Rear Derailleur Spring Tension

I've encountered a few used derailleurs that don't seem to put enough tension on the chain. When this happens, the chain can flop around and fall off the gears more easily. This can even cause "chain suck", which is when the chain doesn't release from the bottom of the front chainrings when you shift the front derailleur. My best guess is that the spring inside the pulley knuckle looses a bit of its "springiness" over the years. There's a relatively quick way to fix this, however. If you see two small holes next to the guide pulley, then you can move the spring over to the next hole to increase the spring tension. This is very easy on derailleurs with aluminum knuckles. First unscrew the small bolt that holds the cage together with a 2mm Allen wrench, shown here:

Next remove the bolt and the cage can be pulled from the knuckle. Then the spring can be gently unwound:

Notice the two holes on the bottom of the derailleur? Move the end of the spring into the adjacent hole to add more tension to the chain:

With the spring in its new position, wind the cage back until the tab on the knuckle is behind the stop on the pulley cage:

Now push the cage back into the knuckle:

And reinstall the set screw:

That's it. On higher quality derailleurs this is a very simple procedure. For cheaper ones with plastic knuckles, you will have to pry out a metal clip and drill another small hole to get the same result. I might show that procedure in a later post. Finally, if your derailleur is especially worn and dirty, you might want to check out this post about doing a full overhaul on it.