Gear · chain · magic
Gear calculator
Pick a gear. Live with it. Gear inches, gain ratio, skid patches, chain length, and every cog that runs a magic gear on your frame. Every setup is a link you can paste into a forum argument.
The setup
compare up to threeSetup A
Length of the crank arm, from the centre of the bottom bracket to the pedal. It’s usually stamped on the inside of the arm — 170 or 175 mm on most mountain bikes.
Distance from the centre of the bottom bracket (where the cranks spin) to the centre of the rear axle. It’s on your frame’s geometry chart. On sliding dropouts, measure it.
Gear inchesOne number for how hard a gear is: wheel diameter × ratio. It’s the size of penny-farthing wheel you’d be pedalling. Bigger = harder to push, faster on the flat. Mid-40s is the singlespeed mountain bike sweet spot.—
- RatioChainring teeth ÷ cog teeth. How many times the rear wheel turns for every turn of the cranks.
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- DevelopmentHow far the bike rolls for one full turn of the cranks. Same idea as gear inches, measured as actual distance — gear inches × π. The metres figure is how most of Europe says it.
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- Gain ratioSheldon Brown’s better number: how far the bike moves for each bit your foot moves. Unlike gear inches it counts crank length — longer cranks make the same gear feel easier. Lower = easier.
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- Skid patchesOnly matters on a fixed gear. It’s how many different spots on the tire end up under a skid — fewer means one spot wears flat fast. “Ambi” is for riders who skid with either foot forward.
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- ChainLinks needed to wrap both sprockets at your chainstay length. Chains only come in whole ½″ links, so the decimals show how far you are from a clean fit.
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Empty
Give us a chainring and a cog. We’ll do the arguing.
Magic gear finder
A chainring and cog that run a standard chain dead taut on fixed dropouts — no tensioner, sliders or eccentric bottom bracket. It only works when a whole, even number of links lands within ±0.5 mm of your chainstay.no tensioner, no sliders, no excusesGive setup A a chainring and a chainstay length and we’ll find every cog from 14T to 24T that runs a standard chain with no tensioner.