Electric mountain bike chainrings

How to choose an electric mountain bike chainring

On an electric mountain bike, the chainring mounts not to a crank but to the motor spindle via Direct Mount, or to a 104 mm spider. Three factors determine which one you need: the motor interface, chainline and number of teeth.

Direct Mount or 104 mm four-bolt

• Direct Mount : the chainring attaches directly to the motor spindle. Each motor has its own interface — Bosch Gen 3, Gen 4 and SX, Shimano EP8 and E8000, Brose S Mag, TQ HPR50, Fazua Ride 60.
• 4-bolt 104 mm : the chainring bolts to a spider or a standard crank. More universal, provided the spider matches the motor.
• The spiders and spider adapters are sold separately: these are the parts that adapt a motor interface to the standard 104 mm bolt circle.

Chainline: get it right

The motor positions the chainring farther from the frame’s centreline. Common chainline measurements are 53, 55 and 56.5 mm, compared with 52 mm on a non-electric mountain bike. Shimano includes it in the product name — the SM-CRE70-B comes in 53 and 55 mm versions — and Race Face does the same. An incorrect chainline causes angled chain wear and makes gears skip on the outermost sprockets. Check the original measurement before ordering.

How many teeth

• Electric mountain bikes use smaller than on a non-electric mountain bike: 34 to 38 teeth cover the vast majority of setups.
• Going down a size gives you more torque on steep climbs and puts less strain on the drivetrain; going up a size lets you pedal at higher speeds before spinning out once assistance cuts off at 25 km/h.
• The motor multiplies the torque passing through the chain: on an electric mountain bike, wear is driven far more by torque than by distance travelled.

Steel or aluminium

Steel — such as Stronglight and SRAM X-Sync 2 steel — lasts considerably longer under the torque of a mid-drive motor. Aluminium saves a few dozen grams but wears faster. On an electric bike, steel is often the sensible choice, and the price difference pays for itself the first time you avoid a replacement.

Narrow-wide teeth do not eliminate the need for a guide

All these models have alternating narrow-wide teeth, which holds the chain in place without an additional accessory. On a heavy, fast bike, the chain develops more slack over bumps: in enduro riding, a chain guide is still useful as well. The two complement rather than replace each other.

To find out more: How to upgrade your electric mountain bike

See also: Electric mountain bike crank arms · Electric mountain bike chain guides · MTB chains and quick links