Cycling Crank Length: 170mm vs. 175mm, and How to Choose

Which is better, a 170mm or a 175mm crank?

There is no universally "better" crank length; the optimal choice between a 170mm and a 175mm crank depends on individual biomechanics, cycling discipline, riding style, and specific goals, with each length offering distinct advantages and disadvantages related to leverage, cadence, and joint angles.

Understanding Crank Length in Cycling

The crank arm is the component of a bicycle's drivetrain that connects the pedal to the bottom bracket, transferring power from the rider's legs to the chainring and ultimately the wheel. Crank length refers to the distance from the center of the bottom bracket spindle to the center of the pedal spindle. While seemingly a small difference, just 5mm can significantly impact pedaling mechanics, efficiency, comfort, and injury risk. This seemingly minor variation in length can influence how your body interacts with the bicycle, affecting everything from power output to joint health.

Biomechanics of Crank Length

The choice of crank length directly influences several key biomechanical factors:

• Leverage and Torque: A longer crank arm (e.g., 175mm) provides greater leverage. This means that for a given amount of force applied to the pedal, a longer crank generates more torque (rotational force) at the bottom bracket. This can make it feel easier to push bigger gears or climb steep inclines at a lower cadence. Conversely, a shorter crank (e.g., 170mm) offers less leverage, requiring more muscular force to generate the same torque.
• Cadence and Power Output: Power in cycling is the product of torque and angular velocity (cadence). A shorter crank allows for a smaller circle of rotation, which can facilitate a higher pedaling cadence (RPM) for a given joint range of motion. Riders who prefer a higher cadence often find shorter cranks more comfortable and efficient. Longer cranks, while offering more leverage, can make it harder to maintain a very high cadence due to the increased range of motion required at the hip and knee.
• Joint Angles and Comfort: This is perhaps the most critical consideration.
  • Knee Flexion/Extension: A longer crank requires greater knee flexion at the top of the pedal stroke and greater knee extension at the bottom. For some riders, particularly those with pre-existing knee issues, patellofemoral pain, or limited flexibility, this increased range of motion can lead to discomfort or injury. A shorter crank reduces these extreme angles, potentially alleviating knee stress.
  • Hip Flexion/Extension: Similarly, longer cranks demand more hip flexion at the top of the stroke. In aggressive aerodynamic positions (e.g., time trial or triathlon bikes), this can cause the hip angle to become excessively closed, leading to discomfort, impingement, or reduced power output. Shorter cranks "open up" the hip angle in these positions, improving comfort and potentially aero efficiency.
  • Lower Back: Changes in knee and hip angles can also influence lower back posture and comfort, especially over long rides.

Factors Influencing Crank Length Choice

The optimal crank length is highly individualized. Consider the following:

• Rider Leg Length and Height: While there's a general correlation (taller riders often use longer cranks, shorter riders shorter cranks), it's not a strict rule. Inseam length and individual limb proportions are more relevant than overall height. However, it is a common starting point for initial selection.
• Cycling Discipline:
  • Road Cycling: Many road cyclists gravitate towards 170mm or 172.5mm cranks, favoring higher cadences for endurance and varied terrain.
  • Mountain Biking (MTB): Cranks from 170mm to 175mm are common. Longer cranks can provide more leverage for technical climbing, but shorter cranks offer better ground clearance, reducing pedal strikes on obstacles.
  • Time Trial (TT) / Triathlon: Shorter cranks (e.g., 165mm to 170mm) are frequently chosen to help open up the hip angle when in an aggressive aerodynamic tuck, improving comfort, power production, and sustained aero position.
  • Track Cycling: Shorter cranks (e.g., 165mm) are favored for very high cadences and to prevent pedal strike on the steep banking of velodromes.
• Riding Style and Preferences: Are you a "spinner" who prefers a high cadence, or a "grinder" who pushes bigger gears at a lower cadence? Your natural preference can guide your crank length choice.
• Injury History and Biomechanical Considerations: Riders with a history of knee pain, hip impingement, or lower back issues may find relief with shorter cranks due to reduced joint excursion and stress. Flexibility and range of motion also play a role.

