 Bicycle drivetrain systems have been developed to convert speed and torque by a variety of methods.Several technologies have been developed to alter gear ratios.They can be used individually, as an external derailleur or an internal hub gear, or in combinations such as the SRAM Dual Drive, which uses a standard 8 or 9-speed cassette mounted on a three-speed internally geared hub, offering a similar gear range as a bicycle with a cassette and triple chainrings.