Methodology & limitations
Understand what is calculated, what is entered, and what remains unknown.
Calculation rules
Use frame stack, frame reach, and head angle for the exact model year and size. These locate the top of the head tube relative to the bottom bracket.
Total steerer rise is measured along the steerer to the stem-axis center. Include the headset cover, spacers below the stem, and the stem clamp-center offset. For a symmetric clamp, the last term is half the clamp height. Do not add spacers above the stem.
The stem then locates the handlebar-clamp center. Compare that center on both bikes, holding bars, bar rotation, hood position, and saddle position constant. This is not a hand-position or complete bike-fit model.
The inverse solution holds candidate steerer rise fixed and solves continuous stem length and angle. Trial options sample 60–140 mm stems in 10 mm steps at −17°, −10°, −6°, 0°, +6°, +10°, and +17°. They are mathematical examples, not a verified parts catalog or installation advice.
Limits that matter
A cockpit calculation cannot guarantee fit, comfort, safe component installation, or handling. Validate the setup with a qualified fitter or mechanic.
Examples, imports, and source evidence
All example setups are synthetic and do not represent verified bicycles. Restore JSON imports a saved comparison locally. Source links explain terminology; they do not verify entered measurements. Use the manufacturer chart for the exact model, year, and size. The calculator does not infer missing specifications.
Reproducible results
Export a JSON plan to preserve the measurements, method version, coordinates, and limitations. Coordinate CSV exports contain unrounded numerical results, while the interface rounds to one decimal place. This is display precision, not measurement accuracy. Saved plans remain on this browser until you delete them. Shared links store measurements in the URL fragment, not on a server.
Corrections and commercial independence
Report calculation issues through the linked source repository using an anonymized reproducible example. Do not include private addresses, serial numbers, personnel records, financial statements, or license-restricted data in a public issue. Advertising cannot change calculations, compatibility warnings, organic order, or source interpretation.