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Belt Drive
From time to time toothed belts are proposed as an alternative to chain drive. A belt may be lighter than a chain, though the pulleys are wider and so, heavier than sprockets for a chain. The major advantage touted for this system is that it is cleaner than a chain.
There are serious disadvantages to this approach, however:
- Belts can't be opened and re-closed as chains can, so it is not possible to fit one to a standard diamond frame, unless you cut the frame open, because the belt has to be looped through the triangle formed by the right chainstay, right seat stay and seat tube.
- Belts can't be lengthened/shortened as chains can, making it difficult to customize the gearing by going to a different sprocket/chainring size.
- Belts bend instead of pivoting, and are less efficient than chains.
- Belts are stretchy. To avoid skipping, they have to be kept tensioned, further reducing efficiency.
- Belts can't be made as narrow as chains, so they do not lend themselves to derailer gearing.
As with shaft-drive systems, all of the benefits touted for this system are also attainable with an enclosed chain drive.
[Update, June 2012: Gates Carbon Drive carbon-fiber reinforced belts are currently in vogue. They don't stretch as much as other belts and so, don't have to be tensioned as much. However, carbon fiber is brittle, and these belts will soon snap if fibers have been damaged -- if the belt is "levered" onto a pulley from the side, or if a twig or trouser cuff is caught between the belt and a pulley. A Gates belt (and probably others) should be fitted over the pulleys loose, and tensioned by moving the pulleys farther apart, so the belt is subjected to stress only lengthwise, as in normal operation. -- John Allen]
Also see R&E Cycles comments on belt drive for the tandem synchronizing chain.
