Wednesday, November 19, 2008

Building a Retro-Direct Drive Bicycle

November is the slowest month in the bike shop and last Saturday night around 5:00 PM Dan, Donald and I were found ourselves in the shop with no customers. Some of the day's discussion had involved internal hub gears, including the old Bendix 2-speed kickback hub. "Yeah, those were nice," I said, "but what I'd really like to try sometime is a retro-direct drive." 'Yep," Dan agreed, "that'd be something." "What's a retro-direct drive?" Donald asked. "The internet knows," I assured Donald and a quick Google search later we were all reading the Wikipedia article and following the various links. On a retro-direct bicycle engages one gear and drives the bike forward, while pedaling backwards engages a second gear that also drives the bike forward. "Weird," said Donald. Weird but also fascinating.

By the time we read on Pierre's page that there are about 8 retro-direct riders in the world Dan and I had decided that we'd be the 9th and 10th. Edison said that to invent you need "a good imagination and a big pile of junk." We weren't inventing, we were just building and while Bike Works isn't exactly a big pile of junk, it is a treasure trove of parts.

Our first attempt, involving threading a single speed freewheel onto a freehub body holding a single cog and some spacers, didn't work. We figured out that we really needed two completely independently spinning freewheels. While Dan worked on re-spacing a rear wheel and scrounging for freewheels, I got a sweet Trek from the warehouse and located an idler wheel I'd spotted awhile back in the attic.

A threaded bottom-bracket cup is the key to getting two freewheels onto a single hub and only a Dicta freewheel works as the rear freewheel. Shimano, ACS and all the other freewheels we found have a lip on them for the removal tool and this lip prevents them from threading onto the cup. But the Dicta freewheels have clear access to the threads.

Saturday night Dan and I stayed until around 8 PM, getting a proof of concept drivetrain running. Our only Dicta freewheel was a 16 tooth, which was the same size as our forward driving freewheel. So our two-speed drive was drove the wheel with the same ratio whether we were pedaling forwards or backwards, but it did work.

For the real drivetrain, we ordered three 22 tooth Dicta freewheels from J&B. One for me, one for Dan and one for the first person who is going to read about this on the internet and want us to build one up for them. The freewheels arrived today. I had my camera handy, some very cool Newk bar ends and a top-tube pad that happened to be perfect for the bike.

The pictures show how things go together. The bike is still undergoing refinement. The chainline needs work and the bike wants to throw the chain. Joe had a good idea to add a larger inner chainring to help keep the chain on, but I'll probably swap the cranks and come up with a better idler arrangement in the next couple of days. But as you can see from the video of the bike in the stand (Joe is turning the cranks while I do the narration), the retro-direct drive works.

And yes, it's pretty strange to ride. When I get the drivetrain more solid, I'll take it further than just around the block.

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