as I understand why the number is "scewed" as you put it. It would be so much better if they measure BS and offset from the same point.

 

[Eagle]

Nobody ever says stock is 5.5" -- it is always quoted as 5.25". Now I'm going to have to measure my stock rims again, because my recollection is when I measured them it was 5.25" to the outer (back) edge of the rim, not to the inside of the bead area.

 

[EricsXJ]

Nobody ever says stock is 5.5"...

I hear it said all the time, but that doesn't mean its correct.

 

[dmillion]

Eagle, did you ever measure your rims? I still haven't figured out, if I have stock, 5-spoke, 7" wide, steel wheels, what is the backspacing and what is the offset?

 

[Eagle]

I hadn't, but you shamed me into it :angel:

I measured both a stock 15x7 steel rim and a stock '88-vintage 15x7 10-spoke "turbine" style alloy rim. In both cases, the measurement was 5-1/4" from the mounting surface to the OUTSIDE edge of the weight flange. In other words, the 5-1/4" backspacing is not taken to the tire bead, but to the extreme inward-most part of the rim.