Drivetrain options that could allow a BMX bike to shift between 2+ gears.

near rear hub

• internal-gear hub
  • criteria
  • durability
  • steel hubshells
• kickback hub
• retro-direct
• Suntour Hole Shot

near crankset

• Truvativ Hammerschmidt
• Browning hinged-chainring

other locations

• internal gearbox — mounted above the down-tube
• dinglespeed (1982 Huffy Pro Thunder Z2)