Single-speed bikes are awesome. They combine the simplicity and efficiency of a fixed gear bike with the ability to coast.
So I'm here to tell you how you can convert that older geared road bike you use to get around town into a single-speed—without resorting to buying one of those new cranks. They might be anodized and look pretty, but they're not a cost-effective upgrade for a beater bike. (It's more worthwhile to invest in a quality lock, instead.)
Road bikes usually have two chainrings in front, the larger one generally having 52 teeth, to generate those high gear ratios you use to hammer down hills. That's too many teeth for single-speed use. The ideal gearing on a single-speed bike needs to strike a fine balance between speed on flat ground and versatility on different terrain types: too high a gear will have you pushing your bike up hills; too low a gear will get you up to speed quickly, but you'll be doing a lot of coasting, especially downhill. We need a crank with fewer teeth.
Enter your garden variety mountain bike. Most MTB cranks have three chainrings, the largest having 48 or fewer teeth. That's the sweet spot for single-speed. And if that bike was originally bought at a store ending in –mart, it's likely that its crank will feature a budget construction wherein the smaller chainrings are bolted or riveted to the largest ring.
Strip the crank off the donor MTB. Remove those bolts or grind the rivets that keep the three chainrings together and you've got yourself a perfectly serviceable single-speed crank.
Now that you've got your single-speed crank, it's just a matter of removing the extra junk you'll no longer need on your bike. You can dispense with the shifters, the gear cables, and the gear shifting mechanisms (also known as dérailleurs, among the cognoscenti). You'll also need to shorten the chain.
The final ingredient for a "legit" single-speed would be a new single-speed wheel. But you can simply repurpose your existing multi-speed rear wheel, by running the chain between the front chainring and over whichever of the rear cogs that results in the straightest chain-line—normally the second- or third-smallest cog. There are a few gear calculator tools you can use to figure out the resulting gear. (In my experience, a single-speed geared to ~70 gear inches is best.)
Congrats! You now have a road bike whose maintenance regimen mostly consists of oiling the chain.
[Lead image via my Flickr]