h a l f b a k e r yThe word "How?" springs to mind at this point.
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It involves a mechanism which slides the front toothwheels and front derailleur sideways when changing gears in such a way that the chain always run into a straight line from the crankset to the cassette.
The pedals themselves don't move sideways
The purpose of this mechanism would be that it provides
a more efficient and more silent chain drive and that it would be less distructive to the chain and to the smallest and biggest toothwheels from the cassette.
this is it
http://en.wikipedia...ki/Derailleur_gears
[zeno, Apr 21 2009]
Speaking of engineering...
Grasshopper_20Cycle
[normzone, Apr 22 2009]
[link]
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Can you describe the mechanism so we can throw rotten tomatoes at you in order to encourage improvements or necessary deconstructionism? |
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Sounds reasonable... does the pedal slide out as well? Would that end up making you a bit unbalanced? |
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In my link you will find a picture of the Shimano XT rear derailleur. The lower part of this alignes the chain to the front toothwheel(s). It does exactly what you want. |
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//a mechanism which keeps the chain in a straight line for derailleur bikes\\ is baked and widely known to exist. |
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The problem you describe does not exist. |
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I don't think you can have understood the problem correctly, [zeno]. The article doesn't say anything about the Shimano XT other than as a caption. If it's the same as a normal rear derailleur, it must twist the chain in some ratios to line it up with the front sprocket. |
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Baked by SRAM, but it is a good idea.. |
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The link is there just for the picture. This derailleur, as indeed any other quality derailleur, guides the chain. It does //twist the chain in some ratios\\ but it is very quiet, very efficient, not destructive to the chain or the toothwheels. |
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If I would have thought this idea existed in this exact form I would have said baked and called for deletion. |
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I am challenging [mr Dries] or you,[Srimech] to explain why this would be better and how exactly it would work better. |
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[Suede] now says it's baked anyway so I will look at that and come prepared. |
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when having a bike with a rear derailleur and the chain is on the smallest sprocket, the chain is not in line with the middle line of the bicycle because the smallest sprocket is not directly behind your front toothwheel. I'm sure many of you have noticed how unaligned a chain can be and this causes a considerable amount of friction and wear and tear on the chain and sprockets. If there was a system which would let the front toothwheels slide sideways when changing gears in addition to the existing derailleur system, this problem would be solved |
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I agree with mr Dries; all conventional derailleurs mean
that the chain is displaced sideways. Suppose you're on
the smallest (outermost) rear sprocket and on the smallest
(innermost) front sprocket. Viewed from above, the chain
is bent (it runs front-to-back over each sprocket, but must
kink sideways between them). I know that on my bike at
least, the extreme ratios do cause greater friction and
wear. |
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As to how you'd solve this, the only easy way I can see
would be to have each gear given by means of a matched
pair of front and rear sprockets; this wold be rather like
the gearing on some belt-driven machine tools, in which
there are two series of pulleys, one biggest at the top and
smallest at the bottom; the other smallest at the top and
biggest at the bottom. The belt can be moved up and
down to change ratios, but is always in plane. |
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a sprocket that slides to relieve the side load? Never be crossed up again! People have been trying for decades to integrate the concept of shifting with the concept of sliding and have failed...... I think a system that moved the gear sets in and out and used the tension of the as yet unrealized shift to lift the chain and let it drop onto the next gear as the shift was achieved would work, but my bicycling friends just laugh and use french words. |
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Cross-chaining, as it's called, isn't going to be solved by this idea. The idea itself just suggests a solution, but doesn't give any method for making it happen. It also doesn't address the fact that the space between the pedals and the chainstay is already as crowded as it can get--there isn't room for sliding sideways. |
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Also, sliding bits from side-to-side is fairly obvious, and has probably been thought of many a time. Only the fact that it isn't at all easy to do has prevented it being built. As I've said before, bicycles are an highly researched area with a long history of both large corporations and garage wackos working on them. |
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Cross-chaining isn't truly a problem, anyhow, although it is bad. Only amateur riders ever let it happen--most kids cruise around using only one gear of their ten-speeds, the gear where both dérailleurs are relaxed, with the chain crossing from the smallest in front to the smallest in back, something serious riders never do. Most bikes are designed so that a similar gear ratio is available with less cross-chaining (a "ten-speed" doesn't really have ten speeds). Most chain-tensioners are not designed to take up the level of slack that cross-chaining involves, anyhow--the idea as described would over-extend the chain tensioning mechanism. |
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The solution to cross-chaining isn't sliding chainwheels, especially without a method for making them slide. Either teach the rider how to shift properly, or get them a simpler bike. |
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Could we have the chain a fixed constant, one piece, not a loop, a straight line performing as a linkage, and have all the work done at the pedals and rear hub? |
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//the chain a fixed constant, one piece, not a loop, a straight line performing as a linkage// |
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You mean a shaft driven bike? Did I get that right? |
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There already exist highly reliable rear hub gearsets, they use planetary gears and I can't remember what they're called right now. But as for shaft driven, then you'll need two right-angle bevel drives, which aren't all that light or efficient. I think you'll end up with more problems than you solve, but as usual I'm happy to be proven wrong. |
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There are bike drives where each pedal is on a lever that oscillates up and down. A chain takes the pull to the rear end, and drives an almost-normal freewheel as a spring-loaded ratchet. Some others have the oscillating pedals and the chain, and very odd back end mechanisms. |
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Oh, I'm not interested in solving problems. I have to do that all day long. |
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I just like overly complicated engineering solutions - it gives me good practice to create what I have to resolve at the day job. |
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I wasn't thinking so much of a worm drive as an incremental transfer - you pedal like a fool and the straight chain, tensioned at both ends, only making contact at the tops of the sprockets, inches back and forth a few links as required to keep the sprockets moving. Guess there will need to be a clutch in there someplace as well. |
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I think it could be done. |
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However, to take up less space, the sprocket cassettes are going to need to be re-designed to be a series of nested splined cylinders. Thus, all of the sprockets move, and there are only two positions - "in" (in-line with the chain) and "out". A splined rod on each side of the cassette supplies the shifting motion - a cable draws a cam along the rod to the appropriate position (selects the gear to be moved), and a bell crank twists the rod, forcing the cam to shove the gear to the other position. |
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The chain tensioner, since it no longer needs to move around to keep up with the chain, can move off the rear dérailleur and onto the frame at the chain's midpoint. I think there should be some advantages to that, as well. |
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The lift-to-shift part can be accomplished by a jump-cog which catches between the chain and the sprocket at the point of impact, and, once caught, perforce circles the sprocket once before releasing. It can be operated in much the same fashion as the knotter needles on a hay baler. |
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<edit> sp- baler. Dang geeks that do these spell checkers, they're not farmboys, neither.</edit> |
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" knotter needles on a hay bailer " ? |
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Damn, now, I need to google. |
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I just see this as splitting the 3D motion of the
derailleur to an up/down motion simple derailleur
and a in/out motion of the cogs( could be front or
back). |
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