Inner Wheel Clearance Calculator
Estimate how far a new wheel's inner edge moves toward the suspension. Compare backspacing of two wheels from width and offset.
Estimate changes at the inner wheel edge.
Inner Wheel Clearance Calculator: Tire Size Inputs
Compare two tire sizes here. Use the formulas and reference tables below for this tool's specific calculation.
How Inner Wheel Clearance Calculator Works?
The Inner Wheel Clearance Calculator works by calculating the backspacing of both wheels and subtracting the old value from the new value. A positive result moves the inner edge closer to the suspension. The tool uses 3 steps.
Enter the current wheel
Type the width and offset, for example 8.0 in ET35.
Enter the new wheel
Type the new width and offset, for example 9.0 in ET45.
Read the change
The calculator returns a 22.7 mm (0.89 in) inward movement of the inner rim edge.
Inner Wheel Clearance Calculator Formula
The Inner Wheel Clearance Calculator formula is Δinner = BS_new − BS_old = (W_new − W_old) × 12.7 + (ET_new − ET_old).
BS = (W + 1) × 12.7 + ETΔin = BS_new − BS_old- BS
- Backspacing (mm)
- W
- Nominal width (in)
- ET
- Offset (mm)
Worked example
An 8.0 in ET35 wheel has 149.3 mm of backspacing. A 9.0 in ET45 wheel has 172.0 mm. The inner edge moves 22.7 mm (0.89 in) toward the suspension.
Inner Wheel Clearance Calculator Conversion Table
The Inner Wheel Clearance Calculator conversion table shows inner rim edge movement for 6 wheel changes.
| Wheel change | Old backspacing | New backspacing | Inner edge change |
|---|---|---|---|
| 7.5" ET+45 → 8.5" ET+45 | 152.9 mm | 165.6 mm | +12.7 mm (+0.50 in) |
| 7.5" ET+45 → 8.5" ET+35 | 152.9 mm | 155.6 mm | +2.7 mm (+0.11 in) |
| 8.0" ET+35 → 9.0" ET+35 | 149.3 mm | 162.0 mm | +12.7 mm (+0.50 in) |
| 8.0" ET+35 → 9.0" ET+45 | 149.3 mm | 172.0 mm | +22.7 mm (+0.89 in) |
| 8.0" ET+18 → 9.0" ET+0 | 132.3 mm | 127.0 mm | -5.3 mm (-0.21 in) |
| 7.0" ET+40 → 8.0" ET+50 | 141.6 mm | 164.3 mm | +22.7 mm (+0.89 in) |
What is Inner Wheel Clearance Calculator?
An Inner Wheel Clearance Calculator is a tool that predicts the movement of the wheel's inner rim edge toward struts, control arms and brake parts. The rim edge is metal, so contact damages both the wheel and the suspension.
The tool measures the wheel itself, not the tire. The Inner Tire Clearance Change Calculator handles the tire sidewall, which often sits further inward than the rim edge at the bottom of the sidewall bulge.
A wider wheel at the same offset gains half its added width on the inner side.
What are the uses of Inner Wheel Clearance Calculator?
There are 4 main uses of the Inner Wheel Clearance Calculator, including upper control arm checks and wheel shopping.
Upper control arm checks
Check room on double-wishbone suspensions.
Wheel shopping
Rule out wheels whose inner edge moves too far in.
Spacer decisions
Size a spacer to recover inner room.
Brake line routing
Check clearance to brake hoses and ABS wiring.
Related Clearance & Stance Tools
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Estimate changes at the outer wheel edge.