Edit : double post
I don’t like your idea at all, and it doesn’t seem accurate enough to keep crank lengths the same, besides being difficult to use that little force for that big of a change…especially without body weight ending up as a factor.
Sorry.
And I’m still trying to figure out how that is “geared,” and what use it would be… your cadence has little to do with what length cranks you need. Shorter is better on flat, longer is better on higher gears, rough terrain, any sort of hopping, etc.
Where does your size factor into desired crank length again?
NEXT!
I like it, but can it be done?
The answer is no, pending miracle.
Seriously though, what use does it serve?
You don’t want longer cranks just because you slow down, partly because that would slow you down even further. Then you’re dredging along at 6mph with monstrous appendages between your hub and pedals, struggling to speed up again because your feet have so far to move just to get each revolution. On pretty much any terrain I see no benefit…when there’s lots of hills, if you speed up and your cranks get shorter that just makes it harder to power up the next slope. On rough terrain, you want a maximum of control… and this doesn’t facilitate that either.
You’d be surprised at what can be made. It’s definitely not impossible to have cranks that change length depending on the speed that you pedal. Impractical, maybe, but not impossible. It actually sounds like a very clever way to approach the problem of gearing a unicycle. (though having it vary length by itself sounds a little iffy… I’d rather shift)
The Dakoroman thing is pretty neat, too. It could probably be built and ridden, but I doubt it would be practical. It seems like a lot of effort would be required to keep the pedals from spinning, which I imagine would wear out your lower leg muscles very quickly. Good concept, but not quite as simple as just getting a bigger wheel with tiny cranks.
Sorry to dig this thread up again, but since the gallery seems to be down a lot at the moment, and somebody recently asked me about my lego model, I uploaded the film to YouTube:
http://www.youtube.com/watch?v=8dr3-P_iY_E
Rob
1 Like
You necro-poster/Lego-master/top-man.
So dakoraman really was a genius?
I really miss him.
john
me too
I’m quite seriously thinking about building a real pair. My fabrication skills and equipment aren’t fantastic, but my wife works at an engineering company where I could get some stuff done (they’ve helped us out with bits for our car before). The biggest problem is the axle interface - cutting either sqare taper or ISIS from scratch is probably not going to be doable, so I think I’ll have to cut the ends off a pair of steel cranks and use those. I’d like them to be ISIS because the only square taper uni I’ve got at the moment is a 20", but I’ve got two old pairs of steel sq-taper cranks lying around. I’m planning on having metal platforms for the “pedals” with screw holes so I can experiment with various lengths of wooden platforms.
I’ll see what I can get hold of off-the-shelf (gears, bearings etc) without it becoming too expensive (I’m assuming these aren’t going to be too practical!) and see how it goes. I’ll probably over-engineer things to make sure they’re strong enough (gearing wise) so I reckon they’ll end up with quite a big Q. We’ll see.
Anybody got a cunning plan for getting hold of ISIS interface without wasting a new pair of cranks? Broken pair of tubular steel cranks from a trials rider would be ideal I suppose.
If it happens I’ll post back.
Rob
I think the truly perceptive/enlightened/majestic recognized that from the start. It’s clear now the ground-breaking/revolutionary upgrade to all human transportation/locomotion has now been established/proved using the most basic building blocks of human engineering/genetics: the Lego.
But Harper and I–early true believers/disciples/liberators now gasp in astonishment/pain/indigestion at the treachery of rob.northcott. We have invested our live’s savings to start a secret dakoroman manufacturing operation in Harper’s garage, but our version–very close to market–is made of a mix of titanium and carbon fibre. We will simply be unable to match the cost profile of the much more economical Lego material, and I fear our joint-venture is lost/ruined/doomed.
I’m not going into mass-production Tom - your operation is safe!
Seriously though, I’m going to try to build a full-size pair of these cranks just for the novelty factor and to see if I can ride with them. They’ll be very heavy and not pretty, but hopefully will be good enough to play around with.
The obstacle at the moment is the cost of the gears… I’m thinking around 150mm cranks with 1:1.5 gearing would be a reasonable size to have, which would mean the two cogs need to have radii of 100mm (axle end) and 50mm (pedal end). Gears of that size from the suppliers my wife’s employers use are seriously expensive in steel (more than £100 each!), and I don’t think nylon ones (much cheaper) will be strong enough. At that price, a pair of cranks will cost me half the price of a Schlumpf hub 
. I’m trying to find a cheaper source of large gears. I could of course use smaller gears with an even number of idler gears along the length of the crank, but that would introduce more slop and make meshing tolerances much more of a problem (although it would look much neater than using a pair of massive cogs). The rest of the construction shouldn’t be a problem (although I nearly forgot to allow for a way of extracting the crank from the axle, which would have been a bit of a cock-up). Once I find some gears at a sensible price I think the project could be a goer, but I don’t want to spend a silly amount of money doing this.
Rob
Rob,
Just an idea, but could you use cogs from an old car gearbox? With all the different shapes/sizes of cogs in a gearbox there must be a couple you could use and I bet a scrapyard wouldn’t charge you much for 4 cogs.
Also, you could get an old set of ISIS bike cranks from ebay for not alot.
Im interested to see the outcome of this. Your Lego model was great!
Thanks Edd - we thought about car gearboxes (got a couple lying around!). They’re a bit course pitch really (so likely to have more slop), but may work if we can’t find anything better. Getting some ISIS cranks to chop up shouldn’t be too much of a problem - I thought about doing it square-taper to fit my 20" (less scary to test ride!) but with such big gears a pair of 150mm cranks would actually be 200mm long, and a bit close to the floor on a 20". I think I’ll probably go for ISIS so it’ll fit my 26" and other unis.
Should be good for a laugh if nothing else
Rob
Rob-
Try these guys for cheap gears. They have thin face, hubless, steel spur gears for $66US for a 100mm dia and $26US for a 50mm dia. That’s about 60 pounds sterling for a pair if you can use them. The gears with hubs are even cheaper. But then you have to ship them and they ain’t light.
We already knew these things would be heavy.
Thanks for that. They actually need to be 100 and 50 radius rather than diameter (to give a 150mm crank length), so 200 and 100mm diameter. It seems that once you approach that sort of size, the prices get silly. I’ll have a look at that site though. You’ve made a few geared machines in your time… do you agree with my assumption that nylon gears (even 10mm wide ones) would not be strong enough? They’re way cheaper, but I’m leaning towards over-engineering in the absence of proper calculation.
Rob
Nylon is really great stuff as long as there is no applied torque. After that, things get a little bit fluid. Plastics do so like to flow. In this case you’re talking about a one tooth nylon contact surface that is 1cm wide, not a six steel tooth contact surface that is 1cm wide. There is an enormous difference in the yield strength.
Sorry about not being able to tell the difference between a diameter and a radius. You would think I would be able to do that by now. Oh, well, what’s a factor of two here and there?
This is exactly why our dakorman operation is so far over-budget/under-funded/screwed. Well that and the fact he’s been funneling our seed money into a screwball idea for a “Home Particle Collider Kit” that he thinks will be a big hit with the high school science geek set.
Recent/current/topical discussions have left me entertained/engaged/amused.
john
Thanks Harper (or repraH sknahT?), that’s what I thought - I’ll try to stick with steel or brass.
Rob