# Bicycle Gearing & Fit Calc

Gear ratios and speed, cadence, gear tables, chain length, wheel circumference, climb grades, saddle height and frame size.

## How to pay

Machine-payable over x402. USDC on: Base (eip155:8453), Solana (solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp), Polygon (eip155:137), Arbitrum One (eip155:42161).
No accounts. No API keys. No signup. The paying wallet is the account.

## Machine-readable

- OpenAPI: https://bike-calc.x402tookit.com/openapi.json
- x402 manifest: https://bike-calc.x402tookit.com/.well-known/x402
- Catalog (free): https://bike-calc.x402tookit.com/catalog
- Health (free): https://bike-calc.x402tookit.com/health
- Uptime (free): https://bike-calc.x402tookit.com/uptime

## Endpoints (8)

### GET /cadence-for-speed — $0.02
Cadence needed to hold a target speed in a given gear, and the chainring-to-cog ratio needed for a speed at a cadence.
Params: speedKmh?: number, speedMph?: number, chainring?: integer, cog?: integer, cadence?: number, wheel?: string, circumferenceMm?: number
Example: https://bike-calc.x402tookit.com/cadence-for-speed?speedKmh=35&chainring=50&cog=17

### GET /chain-length — $0.02
Chain length in links, rounded up to even, from chainstay length and chainring and cog teeth, with the exact figure.
Params: chainstayMm: number, chainring: integer, cog: integer, pitchInches?: number
Example: https://bike-calc.x402tookit.com/chain-length?chainstayMm=420&chainring=50&cog=28

### GET /climb-gradient — $0.02
Gradient percent and angle from rise and distance, plus climbing rate (VAM) and the time to climb at a given rate.
Params: riseMeters: number, distanceMeters: number, vam?: number, timeMinutes?: number
Example: https://bike-calc.x402tookit.com/climb-gradient?riseMeters=600&distanceMeters=8000&vam=1200

### GET /frame-size — $0.02
Rough bike frame size (seat tube cm and S/M/L) from inseam by bike type, with a height-based letter size cross-check.
Params: inseam: number, bikeType?: string, unit?: string, heightCm?: number
Example: https://bike-calc.x402tookit.com/frame-size?inseam=80&bikeType=road&heightCm=175

### GET /gear-ratio — $0.02
Gear ratio, gear inches, development per pedal turn and speed at a cadence for a chainring and cog on a given wheel.
Params: chainring: integer, cog: integer, wheel?: string, circumferenceMm?: number, diameterInches?: number, cadence?: number
Example: https://bike-calc.x402tookit.com/gear-ratio?chainring=50&cog=17&wheel=700x25c&cadence=90

### GET /gear-table — $0.02
Gear inches and speed at a cadence for every chainring and cog in a drivetrain, with the range, steps and duplicate gears.
Params: chainrings: string, cogs: string, wheel?: string, circumferenceMm?: number, cadence?: number
Example: https://bike-calc.x402tookit.com/gear-table?chainrings=50%2C34&cogs=11%2C12%2C13%2C14%2C15%2C17%2C19%2C21%2C24%2C28

### GET /saddle-height — $0.02
Starting saddle height from inseam by the LeMond (0.883) and Hamley (1.09) rules, plus the Holmes knee-angle check.
Params: inseam: number, unit?: string
Example: https://bike-calc.x402tookit.com/saddle-height?inseam=80&unit=cm

### GET /wheel-size — $0.02
Rolling circumference and diameter for common tyre sizes, or an estimate from rim bead diameter and tyre width in mm.
Params: tyre?: string, bsdMm?: number, tyreWidthMm?: number
Example: https://bike-calc.x402tookit.com/wheel-size?tyre=700x25c

## Terms

Bicycle arithmetic computed locally from the numbers given, using standard formulas and common fit rules of thumb, provided as is. Fit formulas (saddle height, frame size) are starting points only: a professional bike fit accounts for flexibility, injury and riding style. Wheel circumference is best measured by rolling the loaded wheel one turn. Not a safety inspection of a bicycle.