Would this pinion with rim-rack drive idea work?

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bigchief   100 mW

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Would this pinion with rim-rack drive idea work?

Post by bigchief » Sep 12 2016 9:23pm

I was thinking of ways to use a high-RPM RC motor on an ebike, and I had an idea to attach the motor to the fork, similar to the way you would attach a dynamo.

Instead of engaging with the tire like a dynamo or a friction-drive motor would, instead I would attach a steel pinion to the head of the motor shaft, and affix a notched "rack" around the brake track of the entire rim, made out of either Nylon or UHMWPE (both lightweight, abrasion resistant plastics).

I did some searching and found something here, a friction drive using the rim:

Image

But, doing it my way seems like it would reduce friction losses, noise and slippage much more than that design, and allow an even bigger reduction in gearing because the pinion could be a smaller diameter.

Is there any reason no one does it the way I'm describing? Here's a metal pinion with a UHMWPE rack to help visualize what would be the motor pinion and the rim-rack, respectively:

Image

Anything fundamentally flawed with the idea? All I can think of is maybe the thing could still overheat and melt the track.

Animalector   10 kW

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Re: Would this pinion with rim-rack drive idea work?

Post by Animalector » Sep 12 2016 10:17pm

There's a bit of info I remember reading about rim-mounted gears.. or belt.. I think I asked a similar question a while back. I think it would work well there's a nice gear reduction available allowing the use of small, high RPM motors.

mrzed   100 W

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Re: Would this pinion with rim-rack drive idea work?

Post by mrzed » Sep 12 2016 11:25pm

Seems like it would be hard to manufacture. UHMWPE does not deform easily, which is the point. So how are you going to radius a strip of milled material to the diameter of a bicycle rim? You could mill the curve itself out of a larger block but that's going to be harder and burn a lot of material. I'm sure sharper minds than mine may have figured it out but seems like a tall order for DIY.

You could reduce the radius issue by mounting the strip on the side of a V-shaped disk rim,or eliminate it by mounting to the inside of a wide flat rim, but you'd lose the motor shaft alignment of the fork mounted solution pictured.

bigchief   100 mW

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Re: Would this pinion with rim-rack drive idea work?

Post by bigchief » Sep 12 2016 11:57pm

Well, if I was going DIY, I suppose I could 3D print a ring in nylon easily enough to test with, or if I wanted to fabricate a piece of UHMWPE, maybe I could CNC a mold from metal, buy a long rod of UHMWPE, and heat it in an oven to try to thermoform it into the mold? I've done stuff like that with polycarbonate but I'm not sure how similar the process would be. That would save on material. But even a big block of polyethylene isn't super expensive or anything.

It's encouraging that no one is saying it's a bad idea mechanically yet...

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ferret   1 kW

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Re: Would this pinion with rim-rack drive idea work?

Post by ferret » Sep 13 2016 8:15am

You will need to true the wheel often because when it goes out of true the gears wouldn't mesh properly.


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bigchief   100 mW

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Re: Would this pinion with rim-rack drive idea work?

Post by bigchief » Sep 13 2016 9:36am

I was thinking of that, I could use an aero tri-spoke or full disk wheel; they are heavier than spokes but don't go out of true. A well built wheel with a high spoke count (like those used for tandem bikes) might be stiff enough too.

If I mounted the motor directly to a brake arm, I could use the normal things you twist to fine-tune brakes, to set the tension on the gear meshing instead.

Punx0r   100 GW

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Re: Would this pinion with rim-rack drive idea work?

Post by Punx0r » Sep 13 2016 9:50am

It's an idea that seems simple at first but will probably be a lot of work to make it effective. Aside from manufacturing the ring, tooth meshing will be a problem as the teeth won't be parallel due to the difference in inside and outside circumference. You'd need a pinion with slightly tapered teeth to suit. Or a straight one and live with partial tooth engagement. You may also find the wheel doesn't have enough lateral stiffness to ensure the gears stay meshed under load. With any slippage a steel pinion will soon destroy a plastic ring gear.

I don't see enough potential advantage over a regular friction drive or a reduction drive (chain or hubmotor) to be worth the various hassles...

bigchief   100 mW

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Re: Would this pinion with rim-rack drive idea work?

Post by bigchief » Sep 13 2016 11:55am

Punx0r wrote:It's an idea that seems simple at first but will probably be a lot of work to make it effective. Aside from manufacturing the ring, tooth meshing will be a problem as the teeth won't be parallel due to the difference in inside and outside circumference. You'd need a pinion with slightly tapered teeth to suit. Or a straight one and live with partial tooth engagement. You may also find the wheel doesn't have enough lateral stiffness to ensure the gears stay meshed under load. With any slippage a steel pinion will soon destroy a plastic ring gear.

I don't see enough potential advantage over a regular friction drive or a reduction drive (chain or hubmotor) to be worth the various hassles...
The advantages as I'm imagining them would be less weight than a reduction drive, but more efficiency than a friction drive.

It seems like this gear design might be the most suitable for keeping alignment:

Image

It's called a herringbone gear; the Wikipedia page suggests that they'd need to be either milled or 3D printed. I guess at this point I should just give it a whirl with something easy like 3D printed nylon and see if it sucks or not.

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Re: Would this pinion with rim-rack drive idea work?

Post by Hillhater » Sep 13 2016 5:49pm

Somewhere on this forum are details and video of a European (German ?) ebike that uses a pair of RC motors with a gear drive to a plastic ring gear attached to the spokes of the rear wheel.
It works well enough to run a motox competition with 10+ kW of power, and reliable enough to sell commercially.
I will try to find a link.
This forum owes its existence to Justin of ebikes.ca

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