Does crank length actually matter for power?
Barely — and that's the most useful fact on this page. The best-known lab work (Martin & Spirduso, 2001) measured maximal cycling power across cranks from 145 to 195 mm and found the differences within about one percent for lengths anywhere near sensible. Your legs adapt their cadence to the lever they're given. So the real questions are the ones power can't answer: does the crank close your hip angle at the top of the stroke, does your knee complain, do your pedals clip curbs and crits, and can you get low over the bars without your thighs hitting your chest. Those all vote shorter when they vote at all — which is why the modern trend runs against the industry's old habit of putting nearly everyone on 170–175 regardless of leg length.
The formulas — and how much they disagree
For an 84 cm inseam, the Machine formula (1.25 × inseam + 65) says 170 mm; Zinn's proportional method (2.16 × inseam) says 181.4 mm — a length barely manufactured. Two respected formulas, eleven millimeters apart, for the same legs: that spread is the lesson. Crank formulas are opinions about trade-offs, not measurements of truth, and the power data above explains why nobody can settle it — there's no wattage penalty to settle it with. The calculator anchors on the Machine number (the moderate opinion), snaps it to a length you can actually buy, and offers the size below for riders chasing comfort or an aero tuck.
| Inseam | Machine | Zinn | Suggested | Comfort / aero |
|---|---|---|---|---|
| 29″ (74 cm) | 157.5 mm | 159.8 mm | 155 mm | 155 mm |
| 33″ (84 cm) | 170 mm | 181.4 mm | 170 mm | 167.5 mm |
| 36″ (92 cm) | 180 mm | 198.7 mm | 180 mm | 177.5 mm |
Computed by the calculator above; suggested lengths snap to manufactured sizes (155 / 160 / 165 / 167.5 / 170 / 172.5 / 175 / 177.5 / 180 mm).
First, your saddle: a longer crank reaches further down, so the saddle must drop by the length difference — our saddle height calculator prices crank length into the 109% method for exactly this reason, so re-run it after any change. Second, your gearing feel: shorter cranks are a slightly smaller lever, which most riders answer with one gear lower and ~5 rpm more cadence — the gear chart shows what that costs you, which is usually nothing.
When to actually change crank length
The real triggers, in descending order of confidence: hip or knee pain at the top of the pedal stroke (shorter opens the hip angle — the most common genuine win); a time-trial or triathlon position you can't hold because your thighs crowd your torso (shorter, sometimes dramatically); pedal strikes on a fixed-gear, crit bike or technical trails (shorter buys ground clearance); and being at either end of the height spectrum on stock 170s — a 5-foot rider on 170s is proportionally riding what a 6-footer would experience on 200 mm cranks, which nobody would call normal. "My formulas disagree by a centimeter" is not a trigger; ride what came on the bike until one of the real ones shows up.
The industry's default spread — 165 to 175 mm across a size run that spans 30 cm of rider height — exists because cranks are expensive to make in many lengths and most riders adapt without noticing. It survived because the power cost of a wrong-ish crank is roughly zero. That's also the liberating conclusion: if your cranks feel fine, they are fine, and this is one of the few components where the calculator's advice is usually don't buy anything.