Monday, July 20, 2009

Retrodirect II

Finally, I bring you the long over due entry on my second attempt at the retrodirect. You may recall that I had this up and running for a bit but it didn't last for long. It is for that reason the this post is labeled 'Retrodirect II'.

Since I had destroyed my larger inner freewheel on attempt I, I needed to replace that. Since the ACS freewheels were the only ones I could find >21 teeth (and we learned last time that those don't work for this purpose) I decided to get a small freewheel that would work for my purposes and just add some teeth. On the right is an 18t Dicta freewheel that has no lip and can therefore have a BB cup threaded through the back of it (without modification). On the left is a 24t sprocket from a multi-speed freewheel. My plan here is to mount the 24t sprocket onto the smaller freewheel so that I end up with a 24t freewheel. The inner diameter of the 24t sprocket is almost perfect to fit over the body of the 18t freewheel with the exception of those four little nubs.
My first step was to grind off the nubs so that the 24t sprocket would fit over the freewheel body.
Now that the sprocket would fit over the freewheel, I lined them up and drilled holes so I could use small bolts to attach them.
To make sure that the sprocket stayed on the freewheel, I laid on a thick application of JB Weld before sliding on the sprocket to bolt it.
Then I tightened down the bolts (with high strength thread locker) and threw it in the vice to cure. Note that I put a sheet of newspaper in the vice to prevent any leakage from welding my new 24t freewheel to the vice.
As you may recall, I need to put a spacer on the hub before I thread on the first freewheel. I chose to use a track cog instead of the typical bottom bracket lock ring so that I would also have a fixed cog on the same wheel to give me even more versatility. I put on high strength thread locker to help it stay put.
I next threaded on my modified 24t freewheel.
Then I threaded the bottom bracket cup into the remaining threads of the first freewheel.
The last piece is the small, outer freewheel which threads onto the bottom bracket cup.
That's what the wheel looks like in the frame. Note that the rim is intentionally not centered between the axle nuts. I had to use an extra long axle to accommodate the freewheels, so I had to re-space the dropouts by asymmetrically coldsetting the frame. I could have re-dished the wheel and done a symmetric coldset, but this was less work, so there it is.
The next thing I needed was a floating pulley so the chain could run directly from one freewheel to the other. For this, I sawed off the cage and jockey pulley from an old rear derailleur, and bolted it to a stiff steel spring. On the other end of the spring I had an aluminum bar liberated from a broken rear bike rack. The bar holding the floating pulley assembly went up between the chain stays and was fastened to the seat tube with a hose clamp. Notice that I put a short section of old inner tube down first to protect the frame. There's a little chainstay bridge that used to have a kickstand attached. This piece of the frame keeps the aluminum rod from getting pulled back towards the rear hub.
Here's the final setup. I posted a teaser of this a while back that shows it in action.
It's a pretty convoluted chainline, but it works.
Except when it doesn't. I learned my lesson from the last build and planned to do small rides around town for a while before trusting this thing on a longer trip. This turned out to be a very good idea. It was perfect for a day or two but then disaster struck. I was going along on a particularly bumpy section of multi-use path and hit a seam in the concrete surface. This seems to have derailed the chain from the floating pulley. The chain then got lodged between the pulley and the sides of the derailleur cage. As I pedaled forward, unaware of this event, the chain pulled the floating pulley assembly back towards the rear hub and then proceeded to wrap it around the hub a few times before totally binding up. It completely stretched out the spring, bent the aluminum rod, and ripped a few teeth off of the idler pulley. I was able to drop down to single speed so that I could make it home, but once again I my new retrodirect had failed me. Not to despair, this means only that I will have yet another opportunity to perfect this all but simple drivetrain. Stay tuned for RDIII coming soon!


  1. I'm hoping so. You'll hear about it when I get there.

  2. some of the fellows from the shop were oogling this page after our volunteer meeting tonight...and smiling and pointing and probably loving every minute.

    that is all.