Shifters and derailleurs are often the most expensive components on a bicycle, and the ones that require the most maintenance. On a bike with only one gearing option the only maintenance you might ever have to do is lube the chain and top off the tire pressure periodically. Also, if you leave your bike locked up outside, there are fewer components to be stolen or damaged. This reliability makes Singlespeed bikes particularly attractive for use in urban areas or on college campuses.
Riding a bike without shifters can also be a lot of fun. While at the high end of the cycling market, the range and complexity of drivetrains has been increasing (they're up to 11 rear cogs now), some riders find it satisfying to ignore the race for the "next best thing" and go for simplicity. Going up a hill? Pedal harder. Going down a hill? Pedal faster, or coast. Instead of worrying about being in the right gear, enjoy the ride.
Of course, the disadvantages of a Singlespeed bike are obvious. In hilly terrain there is no way to get an easier gear ratio, so you have to either stomp on the pedals and muscle through, or get off and walk. If you're trying for speed and can't shift to a higher gear, you'll always be limited by how fast you can spin your legs. Any choice of Singlespeed gear ratio must be a compromise.
Still, if your commute is relatively flat, you're a strong rider, or you don't mind walking up the occasional hill, you might consider a Singlespeed.
|My Singlespeed Commuting/Trail/Touring/Whatever Bike|
There are many types of bikes that fall in the Singlespeed category, in fact "Singlespeed" is probably less of a category in itself than a modifier. You can have Singlespeed Mountain Bikes, Singlespeed Road Bikes, Singlespeed City Bikes, etc. Pretty much any style of bicycle can be found in a one-speed option.
Fixed-Gear bikes are a specific sub-set of Singlespeed bikes. The difference between a Fixed-Gear and any other Singlespeed is that a Fixed-Gear or "Fixie" does not have a freewheel mechanism on the back wheel. The freewheel is what allows the wheel to turn when you stop pedaling, so without it, you can't coast. The rear cog of a Fixie is "fixed" in place on the hub, hence the name.
Early in cycling's evolution, all bikes were made this way, but by sometime in the early 1900s freewheel mechanisms became the norm, and Fixed-Gear bikes were used mostly for track racing. Some racers and club riders would put a fixed-cog rear wheel on their road bikes in the winter, though, in part to save their expensive derailleurs for racing season, in part because being forced to muscle up hills and spin at a high rate of rotation down hills was an excellent workout, and in part because a Fixed-Gear can offer a bit more control on slippery roads.
That last is of interest to the cycle commuter. Having the ability to control your speed by the pressure of your feet on the pedals offers a better sense of traction and control on snow and ice, as well as the ability to fine-tune your speed in stop-and-go traffic. In a sense it's similar to driving a car with a manual transmission.
But while the Fixed-Gear offers a great sense of control (and can be a lot of fun) you also have to bear in mind you can never stop pedaling, no matter how tired you are or what kind of maneuvering you're doing. This, along with the fact that hilly rides quickly become interval workouts, can make the Fixie a poor choice for the novice or casual rider, who might be more comfortable with a Singlespeed that can coast.