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Can't find anything on Google because I guess I don't know what search terms to use.

I am wondering if anyone has ever seen a derailleur setup like I have drawn in the picture. It is NOT for installation on a normal bicycle, but a normal derailleur takes up too much room below the gears and will not work for what I want to do.

(Using a small wheel in larger frame - I have lost all but 4" from gears to ground.)

Please advise on this 'central sprung gear' idea - if you've seen it, if it would work, etc.)

OH_ That three-gear center assembly would slide back and forth sideways on shafts to allow chain alignment.

enter image description here

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    i would think an internal gear hub would be easier to find and to setup. – njzk2 Dec 12 '16 at 19:14
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    No, because it would be very hard to build. Your two "fixed" pulleys have to move laterally as you change gears, but by less than the shift distance at the rear gears. So does the sprung pulley, except that it also has to move vertically. It's a complex design, and making it also robust, lightweight and cheap... I'm not sure it can be done. You asking suggests you don't have a decent metalworking shop available, so I'm going to suggest that it'll be expensive, heavy, fragile, or all of those. An IGH is much simpler. – Móż Dec 12 '16 at 19:53
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    The idea with the derailleur is to have one jockey wheel as close as possible to the the rear cogs as it speeds up the shifting. If the distance is too be big, like in the above design, it is also detrimental to the precision of the shifting and the gear changes will be sluggish. The system you put up could work for a machine or a cooling fan where frequent shifting isn't necessary. – Carel Dec 12 '16 at 21:29
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    I think you'd have trouble getting it to perform well with the pulley so far away from the cogs. That's a large part of why conventional derailers are the way they are. Note that you have at least two options for ground clearance focused small wheel/folder specific RDs, the Shimano Capreo and the Dahon Neos. – Nathan Knutson Dec 12 '16 at 22:42
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    The front fixed pulley is redundant. The rear fixed pulley has to muscle the chain across the rear cogs to change gear. That leaves the VSP as a chain tensioner attached to the frame, not to the derailleur. Honestly its not a lot different to a conventional derailleur. – Criggie Dec 13 '16 at 8:21
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I Designed wire and cable machines for about 10-years and a previous life and have indeed seen your setup many times as a system for maintaining tension on a cable; For that purpose it works great. If all you were trying to do was maintain chain tension at varying lengths, you would have something.

But as Carel points out in the comments (and I've spent years testing this), You absolutely want the application pulley/gear to be as near the point of application as possible. Failing to do so will guarantee that your chain will walk up and down the cogs at will and will not shift when asked to.

Your design would have to be modified such that the fixed pulley nearest the rear cogs would move in/out and forward/back as the gears change. Once you do that, you have something that looks very much like a modern derailleur (funny how that works).

The truth is that a derailleur is a pretty elegant and simple approach that has been tested, proven, and improved over many years. Your best bet, in my opinion, is to play with the placement of a derailleur so that it is less in the way.

  • yeah - good point. I suspect OP is trying to increase both ground clearance, and chain/tooth interaction on the smallest rear cogs. Somehow rotating a normal derailleur upwards would achieve some of that. – Criggie Dec 24 '16 at 9:01
  • My goal is just that- gaining clearance UNDER any derailleur. Looking at standard nes, I don't think I could rotate it far enough up in the front to get the active part above the lower line between the main and wheel gears and still have it function. Also the issue of how much the standard derailleur swing is an issue- they need about SIX inches of movement- which is just a damn lot when you're tryin to keep things compact.... O wish there was a chinese knockoff of the centrally mounted bicycle transmissions - would eliminate the need for the damn drailleur. – 111936 Dec 31 '16 at 8:13
  • @Nixt; Yeah, an internal hub really seems to be what you want. I recall in old bikes, the rear had a 3 or 5 position internal hub that worked great and were dirt cheap. What if you had in internal hub in the front and back, you might get the range you are after. – SteveJ Jan 30 '17 at 22:26
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If you remove the front fixed pulley the remaining two look a lot like a derailleur mounted upside down. Your rear "fixed pulley" has to be the one that does the shifts, so has to move in an out. The vertically sprung pulley could be that or could be a standard jockey pulley that moves fore and aft to take up the chain slack. Perhaps you should try to make a derailleur hanger bracket that positions the derailleur as you want.

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