Mountain Bike Crankarm Length Selection

Crankarms are a critical component of a mountain bike's drivetrain, connecting the pedals to the bottom bracket and transferring rider power to the drivetrain. Optimizing length contributes to pedaling efficiency, comfort, and clearance.

Factors Influencing Appropriate Crankarm Dimension

Rider Height and Leg Length (Inseam): Shorter individuals or those with shorter legs generally benefit from reduced lengths. Conversely, taller individuals, or those with longer legs may find longer options more suitable.
Bike Geometry and Frame Size: A bike's overall geometry and the size of the frame influences effective reach and stack. Crankarm dimensions can be adjusted to compensate for or complement those characteristics.
Riding Style and Terrain: Technical terrain demanding frequent bursts of power and requiring greater ground clearance often favors shorter units. Smooth, flowing trails or uphill climbs may allow for longer options that emphasize leverage.
Pedaling Technique: A high cadence, spinning style may benefit from reduced length. A lower cadence, power-based technique may be more efficient with greater lever arms.
Personal Preference and Comfort: Individual biomechanics and comfort levels significantly impact the optimal dimension. Trial and error, possibly with professional fitting, is beneficial.

Common Mountain Bike Crankarm Dimensions

Typical length offerings span a range, with common sizes expressed in millimeters:

165mm: Becoming more common, especially for smaller frame sizes and riders prioritizing ground clearance.
170mm: A popular choice for riders of average height, balancing leverage and cadence.
172.5mm: Often considered a middle-ground option, suitable for a wide range of riders.
175mm: Traditionally favored for larger frame sizes and riders seeking maximum leverage.

Considerations for Changing Crankarm Length

Altering the existing dimension requires careful consideration of the following:

Bottom Bracket Height: Shorter options may slightly increase ground clearance.
Gear Ratios: A change can subtly affect perceived gear ratios. Compensatory adjustments to cassette or chainring sizes might be needed to maintain similar gearing.
Saddle Height Adjustment: Modifying this parameter is essential to maintain proper leg extension during the pedal stroke. Shorter crankarms will require raising the seatpost.
Q-Factor: While generally constant within cranksets, extreme length changes could necessitate evaluating Q-factor implications on knee alignment.