Bicycle Crank Length & Pedal Motion Calculator
Geometry and mechanics comparison
Compare two crank lengths
Result
Calculation summary
Enter values to see the result
Live pedal-path overlay
Shorter cranks reduce the pedal circle
Crank change
-7.5
Top clearance change
+15
Tangential force change
+7 N
The same cadence and crank power are held constant so radius effects remain visible.
How to use this calculator
- 1Enter current and candidate crank lengths.
- 2Enter a representative cadence, power and current saddle-height reference.
- 3Optionally add leg and tibia measurements to display published maximal-power ratios without treating them as a fit prescription.
Formula
pedal speed = 2π × crank length × cadence; tangential force = power ÷ angular velocity ÷ crank length
The comparison holds cadence and crank power constant while changing crank radius.
Calculation steps
- Calculate pedal-circle circumference and path speed.
- Resolve crank torque from power and cadence, then force from torque and crank radius.
- Compare saddle, top-of-stroke and low-pedal reference changes.
Worked example
Changing from 172.5 mm to 165 mm shortens the crank by 7.5 mm, reduces the pedal circle and raises the low pedal point by the same amount.
Assumptions
- Power is measured at the crank and cadence is constant.
- Tangential force is a cycle-average mechanical comparison, not a biomechanical force profile.
- The saddle-height change maintains the bottom-pedal extension reference only.
- Leg and tibia ratios describe one maximal-power study and do not prescribe bike fit.
Sources
Frequently asked questions
Does a shorter crank reduce power?
Not automatically. Within common ranges, riders can adapt cadence and force; this tool only compares geometry and average mechanics.
Why does tangential force increase with a shorter crank?
At the same torque, a shorter lever requires more tangential force because force equals torque divided by radius.
Should I always raise the saddle after shortening cranks?
The displayed change preserves the bottom-of-stroke reference, but an actual fit may choose a different adjustment.
Is 20% of leg length correct for everyone?
No. It was a maximal-power finding from a specific study, not a universal comfort, injury or fit rule.