All-Axle Hub Motor


Grin Technologies has been slowly working away at making their own direct drive hub motor for electric bikes and is currently in a beta/pilot release program. The idea first took roots in 2010 when we were impressed with the performance of imported direct drive hub motors based around the 23 magnet pair 205mm diameter motors, but increasingly frustrated by their poor mechanical compatibility with modern bicycles. It seemed like there was always some kind of shimming and adapting needed on the axles to get disk rotors to align right, calipers to fit, dropouts filed, torque-arms retroactively installed etc.

We thought, wouldn't it be great to take this basic 205mm direct drive motor standard but make the mechanical side of things fit flawlessly with the standards of modern mountain bikes in North America? That meant first and foremost being compatible with thru-axle forks, but while studying the existing imported hub designs we realized there was lots of opportunity to remove material and shed significant weight without sacrificing motor performance. After retooling every aspect of the motor side covers, magnet ring, stator support, and axle, we were able to eliminate over 30% of the mass, bringing the 27mm wide direct drive motor from 6.2kg down to just under 4kg.


Here are some of the key points which make this motor really stand out

  • All-Axle Compatible. Fits 20mm and 15mm thru axle forks as well as standard 9mm quick release with appropriate insert
    Axle Adapter Inserts for Grin All-Axle Hub Motor
  • Potential use with single side stub axles in quad and trike vehicles
  • Integrated torque arm locks stator securely and means no force on dropouts
  • Large diameter ball bearing allows heavy 12 gauge motor phase cable to pass with ease
  • 18mm disk caliper clearance fits most (but not all) calipers without spacers
  • 3.95 kg motor weight, almost as light as most geared hubs in this power class
  • Drilled for 32 hole rims, vastly more common on the market than 36 hole normally used by ebikes


Of course, not everything came without trade-offs. There were some predicted and other unexpected consequences to the belt tightening motor design

  • Noise: The thinner motor side cover plates act almost like a drum skin and seem to amplify the motor buzz when using a square wave / trapezoidal motor controller, to the point that under hard acceleration the hub can be noisier than a geared motor under similar stress. We really recommend using either a sine wave or field-oriented motor controller with this motor.
  • Thermal Mass: The reduction in metal also meant that this motor had less ability to simply absorb the heat produced under heavy loading, resulting in a shorting time to reach overheating temperatures versus a comparable (and heavier) generic motor. This can be easily remedied with Statorade, and the Grin motor is sealed on the side plates and screws and with a fill port on the axle to inject. In fact, it was investigating alternative cooling methods for the Grin Hub which lead to our discovery of this ferrofluid cooling solution.
  • Bearing Size and Life: Since the motor uses a large diameter 45m axle, we had little choice but to use a thin section (7mm wide) ball bearing to keep the bearing weight reasonable. However, thin section bearings are less tolerant of abuse than normal 12-15mm wide bearings used in hub motors, and this may result in a more frequent bearing replacement on motors subject to rough conditions.
  • Unlacing Required to Disassemble: The spoke flanges of the Grin hub are on the side cover plates rather than the steel rotor ring. This gives maximum flange width for spoke triangulation and helps reduce the rotor weight, but it means that (similar to the BionX and TDCM hub motors), the hub must be unlaced to open it up for any service or repair.


The Grin hub motor has a performance specification similar to the 9C+ 27XX motor series. We have it modeled on our trip simulator web app both with and without Statorade.

Motor Weight  3.95 kg 
 Motor Kv (Slow, Std, Fst)  7.9, 10.4, 12.5 RPM/V
 Winding Resistance  268,145, 100 mOhm
 Hysteresis Drag Torque  0.54 Nm
 Spoke Flange Diameter  214 mm
 Flange Spacing  49 mm
 #Poles for CA  23


Availability and Pilot Program

We are currently in doing a limited scale pilot production of the Grin All-Axle hub motor from manufacturing facilities in Vancouver, and are open to early adopters who want to try out these motors in their specific application that requires some of the unique features of the motor. Pilot users should understand that there could  be unforseen issues with this motor design that will only show up with more extensive field use, although now with over 3 years of testing under our belt and these hubs serving flawlessly in the 2018 Suntrip solar bike race, the risk seems pretty low.

If you think you are a good candidate, please email us with your specific project that needs to use this hub along with a brief background of your experience and familiarity with converted ebikes.

Where to Order