Crank arm length isn’t something most riders think about—until their knees start to ache or their pedaling feels inefficient. Yet this single number, usually between 165mm and 180mm, profoundly affects how your bike fits, how much power you can produce, and how comfortable you feel after hours in the saddle. Choosing the correct crank arm length can improve pedal efficiency, reduce injury risk, and help you enjoy every ride—whether you're climbing mountains, commuting daily, or cruising on an electrical bike.
What Is Crank Arm Length
Crank arm length is the distance from the center of the bottom bracket spindle to the center of the pedal axle. It’s the radius of your pedaling circle. A longer arm means a larger circle, requiring more leg extension; a shorter arm means a smaller, quicker rotation.
This length directly affects biomechanics—hip and knee flexion angles, muscle recruitment patterns, and joint loading. It also influences bike geometry elements like saddle height and bottom bracket clearance. The standard range on most adult bikes is 165mm to 175mm, but there is no “one-size-fits-all.”
Crank Arm vs. Crankset
A quick clarification: a crank arm refers to one of the two arms, left or right. A crankset includes both arms, the spindle (sometimes integrated), and the chainrings. If you're replacing just the crank arm due to damage (like stripped pedal threads), you'll need the correct side and compatible length. If you're changing drivetrain components or gearing, you'll likely need to replace the full crankset.
SEE ALSO What Is a Crank Arm and Why It Matters on Your Bike
Why Length Matters
Crank length affects your knee and hip joint angles, especially at the top and bottom of the pedal stroke. Longer cranks demand deeper flexion at the top of the stroke, which may be problematic for riders with limited mobility or joint pain. Shorter cranks minimize this, offering smoother motion and faster cadence.
There’s also the matter of mechanical leverage. Longer crank arms provide more torque with the same muscle force, aiding climbs and starts—but this comes at a cost to cadence and joint range. Shorter arms offer less torque but faster, more fluid rotation, which benefits endurance and high-cadence riding.
How to Measure Your Existing Crank Arm
Most crank arms are labeled with their length, printed or engraved near the pedal hole on the inside of the arm. If not, use a ruler or tape to measure from the center of the pedal axle to the center of the bottom bracket spindle. Be precise—differences as small as 2.5mm can matter.
Crank Length and Saddle Height
Changing crank length alters your effective leg extension. If you install shorter crank arms, you may need to raise your saddle slightly to maintain full extension at the bottom of the stroke. With longer arms, you’ll often need to lower the saddle to avoid overextension.
Failing to adjust saddle height with crank length changes can lead to poor pedaling efficiency, overuse injuries, and discomfort in knees, hips, or lower back.
General Crank Length Recommendations
Here’s a practical starting point based on inseam length:
| Inseam (cm) | Suggested Crank Length |
|---|---|
| 71 cm | 160mm or less |
| 71–76 cm | 165mm |
| 76–81 cm | 170mm |
| 81–86 cm | 172.5mm |
| 86 cm | 175mm or more |
Note that riding style, flexibility, and bike type may influence what ultimately feels right.
Use Cases by Rider Type
Road Racers: Often choose 170–172.5mm cranks for balance between cadence and leverage.
MTB Riders: Use 165–170mm cranks to avoid pedal strikes and keep cadence high on technical terrain.
Track & Fixed Gear Riders: Favor shorter cranks (165–170mm) for better cornering clearance and high-cadence control.
Triathletes: Commonly use shorter cranks to open up hip angle for better aerodynamics and lower joint strain.
Commuters: Prefer moderate lengths (170–175mm) for all-around comfort and versatility.
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Crank Arm Length on Electric Bikes
Electric bikes present a unique set of demands. Many mid-drive systems like Bosch or Bafang apply force directly through the crank, which means crank arm durability and cadence become even more critical.
Most eBikes use slightly shorter cranks (160–170mm) to:
Reduce pedal strike risk, especially with low-mounted motors or batteries
Support high RPM pedaling, which improves motor efficiency
Minimize joint stress under torque-assisted loads
Taller riders or those riding heavy-duty cargo eBikes may still prefer 172.5mm or 175mm cranks for added leverage—if ground clearance allows. Always check system compatibility and spacing when choosing crank arms for eBikes.
Performance Impact and Power Output
Studies show that crank length has minimal impact on total power output when cadence and saddle height are optimized. However, the feel of your pedaling—how natural or smooth it is—can vary dramatically.
For performance athletes, the real gains from crank optimization come from:
Improved pedal stroke efficiency
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More consistent cadence
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Reduced joint stress over long efforts
For recreational riders, it’s more about comfort and injury prevention than marginal watts.
Mistakes to Avoid
Assuming longer is better: More leverage ≠ more comfort. Long cranks can cause knee pain if mismatched.
Ignoring frame fit: Shorter cranks may improve toe clearance and aerodynamics, especially on compact frames.
Not adjusting saddle height: A 5mm crank change without saddle compensation can throw off your fit.
Overcomplicating upgrades: Small adjustments make a big difference. Start with 2.5mm increments if testing.
Conclusion
Crank arm length is more than a frame spec—it’s a performance and comfort dial you can tune. Whether you’re aiming for smoother cadence, better climbing, or reduced knee strain, finding your ideal length pays off in every ride.
From 160mm for high-speed commuters to 175mm for power-loving climbers, the right crank length balances biomechanics, terrain, and style. For eBike riders, it also ensures optimal motor efficiency and joint protection. So next time you review your bike’s fit, don’t overlook the cranks—they’re the levers that move it all.
FAQs
Can crank arm length affect saddle height?
Absolutely. Changing crank length alters your leg extension. Shorter cranks often require a higher saddle and longer cranks a lower one.
Why are shorter crank arms common on electric bikes?
eBikes often use 160–170mm cranks to avoid pedal strikes, improve cadence efficiency, and reduce joint load under motor-assisted torque.
What happens if I choose the wrong crank length?
Using a crank arm that's too long or too short can lead to knee pain, inefficient pedaling, poor cadence, or frame clearance issues.
