A toothed belt can take the place of a chain on a city bicycle. Gates uses carbon-fibre tensile cords, and the belt itself needs no lubrication. The visible change may look small to a rider; for a designer, it affects the whole drivetrain layout.
A Gates belt forms a continuous loop that cannot be opened like a chain. The arrangement described by the manufacturer therefore needs a purpose-built opening in the frame’s rear triangle. The structure must also allow belt tension to be set. These requirements belong in the bicycle’s design from the outset.
The gears can sit inside the rear hub. Some models in Trek’s District family, for example, combine a Gates belt with a Shimano internal gear hub. This arrangement suggests an interesting conclusion: a simple-looking drivetrain can conceal a more complex mechanism inside the hub. Counting visible parts reveals only part of how a bicycle works.