Crank Length
When looking at crank arm length you would think this would be an easy topic to discuss. However, for years the crank arm length has been the great debate. Some athletes believe in longer cranks while other believe in shorter crank. In the past athletes embraced the idea that longer cranks are good for pushing large gears and climbing at lower rpms, while shorter cranks are good for low gears at high rpm pedaling. The bottom line is the crank arm length determines the size of the pedal circle and the efficiency of the pedal stroke. Crank arm length affects the athletes knee, hip, and ankle flexion. A crank arm that is too long will cause a decrease in your cadence and increase tension on your knee joint. Having the right crank arm length will increase your power, pedaling rate, and have an effect on your efficiency.
The pro’s to a longer crank:
- produce more torque
Con’s of a longer crank:
- decreased cadence
- Increased tension on your knee joint
- Dead spot at 9 to 12 o’ clock position
The pro’s to a shorter crank length:
- Increased cadence (up to the rider)
- improves cornering
- Eliminate dead spot at 9 to 12 o’ clock position
- Decreased acute hip angle
Still thinking of what length to use. Here are a few crank calculators still being used today:
Kirby Palm formula: L(mm) = 5.48 x I(in)
Now, multiply the inseam measurement (in inches) by 5.48. This provides a good estimate of proper crank length, in millimeters, for general road cycling or racing.
Crank Calc -Bicycle Crank Length Calculator
Crank length in mm = (1.25 x Indise Leg in cm ) = 65
Most professional bike fitters agree that if you follow the chart below you will have both a crank length that creates efficiency and increased cadence.
| Crank Length | Height | Inseam | |
|---|---|---|---|
| 160mm | 60 inches | ||
| 165 | 60-65 inches | <29 inches | |
| 170 | 65-70 inches | 29-32 inches | |
| 172.5 | 65-70 inches | 29-32 inches | |
| 175 | 72-74 inches | >34 inches | |
| 180 | 74+ inches | ||
Note Shorter Cranks
Now if you want to take this chart to the next level then decrease it by 10mm.
| Crank Length | Height | Inseam | |
|---|---|---|---|
| 150mm | 60 inches | ||
| 155 | 60-65 inches | <29 inches | |
| 160 | 65-70 inches | 29-32 inches | |
| 165 | 65-70 inches | 29-32 inches | |
| 167.5 | 72-74 inches | >34 inches | |
| 170 | 74+ inches | ||
Having a longer crank would result in a lower cadence that would be less efficient and therefore would require greater force to be applied to the pedals. The diagram below shows long arrows which displays the pedal forces. You can see from the diagram that from the 9 o clock to the 12 o clock position that that there are very little pedal forces. Some researchers believe this could be due to crank arm length. Having the optimal crank arm length and working on pedaling technique will help a rider become more efficient with their pedal stroke and increase their power.
Explore Crank Length in Depth
Gain a complete understanding of how crank length and cleat placement impact biomechanics and performance in Bike Fit Book, 2nd Edition: Creating Optimal Movement — the definitive guide for professional bike fitters and serious cyclists.