I can’t quite get my head round how the system uses enough friction to transfer the torque without either making it hideously inefficient or grinding away the disks or balls, but I can only assume they know what they’re doing.
It sounds like one of the hardest things to learn with a geared hub is changing gear; one of these would make that a non-issue. All you would need then is a torque-dependent ratio and you’d be away…
Unfortunately they seem to have left out “unicycles” from their “Market Size” diagram… I’m sure it’s just a mistake.
You’d still need to rig up some sort of gear change linkage from what I can see on their site. That’s the most problematic thing on a unicycle hub. If it could be made like a self-contained automatic gearbox it would be interesting - probably bizarre to ride though, and automatic gearboxes waste energy. Could be a good novelty machine though!
I suppose you could have a sliding bar through the axle, a bit like the Schlumpf one, but make it variable gearing depending on how far the bar was pushed through…
Or make it change in little steps like a sequential 'box - tap one heel to change up a bit, the other to change down
now, I don’t know much about how this works, but wouldn’t a CVT be easier to run as a ‘freewheel-less’ hub? and couldn’t you just use the same thing as a bike shifter, only mounted under the seat?
The problem is where to have the cable going into the hub. On a bike hub it’s easy because the cable can just come out of the middle of the axle. On a unicycle hub you can’t do that because it would get tangled up in the cranks. It could be done on a chain-driven giraffe though. The Schlumpf method of having a sliding rod with buttons for each heel is a nice solution to the problem, but to make that practical with a CVT it would need to be made to go in steps like a conventional sequential gearbox (click one way to change up, the other way to change down, like on a motorcycle). I’m sure it could be done, but may end up horribly complicated and expensive (not that a Schlumpf is cheap). For practical purposes, two gears is probably enough for a slow-moving thing like a unicycle anyway.
In that case all you need is a torque dependent system that lowers the ratio as you put more pressure on. Low gear for powering up steep hills or accelerating, and high gear for high speed cruising.
ah. I see now, after watching the video. The cable comes out of where the crank bolt would be, and that’s most definitely problematic. Even if it’s got to be hand-adjustable, like via an allen key or some built in mechanism that goes through the axle, it’d be really cool to have something like a 29’er that could be geared from 24-72 theoretical inches. I’m a bit obsessed with the idea of a unicycle that is nearly as good as a bike for transport. Something like this seems like it would be non-freewheeling by default, and therefore require even less components to be turned into a uni hub. And that’d be just awesome.
I know what you mean Phil, but I would think an automatic torque-sensing gearbox would feel very bizarre to ride. Similar things were tried on bikes in the 1980s but were pretty hopeless. On a unicycle it could make balancing very interesting… it would be cool to try it though.
CVT is continuously variable transmission - that means stepless. They put CVT on twist and go mopeds and scooters. It is very different from the automatic gearbox that many cars have, where the car changes gear with a noticeable shift.
The way CVT works on a moped involves a V belt - a strong rubber/Kevlar drive belt that goes around two cones (drums) which change size according to torque and rpm. It’s all done with springs and weights. My gut feeling is that CVT would not be advantageous on a unicycle. It would be like riding with rubber cranks.
the site explains that this is not in fact how this CVT works, and it’s actually directly controlled by the user, there’s nothing really automatic about this. Some saturn VUEs have CVTs though, they’re awesome from a pure coolness of machienery point of view.
Mike: I know CVT can be stepless, but without a lever/cable this wouldn’t be practical as a manual system. It could however be set up to change in even steps when a button is pushed, and behave like a sequential gearbox. Of course it could be a torque-dependent fully variable automatic like Phil suggested, but I reckon that would be very freaky on a unicycle, where you need to use torque to balance. The CVT systems that were tried on bikes (using a sort of variable diameter chainring) were hopeless - if you tried to pedal harder it just reduced the gearing. The mechanism Phil linked to is not a belt-drive system like in DAF cars, it looks like it wouldn’t be “rubbery”, but I’m sceptical that an automatic CVT wouldn’t feel very odd to ride. If it was set up as a stepped sequential box though I reckon it might be possible.
There’ve been various CVTs designed for bikes previously. All of them failed because they weren’t as efficient as a derailleur or a standard epicyclic gearbox.
I’d guess this isn’t that efficient because they seem to be only aiming it at casual cyclists. If it was efficient, they’d be trying to get it onto racing bikes.
You could have the gear (ratio) changing done electrically and wirelessly. There was a high end road bike component company that made a wireless shifter for the rear derailleur. I can’t recall the name of the company right now, but it might have been Mavic. A battery, motor, actuator, and a wireless connection and you’ve got it.
The same kind of wireless shifting for the Schlumpf hub would be cool too. No need to try to tap that button with your heel. Just push a button under the seat and it shifts.
While that sounds possible, it also sounds complicated, and therefore expensive and unreliable. I don’t think I’d want any part of my unicycle that I use at all often to be electronic, I have enough problems with airseats.
It would be cool to hold a button under the seat and feel your legs have to push harder slowly to keep going. If these ever become available in unicycles, I’m going to buy one, if I haven’t made an excuse for buying an outta phaze yet.
The new MINI will eventually have a CVT gearbox, i know 'cause my brother worked on it at one point, the idea being that the engine is always held at the rpm at which it generates most power and thus better acceleration etc. and no jerk like an auto box ‘changing gear’. However, this means that whatever the position of the throttle, the speed or the gradient of the road the engine retains the same rpm and thus the same noise, which freaks people so much that they’ve had to make it only moderately effective, i.e. it only holds it withtin a certain range. Imagine flooring it to overtake, and the revs not picking up atall even though you’re suddenly going much faster i can see where they’re coming from. Back to the real topic at hand, i once saw a bike which used sliding weights in the wheel to change gear of a presumbly epicyclic hub, if you go fast the weights fly out and it shifts up, only problem, if you go fast up a hill and then slow dramatically it can’t keep up and you get stuck in a high gear. Might be applicable to a unicycle.
>I can’t quite get my head round how the system uses enough friction to
>transfer the torque without either making it hideously inefficient or
>grinding away the disks or balls
While they have various lists with advantages etc on the NuVinci site,
they don’t mention anywhere that the power transmission is efficient,
so that must be the answer to your issue: the thing is hideously
inefficient.
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I don’t know what they’re comparing the efficiency to. They may be comparing it to an automatic automobile transmission or maybe what they consider conventional CVTs. I’d be interested in finding out what the efficiency is for their bike hub in various ratios.
??? WOW