Skid patches on a fixed gear

A fixed gear has no freewheel: the rear wheel and the cranks always turn together. A rider who locks the rear wheel does it with the cranks level, so the wheel can only stop in the positions that go with level cranks, and the same stretches of tread meet the road every time.

Counting them is a question about fractions. Write the gear as chainring teeth ÷ cog teeth and reduce it to lowest terms, p ÷ q. After one full turn of the cranks the wheel has made p ÷ q turns, so it comes back to where it started only after q crank turns, having stopped in q different places on the way. A rider who always skids with the same foot forward has q skid patches.

A rider who skids with either foot forward can also stop half a crank turn later. Half a crank turn moves the wheel by p ÷ (2 × q) of a turn. If p is odd this lands halfway between two patches, and the count doubles to 2 × q. If p is even it lands on a patch that is already there, and the count stays q.

This is the rule Sheldon Brown gives, and the calculator's tests check it against a simulation of every gear in its range. Some calculators double the count whenever the number of patches is odd instead. The two rules disagree for an even chainring on an odd cog, where the true count is not doubled, and for an odd reduced chainring on an even reduced cog, where it is.

More patches spread the wear. A cog that shares no factor with the chainring gives the most; a chainring that is a whole multiple of the cog gives one. The wheel size changes how far apart the patches are, never how many there are. This page is about tire wear only; it is not advice on how to stop a bicycle.

Skid patch chart: same foot forward, by chainring and cog

Each cell is the number of skid patches for the chainring in its row and the cog in its column, for a rider who always skids with the same foot forward.

	Cog teeth
Chainring teeth	13	14	15	16	17	18	19
42	13	1	5	8	17	3	19
43	13	14	15	16	17	18	19
44	13	7	15	4	17	9	19
45	13	14	1	16	17	2	19
46	13	7	15	8	17	9	19
47	13	14	15	16	17	18	19
48	13	7	5	1	17	3	19
49	13	2	15	16	17	18	19
50	13	7	3	8	17	9	19
51	13	14	5	16	1	6	19
52	1	7	15	4	17	9	19

Work out your own numbers: Skid patch calculator

Skid patch chart: either foot forward, by chainring and cog

The same gears for a rider who skids with either foot forward. The count is doubled only where the reduced chainring is odd.

	Cog teeth
Chainring teeth	13	14	15	16	17	18	19
42	13	2	5	16	17	6	19
43	26	28	30	32	34	36	38
44	13	7	15	8	17	9	19
45	26	28	2	32	34	4	38
46	13	14	15	16	17	18	19
47	26	28	30	32	34	36	38
48	13	7	5	2	17	3	19
49	26	4	30	32	34	36	38
50	13	14	3	16	17	18	19
51	26	28	10	32	2	12	38
52	1	7	15	8	17	9	19

Work out your own numbers: Skid patch calculator

Used by

• Skid patch calculatorHow many places on the rear tire a fixed gear skid wears.

Sources

• Sheldon Brown: Fixed gear bicycles, skid patches
• BikeCalc: skid patch math