I thought I would post an update on my latest friction drive design. Although the previous design made from 304 stainless has been very successful, there were elements to the design that I wanted to improve on.
Firstly, I wanted to see if I could come up with a 3D printed design that would be robust enough to handle the rigors of hard every day riding.
I also didn't want to be reliant on a commercial laser cutting company to make the required parts as I tend to get an order or 2 a week and really liked the idea of just running off the parts when an order was placed.
The core of this design is the pivoting swingarm that the motor is attached to. The previous design relied on metal to metal pivot point which when clean and lubricated, worked well. However it was important to keep the pivot clean or the pivot action became sticky and less effective.
It also had a tendency to rattle when not engaged. This didn't effect its performance but it was a common complaint. I had built a fully ball raced porotype that I still use on my road bike now. This design always pivots perfectly and due to its smooth action, didn't need a gravity spring. Also there was no rattle from the drive when not engaged.
However building a ball raced version of the previous design added complication and would double the cost of the bracket so I stayed away from this path as I wanted to stick with the $100 USD price point.
So enter the new 3D printed prototype and its new design features.
The positives:
Firstly, the swingarm has dual 8mm sealed bearings fitted providing a super smooth rattle free and maintenance free pivot.
No gravity spring is needed to assist with engagement due to swingarm moving so freely. Added a cam style endstop adjustment making setup much easier.
Added a built-in cable guide for installations that want to be able to lock the drive in the fully engaged position from the handlebars.
Lowered the parts count to 4 x 3D printed parts, 2 bearings, 2 washers, and 7 off the shelf fasteners.
Halved the weight of the bracket. (from 200g to 100g)
Still working though a few things. So far I have been making the bracket from PLA. I need to do some warm weather testing to see how the PLA stands up. I may need to switch to PETG or some sort of composite filament in the future.
I am fairly new to the world of 3D printing and so far my prints in PETG have not been great. As anyone who 3D prints would know, its a hobby in its own right.
And now the negatives.
The attachment on the bottom bracket now moves the Q factor out by 3mm. The 304 stainless design moved it by 2.5mm. I will need to see if this will be an issue.
There is more bulk to main support bracket which may effect clearances on some bikes. So far on the bikes I have tested, it has been fine but it may be problematic on carbon frames with bulkier bottom bracket areas.
Anyway, I am really pleased with the design in its current state and think it has a heap of potential. Below are some renders and a photo of an assembled drive.
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- preview4.JPG (79.51 KiB) Viewed 1255 times
- 20210926_174420.jpg (318.96 KiB) Viewed 1254 times
- 20210926_174222.jpg (316.55 KiB) Viewed 1254 times
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