A fixed-gear bicycle, known also as fixed-wheel bicycle or fixie, is the simplest in terms of construction type of bicycle that has a drivetrain with no freewheel mechanism.
Name “fixed gear” may refer to the mechanism itself as well as to the whole bicycle based on it. The main rule of it is lack of freewheel and derailleurs – drivetrain has the drive sprocket bolted stiffly to the hub of back wheel. Fixies have only one gear and ride comes along with constant pedaling. Such solution used to be applied to children bikes, though it originates from technology used in track bikes. This type tends to get more and more popular among bikers who need to move smoothly around crowded streets in the city.