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.










video

25 comments:

Tarik Saleh said...

Well done buddy! That looks great. Chinese cargo trikes either come with retrodirect drive:
here
Or have enormous amounts of extra chain hanging as the retrodirect drive has been disabled:
here

Robert H said...

The "Retro Power" top tube pad is a nice touch. What is the difference in the gear ratio? I can't wrap my head around trying to calculate it.

Chad said...

That is really great!

I'm curious why you chose to not use 2 chainrings with 2 separate chains to the freewheels. Too obvious? Too simple?

Kent Peterson said...

The chainring has 38 teeth and the freewheels are 16 and 22 and the bike has 26" wheels. Forward pedaling engages the 16 tooth freewheel, so the math is:

38/16*26 = 61.75

Pedaling backwards engages the 22 tooth freewheel, so it is:

38/22*26 = 44.9

Kent Peterson said...

Chad,

The original thought was that the single chain plus an idler was cleaner, but it's gotten kind of ugly getting the chainline and tension right, so I may try the dual ring, dual chain thing. The back-driving chain has to run in a figure eight but that should be doable. And having 2 chainrings will give me more choices in terms of getting good ratios.

nollij said...

Wow... uber bike geekdom. Now *I* want one. :)

celeriac said...

A very simple way to set up retrodirect would be a double side drive -- get a flipflop hub and put two freewheels on, and fit a tandem crank arm. The left crank driving the freewheel on the left would engage when pedaling backwards.

Jeremy said...

That's freakin' awesome! Your next challenge: ride a 200k with one. Although I would imagine extended climbing while pedaling backwards might wreak havoc on your leg muscles. Then again, I've heard of people riding PBP with them, so maybe it's not a big deal.

Jason Nunemaker: said...

Nice work, guys! Bike geekery of the highest order! I'd like to think that wherever Sheldon Brown is right now, he's smiling at this nifty little hack.

Boxer Bikes said...

I spent long hours in the basement shop hacking my Bridgstone XO-2 to get this thing rolling. My gearing is a little higher; same freewheels (16/22t) matched to a 44t TA track ring and 54T chainguard with the teeth filed off.

My rig wants to throw the chain pedaling forward, so I'm going to build in a little guide.

I brazed a little armature to hold the hacked huret duopar derailleur pulley in place. Now I wish I'd included a provision for tensioning the chain. Maybe MKII will get that. I guess a frame with horizontal dropouts would have solved that issue. Maybe a half-link will work? Hmmm.

Photos to follow...

jimmythefly said...

Nice! I suppose you could drill and bolt a larger cog to the 22t freewheel if you wanted a larger difference in your ratios?

Erik said...

This is outstanding! What an interesting and fun project. I'd also imagine that power pedaling backwards is going to strengthen leg muscles in an entirely new way (like running backwards to increase forward sprinting). I only wish my wife would allow me to have another bike! I will definitely be linking to this post on my NorCal Bikers (http://norcalbikers.blogspot.com/) blog.

Cellarrat said...

I'm gonna have to get one togther!

Nicely done!!

Boxer Bikes said...

So, tensioning the chain helped, but a days commuting managed to unthread the bottom bracket cup from the larger freewheel (a block and a half from home).

Loctite applied to the cup and outer freewheel should help with that, but I torqued the whole assemblage further on the freewheel threads on the hub. Chainline was affected and I threw my chain three times within a couple blocks from the house, this morning. Back home, switched bikes and finished my commute.

I promise, photos to follow...

Neve_r_est said...

FYI 1/8" Shimano freewheels are also threaded all the way through.

DG

gpickle said...

Here in Iowa City a clever fellow with a welding torch built a tall bike with 2 rear wheels stacked one on top of the other that worked as retro direct. It is a cool project however you approach it and a blast to ride.

Boxer Bikes said...

Solved the chain tension/chainline issues. Commuted 40 miles on the thing this week.

It's a little rough on the knees/muscles since I never use them in this way. It's extra strange to pedal backwards standing - the nose of the saddle is in the way. Perhaps this would be an appropriate project for "THE Seat".

Photos of the project here:
http://www.flickr.com/photos/drboxer/sets/72157610220540976/

Anonymous said...

Awesome. There are a couple photos on this post from Zoobomb.net that show another implementation if you are interested:

http://zoobomb.net/forums/index.php?topic=19332.0

Donna

Anon of Florida said...

Wow, that Zoobomb build is so clean.

How is the chainline with the idler worked out? Is the idler lined up to a particular cog, or is in some intermediate position?

Flower said...

This is just brilliant! I was building a fixie from assorted recycled parts that I've been hunting down over the last few months, but was looking at gearing options, found out about the Hirondelle & now I'm 100% converted. This one-ups the fixie in efficiency. Very impressive conversions- the build on zoobomb is particularly handsome.
Can't wait to implement the retro-direct transmission on my own build!

Fish said...

I built one based on yer pics. Thanx... We use those big freewheels on adult trikes so I had one layin around... Wondered what I'd ever do with that odd beast. Just pulled an old BB of the same cup style outta the trash the other day too. Whole build cost me around 40 bucks. Took about 3 hrs... Works flawlessly... Blows minds everywhere I go...

Anonymous said...

where did you find a 22t dicta freewheel. the only dicta freewheels i could find only goes up to 18t

Kent Peterson said...

I got my freewheels from J&B Importers. A lot of bike shops have accounts with J&B and they have some things you just can't get anywhere else. The Dicta freewheel is also sometimes listed as SUNLITE (J&B's house label) and I just found one listed on Amazon.

Prodromal said...

can you send me a picture of the two free wheels with bototom bracket cup for my blog?
the second picture.

I know Bob Bryant and think I have seen you at the shop.

Kent Peterson said...

Prodromal,

I don't have your email but you can right click on the images from this post to save them to your computer.