The GMAC hub motor is a collaboration between Grin and MAC motors to produce a unique hub motor in the electric bike space: a geared motor with powerful regenerative braking, integrated no-play torque arm, and near perfect bike frame compatibility. We started this co-development in earnest in late 2017 along with Bike Swift and after much testing and refinement are happy to indroduce this hub in our 2019 motor catalogue.

What's Special?

The core motor performance is the same as you would expect from the latest MAC motors. That includes a modern cassette freehub system instead of a screw on freewheel and a powerful 20mm wide stator core made with thin 0.35mm laminations, but the following differences make it really stand out

Locked Clutch:

It's been the norm that geared hub motors have a built in freewheeling clutch so that the motor coasts in the forwards direction with no drag, but this prevents regenerative braking, reverse operation, and the clutch itself becomes a common point of failure with high torque mtors. The GMAC motor does away with the freewheeling clutch entirely, allowing powerful and efficienct regenerative braking and one less component to fail.

Integrated Splined Torque Arm:

A powerful motor requires a strong torque arm, and a powerful motor with regenerative braking requires no play between forwards and reverse torque. We designed a splined torque arm interface inside the dropout which provides several times the spinout strength of the conventional axle flat approach, with a snug fit for handling torque reversals without any slop.

10mm Round Axles:

By placing the integrated torque arm inside the dropout slot, the remaining axle could be made to match the standard round M10 x 1.0 convention of bicycle hubs. This allows the motor axle to seat fully into a bicycle dropout slot with correct positioning of the disk rotor and cassette cluster, unlike M14 axles which locate the centerpoint about 3-4mm too low.

Side Cable Exit:

In conventional MAC motors the cable comes out the end of the axle, leaving it vulneratble to damage in accidents and complicating the installation and exchanging of axle hardware. Our design of the splined torque arms allows for a side cable exit that bends downwards out of the dropout slot, nicely shielding the cable exit from mechanical damage in any mishaps.

L10 Connector:

Finally, we've had these motor cables terminated with the 10 pin high current locking motor connector which integrates the hall wires, temperature sensor, internal speedometer sensor, and phase leads into a single reliable and water right plug. The motor phase wire is heavy 4mm^2 copper (larger than 12 gauge) and capable of handling 100A currents for short times.

Applications and Uses

The primary use case for this motor was for heavy hauling applications and people riding on steep hills. These are scenarios where the motor is in use almost all the time, and the lack of freewheeling is not an issue since the rider is rarely pedaling the bike unassisted. Traditionally this application involved a toss-up between choosing powerful geared hub motors or mid-drives for good efficiency at low speeds and high torques, and giving up on regenerative braking, or using a large and heavy direct drive motor for regen but at the expense of reduced efficiency on the slow climbs.

The GMAC motor eliminates this trade-off, providing a relatively light motor option that has good low speed torque and efficiency plus regenerative braking that is nothing short of amazing. In cargo applications where riders will be frequently burning through brake pads, this hub is a game changer for reducing system maintenance and improving vehicle handling.

The GMAC motor without a clutch has a freewheeling drag that increase from 1.0- 1.5 Nm, about twice as much as a direct drive hub motor. For most riders this would be a noticeable amount of extra resistance when pedaling the bike, and it's one of the reasons that geared hub motors have all tended to have a freewheel built in.

However, even if you are pedaling the bike without assist, the energy that is expended overcoming this rolling drag in most cases much less than the energy recaptured from regenerative braking. This point is really interesting, since it means that the motor could be run all the time at low powers to overcome the rolling drag using just the energy captured from regen, and still put more energy back than expended.

We are working on this "electronic freewheeling" function for the Phaserunner / Baserunner controller series and hope to have it available via firmware update by summer 2019. At that point, we could say that the GMAC motor is a great choice even for commuter and touring applications where a freewheeling hub is often considered vital since as long as there's a battery on the bike it can be made to feel just like a freewheeling setup.

Winding Speeds

We're stocking the GMAC motor in two winding speed options which should cover all bases; a standard 10T wind (8.1 rpm/V) generally for 24" and larger wheels, and a fast 8T wind (10.2 rpm.V) mostly intended for 20" wheel sizes. Of course it's possible to swap this around depending on your speed requirements and battery voltage.

The very slow 12T motor winding did not have any advantage given the large size phase wires available, while the very fast 6T motor requires more phase current to achieve maximum torque than is available from 6 fet controllers and the L10 plug standard.


The GMAC motors are available as stand alone hubs, or as part of a kit with an L10 Phaserunner motor controller.