I’ll try to keep this simple. I have an old 2006 Giant Yukon and i just did an overhaul on the Bike back in June. New rear derailleur, chain, wheels, tires and driveshaft to now make it single front sprocket. I MTB about 3 times a week since June at the local Cincinnati CORA trails fyi.
I have not had any problems until today, in the highest gear(fastest gear) the chain breaks a link right off after about the third rotation, smooth first and second rotation then it quickly locks up and breaks the chain. The chain is then tightly seized around the crank but that might be because I’m practically spinning the pedals 100rpm once it breaks. I adjusted the front derailleur to make sure that it was not forcing the chain off the sprocket since its in the highest gear and its still braking links.
The Chain tension was on the loose side during the first break. Now since I shortened the chain from taking out broken links it is very tight but it breaks all the same. The derailleur is aligned perfectly with my laser sprocket aligner from my old shifter kart days. I think that’s the best way to align the sprockets?
In all the other gears the chain does not break, but once it drops to the highest gear it just snaps. ANYONE HAVE ANY SOLUTIONS OR NEXT STEP IN TROUBLESHOOTING????
Maybe check the teeth on the cassette and the chainring to make sure none of them are mangled. Also, be careful about having a chain that is too short. If your chain is too short and you shift into your easiest gear, you could seriously damage your rear derailleur.
If there aren’t any teeth that are obviously mangled, I would recommend a new chain.
As others have said, definitely check for chain stretch and check the cassette for wear. As Zyante says, chain line is critical so it’s generally best to be on the biggest ring when shifting to the smallest cog. As to derailleur alignment, I still do it the old-fashioned way. Put the bike on a stand and pedal the cranks with one hand while you shift the rear derailleur with the other hand. Then adjust the shifter cable tension slightly and do it again, watching and listening for how quickly and smoothly it shifts to the next cog. If it’s worse, adjust the shifter cable tension the other direction. Repeat ad nauseam, although it shouldn’t take long to find the sweet spot. I prefer to do this between the two cogs I use the most.
Not sure if you are fixing your broken chain with a chain tool? Most modern chains are flush pin and are not designed to be put back together with a chain tool. You likely need to buy another chain or add quick links (not recommended). You can try to use a chain tool and sometimes get away with it if you’re very precise when setting the pin. It could also be breaking If the link you’re attaching is not loosened after it’s pushed through. Leaving the link stiff like that could explain your breakage problem. Once your chain is broken and put back together w a chain tool the horizontal flex created in your highest (and lowest) gears might be enough to break the chain.