bike-fit-estimator

Use when buying a bike, setting one up, or diagnosing ride pain — computes frame size and stack/reach from your body (inseam×0.885 saddle height, Holmes 25–35° knee angle, crank length, standover clearance by discipline), dials position by flexibility and ride type, and routes pain symptoms (numb hands, front/back knee, hot foot, saddle numbness, neck) to ranked fit causes with the exact adjustment to try first.

A bad bike fit turns a healthy activity into a pain generator: knees, hands, neck, perineum. Professional fits cost $150–400; a lot of what they do is geometry you can compute from two measurements (inseam + height) and a symptom table you can follow.

This skill sizes the frame, sets the contact points (saddle height, setback, bar drop/reach, cleat fore-aft), and routes ride pain to its most likely fit cause with the adjustment to try first.

When to Use

• Buying a bike (online or used) and decoding size charts
• Setting saddle height/setback on a new or inherited bike
• Diagnosing pain that appears during or after rides
• Adjusting for flexibility changes, aging, or switching ride type
• Deciding crank length and checking standover clearance

Don't use for: post-crash pain, pain at rest/night, or any neurological symptoms (numbness that persists off the bike) — medical.

The Method

1. Measure once, properly. Barefoot, feet ~10 cm apart, hardback book pulled into the crotch, measure floor→book top. That's inseam. Height without shoes.
2. size — frame + contact points: saddle height (BB-to-top = inseam × 0.885, the LeMond method; cross-check Holmes 25–35° knee angle), setback (KOPS starting point), crank length, standover check, estimated stack/reach class.
3. position — bar drop and reach by flexibility × ride type: endurance/commute (comfort-first) → sportive → aggressive. Includes saddle tilt and transition protocol (change ONE thing, 2–3 rides).
4. pain — symptom → ranked causes → first fix. Change one variable at a time, 2–5 mm increments; log rides.

Quick Start

bash

# Frame size + contact points: 178 cm, 84 cm inseam, road bike
python3 scripts/bike_fit.py size --inseam 84 --height 178 --type road

# Position by flexibility
python3 scripts/bike_fit.py position --height 178 --flexibility medium --type road

# Diagnose: hands go numb after 45 minutes
python3 scripts/bike_fit.py pain --symptom numb-hands

# Check a listing: 56cm frame, your 84cm inseam
python3 scripts/bike_fit.py standover --inseam 84 --standover-height 80 --type road

Stdlib-only Python 3. Offline.

Key Numbers (what the script computes)

• Saddle height (crank axle to saddle top along seat tube): inseam × 0.885. Verify: at bottom-dead-center, knee angle 25–35° (Holmes).
• Setback: knee-over-pedal-spindle as STARTING point, then adjust by hamstring flexibility; saddle nose behind BB axis 0–3 cm typical.
• Crank length: ~ inseam × 2.0 → mm (168 cm → 170 mm); shorter cranks (165) for tight hips/close fit or aero positions.
• Drop (saddle-to-bar): 0–2 cm low flexibility / endurance, 2–5 medium, 5–8+ high flexibility / race. Reach: bars at natural elbow-to-hand span.
• Standover clearance: road ≥ 2 cm, gravel 3–5, MTB 5+.

Pain Routing Table (summary)

Symptom	Most likely (try first)	Then
Front of knee	saddle too LOW or too far forward	+2–3 mm height
Back of knee	saddle too HIGH or too far back	−2–3 mm height
Outside of knee	cleat rotation/angle	float check
Numb hands	weight too far forward: bars low/reach long, saddle nose down	raise bars, shorten reach, level saddle
Numb/lady-parts	saddle width/shape, too much drop	level saddle, wider/cutout saddle, less drop
Hot foot	cleat too far forward, shoes tight	move cleat back 2–4 mm
Neck/traps	drop too deep for flexibility	raise stem, shorter reach
Lower back	reach long, drop deep, saddle nose down	shorten reach, level saddle
Hip rocking	saddle too high	lower 2–3 mm until rock stops

Common Pitfalls

1. Changing three things at once. You fix the pain but learn nothing — and can't revert. One variable, 2–3 rides, log it.
2. Saddle height from the floor or from old muscle memory. Only BB-axis→saddle-top is a stable reference; it survives frame changes.
3. Level the bike wrong. Set heights with the bike on a trainer or measured against a wall on level ground, tires equal pressure.
4. Fixing knee pain with a bigger gear / cadence change first. Geometry first: 3 mm of saddle height fixes what a season of "spinning easier" won't.
5. Buying by top-tube size alone. Modern geometry varies: compare stack and reach, not "56 cm" — a 56 from two brands can differ 2 cm in both.
6. Copying a pro's position. Their flexibility and weekly hours aren't yours. Drop is earned, not chosen.
7. Ignoring cleats. Feet are the fixed point the whole chain rotates around; cleat rotation causes "mystery" lateral knee pain no saddle change touches.

Verification Checklist

• Inseam measured with book method, barefoot (±5 mm matters)
• Saddle height set and marked (tape on seatpost)
• Knee-angle check at BDC: 25–35°
• One change at a time, ≥2 rides between changes
• Pain persisting after 3–4 fit iterations → professional fit / medical

References

• references/fit-science.md — the geometry, angles, formulas, flexibility tests, cleat science, and the full pain table with reasoning
• references/used-bike-buying.md — decoding listings, stack/reach comparison, contact-point refresh budget (bars/saddle/tape are wear items)