Someone on the unicycle discord (TinyCrankz) was looking for a Schlumpf to put on their 20”, I know that Schlumpfs are hard to come by so I’ve been thinking about multi geared unicycles and came up with this. I need to play around with geometry to get everything working but overall are there any problems or suggestions?
There will always be a ton of play between cranks and wheel when you use a derailleur type setup on a unicycle.
Correct but that’s just on idea of many that I have and currently one of the most feasible options for me to make. I’d be happy to share my other ideas though, learning as group and seeing new perspectives is the best way to learn and make progress
A freewheel derailleur unicycle would work – build one of those and learn how to ride it and shift on it, that would be impressive (with a pretty significant chance of getting hurt in the process though).
A fixed (ie conventional) unicycle with a derailleur – that is just not going to work. Go and look at the chain-line on a bike with a derailleur and see how the torque from the chain ring is transferred to the sprocket on the wheel – the chain is flexible, it must be under tension to transfer the linear force from the chainwheel to the sprocket. The ‘bottom’ path of the chain with the derailleur/tensioner in it is just there to move the chain to the appropriate sprocket/chain-ring and tension the chain enough so it can return to the chain ring, it cannot transmit any significant force – any force which is transmitted in that direction would be against the springs, so even very stiff springs would have movement.
The suggestion with two geared drive trains at either side running in opposite directions basically won’t work for dynamic shifting either as it would have to have perfectly synchronised simultaneous shifting on both sides, which would be pretty difficult to achieve. In this case, stopping and statically moving the chain at each side could perhaps work. However since you are driving the crank on the opposite side through a jack-shaft going to the other crank through a derailleur running in ‘reverse’ (see argument above) you will essentially only have ‘drive’ from one side – with one side giving you torque in the ‘forward’ direction and the other torque in the ‘reverse’ direction. So it would be like riding a freewheel one-footed – albeit you could do it forwards and backwards 
So basically, it won’t work.
If you want a multiple speed drive like this you will need to have a non-sprung/rigid chain tensioner. You would essentially have a multi-speed Hunirex type arrangement – search on here for the issues there were with chain tensioning on that. The problem with using chain rings is the relatively large difference in chain length you have accommodate hence the chain path with a derailleur changing direction to take this up.
So, thinking out loud, so I don’t seem quite so negative about new ideas – a cheaper (still not cheap) solution which could give you 9 gears instead of a 2 speed Schlumph, and be shiftable on the fly, would be to build a custom frame with a Pinion (or similar) gearbox – two identical chain drives up to the gearbox, connected via the gearbox input-shaft, then the output shaft taken down to a single drive sprocket on the wheel….
I was thinking a Red Menace style jackshaft uni so the cranks have their own axle. Not my uni, this is red menace
Do they make a fixed version of the Pinion?
I think you’d end up with a many speed freewheel.
I’ve been thinking about gearbox hub that can turn a one way hub into a fixie. Like you said an outboard multi speed hub or CVT with a chain down to the wheel like Justin lemires Sturmey 3 speed, but with a freewheel hub. The gearbox hub utilizes a differential and a “push rod?” To choose which side powers the wheel. But the only problem is that it requires 3 chains two from the cranks one regular and one crossed chain to make it go backwards both are on opposing freewheels but the problem is that most multi speed hubs don’t have any reasonable way to add another freewheel. 3rd chain is the output from the mid drive hub to the gearbox hub where the magic happens
Do we need a “hypothetical unicycle ideas” thread?
I keep being really disappointed coming in here to see no new projects 
That is a good point, I had something in mind that it (or maybe some other brand) did need an external freewheel but I didn’t actually check. Alee Denham (cyclingabout) has some good videos on gearbox drivetrains, I’ll rewatch some of that. Either way, a ~550% range is probably a bit excessive.
It is very excessive for unicycles but make sense for cargo bike MTBs, anyway I did find that shamans used to make freewheeling cranks so if I can add a second sprocket to my nuvinci and make a pair of freewheeling cranks with opposing freewheels then make the “Corrector hub” then we could have a fixed and somewhat lightweight CVT uni. But why worry about 150g if I ate 400g of cookie while writing this
That would be a good idea, some people have wonderful ideas but don’t have the skill or tools to make them happen.
I didn’t realise you were thinking on having a concentric pedal-spindle/hub as to work a derailleur you’d need a much longer chain-line than there is here, so I assumed the idler shaft would be outside the wheel. Either way, the same arguments on derailleurs apply. However, if you went with the concentric approach as per RedMenace and you had an external block on a lay-shaft (say 3 sprockets) and a suitable method of applying fixed tension you could probably make this work (if you manually changed gear and tensioned afterwards).
But in some cases that can be a good thing. For example, if the obvious solution is welding but I can’t do that then I’ll take more time to analyze the problem and I might find that welding locks me out of serviceability or adjustability
I got this idea from a video of a guy who wanted two gears on his singlespeed so he put a two by up front and a chain tensioner in the back and he would hop off and manually switch gears and the tensioner would pick up slack
@Canapin, not sure how best to flag a split, but maybe you could do such a thing? , see Post Your Current Projects Here - #928
done 
Nice! I’ll be posting here a lot
May we please see the insides?
