Sorting Rear Derailleurs
Rear derailleurs are all a lot alike, but there are two ways that they can be different.
Bolt on or Hanger?
This is how the derailleur attaches to the bike. The two different types are made for different types of bike frames.
On the right side of the picture below is is a rear derailleur with a hanger. The hanger is the thing at the right in this picture that is sort of C-shaped.
On the left side is a bolt-on derailer. You can see from the picture that it has a bolt coming out where the other one has a hanger.
Short or Long?
Rear derailers are also different sizes. But the only size that counts is the length of the cage. That's the distance between the two wheels (pulleys) that are part of the derailer.
If you look in the picture above, you can see that the cage is much shorter on the derailer on the left. It is (barely) a short cage derailer. The derailer on the right is long cage.
If think you could fit two fingers between the wheels, it's a "long cage" derailer. If not, it's short. If it's close, put it with the shorts.
Four Categories
You have two choices, so there are four types of rear derailleurs:
- Short Cage with Hanger
- Short Cage Bolt-on
- Long Cage with Hanger
- Long Cage Bolt-on
All rear derailleurs will be one of these, so you can sort them into four groups.