Pros and Cons of Shorter (170mm) Cranks

Choosing a 170mm crank can offer specific benefits for certain riders and disciplines:

• Pros:

  • Higher Cadence Potential: Easier to maintain a high pedal RPM, which can be more efficient for some riders and reduce muscular fatigue over long distances.
  • Reduced Joint Stress: Less extreme knee and hip flexion/extension can alleviate pressure on joints, potentially reducing the risk of patellofemoral pain or hip impingement.
  • Improved Aerodynamic Position: Allows for a more open hip angle in aggressive aero positions (TT/Tri), enhancing comfort and power delivery.
  • Better Ground Clearance: Particularly beneficial for mountain biking, reducing the likelihood of pedal strikes on rocks or roots.
  • Smoother Pedal Stroke: Can lead to a more fluid and less "jerky" pedaling motion for some individuals.

• Cons:

  • Less Leverage: Requires more muscular force to generate the same torque, potentially making it harder to push big gears or climb steep hills at low speeds.
  • May Feel "Spinny": Riders accustomed to longer cranks might initially feel like they're pedaling in circles without much resistance.
  • Reduced Peak Torque: For riders who rely on high torque at lower cadences, a shorter crank might feel less powerful.

Pros and Cons of Longer (175mm) Cranks

The 175mm crank remains a popular choice, especially for riders seeking maximum leverage:

• Pros:

  • Greater Leverage: Provides more mechanical advantage, making it easier to push bigger gears, accelerate quickly, or climb steep gradients at lower cadences.
  • Higher Peak Torque: Can feel more powerful for riders who prefer to generate force through lower cadence and higher torque.
  • Familiarity: Historically, 175mm has been a common standard for many bikes, especially larger frame sizes.

• Cons:

  • Increased Joint Excursion: Requires greater knee and hip flexion/extension, which can exacerbate existing joint issues or lead to discomfort for some, particularly at the extremes of the pedal stroke.
  • Closed Hip Angle in Aero Positions: Can make it challenging to maintain an aggressive aerodynamic tuck comfortably and powerfully.
  • Reduced Ground Clearance: More prone to pedal strikes in technical off-road riding or when cornering aggressively.
  • Harder to Spin High Cadences: The larger circle of rotation can make it more difficult to maintain very high pedal RPMs for extended periods.

Making the Optimal Choice

Given that there is no single "better" option, how do you determine what's right for you?

• Experimentation: If possible, try bikes or setups with different crank lengths. Even a 5mm difference can be noticeable. Many bike fit studios or demo days may offer opportunities to test.
• Professional Bike Fit: This is arguably the most effective approach. A qualified bike fitter will assess your unique biomechanics, flexibility, riding style, goals, and injury history to recommend the optimal crank length, along with other critical bike setup parameters. They can also observe your pedaling dynamics with different lengths.
• Listen to Your Body: Pay attention to how your knees, hips, and lower back feel during and after rides. Discomfort, pain, or a feeling of being "locked out" at the top or bottom of the pedal stroke are strong indicators that your current crank length might not be ideal.

Conclusion

The choice between a 170mm and a 175mm crank is a nuanced decision, not a matter of one being inherently superior. Both lengths serve different purposes and suit different riders. While 175mm cranks offer more leverage, potentially suiting riders who prefer lower cadences and high torque, 170mm cranks can facilitate higher cadences, reduce joint stress, and improve comfort in aerodynamic positions. Ultimately, the "better" crank length is the one that allows you to pedal comfortably, efficiently, powerfully, and without pain, aligning with your specific cycling discipline and personal biomechanics. A thorough understanding of these principles, combined with personal experimentation and professional guidance, will lead you to your optimal setup.