Skid patch calculator
Skidding with the same foot forward wears 8 places on the tire, 26.4 cm apart. If you skid with either foot forward it is 16 places, 13.2 cm apart. In lowest terms the gear is 23 to 8.
Result
Skid patches, same foot forward
Skid patches, either foot forward
Chainring in lowest terms
Cog in lowest terms
Distance between patches, same foot
cm
Distance between patches, either foot
cm
Gear inches of this gear
in
How it works
On a fixed gear the rear wheel can only stop where the cranks stop. Riders lock the wheel with the cranks level, so the tire meets the road in the same few places every time: fixed gear skid patches. How many there are depends only on the two tooth counts. Reduce the gear to lowest terms and the reduced cog is the number of patches for a rider who always skids with the same foot forward. Ambidextrous skid patches, for a rider who skids with either foot, are twice as many when the reduced chainring is odd and the same when it is even. Several calculators double on an odd cog instead, which gives wrong answers for gears such as an even chainring on a prime cog. The best gear ratio for skid patches has a cog that shares no factor with the chainring; the skid patch chart on the method page shows the common combinations.
Formula
Reduce chainring ÷ cog to lowest terms, p ÷ q. Skidding with the same foot forward: q patches. Skidding with either foot forward: q patches when p is even, 2 × q when p is odd. Distance between patches = wheel circumference ÷ patches, with circumference = π × (rim bead seat diameter + 2 × tire width).
Worked example
The calculator opens with this example; the result above is computed from it.
| Input | Value |
|---|---|
| Chainring teeth | 46 |
| Cog teeth | 16 |
| Rim size | 622 = 700C, 29 inch |
| Tire width | 25 mm |
| Skid patches, same foot forward | 8 |
| Skid patches, either foot forward | 16 |
| Chainring in lowest terms | 23 |
| Cog in lowest terms | 8 |
| Distance between patches, same foot | 26.4 cm |
| Distance between patches, either foot | 13.2 cm |
| Gear inches of this gear | 76.1 in |
Common cases
| Case | Starting values | Skid patches, same foot forward | Skid patches, either foot forward | Chainring in lowest terms | Cog in lowest terms | Distance between patches, same foot | Distance between patches, either foot | Gear inches of this gear |
|---|---|---|---|---|---|---|---|---|
| Popular street gear | Chainring teeth 46; Cog teeth 16; Tire width 25 mm | 8 | 16 | 23 | 8 | 26.4 cm | 13.2 cm | 76.1 in |
| A gear with a single patch | Chainring teeth 48; Cog teeth 16; Tire width 25 mm | 1 | 2 | 3 | 1 | 211.1 cm | 105.6 cm | 79.4 in |
| Prime cog with an odd chainring | Chainring teeth 47; Cog teeth 17; Tire width 25 mm | 17 | 34 | 47 | 17 | 12.4 cm | 6.2 cm | 73.1 in |
| Prime cog with an even chainring | Chainring teeth 48; Cog teeth 17; Tire width 25 mm | 17 | 17 | 48 | 17 | 12.4 cm | 12.4 cm | 74.7 in |
| Sheldon Brown's own road gear | Chainring teeth 42; Cog teeth 15; Tire width 28 mm | 5 | 5 | 14 | 5 | 42.6 cm | 42.6 cm | 74.7 in |
Questions and answers
How many skid patches does my fixed gear have?
Reduce the gear to lowest terms and read the cog. A chainring of 46 teeth on a cog of 16 reduces to 23 over 8, so the tire wears in 8 places with the same foot forward and 16 with either foot.
Do ambidextrous skidders always get twice the skid patches?
No. The number doubles only when the reduced chainring is odd. A chainring of 48 teeth on a cog of 17 is already in lowest terms and the chainring is even, so there are 17 patches with one foot forward and still 17 with either foot.
Assumptions and limits
- The count assumes you lock the wheel with the cranks level. Same foot forward means the cranks are in the same position at every skid.
- Each patch is a short stretch of tread, not a point, so patches that are only a few centimeters apart overlap and wear like one band.
- The wheel size changes only the distance between patches, never their number.
- This counts where a tire wears. It says nothing about stopping a bicycle, which is a matter for brakes and local law.
The result is an estimate for planning, not professional advice.
Sources
- Sheldon Brown: Fixed gear bicycles, skid patches (the rule and five worked examples)retrieved 2026-10-13
- BikeCalc: skid patch math (the same rule; counting patches by marking the tire)retrieved 2026-10-13
- Wikipedia: ISO 5775 (bead seat diameters of bicycle rims)retrieved 2026-10-13
Last updated:
Method in depth: Skid patches on a fixed gear
Cite this page: Crank Math, “Skid patch calculator”, https://crank-math.pages.dev/wheels/skid-patch-calculator/ (updated 13 October 2026).
Found a mistake? Email handycalc.sites+crankmath@gmail.com.