GMAC on a new Mountain Bike?

Hi all -- I'm new to bikes but I built a generic ebay kit on an old bike and now I want to build a trail worthy e mountain bike. Not for extreme mountain biking per say, but normal local mountain biking stuff, definitely off-road oriented. I'm new to the scene but have found there are a ton of trails in my area.

Sooo, I was thinking of getting a nice bike like a Trek Roscoe, but realized that the dropouts on a lot of mountain bikes won't work with hub motors. If I lower my price point, I can get a 2021 Trek Marlin 7 which has standard 135mm dropouts and woulds support a rear hub motor. I like this bike in particular because it has the "old school" dropouts, but still has a single front chain-ring/casset combo that the newer, higher-end bikes have. I'm planning to use a GMAC motor, Phaserunner, CA, Torque sensor, etc all from GRIN. About $1k in gear, and more expensive than the bike.

So my questions are:

1. Is this a good bike to start from? I would spend more $$$ on a better bike, but I'm not seeing many options that fit a rear hub motor
2. Is the GMAC a good hub motor for a mountain bike? 8T or 10T? I'm not looking for top speed, I have another bike for that. I'm looking for torque.

Thanks!!

Reap3r said:

Sooo, I was thinking of getting a nice bike like a Trek Roscoe, but realized that the dropouts on a lot of mountain bikes won't work with hub motors.

Click to expand...

You can usually make torque plates that fix that. See some of the examples here:
https://endless-sphere.com/forums/viewtopic.php?f=2&t=26444
and there are others around the forum in ohter threads.


Regarding teh GMAC, keep in mind that it doestn' have a freewheel in it, so without power it may be very difficult to pedal, possibly much harder than a DD hubmotor due to the planetary gearing

If you don't need regen braking, you could use the regular MAC, which has a clutch (freewheel) so it doesn't have much drag when unpowered.


If you're looking for torque, you get the higher winding. (more turns = more torque, less turns = more speed).

Custom torque plate seems quite involved. I'm not sure where I'd start without actually having to buy the bike first, and then get into unknown territory.

I have been concerned about the freewheel a bit, but with the ability to have a low power software-based freewheel, I assumed that I would always have SOME amount of juice left in the pack and could just disable all assist and ride in freewheel only mode? I can see how it would restrict you to only riding while having a battery though which could be problematic if I end up out-riding the battery on a regular basis.

I did come around to liking the MAC 12T motor for the mountain bike, but I reached out to MAC alibaba store (only place I could see to order) and asked about cost of ordering a motor. They said MOQ is 8 motors and I should just buy from ebikes.ca store instead. But ebikes.ca only carry the GMAC now. So I think I will settle on the 10T GMAC. I don't know how much difference the 12T would have made, but does not seem like an option.


Anyone have thoughts on a better base bike to start from?

EM3EV.com should have the 12T; they've had them listed before.

LUna might; I remember someone saying they bought a MAC kit from them, dunno what winding.

BTW, here's a thread that might interest you
https://endless-sphere.com/forums/viewtopic.php?t=60443

I've also been keenly looking for 10T or 12T MAC hub.

em3ev are out of stock of all MAC (rear) hub motors and won’t be getting any more stock in the immediate future.
Eunorau stock 8T but no (10T or 12T) and i'm not sure what they are like.

If anyone knows any other suppliers please advise.

We've been running MAC10T's with well over 3500 miles on each of them on trikes. Gears & clutch still in great shape, despite pushing them sometimes for sustained 1500 watt runs.

I've heard some issues in the past with geared hubs on mountain bikes that get some airtime with the wheels. Not sure if thats still a problem, just mentioning that. Sudden shocks are pretty rough on gear teeth, especially as they become worn.

Maybe someone who's had some experience with long term on a Mountain Bike can share their experience.

amberwolf said:

Regarding teh GMAC, keep in mind that it doestn' have a freewheel in it, so without power it may be very difficult to pedal, possibly much harder than a DD hubmotor due to the planetary gearing

Click to expand...

No, it's not. I have a Krampus Surly with a GMAC and use it sometimes without battery, it's easy to pedal at 18-20 km/h on flat terrain. The difference with a freewheel motor like the Bafang G310 (I had this one before on the Krampus) is noticeable, but not huge.

What voltage are you ready now and what voltage are you going to run

Before you buy any bike and want to put a rear hub on it make sure the nut of the rear hub can fit with some bikes they put derailleur so close to the axle that you can't get a nut on it.

I don't know what's happened with the regular MAC motors lately. Maybe Grin is buying all the components necessary to build a regular MAC for their GMAC production? Glad I was able to get my 12t when I did!

As far as building a GMAC bike is concerned (have plans to do this as well), there's a massive new torque arm setup that comes standard with them, and rather than trying to get the hollow axle to fit/seat fully in standard drop outs, the GMAC uses a solid round axle that makes this part of the install much easier in my mind. Wires now come out the side plate. Point being, I see fewer drop out/torque arm related issues if going GMAC.

As far as what wind you would want, Grin is now saying that the 10t will do anything the 12t will when it comes to torque, so the 12t is not being offered in the GMAC line up. So to answer the question, if there is no concern over top speed, you would want the 10t.

The difference between 10T and 12T yields no appreciable difference in the raw amount of available torque. What the 12T will allow you to do, is to produce the same high torque but with lower current at the expense of higher voltage; that's a wash. The joules of energy (kWh) used from your battery and the amount of torque you will get, are going to be almost exactly the same for any given hill. The 12T motor will only make a difference if you have a crappy controller with high internal resistance or thin phase wires. Then you might see a small advantage to the 12T due to lower overall system losses. Losses in the actual motor will be materially the same regardless of T10 vs T12. You can try this on the Grin motor simulator and you will see this is the case. Furthermore, the load on the battery pack in terms of minute to minute current draw will be almost exactly the same for any motor wind because the controller will translate instantaneous current from instantaneous power being burned by the motor, irrelevant of actual motor current.

Having said that, I too would prefer the 12 GMAC so that I can use a cheaper controller under high loads. Speed is not my thing. Are you listening eBikes.ca ?