The bike world is broken up into two basic categories when it comes to drivetrain: multi-gear and single speed bicycles. Single speed bicycles are attached to a freewheel, allowing coasting, or a fixed gear — no coasting — hub, and do not allow you to change gears. The advantages to this style of bike are many, but they do have one major drawback for some riders.
Single speed bikes have far fewer components than their multi-gear counterparts. Derailleurs, shifters, multiple cogs and chain rings are all moving pieces that need to function together on a multi-geared bike. If one thing breaks, the entire bike will not work. A single speed bike is easy to work on because it only has one cog, one chain ring and no derailleur. Some single speed bikes also have simplified coaster brakes, which eliminates the need for a rear drum or disc braking system.
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Because they have such a lack of components, single speed bikes often are much more affordable than multi-geared bikes. Lower cost also comes in the form of maintenance. Many of a single speed bike's problems are capable of being fixed by the average rider. If they aren't, most bike shops and mechanics can fix problems quicker on single speed bikes than on multi-gear bikes, meaning less labor time and lower overall cost.
The reduction in components also affects the bike's weight in a positive way for single speed riders. The multiple chain rings, gears and brake systems on multi-gear bikes add up fast. Single speed bikes will save you several pounds in weight, beneficial both on and off the bike. A lighter bike means that your overall weight is less, thus requiring less power to move the bike forward in a given gear ratio. Plus, lighter is easier to carry. If you live in an apartment or frequently have to carry your bike inside, you might consider the weight advantages of a single speed.
The main disadvantage of a single speed bike is versatility. Multi-gear bikes are unparalleled in terms of pedaling efficiency and power output given a variety of terrains. On hillier surfaces, a multi-gear bike is perfect for matching your ideal pedaling speed with the slope of the hill you are facing. If you value easy, speedy rides on your bike and you live in an area with varied topography, a multi-gear bike is probably your best bet.