Quote:
Originally Posted by doughboy
Great write up Tony 
Just a point - changing the offset doesn't have to move the spokes in relation to the mounting face or change your brake clearances...
If a rim width goes up by 1inch (call it 25mm) then if they add 12.5mm to each rim lip the offset would be the same, however for example, if they added all 25mm to the inside lip, the offset would move by inwards 12.5mm, but the spokes would not move in relation to the mounting face, only the tyre/rim position would change |
That is correct. However in practice that isn't how a wheel is made and if the wheel design changes from a 9J to a 10J, the extra 1J isn't just added to the inside of the wheel as that would change the fundamental rotational balance of the wheel drastically. Going up a from a 9J to a 10J will in 99% of cases result in a equally distributed increase around the centre line.
Quote:
Originally Posted by doughboy
But I suppose with different offset versions of the same wheel a manufacturer would just machine more off the mounting face to increase the offset, thus moving the whole wheel and spokes toward the brake.
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Again this is correct. Most ET adjustments are in the range of +/- 5-10mm. A particular wheel design will generally only offer ET within a 10mm range, because any more than that and the balance of the wheel changes too much. The easiest way for a manufacturer to offer a range of ETs is to, as you have said, machine off the mounting point. They won't cast a separate wheel for each ET as it will cost too much money. Hence why the position of the spokes might change relative to the hub surface and brake calipers.
Quote:
Originally Posted by doughboy
So it is not 100% that a wheel will fit based on offset, width and diameter. Brake clearance could still be compromised even if the other parameters are met - is that right?
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Correct again. Even if you have the offset, width and diameter all sorted, you still need to make sure the spokes clear the brake calipers sufficiently. This is even more important if you have upgraded brakes, like APs, which take up more room than the standard OEM calipers.
Quote:
Originally Posted by doughboy
Also,
As I'd guessed from looking, BMWs have a positive offset wheel, but that is not clear in the 'ET' value, or is positive assumed with no sign?,so negative offset wheels would be 'ET-xx'?
Cheers!
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Positive is assumed with no sign - ET35 = ET +35mm.
If you have a negative offset wheel (very rare) then it will be signed -ET35 or ET-35