This page explains exactly how the calculator works, so you can judge how far to trust it. The short version: the geometry is sound, the comfort ranges are reasonable starting points, and the biggest uncertainty is where each bike’s handlebars and footpegs are. Every number on the site shows its source.
1. Your body
We build a 2D side-view model of a rider from body-segment ratios (Drillis & Contini): thigh ≈ 24.5% of height, shin ≈ 24.6%, upper arm ≈ 18.6%, forearm-to-grip ≈ 20%. If you enter your inseam, arm length or seated shoulder height, we scale the matching segments to your real measurements.
2. The motorcycle
For each bike we store the seat reference point (the lowest point of the seat), the handlebar-grip centre and the footpeg centre, plus wheelbase, rake, trail, weight and tyre sizes.
- Specifications (seat height, wheelbase, weight…) are transcribed from manufacturer publications.
- Grip and peg positions are not published by any manufacturer. Most bikes currently use a template for their category with per-model adjustments, labelled Estimated. We replace these with photo-traced and owner-measured values over time.
3. Riding posture
Your hip sits on the seat, the ball of your foot on the peg, and your hands on the grips. We solve the knee position from thigh and shin length, then find the most upright back angle at which your arms reach the grips with a natural elbow bend. From that pose we read knee, hip, elbow and back-lean angles.
4. Comfort ranges
Ranges are informed by seated-comfort research (Porter & Gyi, 1998) and published motorcycle posture studies, adjusted per riding style. They are reference ranges for comparison, not medical limits.
| Style | Knee | Hip | Elbow | Back lean |
|---|---|---|---|---|
| Standard / naked | 66–100° | 88–118° | 118–165° | −3–22° |
| Sport | 52–85° | 65–98° | 118–165° | 18–45° |
| Touring | 66–105° | 92–122° | 120–168° | −5–18° |
| Cruiser | 72–120° | 85–130° | 125–176° | −12–10° |
| Adventure | 66–105° | 92–122° | 118–165° | −6–16° |
The "relaxed" preference shifts knee and hip ranges 6° more open; "sporty" shifts them 6° tighter.
5. Ground reach
We compare the leg length a bike needs with your inseam plus boot sole. The leg length needed is the seat height with rider sag, plus a seat-width allowance, because your legs wrap around the seat’s front edge. Where BMW publishes its inner leg curve, we use that instead of a width estimate. Laden seat heights (as published by Harley-Davidson) are used directly.
6. Fit score
The score starts at 100. Angles near the edge of their range cost a little; angles outside it cost more per degree. Arms that can’t reach the grips, and every millimetre short of flat feet, also cost points; the ground-reach penalty grows with the bike’s weight. 85+ is a great fit, 70–84 good, 55–69 compromised, below 55 poor.
7. Data confidence
| Label | Meaning |
|---|---|
| Verified | Two or more independent physical measurements agree within 10 mm |
| Measured | One physical measurement with evidence |
| Manufacturer | Transcribed from published specifications |
| Photo-traced | Traced from a true side-view photo, scaled by the wheelbase (≈ ±15 mm) |
| Estimated | From a category template or sibling model; compare bikes, don’t rely on small differences |
Limitations
- The model is two-dimensional: it doesn’t yet account for bar width or tank width at the knees.
- Seat shape and foam softness vary; we treat the seat as a point.
- Comfort is personal. Flexibility, injuries and riding experience matter.
Found a mistake? Send a correction or a measurement.
References
- Drillis, R. & Contini, R. (1966). Body segment parameters. New York University.
- Porter, J. M. & Gyi, D. E. (1998). Exploring the optimum posture for driver comfort. International Journal of Vehicle Design, 19(3), 255–266.
- BMW Motorrad: seat height and inner leg curve.