E-MTB crank arms

How to choose crank arms for an electric mountain bike

On an electric mountain bike, the crank arm does not attach to a bottom bracket spindle: it mounts directly onto the motor’s output shaft. The motor, and the motor alone, therefore determines which model is compatible. A crank arm for a non-electric mountain bike will not fit, even if it is the right length and brand.

First, the motor interface

• ISIS : the splined interface used by Bosch (generations 1 to 4), Brose and most mid-drive motors. The most common standard in the range.
• JIS square taper : Yamaha, Panasonic and Ananda.
• Hollowtech II : Shimano’s original crank arms — FC-E5000, FC-E6100, FC-E8000, FC-EM600, FC-EM900 — specific to the STEPS system.
• SRAM DUB, Bosch SX, TQ HPR50, Fazua Ride 60 and DJI Avinox each have their own dedicated models.

Offset: the detail that makes the difference

Two Bosch-compatible ISIS crank arms can have a different offset — 5, 10, 13 or 26 mm. The offset moves the crank arm away from the frame and, more importantly, positions the chainring on the chainline. Get it wrong and your heel may rub against the chainstay, or the whole drivetrain may be misaligned. Stronglight and E.Thirteen include the offset in the product name: check the value on your current crank arm before ordering.

Shorter than on a non-electric bike

• The motor lowers the bottom bracket: ground clearance decreases and the crank arms strike obstacles sooner on technical sections.
• 165 mm has become the standard length for trail and enduro electric mountain bikes, 160 mm is becoming more common on long-travel bikes.
• Switching from 175 to 165 mm makes little difference to efficiency on an assisted bike, as the motor supplies most of the torque.

Aluminium or carbon

Forged aluminium withstands pedal strikes, which are very common on an electric mountain bike: short crank arms, a heavy bike and high speeds through technical sections all play a part. Carbon — E.Thirteen E-Spec Race, Praxis — saves a few dozen grams but does not cope well with hard impacts on its edge. On a bike that already weighs over 22 kg, the weight difference is barely noticeable.

The parts that go with them

Crank bolts, crank arm protectors, spacers and chainring spiders can be replaced separately. The crank bolt is specific to the motor, not the crank arm: it is often the part people forget to order, and without it you cannot complete the installation.

To find out more: Guide to choosing an e-bike motor

See also: Electric mountain bike chainrings · MTB cranksets and crank arms · Electric mountain bike chain guides