I recently purchased a used 2008 Trek 6000. However, I've broken 2 chains within 2 days of each other, the factory chain as well as another chain I purchased at my local bicycle store. Please help ASAP! I need to be able to get my money back from the guy if there's something very wrong with it! Thanks!
The first time the chain popped, I did not experience any skipping. However, the second time I did. The chain was definitely installed correctly so I wouldn't worry about that. I will post pictures of the break soon. But until then, it looks as though the chain broke when a link was forced out the side of it's connecting link right at the pin. Basically, the inside link is fine but the outside link with the pin is bent out on one side.
It is possible you are shifting with way to much load on the pedals (you need to lighten up a little as you shift.) However, I think it is more likely that the chains just weren't installed right. If there was something else going on serious enough to break a chain, you'd think the bike would ride very poorly. Good luck.
Check all sprockets front and rear for damage, bent or worn teeth. It takes a lot of force to snap a chain.
Also what gear were you in and type of riding at the time?
Are you a physically strong person?
What was the chain you bought? Did you use a quick connect link? Who installed the chain?
I haven't broken a chain in awhile, I bought cheap chains with a 7 speed cassette and rarely broke a chain. I have a new 9 speed cassette and I broke the same twice and the chain is the best Sachs. I was hammering up a long hill. I pedal up steep hills all the time. Most of my rides are around 100 miles each. I wonder if a good chain that needs lubed has a better chance of breaking. I just learned that a 9 speed chain is the same as a 7 speed chain, I measured it. I have been using new 7 speed links that were extra that I took off new chains. I am a strong rider, I snapped a Shimano 105 crankarm more than once, they are hollow inside.
I am thinking the chain may need lubed more and that may cause a link to be stiff and I shift just as the stiff link is off a gear and this puts a side to side strain on a weak link and this causes the chain to brteak. I am shifting when the chain broke the second time, the first time was at the quick connect link.
It sounds to me like you are putting a lot of strain on the drive chain while shifting. While 105 stuff isn't the best, you shouldn't really be breaking crank arms unless you are really abusing the drive train somehow. Shifting under heavy load while climbing takes some finesse not to overstress things. One of the nice things about new higher end systems is that they shift so quickly. But you need to back off the force for that 1/2 second while the chain is moving over. Unfortuantely, most companies focus on light weight instead of durability. But pros are using stuff not that much different than this and they don't seem to break drive trains much.