This provides a second gear ratio when pedalling backwards for climbing steep inclines.
A single chain (represented by the black line) wraps around pink chainring, violet idle sprocket and two sprockets (in blue and green). The two latters belong to two freewheels mounted in the same direction. The frewheel hubs are fixed to the bike rear wheel.
Only one freewheel is engaged at a time, while the other spins backward freely. Since the chain wraps around the second sprocket in the opposite direction to the first sprocket, the cyclist needs only to pedal backwards to engage it.
There is a small amount of out-of-line for the chain.
The video shows:
1. Pedalling forwards: the rear wheel turns forwads fast.
2. Stopping bicycle.
3. Pedalling backwards: the rear wheel turns forwads slow.
In fact it is the mechanism for converting two way rotation into one way rotation of different speeds..
Disadvantages:
- Noise when pedalling, because one of the two cogs is always freewheeling while the other drives the bike.
- It’s not possible to position the pedals without lifting the rear wheel. That’s because both pedalling directions are used.
- The bike will also refuse to be rolled backward. That’s because both freewheels will engage and fight each other through the chain and the chainring.