Friday, February 17, 2012

clever 3 speed unicycle rig

So when I was informed a few months ago that Sturney Archer had just released an internal hub for fix gear bikes I ordered one immediately!

The S3X has 3 speeds of 1.00, 0.75, and 0.625, so nowhere near the nuvinci's current 360% range but on the plus it's just $140 and it works bidirectionally out of the box.

My goal was to try and design a drive around this that could be attached to a standard uni frame with no modifications. Here is what the spacial concept was like:

29" KH frame, both chains on the inside, with one going from the pedal drive sprocket up to the input of the S3X hub, and the other from a sprocket on the flange of the S3X hub down to the wheel.

A cross sectional view of the hub itself should be pretty self explanatory. The spindle and cranks naturally spin independently of the hub body:

I had a few bike sprockets pulled from cassette freehubs kicking around, so with the pickings from that I designed it for 28 teeth on the pedal, down to 15T on the S3X. Then the bolted to the flange of the S3X was a 21 tooth, which connected to an 18 tooth on the wheel.

So the overall chain ratio was 28/15 * 21/18 = 2.18

Combine that with the 1.0, 0.75, 0.625 ratios of the S3X hub and the 24" wheel and the final result should be a uni with the physical size of a 24" but 3 speeds of 52", 39", and 33"


D - very nice! Maybe my mid-steer thru-front-wheel-pedal will need to use this layout initially. Until the gear hub companies take notice and build them for this layout.
Though a hydraulic pump so it is RWD might be simpler, ultimately.
The mid-steer with FWD will need at least a lite 12V rear e-assist for moving uphill on loose gravel.

D - to be fair, with Justin's clever layout, there is really no limit to what gear hub could be used. A Rohloff becomes practical.
The hub on my bike layout would make sense as far forward as possible, to add ballast to the front tire on a FWD bike.
Ditto a battery pack for the rear tire E-assist.
As a bonus, my layout would not flip forward like a standard SWB. I had to loosen the front tire to prevent that. My Cruzbike became a unicycle briefly, when I most needed 2 tires!

Um, is Rohloff ever gonna release that new lighter 580% gear range gear hub? I don't mind that it cannot handle too much torque. After all, the lack of weight on the FWD front wheel of my proposed mid-steer means the tire will slip well before the gear hub fails.

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