"Why does my chain keep falling off?"
First of all I want to thank you for the Weekly Wrench. I've learned a lot from this site about basic bike repairs.
I want to ask you about my problem. Every time I ride my bike and bumps into something - like speed bumps, my chain keeps on falling from the big chain ring to the middle chain ring and from the middle to smallest chain ring. I have a Truvativ firex GXP crankset, a shimano deore xt shifter (M760) and an xtr front deralleur. Are these three compatible with each other? Is it my derailleur or my shifter that is causing this problem?
Also, I noticed a rattling sound in my front shifter everytime the indicator is on the middle, but the sound disappears when the indicator is either on the top position (big chain ring) or lowest position (small chain ring).
Thank you. Hope you can shed some light into my problem.
I'm glad my site has helped you so far! Hopefully I can help with this...
The good news is that the problem isn't uncommon - chains are always dropping off the rings. It happens on rough trails even with a perfect setup, but it shouldn't happen on smaller impacts like speed bumps.
In your case it could be a few different things:
- The chain might be too long. If it's not pulled tight enough around the rings, it could drop more easily.
- The front derailleur might need adjusted. The lower limit screw might be allowing too big of a gap on the inside of the ring.
- Gears. If you are in the big ring and a big cog (cross-chaining) it's likely that the chain will drop like this. Try to keep the chainline as straight as possible (i.e. big ring and smaller rear cog) and see if that helps.
- You are hitting bumps too hard. Shouldn't be the problem, but riding and shifting smoother always helps :)
Here are some useful links for this:
Chain length guide: http://www.parktool.com/repair/readhowto.asp?id=26
Front derailleur: http://www.parktool.com/repair/readhowto.asp?id=75 and http://bloombikeshop.com/repairmaintenance/derailleurshifter/adjustfrontderailleur.php
As for the noise in the shifter, if the shifter still shifts fine, I wouldn't worry about it. The rattling is probably just the indicator itself and not part of the shift mechanism.