Crank Length - A Bike Fitters Advice

Crank Length in Bike Fitting

Crank length is a crucial factor in bike fitting that’s often overlooked. It affects both comfort and pedaling dynamics/performance.

The speaker notes that crank arms (the arms the pedal attaches to) come in many lengths. Commonly, commercially available options fall around 165 mm to 175 mm, with examples like 165 mm, 170 mm, 172.5 mm, and 175 mm.

How crank length is measured

Crank length is measured from the:

• Center of the spindle to the
• Bottom bracket axle, then to the
• Middle point where the pedal joins.

Starting Point: Use the Bike’s Stock Crank Length

The speaker suggests that the crank length included on an off-the-shelf bike is usually a reasonable starting point, because crank length often scales with bike size.

However, they argue this scaling can be slightly off, giving an example:

• 52–54 cm bikes may come with a 172.5 mm crank
• The speaker often considers this too long
• They suggest 170 mm is commonly better (and mention it as their best-selling option)

“Optimal” Crank Length Varies by Rider

There isn’t one single “optimal crank length” for the general population. The speaker emphasizes that optimal choice depends on the individual’s needs and limitations, rather than broad averages.

They note:

• Research may focus heavily on elite athletes with high mobility and strength
• Average riders are often more sedentary and may have tighter hips
• There is a trend toward shorter cranks, but shorter is not always better, especially at the extremes

When the Crank Is Too Long

A crank that’s too long can contribute to:

• Hip closure, making it harder to effectively reach the top of the pedal stroke

The speaker says that even small changes can help:

• Changes as small as 2.5 mm can noticeably improve pedaling dynamics and saddle interaction.

Potential knee-load issues (in larger mismatches)

In more extreme mismatches (e.g., a small rider on a 175 mm crank), the speaker suggests it may increase:

• Knee flexion load, stressing the knee joint

They cite a recent example of a rider around 5’6”–5’7” with a 175 mm crank causing major issues, but they suggest this is relatively uncommon—more typical on budget setups such as:

• 54 cm bikes paired with 175 mm cranks

Practical Rule-of-Thumb Guidance

As a quick baseline, they offer ranges based on size:

• Extra small / small: ~ 165 mm
• Small / medium: ~ 170–172 mm
• Large / extra large: ~ 175 mm

If crank length falls well outside these ranges, the speaker implies something may be off.

When the Crank Is Too Short

From the speaker’s testing experience (using an adjustable crank on their jig), cranks that are too short can cause riders to:

• “spill over the front” of the bike

While pedaling dynamics may not necessarily get worse, the speaker notes it can affect handling.

They also challenge the “shorter is always better” idea by stating that very tall riders with very short cranks are unlikely to be optimal—for example:

• A 6’6” rider with a 165 mm crank

They claim this mismatch is more common among short riders than very tall riders.

Alternatives to Shortening Cranks

Instead of reducing crank length solely to open the hips, the speaker highlights other ways to improve hip function, such as:

• Lowering pedal stack with certain pedal systems
• Using a thinner shoe sole
• Increasing cleat / “star” adjustments
• Reducing saddle setback

Saddle-Height Relationship

The speaker recommends that if crank length is reduced by 5 mm, the rider may need to raise saddle height by roughly 5 mm.

They strongly emphasize that:

• Testing is necessary
• Either by trying multiple crank lengths (adjustable cranks exist)
• Or via a professional bike fit

They discourage relying on blanket rules.

Availability and Brands

The speaker emphasizes that bike fitting should not push one “correct” crank size. Instead, they want more crank length options.

They mention working with crank lengths from about:

• 150 mm up to 180 mm

They also note limited availability for longer cranks—especially 180 mm—and mention Dura-Ace Power Meter as an example someone had to purchase due to crank availability.

Shimano and broader options

They praise Shimano’s direction, stating that in some higher-end groups (e.g., 105 Ultegra and Dura-Ace) cranks as short as 160 mm are available.

They express concern about setups using cranks shorter than 170 mm, specifically cautioning at around 165 mm.

Closing Message

Ultimately, crank length should be selected for the rider’s individual needs, not generalized averages—especially for riders outside typical ranges. They encourage viewers to ask questions or book a bike fit.

Presenters / Sources Mentioned

• Quintilian (as a rhetorical example/source)
• Shimano
• SRAM
• Campagnolo (Campy)
• Race Face (implied via “crowder,” possibly misheard; context suggests another crank brand example)
• James (the fitter mentioned in the closing)
• Zed crank (mentioned as an adjustable crank example)
• Shimano (again, emphasized for recent availability)