# BikeFitDelta

> BikeFitDelta compares two bicycle frame-and-cockpit geometries in millimeters. It is a local geometry aid, not a professional fit, medical, handling, or safety assessment.

## Entry points and public routes
- Tool: / - the calculator; live recalculation, no hosted API
- Contract: /tool-spec.json - input schema, field selectors, executable example
- Calculation method: /methodology/
- Full text (methodology + FAQ) for quoting: /llms-full.txt
- Pure source module: /assets/domain.mjs (compute(state), analyze, and config)
- /methodology/
- /privacy/
- /about/
- /contact/
- /guides/how-it-works/
- /guides/verification-checklist/
- /guides/stem-angle-and-spacers/

## Long-tail worked examples
- Comparison library: /compare/ (30 precomputed stem/spacer worked examples)
- Stem length change: /compare/stem-length/
- Stem angle change: /compare/stem-angle/
- Steerer rise / spacer height change: /compare/spacer-height/
- Machine-readable data: /data/compare.json (asOf 2026-09-12, every row's deltaX/deltaY/clamp offset)

## Browser operation
Wait for html[data-app-ready="true"], then use the 12 labeled numeric inputs with the stable field-* IDs listed in the contract. Inputs calculate on input events; read #input-error and aria-invalid before accepting results. Invalid drafts clear stale numeric output. [data-trial] changes only the candidate stem length and angle. There is no hidden agent-only control or hosted compute endpoint.

## Prefilled links
Every page under /compare/ links into the tool with a stable deep-link format: /#plan=<url-encoded JSON {version:1,fields,demo}>. Parsing this fragment on load prefills the 12 fields; it never submits to a server.

## Units, method, and limitations
All lengths are millimeters and angles are degrees. TOTAL rise along the steerer: refSpacers and newSpacers are the total rise from frame top to stem-axis center, including cover, below-stem spacers, and clamp-center offset. Positive deltaX is farther forward; positive deltaY is higher. The model transforms measured points and reports coordinate differences; it does not infer compatibility from a small offset, clamp stem limits, cable routing, saddle/hood position, handling, or a comfort score. All examples are synthetic.

Reference material explains terminology but does not verify a bicycle or entered measurement:
- Canyon: frame stack, reach, and effective fit: https://www.canyon.com/en-us/blog-content/road-cycling-news/road-bike-geometry-explained/b19102022.html
- Bike Insights: stack and reach definitions: https://bikeinsights.com/cyclopedia/stack-and-reach
- Trek bicycle sizing and fit guidance: https://www.trekbikes.com/us/en_US/size-finder/

## Persistence and sharing
Only an explicit Save action writes browser storage. JSON export preserves fields, tables, method version, and limitations. Share links use a #plan= fragment and contain measurements; they are not cloud-synced. Review exports before sharing and never claim a low offset guarantees a safe fit or available component.