3D Printed Bike Motor Mount

31 May 2018

A small journey to build a mid-drive electric bike - The motor mount

During the late Winter and early Spring months that I spent in Berlin, I slowly came to the conclusion that, in addition to my most recent electric longboard build, I was going to need an electric bike for my upcoming Junior semester at Lafayette College. After doing quite a bit of internet research from my room, I decided that, honestly, hub motors are the most uninteresting way to complete this task. Therefore, I've opted to make my bike, what the public seems to call a mid drive bike. So, I went online and ordered a bunch of parts from China to hopefully arrive by the time I landed back in NY in late April.

Now, this post is really only going to contain information related to the process of me designing a mount to attach to my bike. The project is far from finished and I am back again in Germany completing an internship at the moment. Hopefully, more updates will come to the Jamaisvu project on this blog; I think I have made an interesting breakthrough there.

3D Printing with my MP Mini v2

Since I got my received my first 3D printer last August, I haven't had many proper uses for it. I've made only a few simple prints for various miscellaneous things. This printer is the MonoPrice Mini v2. This is a fantastic printer to start with. It handles fairly complex prints well and has been very reliable to me. Also, the modding community has many resources so that one can mod and improve their printer very cheaply. In the end, however, this printer could not supply me with prints that where as strong or as complex as I would need for this project. I ended up buying a FlashForge Creator Pro which is basically a MakerBot Replicator V1 clone.

Motor mount design

It is quite easy to get pre-made motor mounts for electric longboards in the form of kits, and while these work well, they rely on a certain truck style to attach properly without welding. Bikes vary much more in their designs and so I knew I was going to require a custom solution.

In this bike I am currently using a Turnigy Aerodrive SK3 - 6354-260kv, which I am actually planning on replacing (lack of torque). Nonetheless, what is important here is the mount. This motor as 4 M4x7 treaded holes in the front (near where the shaft protrudes) to which you can fix the motor to a mount. These holes are about 43.5mm apart from each other (diagonally). It's quite simple to model and it only took me around a half hour to get a working model.

This would lead me to my first design, where I have two pieces that would clamp around the bar between the seat and the handlebars and a third piece would mount the motor to that.

Attempt 1

First Motor Mount Assembly on bench First Motor Mount Assembly on bike

Attempt 2

Here, I moved from trying to grip the bike itself, to using two bolts that were already in the bike and near the sprocket set. The preivous method would spin quite easily under the torque from the motor and detacth from the chain. Here I still used the ABS motor mount part bolted onto a white PLA part which attached to the bike and can be adjusted to make the chain tought. The issue that would occur with this design was that the part where the motor mount and the bike mount where attached would allow too much flex under torque from the motor and the chain would detatch.

Second Motor Mount Assembly on table Second Motor Mount Assembly on bike front Second Motor Mount Assembly on bike back

Attempt 3

In this attempt I decided to slightly rework the motor mount so that it would much more snugly fit the bike mount and hopefully prevent the flew that was occuring before. This would unforunately fail as the torque from the motor would snap the somewhat weak PLA print.

Third Motor Mount Assembly in hand Third Motor Mount Assembly failure

Attempt 4

This attempt I decided to combine the motor mount and the bike mount into one solid piece (still contained infil at 50%) in hopes of fixing the break. Nonetheless, this broke in roughly the same spot as before sadly...

Fourth Motor Mount Assembly in hand

Attempt 5

Here, after some research on the internet and advice from some of its users, I settled on purchasing some ePC (Polycarbonate) filament for its added strength. However, I believe this is mostly beneficial for impacts and not torsional strength which is primarily what I needed. Nonetheless, I would need a filament which would deform at a higher temperature than PLA for when the motor would get hot anyways. I had an infil of around 40% for this print after hours of tweaking my FFCP to get it to properly print. Sadly, this failed in the same place as before.

Fifth Motor Mount Assembly on bench Fifth Motor Mount Assembly failure

Success - The 6th attempt

Finally, after much frustration and several days worth of print time. I completed a near 24 hour print which has continued to withstand the torque of this motor. Sadly, I do not have a photo on hand, however, I increased the thickness of the part of the piece that was breaking by around 4mm and set the infill of the piece to 100%. This made a solid ePC plastic motor bike mount. In addition to the solid piece and thicker motor mount section, I added two M4 sized shafts that run up from the bottom to the top of the motor mount which pass through the fault point. I installed two 65mm M4 bolts from beneath to provide even more support. More to come in the coming months. Stay tuned!

Published on 31 May 2018 by Clement Hathaway