When I took my newly singlespeed bike for its first ride as a singlespeed yesterday, I had a pretty major problem. It dropped the chain. A lot. Especially if I stood up and hammered or swung the frame. Since the bike's a singlespeed, that's a required move to accelerate rapidly from a stop, so just riding more smoothly was not an acceptable solution. At first I thought the tension might be too low, so I tried riding a larger sprocket on the freewheel cluster, and then removing a link and riding the sprocket I actually like. While this helped a little, it didn't actually solve the problem. I was considering buying a chain tensioner, as these are specifically designed to aid in conversions like mine, but they all attach to a derailleur hanger, and Skank doesn't actually have one. The original derailleur attached to the rear axel and a hole next to the dropout.
This is yesterday's first configuration, using the 20 tooth sprocket and a 3/32 road chain, shortened to have no slack. The smaller hole immediately above the adjusting nut on the quick release skewer was the one used to prevent inadvertent rotation of the derailleur; the other is for mounting a rack or fenders.
Further research suggested that by changing to a cog actually intended for singlespeed use, I might fix the problem. The hyperdrive cluster I was using is designed to shift well, and when used with a derailleur it's a huge improvement over previous systems. However, without a derailleur it still performs as designed - it shifts - but without a derailleur to pick up slack and maintain tension, the chain drops. It turns out that BMX hubs use the same threading as road hubs, which means that a BMX singlespeed freewheel will go onto a road hub with no adapters, etc. required.
On my way to work this morning, I stopped at Gotham Bikes to see if they had a BMX freewheel - I figure this is a really common part, and since I don't have the tool to remove a freewheel, ordering everything for the next phase of the conversion online would probably be more expensive than letting them take care of it. They had the part, but only for a 1/8" chain, the standard singlespeed size, usually used on BMX and track bikes as well as many urban singlespeed rigs and probably cruisers. D'oh! Anyway, I figured that the bike's not working very well for me as it is - I want to be able to accelerate rapidly - so I went ahead and bought a new chain too.
This is the new freewheel, and the new chain.
Finally, here's the new iteration of the drivetrain. The only original parts left are the bottom bracket, crankset and pedals (not including toeclips, which I had the shop throw in when I bought the bike.)
I'd be lying if I said I didn't enjoy the black and silver finish on the chain, or how clean this setup is. The gear ratio is 52/20. According to the gear ratio calculator on Sheldon Brown's web site, this means 69.6 gear inches, whatever that means, or 18.6 mph if I spin at 90 rpm. Which means that my cruising speed on the flats is terrifying by casual cyclist standards and not half bad by mine. If I get strong enough riding this way, I might get an 18-tooth cog, which would bring my speed to over 20mph; I'm concerned that any larger and I wouldn't have the torque to accelerate effectively or cruise comfortably.