Formula
Nominal width is the bead width. Overall width is nominal plus 1 inch, about half an inch of flange on each side. Backspace is half the overall width plus the offset. Frontspace is half the overall width minus the offset. Positive offset is ET outward from the centerline, which adds backspace and removes frontspace.
overall = nominal width + 1 in
backspace = overall ÷ 2 + offset mm / 25.4
frontspace = overall ÷ 2 − offset mm / 25.4
poke change = new frontspace − old frontspace
inner loss = new backspace − old backspace
An 8 inch wheel at ET 0 has 4.5 inches of backspace because (8 + 1) / 2 = 4.5. Moving it to ET 35 changes poke by −35/25.4 inches. The diagram looks from above: the dashed line is the mounting face, inner clearance is to the left, and the fender is to the right. Suspension, brake calipers, and the tire section are not in the drawing.
Worked examples
8 inch ET 0 compared with 8 inch ET 35. Backspace goes from 4.50 in to 5.88 in. Poke change is −1.38 in. Inner loss is +1.38 in.
8 inch ET 35 compared with 9 inch ET 20. The new wheel is an inch wider and 15 mm less offset. The outer lip moves out. The inner lip barely moves. The readout is that pair when the page loads.
Same ET, one inch wider. Half an inch appears on each side: poke +0.50 in and inner loss +0.50 in, before you change the offset.
Reference
Backspace for an ET 0 wheel, using nominal width + 1 inch overall.
| Nominal width | ET 0 backspace | ET 35 backspace |
|---|---|---|
| 7 in | 4 in | 5.38 in |
| 8 in | 4.5 in | 5.88 in |
| 9 in | 5 in | 6.38 in |
| 10 in | 5.5 in | 6.88 in |
Questions
What is the backspace of an 8 inch wheel at ET 0?
4.5 inches. Nominal width plus 1 inch of flange is 9 inches overall, and half of that is 4.5 when the mounting face is centered. That 1 inch is the assumption this page uses for every width.
What changes when I go from ET 0 to ET 35 on the same 8 inch wheel?
The wheel moves inward by 35 / 25.4 = 1.38 inches. Poke decreases by 1.38 inches. Inner clearance shrinks by 1.38 inches. Width did not change, so the two shifts are equal and opposite.
Does a wider wheel with less offset stick out?
Often. An 8 inch ET 35 against a 9 inch ET 20 pokes about 1.09 inches farther out, because the extra half inch of width and the 15 mm less offset both move the outer lip. Inner clearance is almost unchanged. The diagram draws both.
Sources
- Backspace = (nominal width + 1 in) / 2 + ET / 25.4. The extra inch is the usual flange allowance, which makes ET 0 on an 8 inch wheel equal 4.5 inches of backspace.
- Poke change = new frontspace − old frontspace. Inner-clearance change = new backspace − old backspace.