Bike Fit Estimator
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
- Measure once, properly. Barefoot, feet ~10 cm apart, hardback book pulled into the crotch, measure floor→book top. That's inseam. Height without shoes.
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.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).pain— symptom → ranked causes → first fix. Change one variable at a time, 2–5 mm increments; log rides.
Quick Start
# 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
- Changing three things at once. You fix the pain but learn nothing — and can't revert. One variable, 2–3 rides, log it.
- Saddle height from the floor or from old muscle memory. Only BB-axis→saddle-top is a stable reference; it survives frame changes.
- Level the bike wrong. Set heights with the bike on a trainer or measured against a wall on level ground, tires equal pressure.
- 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.
- 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.
- Copying a pro's position. Their flexibility and weekly hours aren't yours. Drop is earned, not chosen.
- 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 reasoningreferences/used-bike-buying.md— decoding listings, stack/reach comparison, contact-point refresh budget (bars/saddle/tape are wear items)