|The Hirondelle retro-direct shown here is probably the|
definitive version of the system.
|As seen in this little animated file, when pedaling|
forward, the smaller cog is engaged, while the larger
one "free-wheels"backwards. When backpedaling,
the smaller cog "free-wheels" while the
larger one is engaged for a lower gear.
The simplicity and reliability of the retro-direct system comes from the fact that without derailleurs and shift levers, the only "shifting" a person needs to do is to reverse pedaling. Some have even believed the system to be beneficial because it would develop more and different muscles than forward-pedaling alone. That would seem to make sense, though I don't know of any studies to prove it. In contrast, from what I've read, it can be very difficult to generate the same kind of efficiency when pedaling backwards. Bicycle Quarterly's Jan Heine has tested a few retro-direct bicycles and found it difficult to back-pedal at any more than 45 rpm. I've read other impressions around the internet from people who have built their own retro-direct systems, and they seem to confirm that pedaling backwards is generally an awkward endeavor, and even more so when trying to do it out of the saddle. By the way, I have also read that a possible unexpected problem can present itself with an R-D drivetrain -- that pedaling backwards for an extended period can unscrew pedals from the cranks! A healthy application of loctite may be in order.
|A 1920s Hirondelle, with double chainrings and |
a unique front derailleur. (from M-gineering.nl)
|The only dedicated R-D hub on the market today.|
Made by Curtis Odom.
Luckily for those interested in this alternative drivetrain, someone else who has caught the retro-direct bug is component maker Curtis Odom, whose vintage-inspired hubs are well-engineered things of beauty. Curtis was first commissioned to build a retro-direct hub for Hojmark Cycles in Germany, though he has since made several others. In fact, Curtis Odom is almost certainly the only person out there today who makes a dedicated retro-direct hub. Unlike the DIY versions out there, Odom's hub has a much longer threaded section which fully supports the two independent freewheels. Not only that, but just as with modern cassette hubs, the right side of the axle is well-supported by outboard bearings. Odom's R-D hub kit is available with an arm to hold the return pulley in position, which is a smart touch that makes it reliable when retro-fitted to bicycles that don't have a brazed-on attachment point for the pulley (as in any bike not custom-built for a retro-direct setup). See Curtis Odom's website HERE.
|A modern, and beautiful, retro-direct bicycle -- built by Hojmark Cycles, with hubs by Curtis Odom. Note that the return pulley is attached to an elegant little brazed-on mount on the chain stay.|
Retro-direct is an interesting curiosity of an alternative drivetrain. I don't think it's a system that is exactly poised for a comeback, but clearly there are plenty of people interested in it, and plenty of do-it-yourselfers who are keeping the idea alive. It's not something I see myself building, but certainly, any bike at a club ride with an R-D drivetrain would be the topic of much conversation and would be a blast to try out.