elliptical chain rings were all the rage back in the late 80’s from memory - one of my first MTB’s back then had them. Wasn’t the theory to give you a long power stroke? lol… that doesn’t sound good. It didn’t work for me anyway
The idea of Biopace was to dig up an idea that had been tried and discarded 100 years ago, and about every 20 years since then, and market it as something new so they could generate more sales. And then generate still more sales when people switched back to round chain rings because elliptical chain rings suck.
Sorry to dig up an old thread, but I was thinking about offset cranks the other day and my thoughts wandered back here… I had this idea that’s kind of crazy and probably impossible to ride but I want to try!
So how about this: cranks offset from the hub a bit with a single chain on each side connecting crank to hub. No spindle connecting the cranks, they’re independent. Put a sprocket on each crank shaped so that you’re in really high gear when the crank is in front, and really low gear when it’s coming back up around the back.
You’re feet would each be rotating at varying speeds through the stroke, and out of sync with one another. They’d spend more time in front than in back.
Ultimate accelerating machine or stupid death trap?
Seems like this would be possible at least on a bicycle since chain tension is less of an issue.
I had a single speed bike on which I put a front elliptical chain ring and no deraileur. The chain tension goes from too-tight to too-loose on each rotation–twice. The chain is not designed for this and eventually the chain will stretch enough to jump the sprocket. Biopace rings can only work if there is a spring loaded chain tensioner to absorb the varying length of chain required based on the elliptical requirements of the sprocket.
I used mine until it failed–ruined chain, ruined sprocket, and bent axle before I gave up on it. Round chain rings are critical with a fixed-length chain.
Aside from the slop necessary to make an eliptical chainring work on a fixed gear I see one problem with this concept. If you have a lower gear as the crank comes around the back you will have more crank travel to check your balance around the back than you have in the front. I think that would add to the dificulty, but it wouldn’t make it impossible. After all, even an impossible wheel can be ridden, can’t it?
I used to have a non-round chainring on my old Recumbent bike (1985). It seemed to work all right on there, but I never tried it with round chainrings. Mine were not the Biopace brand, nor were they elliptical, the were more squarish.
On a giraffe it would be interesting to try, though apparently it won’t work unless both sprockets are non-round. A fun machining project for someone who’d find that interesting…
The idea of the two-drive train giraffe described above sound interesting; I didn’t completely follow the intent, but it would be fun to try. Tom Miller (The Unicycle Factory) made something along those lines; two separate drive trains with a one tooth difference between the sides. Start off normally, and after a dozen rotations or so you’re riding a kangaroo giraffe (both pedals facing the same way). Another dozen rotations or so and you’re back to normal. Very interesting to ride!
This is actually pretty interesting in that it combines the concepts of gear ratio and crank length in a way I haven’t see before, though I won’t bet that it wasn’t done in the late 1800’s. World’s Fair Crazies.
More recent examples of similar designs:
Rotor cranks - use a cam mechanism to provide cranks with uneven rotation speed.
Revelox cranks - use cranks which change length depending on location.
I don’t think they are actually in production (surpirse!) but they seem like they might not be a terrible idea on a unicycle, though I can’t say I see them improving speed or efficiency either. :o Especially since crank lengths are short enough that spinning isn’t really that hard.
Blah blah blah… Your right, but it is still an interesting avenue to investigate provided one doesn’t expect to strike engineering gold. There was some interest in using non-round crank systems on faired HPV’s because it allowed for a smaller nose cone, though ultimately the aerodynamics probably didn’t pan out. Though perhaps for rearward facing riders… The point being that it’s one thing to recognize why something doesn’t work for a given application and another to just label it as useless. I only have a problem with the people who try to market it regardless.
This could be solved if both the chainring and cog are non-spherical and in an alignment with inverse changes in chain wrap. So that as the chainring gains in diameter the cog loses the necessary diameter to maintain chain tension. Sort of an interesting exercise to figure out the arrangement necessary.
Oval chain rings are good for climbing hills (mountainsides) at low (60) rpm. they’re funky for spinning (90+) and will rip up a fixed gear system.
I thought Unicycles use shorter cranks so we can spin faster than say, 120 rpm smoothly to attain higher speeds since we’re pedaling as fast as the wheel is spinning. Oval rings seems to do the opposite of the premise of smaller cranks for speed. Do you intend to use long cranks and pedal at lower speeds and travel up silly steep hills? The only local event that really deems such rings is the Mount Washington Hill Climb… and as soon as it’s over, all the mechanics are pulling them off the customers bikes until next year!