- Most bikes use a chain drive. Derailleurs move the chain between different front and rear gears to change the gear ratio of the bike. The chain is exposed to dirt and water, which can damage it and hurt efficiency. The gears can get bent, the chain can jump off and oil from the gears can get on your pants. A hub transmission, by contrast, has no exposed parts. It requires little maintenance, doesn't damage pants and isn't affect by the weather.
- The pedals are attached to a small gear. That gear meshes with a gear attached to a driveshaft, which transfers power to the rear wheels. When the pedals are turned, they transfer power directly to a sealed gearbox sitting on the rear hub. This gearbox contains a device called a planetary gear system, which changes the gear ratio and powers the rear wheels.
- The planetary system has three layers of gears. At the center is the "sun" gear, which is driven directly by the drive shaft. Around the sun are "planets."Finally, around the planets is the "annulus"--a ring-shaped gear attached to the hub of the bicycle. The hub transmits power to the planets, which transmit that power onto the annulus, which turns the wheel. When the rider shifts gears, the gear box slides in planets with more or less teeth. By changing the planets, the gearbox can increase pedal speed for difficult uphill biking or decrease the speed for high-speed cruising.
