Rear derailers sometimes wear out, but usually if they are bad it's because they're missing parts. Check for these parts, and if they're there, you can say it's OK.
In the lower left is the barrel adjuster where the cable housing stops. Some derailleurs don't have this part, but will instead have a hollow area where the housing fits. If there is a hole with threads on the inside, and nothing screwed into it, the derailleur is missing something.
The two similar screws limit the derailleur's travel, and the other circled screw holds the cable in place.
Also very important, although it isn't circled, is that the derailleur must have both of its pulley wheels.
There are also some rear derailleurs which have removable bolts to attach them to the frame. The bolt in this picture is at the upper left corner of the derailleur. If this part of the derailleur is hollow instead of having a bolt, the derailleur is missing a part.
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.
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.
- If your chain falls off the cog or chain ring
- If you cannot go into your highest or lowest cog or chain ring
- If your derailleur is skipping cogs or chain rings
Cambio Trasero (m)
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