Bike chain rattles

Question

I've got this issue with my bike: the chain rattles an unusual lot when the bike is subjected to usual shocks during a very usual ride (or when I replicate that myself).

Background

I've got a 21 speed bike with a Shimano tourney derailleur-shifter set and a KMC chain that's the same as what came with the bike (I also recently installed a KMC missing link, but the issue with the rattling predates that development). The chain rattles and touches other front chainrings when I ride on 2*x combinations (and is very annoying too).

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Does the problem has something to do with the derailleur tension? I observed that when I pulled the derailleur back, the chain was more tense, so this could be a thing. If this is the case, then I would be tweaking the B-tension screw.

Or can it be something with the chain length? (Remember: the chains the same that came with the bike and when I measured the gap between 12 links, it came out to be less than 12 inches !)

I would really like this issue sorted to make my bike a nicer experience (I've taken up biking after a lag of an year or so and I really intend to keep it up and improve my performance) :)

Answer 1

Chain 'stretch' due to wear is not going to make the chain appreciably longer and affect tension.

You imply that the chain has been replaced. Could it have been incorrectly sized?

The 'A' pivot (where the cage rotates relative to the derailleur body) or 'B' pivot (where the derailleur body rotates relative to the frame) may be stiff or seized. Corrosion or dirt may have enter the pivot and is causing the spring to bind and hence not putting enough tension on the chain. Another possibility is that one of the springs may have broken.

If you want to diagnose it yourself, get the tension off the chain by removing the rear wheel, slipping the chain off the chainrings, or even shifting to the smallest chainring and smallest cassette sprocket. You should then be able to tell if one or both of the derailleur pivots is stiff, seized or has no spring tension.

If there is a problem with the derailleur, there is no point trying to repair it. Tourney derailleurs are very inexpensive so just go ahead and replace it.