Retro-direct

First you think it could never work. Then you realize it does work, but you think it's such a crazy and immature technology that it could never work well. But for a "pure" retro-direct, the chainline doesn't move at all, so it should be possible to make it almost as reliable as a normal singlespeed?

• en.wikipedia.org/wiki/Retro-Direct
• videos
  • hipster chain, though it's actually illustrative here, since it makes it easier to see which directions the chain is going
• "pure" retro-direct (2-speed)
  • myspace.voo.be/pcoupard/retrodirect_drive/
  • bicicletasantigas.com.br/arquivos/portal/galeria/m10.htm
  • twoshort.net/bikes/retrodirect/
  • cykelhistoriska.se/hirondelle.htm
  • velospace.org/node/8517
  • bikerodnkustom3.homestead.com/Lingo_retrodirect.html
  • flickr.com/photos/gabrielamadeus/sets/72157623278355896/
    • the tensioner design on this is pretty interesting — a lot of people admire it
  • circleacycles.com/projects/2009/01/16/retro-direct-experiment-complete/
  • kensbike.blogspot.com [1] [2] [3] [4]
• retro-direct with a front derailleur (4-speed or 6-speed)
  • A "pure" retro-direct has a fixed-position idler pully; its position should never need to change. Chain tension is set via horizontal rear dropouts or other usual fixed-gear solutions.
    A 4-speed or 6-speed retro-direct must have a spring on the idler pully to make up the difference in chain length between the front chainrings.
    
    
  • m-gineering.nl/retrog.htm
  • members.westnet.com.au/rjharrop/recumbents/retro-direct/Retro-Direct2.htm
  • frankb.us/gears/
• retrodirectcycle.wordpress.com/
• discussions: [1] [2] [3] ongoing

caveats (that can be worked around)

• It's not possible to position the pedals without lifting the rear wheel (no different from a fixie).
• Pedaling backwards is qualitatively different than pedaling forwards, and may have DIFFERENT optimim crank arm lengths. Usually the difference between optimum-forward-crank-length and optimum-backward-crank-length isn't so great that you can't find a happy medium.

downsides

• noise? I can't see why this is such an issue, but this page lists it as the primary problem (doesn't seem that bad)
• the bike will refuse to be rolled backward

real-world gotchyas / practical improvements

• figuring out how to get the "when pedalling backwards" half of the chain to avoid intersecting the chainstay seems like it can be a major issue, unless you happen to find the perfect frame
  • a lot of solutions seem to use another idler pulley there, to deflect the chain out of the way of the chainstay... but do you really want an idler pulley in the path of the main driveline? You're putting a lot of force on that chain. I assume that particular idler pulley MUST be welded on, not just clamped-on?
• his pulley ended up getting caught in the spokes
  • yeah, scary stuff
• Cranksets aren't always happy being driven backwards. Lock-tite EVERY screw on the crankset. The crank threads are a particularly difficult problem — settle on some cranks/pedals, and then epoxy them in for good.
• if you want to have a hope of getting the freewheels off, make sure you put lots of grease on before installing them
• it can be important to manage the chain's desire to deflect side-to-side, since the chainline is never totally straight
  • there are really two chainlines operating simultaneously, so it's not possible to make both straight