A fixie bike, or fixed-gear bike, is a type of bicycle that has a drivetrain with no freewheel mechanism, meaning that the pedals always spin together with the rear wheel. This design was originally developed for track racing, but has become popular among urban cyclists due to its simplicity compared to multi-geared bicycles. Some key features of fixie bikes include:

• Simplicity: Fixies are among the simplest of bicycles because they dont require derailleurs, shifters, double- or triple-chainring cranksets, or, in some circumstances, even brakes.

• No coasting: Because there is no freewheel mechanism, riders cannot coast on a fixie bike and must continue pedaling whenever the bike is moving.

• Customizability: It is possible to build your own fixie starting with a bike with standard road geometry and clearance for wide tires and even fenders. These rigs are available complete and can be built from older road bikes, too.

Fixie bikes are popular among some cyclists because they are simple, customizable, and fun to ride. However, they may not be suitable for all types of riding, particularly if you need to tackle steep hills or ride long distances.