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- A goniometer measures joint angles — the key ones are knee extension (140–150° at bottom dead centre), hip angle (40–50° at top of stroke), and shoulder-to-elbow (90–110°). Get these right and 80% of fit problems disappear
- A plumb line drops from the tibial tuberosity (bump below the kneecap) to the pedal axle at 3 o'clock — Knee Over Pedal Spindle (KOPS). Fore-aft saddle position tunes this and affects both power and knee strain
- Cleat wedges (varus/valgus, 3° per wedge) and leg-length shims (1–3 mm) correct the foot-knee alignment. Knee tracking in a straight line is the difference between a season injury-free and IT band syndrome by May
- DIY fit using a goniometer and a friend can get you within 5 mm of a pro fit; a professional fit ($200–400, 2–3 hours) earns its cost when you have a specific pain (back, knee, neck, numb hands) that won't resolve with basic adjustments
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The four essential fit tools
A bike fit reduces to measurements and adjustments. The measurements are angles (knee bend, hip angle, shoulder) and distances (saddle height, saddle fore-aft, reach). The adjustments are saddle height, saddle fore-aft, stem length, cleat position, and shims/wedges under the cleat.
The fitter's toolkit is small. A goniometer measures joint angles in real time. A plumb line confirms the saddle's fore-aft position relative to the bottom bracket. Cleat wedges correct foot and knee tracking. And a level plus a tape measure handle the basic saddle height and reach numbers. None of these tools costs more than a single tyre.
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What each tool measures
The angles, distances, and corrections a fit targets. Numbers shown are common starting points, not absolutes.
| Tool | Measures | Target |
|---|---|---|
| Goniometer | Knee extension at BDC | 140–150° |
| Goniometer | Hip angle at top of stroke | 40–50° |
| Goniometer | Shoulder-to-elbow angle | 90–110° |
| Plumb line | Tibial tuberosity over pedal axle (KOPS) | 3 o'clock position |
| Tape + level | Saddle height (BB to saddle top) | Inseam × 0.883 (LeMond) |
| Cleat wedges | Foot varus/valgus correction | 1–3 wedges × 3° |
| Leg-length shim | Structural leg-length difference | 1–8 mm in 1 mm steps |
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The goniometer: where fit lives
A goniometer is a 360° protractor with two arms. You align one arm with the long bone above the joint and the other with the bone below, and read the angle off the dial. The three angles that matter are knee extension (measured at the bottom of the pedal stroke, between hip-knee-ankle), hip angle (between torso and thigh at the top of the stroke), and shoulder-to-elbow (between upper arm and torso).
Knee extension is the most important and the most diagnostic. At bottom dead centre (3 o'clock for the foot), the knee should bend between 140° and 150° (180° is fully straight). Below 140° and the saddle is too low — knee pain at the front, loss of power. Above 150° and the saddle is too high — hamstring strain, hip rocking on the saddle, loss of smoothness. The sweet spot, 145°, balances power and injury risk.
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Plumb line and KOPS
Knee Over Pedal Spindle is the rule that decides saddle fore-aft. Drop a plumb line from the tibial tuberosity (the bony bump just below the kneecap) with the crank at 3 o'clock. The line should pass through the pedal axle. If it falls in front of the axle, slide the saddle back; behind the axle, slide it forward.
Fore-aft matters because it shifts the knee's lever arm over the pedal. Knee too far forward (saddle forward) loads the patella; too far back (saddle back) reduces sprint power and stretches the IT band. Triathletes often move 1–2 cm behind KOPS to open the hip angle for the run; sprinters ride 1 cm in front for snap. For general road riding, start at KOPS and adjust 2 mm at a time.
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Wedges and shims: fixing foot and leg
Most riders' feet pronate or supinate slightly — the knee tracks in or out instead of straight. Two wedges under the cleat, each angled 3°, correct this so the knee pedals in a straight line. The test: film yourself from the front, watch the knee. If it dives inboard on the downstroke, add a varus wedge (thick side inward). If it tracks outboard, a valgus wedge (thick side outward).
True leg-length differences (structural, not functional from a tilted pelvis) are shimmed. Add shims in 1 mm increments under the short-leg cleat, up to about 8 mm total. Beyond that you need a crank-length correction, not a shim — and that's a job for a fitter with a motion-capture system.
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DIY fit vs professional fit
DIY fit covers saddle height, fore-aft, and stem length with a goniometer, a plumb line, a tape measure, and a friend with a phone camera. You will get within 5 mm of where a pro would put you, and for many riders that's enough. The cost: a couple of evenings and the price of the goniometer (~$40).
Pay for a pro fit when: you have a specific pain that hasn't resolved with two weeks of small adjustments; you are buying a custom or expensive frame and want exact numbers; you are new to clipless pedals and have knee tracking issues; or you race at a level where a 2% power gain matters. A motion-capture fit (Retül, Shimano Dynamics, Specialized Body Geometry) costs $200–400, takes 2–3 hours, and delivers a report with every angle, distance, and wedge recommendation.
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