3 Speed Unicycle Build

Bit of a background. In early 2008 or I got a Nuvinci CVT hub and was convinced I’d be able to hack this to work bidirectionally in a unicycle application with a jackjaft. Unlike the conventional internal hubs, the friction drive against the spherical rollers should fundamentally be able to alternate between forwards and reverse torques with no slop at all.

Then you’d have the dream setup, a 300+% gear ratio with no sudden shifting.

Here’s what the whole thing looks like taken apart on the bench.

The hub casing is on the left, in the center of which is the pedal drive sprocket that connects to the input ring, which then spins the roller balls, in turn rotating the output ring which is directly connected to the hub. The shifting mechanism tilts the axis of the roller balls.

What I didn’t realize is that the compressive force that holds the rings against the balls is generated from the input torque of pedaling, I has assumed it was just pre-loaded. So as you pedal, that causes the flat cylinders here to roll up the ramp and squeeze the parts together:

This is clever since it means that you don’t have excessive friction loss with light pedal effort, there is only ever just as much friction as needed to prevent slippage. But it’s designed to work only in the forwards direction. To work both ways, the ramp plate would have to be machined so that it slopes equally on either side of the cylinder, but there was no way I could machine that on the precision ground hardened steel rings.

What I tried to do is instead is a spacer shim that would enable a certain amount of preload when the parts were all put back together. That kinda worked but not great.

It turns out that Nuvinci actually put some development effort into a bidirectional hub model for electric vehicle and scooters, where regen braking and reverse are desireable, though I got the impression it’s not currently on their program.

I pleaded with some of their engineers at the interbike trade show to get back on this as a whole unicycle community is craving it! (Apparently they get a tech inquiry about twice a month from someone wanting to use their CVT in a uni application.)

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Justin, I had similar analyse of NuVinci with my engineering school 2 years ago but prefered to switch to gears hub …

Cannot see your photos ! may be there is a concern somewhere

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"

Hub Cross Section.jpg

24" wheel is very small for such ratio (2,18), 24" inertia would not be enough for hight speed stability, I would prefer 26" or better 29" wheel and shorter ratio

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Hmm, is anybody else not viewing the images too? I was having some troubles with the attachment manager and things seemed a bit screwy, though it display OK on my computer. I also can’t seem to go back and edit the first post to relink the images which is frustrating, and embedding an attached image as in the last post causes it to display twice, once inline and once as an attachment, which is retarded.

In any case, here is the images of the hub all taken apart:

The compression roller:

and finally the spacer rings to produce a preload:

Nuvinci Parts.jpg

Compression Roller.jpg

Spacer Rings.jpg

Machining the Spindle

The ISIS spindle for this project came from a Nimbus Eclipse Hub, used because it’s aluminum and hence easier to machine:

The bulk of the hub body was removed on a lathe and turned to exactly 22.0mm for ball bearings, and one of the spoke flanges was left in place and squared off for attaching the pedal drive sprocket.

Eclipse Machining.jpg

Finished Spindle.jpg

Wow… this is a really cool attempt at getting more than 2 speeds on a uni!

Let us know how you make progress here over the next days or weeks!

Congratulations for this ambitious project.

Cheers, MadC.

Hub Body Machining

The free spinning hub body was hogged out from a piece of 2.5" aluminum round stock on a lathe. The outside had a pair of flanges made for the spokes, and the inside was bored to exactly 42.00 mm so that it could be supported by standard ISIS unicycle bearings.

Drilling Aluminum Stock.jpg

Boring Hub Inside.jpg

Finished Hub

After drilling out the 36 spoke holes in the flange and tapping holes for the sprocket, this is how she looked:

Finished Hub.jpg

S3X Sprocket

Here’s the 21 tooth sprocket having the inside machined out to fit over the S3X hub and bolt directly to the spoke flange. The steel is really hard and I broke two milling bits doing this. Afterwords I annealed all the steel gears with a torch before doing any machining and that made things cut much easier.

Milling Sprocket.jpg

Just before Assembly

All the drive related parts with sprockets installed ready for assembly. Not shown is an aluminum spacer tube that was fit between the two middle ball bearings to keep them in position.

Drive Parts Ready for Assembly.jpg

It mostly already happened over the past week, so I will let you know the results over the next couple hours!

Thanks! This is only half of the project. My ultimate goal of course is a multispeed geared electric assist unicycle.

Once I have that I will finally be happy.


Built Wheel

After putting the parts of the hub together, I had An’So lace then it into a 24" rim for me because she does such a good job of wheel building :wink: And after a couple late nights in the shop I needed to catch up on sleep.

I was a bit worried that with the eclipse hub being designed for narrow 90mm frames, I might have some trouble getting the bearings and the cranks to all fit in the KH frame which uses the 100mm standard, but it actually went together just fine. The Nimbus Venture cranks don’t rub against the frame even through each side is 5mm closer in than they’d be with a standard hub.

Wheel Closeup.jpg

3 Speed Transmission

Further up the frame, I made an adapter to hold the 3 speed hub from two pieces of angle aluminum that bolt to the brake mounts. The hub is inserted through slotted dropouts for chain tension adjustment. By total luck of the draw, the gear ratios that I chose resulted both sides of the chain being tight together, so there was no need for a chain tensioner on one side.

You can see the shifting cable on the right. Shifter cable all the way in is direct drive, all the way out is 0.625 reduction. Somewhere in between is 0.75

S3X Closeup.jpg

amazing…!!! <3

Shift Lever

The S3X kit came with a shifting lever that is designed to fit in the ends of a handlebar. We were able to make this work with a KH seat brake mount by drilling out the ID a bit. The lever sits a tad too close to the seat handle for comfort, but it did the job and keeps it protected in the event of a bail.

So now you just reach down with your finger, flip left or right, and switch gears!

It would work really nice on a uni with handlebars so that will be the next step.

Shifter Details.jpg

That’s incredible!

Brilliant! We’d talked about a design like this but it’s great to see it beautifully executed. On a 29er (Coker frame, presumably) you could have 39", 47", 63" gears with the same cog ratios; that would be a serious road machine.


So at about 2am on a Thursday night, everything had come together and I was ready to hit the streets. Or at least, cautiously hit the sidewalks. The goal was to have this built in time for the Vancouver Mini Maker Faire on Sat. June 24th and everything seemed on track.

Ready to Ride.jpg

I love it. brilliantly executed! I could almost see it, and wish I built it… Kudos!

it needs a disc brake.