To start off, what is retrodirect? This is a two-speed gearing system that allows the rider to engage one gear by pedaling forward, and the other by pedaling backwards. It's elegant in that it eliminates shifting. For a nice explanation of the general principle see the Wikipedia entry. Although they were once commercially manufactured, retrodirect bikes are now just another one of those obscure (although decreasingly so) custom DIY freak-bikes. I would suggest that they are rare in the modern bike world for two main reasons. First is the reason they originally went out of style; highly effective multi-speed bikes became available and still remain the easiest way cover diverse terrain on a bicycle. Second, to manufacture a retrodirect drive train requires modification or custom fabrication usually dictated by the parts available. Because of this second reason, there is no formula to follow, or standard set of parts to use. Each retrodirect bike out there will be different and reflect the unique construction challenges presented by the particular set of parts used.
What I did find when I set out to select my parts and plan my design, was that there was a significant lack of useful information available online. I found that most of the resources I came across focused on explaining the general concept of the drive train, but few contained insightful comment on the details and caveats of construction and how to overcome the common pitfalls. I tried to capture things that I learned the hard way in the hopes of preventing my fellow bike-hackers from repeating my mistakes.
The first part I collected was a frame. I selected an old steel road frame that had semi-horizontal dropouts to allow adjustment of chain tension. I've found that most other folks choose to use mountain frames and for good reasons. As you will see, a retrodirect rear wheel typically needs to be wider than 120mm, so a mountain frame with rear dropouts spaced for a 135mm wheel nicely accommodates the bulky hub. To overcome this challenge, I had to asymmetrically cold-set my frame (more on this later). Wider-set chainstays also help to reduce conflict with the chain. I've seen some folks that actually chop and offset their chainstays to move them out of the way of the complex chainline. I decided that it was easier to move the chain to accommodate the frame... but that's just me. In any case, know that you will probably have an easier time if you use a MTB frame.
The next part that was going to require some special attention was the rear wheel. I wanted a 700c wheel for the 27" frame I had chosen so that I'd have greater clearance for my fenders, but that's not specific to the retrodirect setup. What you will almost invariably need is a hub for a multi-speed freewheel. My original plan was to use a fixed-free flip-flop hub so that I could just flip my rear wheel, pop out a few chain links, and have a fixie. The huge flaw with this (as I immediately discovered upon attempting to stack two single-speed freewheels onto a hub designed for only one) is that the two stacked freewheels extended almost 14mm past the locknut on my axle. I did a little thinking about how I could still use the wheel, but every solution meant re-dishing my wheel each time I flipped it. The point is: USE A HUB DESIGNED FOR A MULTI-SPEED FREEWHEEL. I still wanted to have the versatility of switching to a fixie and ultimately came up with a much better way to accomplish it anyways, so don't despair oh ye of little coasting.
Here's a schematic representing half of a rear freewheel hub as seen from the back of a bike. I've omitted the axle for clarity so all that's pictured here is half of the body, the flange and the threading for the freewheel.
The first thing to assemble onto the hub is some type of spacer. The lockring from a standard English bottom bracket is the typical choice since it has the same threading as a standard English treaded freewheel (1.37" x 24 TPI). The reason for this will become clear soon.
Next, thread on the first freewheel. Because of the spacer, it won't thread all the way on, so some of its threads will still be exposed. Having these threads exposed will become important in the next step. If you were able to find a freewheel that was threaded all the way through, then you probably wouldn't need the spacer. This would be ideal because then the whole assembly would be attached to the hub by more threads. Because such a freewheel is typically unavailable, most people just use the lockring spacer trick.
Because you've used up all the threads on the hub, you need to artificially extend the hub threads THROUGH the first freewheel and beyond so that you can thread on a second, independent freewheel. This is typically accomplished with an adjustable cup from an English bottom bracket that will thread into the extra exposed threads from the first freewheel and serve to splint on the second one.
Finally, thread the second freewheel onto the exposed threads of the bottom bracket cup. In theory, it's very simple.
To review:1. Multi-speed freewheel hub
2. BB lockring as a spacer
3. SS freewheel 1
4. BB adjustable cup
5. SS freewheel 2
Because the stack of two freewheels was a little longer than the original 120 mm QR axle on the wheel, I replaced it with a longer, bolt-on axle and put an extra 10 mm spacer under the locknut. To accommodate this extension, I used a broom handle to spread the right (drive side) dropout out by 10 mm. By spreading only the right side, I can use the wider axle in the same frame without having to re-dish the wheel. BEFORE ATTEMPTING TO COLD SET A FRAME, READ SHELDON BROWN'S ARTICLE ON THE SUBJECT AND BE SURE THAT YOU'RE WORKING WITH A STEEL BIKE.
Here's the BB lockring I used as a spacer for this setup. Just thread it on the hub.
For my low gear, I got the biggest freewheel I could find (ACS CLAWS 22t pictured here). What I realized when I got it home was that it had a major problem. I was going to need to thread a BB cup out the back of it, but there was a lip that extended inwards from the edge which would prevent me from passing something all the way through. The lip I'm describing is hard to see in the picture below, but is pictured on the second freewheel (#5 in the diagram above). So, my word of caution on freewheel selection is: for the first freewheel (#3 above) BE SURE THAT YOU HAVE ONE WITH NO LIP AT THE END. I've seen some that are actually threaded all the way through, but at a minimum you need to avoid ones with a lip. I think that Dicta, Pyaramid and Shimano all make SS freewheels with desirable properties. The ACS ones WILL NOT WORK.
Well I didn't know any better at the time, so I just figured I could grind away the lip and then go on my merry way. After about 5 hours with my rotary tool, I had removed enough of the freewheel body to get the BB cup through.
This is basically what it looked like after grinding away the lip. I assembled the freewheels onto the hub as depicted above. I got the chain and the floating idler pulley all set up too, but that will have to wait for another post as you're about to find out.
Unfortunately (and the reason this post is just number 1) the removal of so much metal from the FW body weakened it to the point of failure. My first big ride on the bike was a disaster. I had to drop down to single speed on the big freewheel so I was doing about 150 RPM for 30 miles. What's worse is that because the freewheel had broken, it wasn't freewheeling anymore, so I couldn't really coast either. My legs were Jell-O by the time I finally made it to my destination.
Also, because I had to grind away the teeth normally engaged by the FW removal tool, to get this one off, I had to disassemble it as guided by the Park Tool site. Disappointing though it was that I would have to try again to get the RD working, I was now granted the greatly educational opportunity to dismantle a freewheel and see what all the guts look like.
Once you remove the top cone (with a pin spanner) you can see all the insides. The first thing to note is that a freewheel has many tiny bearings, so if you plan to dismantle one to be put back together or serviced, you need to be extremely careful not to lose any of them... it's very easy. The next thing you'll see is the pawls. On this particular freewheel, there are 4 little spring-loaded hinged paddles that engage little teeth on the inside of the sprocket in only one direction. When the sprocket is spun opposite its threading, the teeth just glide over the pawls, but in the other direction, they catch and drive the inner FW body which is presumably threaded to a hub. As you can see it was the inner FW body that ultimately came to grief in my case.
This is just the sprocket part. You can see the teeth that get engaged by the pawls.
Once you remove the sprocket, there's another set of bearings on the other side. You can see here, the little hole that the pivot point of the pawl inserts into.
So that's the end of this sad story. Stay tuned for the report of my second attempt. I promise that I will eventually get it right.
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