|Image from freewheelburn.com|
|A closer look at the two derailleurs and sprocket-cassettes.|
Diagram by Keithonearth
Cyclists operate these using shifter levers at the front of the bike, which put tension on chain-guides with a Bowden cable system. The change in cable tension on the chain-guide pushes the chain to one side, at an angle to the sprocket. When the angle becomes sharp enough, the chain will stop meshing with the sprocket teeth and either fall onto the next smaller sprocket (if shifting up to a smaller size gear) or be caught by the teeth of the next larger sprocket (if downshifting to a bigger gear). This process of displacing or "derailling" the chain is where the name comes from.
|Components of the rear derailleur controlled by a shifter|
Image from infovisual.com
The rear derailleur is extra-interesting. Not only does it control the gear ratio on the rear hub, it also has a swingarm (consists of spring-loaded pulleys) that takes up slack from the chain.
|Tension on the bowden cable shifting up from the big gear to a smaller gear contracts the chain guide, moving it up and and away from the wheel (green arrows left and right). |
At the same time, the roller sprocket on the swingarm swings backwards, taking up slack (red arrow).
Image from bike-advisor.com
Some extra facts about derailleurs - they require the chain to be moving in order to shift sprockets, and the transmission loses efficiency the more diagonal the chain is running relative to the sprocket plane. Therefore, cyclists avoid combining the biggest drive sprocket (front gear hub) with the biggest driven sprocket (rear hub) and similarly avoiding smallest front with smallest rear.
In general the derailleur system has an efficiency of around 95%, which is really good compared to most gears.
But if that's the case, why isn't this transmission system seen anywhere else except for bikes? We see chain-driven transmissions in plenty of other applications - usually in heavy industrial equipment. In addition, chain drives have plenty of advantages over other kinds of transmissions:
- Don't slip or creep, unlike belt drives
- More compact than belt drives
- Better for high temperatures
- Weather resistant
It's likely that derailleurs fulfill a niche set of requirements that only emerges in human-powered vehicles. Motors generally spin faster than their outputs; human legs generally spin more slowly than the desired output wheel. Furthermore, humans likely want to tune the variable-transmission for energy efficiency more often than a battery or liquid-fuel engine since humans feel laziness and engines don't. Finally, derailleurs have a possible set of disadvantages that lead them to not be used in a wide variety of applications:
- They require the chain to be moving and ram the chain side to side to switch sprockets, so probably can't be used for high-speed applications
- They maintain tension with a spring-loaded swingarm that greatly limits the load capacity of the chain to still allow gear changing
- Load capacity of the sprocket is also limited by allowed horizontal force on the chain before the weaker, thin side-walls buckle
Perhaps it's not that surprising that only bicycles use this transmission system.