So I'm pedaling along the other day on my 4 month old Trek Mondane 4.5 when without warning the rear deraileur broke and fractured the seat stay on my frame. I stopped right away and looked to see what happened. The metal that hold the lower gear had snapped off and the main body of the deraileur also had a break in it. Curious if this had happened to anyone else? I was actually taking it easy at the time it happened tooling along at around 12-13 mph on a level section of road. Am not aware of hitting anything and like I said it was totally without any warning. The LBS is filing a warranty claim for me but I'm not sure how it will turn out. Trek covers the frame but the Shimano components are covered by Shimano. I'm afraid this is going to turn into a fight between the two manufacturers. The other joker in the deck is that the bike had just been serviced by the LBS the day before. Now I did put 20 miles on it the day before and was 9.5 miles into another day's ride when the part failed. Is it possible that something was not put back together. They did disassemble and soak the cassette and chain as part of the service. I really just want to know what could have caused this too happen and am curious as to if someone else had the same kind of failure.

I've seen this happen on a new bike when a tight link in the chain failed to pass through the rear dérailleur.

Check to see if one of the chain pins is sticking out too far.

Sounds very similar to what happened to a friend's bike. It was also quite new (~ 3 months) and had been serviced at the LBS where he bought it. His warranty claim was initially denied, but that decision was reversed when he pointed out that:
- the bike was almost new and had no modifications
- had only been maintained by the LBS (an authorized Trek dealer)
- he and his wife had bought a number of new Trek bikes in the last few years.

He was scheduled to receive his replacement bike today - don't know if that happened yet or not. AFAIK he still doesn't know exactly what initiated the failure.

This also happened to a guy that I know. Derailler either failed or shifted into the spokes. I'm not really sure what happened exactly, but his derailler tore away and gashed his right seatstay. Huge crack in his carbon frame. Basically unsafe and unridable. Trek did not accept liability. He phoned them and eventually got to someone who said that it was most likely a mechanic error and the derailler limit screws were not set properly, thus causing the derailler to shift out of its intended range and into the spokes. He then went to the bike shop and they refused to accept responsibility even though the frame was new. Not sure if the guy ever got the frame warrantied or not. Sorry that happened to you. It sucks especially considering that nobody wants to accept responsibility on a $5000 bike.

Unless you were in low gear at the time, it's doubtful that the RD went into the spokes. It could be, as AL1943 pointed out, a stiff section of chain binding as it went through the cage and pushing the RD back, or it could not be a mechanical issue at all. Once in a while the wheels kick up a stick or other bit of road debris which jams into the chain and lower RD pulley.

The big variable here is the coincidence of it happening so soon after service. I scuba dive, and know that the greatest risk of a sudden regulator failure is during the first dive after service. I doubt there's any way to prove the exact sequence of events, and wish you good luck on the warranty. My fear is that the three parties will each blame the other.

Any broken spokes? I had one break which jammed the chain and derailleur, breaking the derailleur hanger and bending the RD itself. It left a gash on the seat stay but fortunately didn't compromise the CF frame.

I would say that it probably was an unlucky break, spoke chain, etc. It could be mechanic related, if it could have been prevented. Trek will probably say that there was nothing inherently wrong with the frame that could have caused this, unless you started out with some waked out bent derailed hanger that caused a shifting err, but that probably isn't likely because any competent mechanic would snuff that out. I don't see how Shimano could be liable. I think it's going to come down to either the bike shop or the Trek. Could be just bad luck, but things like this don't generally happen on new bikes. Sorry about your frame. Hope you can get Trek to replace it.

Photos? I once had a Shimano RD fracture due to a casting flaw in the metal; fortunately, I was only going about 2mph at the time.

...and this is why I think that aluminum frames aren't so bad.

I hope your claim goes well.

Thanks, I just bought a 1999 Schwinn Circuit with a Reynolds 853 Steel Frame as a backup. Hopefully a similiar failure won't hurt this frame.