Road Bike Crank Arm Length Guide: The Science of 165mm to 175mm

Crank Arm Length: The Overlooked Key to Fitting

When shopping for road bike components, crank arm length is often overlooked. Many riders spend a large budget on frame size, saddle, and stem, yet stick with whatever crank length came stock on the frame, never considering how much this number affects their ride. In fact, crank arm length directly affects your pedaling efficiency, joint load, and riding comfort.

The Basics of Crank Length

Crank arm length refers to the distance from the center of the bottom bracket (BB) axle to the pedal axle. Common options include:

Note: The height reference in the table above is only a rough guideline. Actual crank length selection should be based primarily on leg length (inseam).

How Crank Length Affects Pedaling

Lever Principle

A longer crank provides more torque, theoretically producing more force with each pedal stroke. But at the same time:

• A larger circular pedaling range: your legs must perform a bigger circular motion
• A higher top dead center: greater knee flexion angle, increasing joint stress
• A lower bottom dead center: the hip joint needs a greater range of motion

Cadence vs. Power

• Short cranks (165mm and below): favor high-cadence riding, with less knee strain
• Long cranks (172.5mm and above): favor high power output, but with a lower cadence ceiling

What the Scientific Research Says

Recent trends in sports science research tend to support using shorter cranks:

1. Kody 2008 study: Testing across a 120-220mm range, found crank length affects maximum power output by only about 1-2%
2. Martin & Spirduso 2001: Found that at high cadence (>100rpm), shorter cranks are more efficient
3. Barratt 2012: Pointed out that shorter cranks reduce knee stress at top dead center, lowering injury risk

These studies suggest: most riders may be using cranks that are too long.

A Leg-Length-Based Selection Method

The most scientific approach is to use inseam length as the baseline:

Note for Taiwanese riders: The average height in Taiwan is about 172cm for men and 160cm for women, with corresponding leg lengths typically between 76-82cm and 70-76cm respectively. As a result, 170mm is a reasonable choice for most Taiwanese male riders, while female riders should seriously consider 165mm or even 160mm.

The Trend Toward Shorter Cranks

In recent years, a clear trend toward shorter cranks has emerged in professional cycling:

• Chris Froome: At 186cm tall, he has used 172.5mm cranks (a rider of this height would typically be expected to choose 175mm)
• Tadej Pogačar: At 176cm, uses 170mm
• Time trial trend: Many riders use even shorter cranks on TT bikes to lower their body position

The advantages of shorter cranks are gaining consensus in the bike fitting world:

• Reduced knee stress: a more open knee angle at top dead center
• Improved aerodynamic position: allows for a lower upper body position
• Improved pedaling efficiency: less time spent at dead center
• Reduced hip joint conflict: especially in an aggressive, low riding position

Things to Watch When Changing Crank Length

Saddle Height Needs Adjusting

After changing crank length, you must readjust your saddle height:

• Crank shortened by 5mm → saddle should be raised by about 2.5-5mm
• Crank lengthened by 5mm → saddle should be lowered by about 2.5-5mm

Adaptation Period

Switching from 172.5mm to 170mm typically requires 1-2 weeks of adaptation. At first it may feel like you “can’t generate power,” but once your muscles adapt, you’ll usually notice smoother pedaling.

Consider the Impact on Pedals

Don’t forget that pedal Q-factor (the distance between the two pedals) also affects how pedaling feels. When changing cranks, it’s a good habit to review your overall pedaling geometry at the same time.

Crank Length Options by Brand

Bike Fitting Resources in Taiwan

If you’re unsure which length suits you, it’s recommended to seek professional bike fitting help:

• Professional fitting studios: Both Taipei and Taichung have professional bike fitting services
• Retül / Guru systems: 3D dynamic fitting can precisely analyze your crank length needs
• Try before you buy: some shops offer test rides with different crank lengths

Practical Recommendations

1. Don’t blindly follow trends: shorter cranks are trending, but that doesn’t mean everyone needs to switch
2. Get a fitting first: investing in a professional fitting is worth more than blindly buying parts
3. Adjust gradually: it’s recommended to change only 2.5-5mm at a time, not jump straight from 175mm to 165mm
4. Track how it feels: after switching, record your riding sensations and power data to objectively compare results

Conclusion

Crank arm length is an often-overlooked but far-reaching factor in road bike fitting. Given the average height and leg length of Taiwanese riders, many may well be riding cranks that are too long. Taking the time to reassess your crank length could bring an unexpectedly noticeable improvement to your ride.

Related Reading

• The Science of Crank Length: How 165mm, 170mm, and 175mm Actually Affect Your Ride
• The Science of Choosing Crank Length: More Than Just a Leg-Length Question
• How Bicycle Crank Length Affects Pedaling Biomechanics: Research Evidence for Individualized Selection
• The Complete Guide to Choosing a Road Bike Saddle and Measuring Sit Bone Width: Ending Saddle Pain for Good