CVT hub

In the never-ending search for a continuously variable transmission (CVT) hub I stumbled upon a mock-up of a design that I like. Maybe because I can understand it fairly well. :stuck_out_tongue:

A “working” mock-up is shown in this video:

If it worked it would offer a gear ratio from 2:1 to 1:2 and everything between, with smooth shifting. Perhaps ideal for a uni.

Its relative simplicity makes me think it could be made lighter and cheaper than a Schlumpf or Nuvinci. It’s not so packed full of gears and oil.

One big concern that stands out is the belt slipping under high torque. Of course the weak rubber band in the video wouldn’t work, but with the right material and belt design could slippage be avoided in a uni hub? Is it a deal breaker or can some further engineering solve that? Perhaps its an easier problem to brainstorm than the complexity of gears.

And of course there is no shifting mechanism in this mock-up. I can envision some kind of vertical guide with rollers that pushes the band to the desired position. How exactly that mechanism gets controlled from the outside is another problem.

Several days ago I tried to email the maker of this mock-up but got no response. He might have some findings to share.

I’m excited to think this could be the basic design of a CVT uni hub.
What do you think?

And don’t forget the axle in the model is broken into two halves, one for drive the other for output.

Isn’t the point of a CVT is that it doesn’t require a shifting mechanism? it just shifts on its own?
I wonder what it would be like to ride a CVT unicycle. How well could it handle back pressure?

A shifting mechanism is still required in a CVT. It can be some kind of automatic system, but I think most people want manual shifting on a bike/uni. In a car the typical driver probably doesn’t care about the engine’s RPMs too much, as long as it’s in a reasonable range and isn’t going to explode. A cyclist is much more finicky and might want a difference cadence based on current terrain, upcoming terrain, or just because they feel like it, regardless of speed.

From the wiki:
A continuously variable transmission (CVT) is a transmission that can change seamlessly through an infinite number of effective gear ratios between maximum and minimum values.

The “Nuvinci” hub is the only production CVT I know of.
It is a brilliant piece of kit, which I have fitted to my recumbent trike.
I’ve used it for about three years. Totally maintenance free, with a 360% gear range.

The downside, as with any CVT, is that it is quite heavy.

Neat idea, but I cant see the band standing up to the torque applied to a uni… its bad enough when the bearing housing on a schlumpf lets slip and you get spat off the back!!

Actually looking at this then it could be feasible if you could turn the coned gears a few degrees so the faces were parallel and had 2 gears between them that with one sliding over the input shaft. They would need bevelled gears on the end to sort the angles out again… its all doable :slight_smile:

Gears with teeth? Which engage with teeth on the cones? Presumably the number of teeth on the cones continuously varies from one end to the other…

Depends on the shape of the teeth and the radius at the points of contact… very complicated!!

Supposedly Argo cats have this system - we’ve got a very old one however supposedly it randomly fluctuates the gear ratios, definitely an issue on a unicycle. On the other hand, it does mean there will be a way to prevent it slipping though :slight_smile:

An Argo Cat of the same era as ours:

Argocat vehicles probably use a variable diameter pulley (VDP) type CVT. I’ve also seen these on treadmills and drill presses.

There are different types of CVTs. Some times may be easier to configure for a unicycle than others.

Random web surfing about geared hubs brought me here.
I find the NuVinci hub very ingenious and was telling to myself that Justin could easily adapt this system to a unicycle, attaching the NuVinvi externally to the wheel, and guess what…

He actually did it, look at this video

And HERE is the thread

Unfortunately, because of the freewheel/back pedal forces issue, it appears that the NuVinci hub is not fully transposable to unicycles, at least as it is set, I’m not sure to understand why (or if) the “balls” idea isn’t transposable to a non-freewheel unicycle hub, but it would be so great to have an infinite amount of gears between low and high gear on a unicycle, especially with a shifting system on the handlebar.