Well, essentially they are there so that the massive amount of power we all possess (roughly equal to a weedwacker!) can be put to the best use. It turns out that we all operate best at a given pedaling speed (known in bike-speak as cadence). All those gears help us match our preferred cadence to our terrain, fatigue, fitness and speed. It works really well, which is why some of us do something else. And that something else is either a single speed or a fixed gear (read more at http://en.wikipedia.org/wiki/Track_bicycle).
Single speeds can be freewheeling or fixed, road or off-road. Freewheeling means you can stop pedaling and coast down the road. Fixed means you can never stop pedaling unless your rear wheel stops turning, therefore you can never coast. Single speeds are hard enough, especially in hilly areas. Lately there has been growing popularity in single speed mountain bike races (check out www.youtube.com/watch?v=cmgDrxEklv0&NR=1). Fixed gears are downright difficult to ride and require retraining your riding style. They will also make mush of your legs in no time flat because you can’t stop pedaling and because you push against the pedals to stop as well as to go. The rider becomes the engine and the brakes. In some cases, people take off their brakes completely.
Professionally, fixed gear bikes are used on closed tracks (www.youtube.com/watch?v=wFfAAnXsB9U), either side by side like the Kentucky Derby or to set speed records. Madison Square Garden was once used to hold six-day races on these bikes! Today you are most likely to see them under a bike messenger or on campus. When ridden well, (www.youtube.com/watch?v=F0AFpq6jFok) they become an artist’s tool, but while you’re learning they will probably throw you off the bike at least once.
If you feel the need to venture into single speed and fixed gear land, here are a few considerations:
To buy one or build one depends on your wallet and interest. They are fun either way.
Keep your brakes on! Rarely do cars pull out in front of bikes on a closed track. The daily commute is another matter. Besides, most of your braking power comes from your front wheel anyway.
Start with a freewheel. Convert it to a fixed gear later. It’s cheaper and much safer.
Try it with the bike you have now by picking one gear and never changing it. Once you decide on a gear you can live with, you can duplicate that on your single speed.
Remember, single speeds are “simply” harder. Enjoy the simplicity.
Erik West is a certified bicycle riding instructor and mechanic, who teaches alternative transportation for the Greater Portland Council of Governments, the Bicycle Coalition of Maine and The Bicycle Workshop. He generally has his nose in most things related to promoting pollution-free transportation in Maine. To learn more check out www.thebicycleworkhshop.com or email him at erik@thebicycleworkshop.com.