Originally Posted by
wgscott
Looks like the dork disc saved your wheel. Bummer about the frame. That is a pretty major derailleur hanger bend. It is hard to tell from the picture, but I am guessing you are right -- not a coincidence.
The chain is no where near the wheel so the dork disc did nothing. If the lower part of the derailer had contacted the spokes...not saying it did..., the dork disc wouldn't have prevented that anyway.
Originally Posted by
trailangel
Well that's strange. Was the chain routed correctly through the DR cage.... with regards to the little tab between the 2 jockey wheels...? I don't know what you call that tab.
It's clear in the second picture that the chain is routed correctly around the tab.
Originally Posted by
woodcraft
Chainsuck can happen & maybe more likely w/ new chain, old cassette,
so may not have been error on the mechanic's part.
Chainsuck has nothing to do with a cassette. The chain is under tension as it goes over the top of the cassette and can't be "sucked" downward since the chainring is pulling on the chain.
Originally Posted by
woodcraft
I agree, doesn't look that worn. IME, chainsuck can be quite mysterious and unpredictable
and can include loose chain jamming between the lower jockey wheel and the RD cage.
Anyway, the point was that there are possibilities that don't involve error on the mechanic's part.
It's not mysterious and is almost predictable. Chainsuck is where the chain sucks up into the frame as the chain leaves the bottom of the chainwheels. The chain holds onto the chainring too long and it gets pulled up. The rear derailer usually has enough tension to pull the chain off the chainwheel but if the chainwheel is damaged, the chain can stick onto the wheel. Even small burrs that develop from a shifting error or poorly adjusted front derailer...like when you hear a "grindy" or "clattery" shift.
I would also disagree that the chain can jam between the lower jockey wheel and the rear derailer cage. There's just not enough room for the chain to jump off the pulley and jam. It would rerail rather than jam.
I do agree that lots of stuff can happen that might not be the mechanics fault.
Originally Posted by
Pavol Stromcek
I went back through the photos I took and found this one, in which you can kind of see the busted chain link: if you look just above the cassette, where the chain leaves the pulley, you can see it. Not sure if this really proves anything, ultimately, but the more I think about it, the more it seems that either the mechanic didn't insert the pin properly, or the chain was defective, because I don't think the chain could have broken like that from the impact of it being slammed into the cassette when the derailleur went.
That may just be an artifact of the derailer twisting around the cassette. The Shimano pin is peened on the front side of the pin so that the force of dragging the derailer up the cassette doesn't pull the chain apart. The back of the pin doesn't need to be peened because there is much less force on the chain when upshifting. The spring on the derailer is enough to make the chain move.
I can't absolve the mechanic entirely but there are other possible causes that have nothing to do with the mechanic. You said you were in the smallest chainring and went to make a shift. The smallest chainring is the one most likely to have chainsuck problems. The chain is under less tension from the rear derailer spring because the chain is effectively "longer" and more slack in the inner ring. The derailer arm is also flatter so it puts less tension on the chain. If the chain catches on the ring and starts to "suck" upward and suddenly releases, the energy goes into the rear derailer which swings that flatter arm upward and allows it to contact the cassette or catch the chain. That pulls the spring on the hanger upwards and the next thing you know the system jams and goes for a wrong way trip around the cassette.
The chain pulling apart is just a result of the having the connecting pin in the wrong place at the wrong time. That wrong way trip around the cassette puts a lot of stress on the chain and can even twist it. It looks like your chain is twisted just above the cassette. That twisting would separate the plate.
Bottom line, I wouldn't be quick to say that it was the mechanics fault. Have you been hearing mysterious chain slapping noise occasionally? It's hard to put into words but, occasionally, the chain will make a slapping noise or you can feel the chain move when you haven't made a shift or it may make the noise after you've shifted. That could be caused by a burr on the rings. Are the rings old? Do you see any signs of the rings having damage...it would show up as a shiny scrape on the ring? All these could have caused the problem. It could still be the chain but it could be other causes as well.
Finally, I hate to be the bearer of bad news but your frame is probably gone. Look at your second picture. The hanger is bent way back behind the axle and the slot for the axle is significantly wider than it should be. You might be able to straighten the hanger but it is more likely to just break off than bend back. And getting the dropout slot back to the width it needs to be would be next to impossible.