Advise Please: Fitting MAC hub motor to thru axel ends

[TJP]

I would like to convert a new dual suspension Aluminum 27.5 Plus boost MTB frame with (2) 4 kg hub motors. The plus boost frame will allow the use of 3.0” wide tires.

How can I install a MAC/BMC/Ezee rear hub motor without cutting out the thru axel rear frame holes?

I found these 2 threads which show torque arms.
https://endless-sphere.com/forums/viewtopic.php?f=1&t=11570
https://endless-sphere.com/forums/viewtopic.php?f=2&t=26444
I understand how a slotted torque arm and a clamping torque arm are used.

How is the wheel and motor hub axel fitted through the round holes without spreading the rear triangle?

Thanks in advance for your help.


Hi speed, light weight, high strength dual suspension conversion
Max speed on level ground: 60 kph
Max range: 50km at 50 kph
Brakes: Hydraulic Disc
Terrain: 80% asphalt 10% gravel, 10% trail with moderate grades
Wheels: 26" X 39mm
Tires: 26” X 2.5” to 3.5”
Frame: 27.5” AL Plus Boost Full Suspension
Rear Hub: 4kg MAC, BMC or Ezee
Front Hub: 4kg Grin All axle or other Direct Drive
Max Bike Weight: 35kg, 77 pounds
Rider Weight: rider 75kg, 65kg passenger approximate 10% of time.

 

[Drunkskunk]

You can't. First, unless there's been a design change, the threads are 14mm. a through axle is 12mm. No amount of lube is going to make a 14mm shaft go into a 12mm hole without wrecking it.

Second, you can't get it in there without cutting one hole off anyway.

Third, there wouldn't be a way to safely lock down the axle. It's not just about keeping it from rotating, it also needs to be tight to stiffen up the frame and keep the rear arms from bending. the axle is essentially a structural member of the bike.

 

[TJP]

You can't. First, unless there's been a design change, the threads are 14mm. a through axle is 12mm. No amount of lube is going to make a 14mm shaft go into a 12mm hole without wrecking it.

Second, you can't get it in there without cutting one hole off anyway.

Third, there wouldn't be a way to safely lock down the axle. It's not just about keeping it from rotating, it also needs to be tight to stiffen up the frame and keep the rear arms from bending. the axle is essentially a structural member of the bike.

Thank you for that confirmation. That was my impression, but I wanted someone with experience to confirm.

It seems most unfortunate that all rear hub motors and conversion kits are incompatible with new high end mountain bike frames without extreme frame modifcation which make the frame unusable as a conventional bicycle.

 

[amberwolf]

If you make a pair of torque plates that bolt to the dropouts and extend them backward, you can use a thruhole frame for a hubmotor. The plates would be mounted to the motor first, then bolted to the frame, so there doesnt' have to be a slot, just a perfect axle-shaped hole.

You can even add disc caliper mounts to the torque plate so that you can use a disc brake with the motor if you like.

 

[goatman]

Grin makes those thru-axle 4kg motors

https://www.ebikes.ca/shop/electric-bicycle-parts/motors/direct-drive/grinhub-fst-bk.html

 

[amberwolf]

AFAIk it's only a front motor, though...so if you don't need to pedal that'd be fine for a rear, but youd' have to make something to add a freewheel or sprocket to it to use it as a rear and still pedal.