A single speed bike has just a single gear. There are no gear levers and there are no front or rear dérailleurs. You just have just a single chain ring on the front and a single sprocket on the hub of the back wheel, connected together by the chain. It’s a beautifully simple set-up that is easy to use and maintain and makes your bicycle lighter as a result.
The front chain ring is attached to the cranks (it’s called the chainset) and the pedals are attached to each crank. The two cranks are bolted to the bottom bracket – the spindle type thing that’s inside the frame of the bike.
For a single free geared bike, the bike rolls forward when you pedal. You can stop pedalling or pedal backwards whenever you like and you will free-wheel until the bicycle comes to a stop.
For a fixed wheel bike (or a fixie as it’s sometimes called) if the back wheel is going around then so do the pedals. If the back rolls backwards the pedals go backwards too – so you’ve always got to pedal when the bike is moving, you can’t free-wheel. This is how a track bike works in a velodrome and that’s how the sprinters manage to balance at a standstill by slightly turning the pedals backwards and forwards.
Flip Flop Hub
The hub is the middle bit of a bike wheel, the bit that fits in the front and rear forks and where all the spokes at attached to. To get the best of both single free and fixed wheel you can get a flip flop hub. With a flip flop hub one side has a free wheel sprocket (cog) and the other side of the hub has a fixed wheel sprocket. You simply swap the rear wheel around depending upon whether you want to ride fixed or free.