Major Rear Derailleur Failure w/ New Chain
 

Even though I've been riding for over 10 years, I've never been very mechanically inclined, and so there are some pretty basic, stupid maintenance things that I sometimes leave to the local bike shop rather than deal with myself. But after what happened this weekend, I think I'm going to have to take care of these sorts of things myself from now on:

I brought my two-year-old Bianchi Vigorelli in to a LBS yesterday to have the (worn) chain replaced. The next morning, about 3-4 blocks into my ride, I heard a loud crunch, my drivetrain locked up, and I looked down and saw that my rear derailleur had flipped up into the spokes, bending the derailleur hanger in the process, and munching the chain.

Seems to me that the chain was the likely culprit, and I'm wondering if either the mechanic didn't install the chain properly (like it was too short, or maybe he didn't insert the pin correctly), or if the chain was somehow defective. I've never had a single issue with the bike before this.

I took it back to the bike shop today. The mechanic who did the chain wasn't there, but I left the bike with them and they said they'd assess the damage and get back to me in a few days. I'm still not sure how much (if any) responsibility they're going to take for this.

I'm attaching a few pics of the damage. Just wondering if anyone else thinks the chain is the culprit or if this is just a crazy coincidence?
https://cimg2.ibsrv.net/gimg/bikefor...9368d1bfcd.jpg
https://cimg3.ibsrv.net/gimg/bikefor...9c01d397ff.jpg

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.

Hard to say for sure but yes it's a possibility the mechanic did something wrong such as incorrectly changed your derailleur limit screws as part of the installation, installed the master link wrong, etc., etc., or it could just be coincidence. That's why you'll be encouraged to do your own work by posters here. Nobody cares about doing it right more than you, the owner of the bike and you can spend the time necessary to double check everything and it will give you the knowledge to sometimes spot trouble before it becomes catastrophic. Just sayin'.

Yeah, I'm glad I left the dork disk on there because you're probably right about it saving the wheel. As for the frame, I know that bent steel can be straightened, but I just wonder if this particular bend can be straightened without compromising the strength of the steel. It's a pretty serious bend, after all.

Well, what gear were you in when it happened?

If the chain had a pin (as opposed to masterlink) they may not have inserted it fully. or they re-opened it. It is just a bad design and why i only use masterlink chains (since it cost me a RD and hanger...).

I don't expect them to take the blame and buy you a new frame. You should have kept the bike as evidence and investigated yourself (with pictures here etc.) You basically charged the defendant with investigating and also made them the judge. Don't expect them to plead guilty.....

Small chain ring in the front, and had just shifted from the center cog in the rear to the next biggest gear.

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.

The chain is what comes with the Shimano 105 groupset, which to my knowledge does not have a master link. I did notice that one link (obscured in the first photo by the pulley closest to the cassette) looked like it had burst apart, but I wasn't sure if that link came apart and caused the failure, or if it burst apart after the RD snapped from slamming into the cassette or something.

But you mention the prospect of having to buy a new frame - does it look like my frame is toast? Or is that something that can straightened? I know that bent steel can be straightened; I just don't know if this particular bend is too severe for it to be reliably straightened.

This shop I took it to has been around for decades and has a pretty solid reputation, and in all my dealings with them in the past, I'd say 98% of the staff has been friendly and honest. It wasn't like I took it to some shady fly-by-night place that I'd never been to before. I guess we'll see how it goes. I investigated as much as I could with my limited mechanical knowledge, and sent photos to a cycling buddy of mine who used to work as a mechanic, and he was convinced the chain was at fault and strongly urged me to take it back to them and insist that they deal with it.

I can't be certain because it didn't even occur to me to check. (The mechanic who installed the chain has been working at this place since the mid 80s, and I've dealt with him several times in the past, so I just didn't even think to look it over.)

For the first few blocks that I rode, the pedaling felt normal, at least, so I don't believe the chain was routed incorrectly. I was hearing a mild sort of popping sound from the drivetrain, however, and the chain froze up for a second shortly before I shifted to a larger rear cog and the failure occurred.

Chainsuck can happen & maybe more likely w/ new chain, old cassette,

so may not have been error on the mechanic's part.

The cassette didn't appear worn, and this was the mechanic's opinion as well. He said the cassette didn't need replacing, so in a way, it might kind of be on him.

The link coming apart seems like the most plausible explanation.

As for the derailleur hanger, that'll buff right out.

(Nothing to lose bending it back. If the shop won't do it, get an Abbey HAG.)

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.

If you weren't shifting to the large cog it can't be chain length (and it's quite easy to check for proper length) or limit screws, and a bad chain pin or master link is not only unlikely but quite easy to spot on the chain. You state you had just shifted, which sounds like the chain had seated on the cog properly. I don't see any way to apply blame to the shop, especially if your shifter is not in the large cog position. It's possible something flipped up from the road/path into the drivetrain.

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.
https://cimg8.ibsrv.net/gimg/bikefor...51b7570e18.jpg

I don't know, I think that's extremely unlikely. The chain was making a strange popping sound from the time I began riding, so I honestly think something was amiss from the get go. I thought it might have been a stiff link or something, so I kept riding another block or so to see if I could figure it out or if it would go away. Then the chain froze for a second, I eased up on the pedals, the chain moved again, I pedaled a bit more, then I shifted gears, and a few pedals later - bam.

(And if something would have flipped up from the road into the drivetrain, I think it would have to have been a very hard object, like a pebble, to cause that kind of damage, and I would likely have heard the resultant pinging or ticking sound.)

Hard to tell for sure--was the broken link the one with the joining pin in it? That would be telling. The probability of the chain breaking in a short period of time under what sounds like modest load elsewhere is low. If the link came apart it could jam in the derailleur and the force could pull on it. Mistakes happen but the shop has control of how they handle this. Give them the chance to make this right--they have an opportunity to prove to you how they stand by their work and their standards for service.

Mechanically, I would carefully bring the hanger closer to sanely straight with knipex wrench pliars so as to not too much force on the hanger threads before aligning it with a hanger alignment tool. It'll probably be mechanically OK, although the paint on the hanger will likely be a bit messed up, and there's a small chance that it will break. If it comes back straight it'l be fine for use, though the odds of it braking if it gets bent again are much higher than before. Obviously new derailleur and chain.

Looking at the first photo above, it looks like the chain is currently resting on top of that guide tab.

I'm having troubles imagining how the chain could or would have popped out of the proper chain routing.

Inspection of the derailleur should indicate whether there is wear on the top side of the tab.

One can ride the bike just fine with the incorrectly routed chain, but it will cause the chain to sound a bit noisier than normal.

I can't say that would have definitely caused the bike to eat the derailleur, but if the chain was, in fact, routed incorrectly, then a large part of the blame would have to fall back on the shop.

This is probably what happened. I hope you also bought the chain from them or they could claim YOUR chain was faulty. If you bought the chain from them, it is pretty clear who is responsible.

I'm not a pro mechanic anymore, but I do turn wrenches on other peoples's bikes from time to time. I always try to give it a test ride on it's own wheels, not just on the stand, to make sure everything is OK before I give it back. Most of the shops I've been in do this too. Time or weather may have prevented it in this case though.

Bring the pictures back when you go to talk to them, esp, the ones that show the busted link, and that first one, where it looks like the chain is over the tab.
That way, they know that you know the condition / routing of the chain when it happened. Important, since the bike is back at the shop now.

Be cool, and give them the chance to own up to it first and make it right.

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.

It's clear in the second picture that the chain is routed correctly around the tab.

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.

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.

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.

That's a pretty important detail you left out of the first post. The shop can say that you should have stopped and determined what the problem was before continuing on riding. It does sound as though there is a good chance that the chain was coming apart at that point and shifting put enough sideways stress on it to make the link come apart but still, not absolute proof, could have been something else. Was it just popping or was it skipping as well? No matter how good any mechanic is mistakes can happen. So if you've been happy with this shop I see no reason not to continue using them if you end up coming to an agreement about the fix of your bike. Changing your own chain is something most can learn and takes much less time than driving to a shop to drop off and pick up your bike not to mention however many days you have to leave it there. You basically just need to get a chain tool, remove excess links from new chain, install chain with a quick link (available separately if not included with your chain) and you're done.

Actually, you can't see it from the photos I posted, but the pulley that's closest to the cassette is actually pressed intro the dork disk pretty tightly - enough to have left a dent in it.

The chainrings probably have at least 3,000 miles on them, and they honestly do not look worn. I wasn't hearing any sort of slapping noise like you describe with the previous chain on any recent rides.

The thing is, years ago, on a completely different and much older bike, I had different shop install a new chain, and the very first thing they said was that the cassette will have to be replaced as well (a few cogs were visibly worn), and possibly the chainrings. (In the end, the cassette was replaced, and the chainrings were deemed fine.) This particular mechanic was adamant - and rightly so - that thoroughly checking both cassette and chainrings was mandatory when even considering a new chain. That being said, should the onus be on every mechanic who is asked to replace a chain to thoroughly inspect both the cassette and chainrings?


Interesting that I'm getting some differing opinions on this. I suppose this sort of thing might be difficult to determine definitively - even with a good, clear photo - without seeing it in person. The frame being toast is clearly my biggest concern. And if the shop does successfully straighten it, I do worry that that part will forever be compromised (not to mention, the value of the frame itself will have decreased significantly).

I suppose, though I really didn't get far - I mean, we're talking literally three (mostly uphill) blocks, and scarcely more than a minute into my ride. Also, in hindsight, yes, I really should have stopped after the first 3-4 times of hearing that sound, but it wasn't super loud or egregious sounding, just an odd sound I'd never heard before. I recall too that I was thinking about stopping after another block or so anyway if that sound had continued.

But for argument's sake, I think when you have a sound like that, that's not glaring or alarming, you can't necessarily put the blame on the customer if it is discovered that the mechanic is at fault. I mean, speaking generally, I don't think a shop can expect every rider to have instant knowledge of what a particular sound could indicate and what they need to do in that moment.

Just popping, not skipping.

Well, this shop does walk-ins on weekends, and on this morning, there was no wait, and I was in and out of the place in less than 10 minutes, so it actually was quite convenient. But yes, I'm fully aware there's no excuse for me to have not bought a chain tool by now and spent time with YouTube videos and learn how to do this myself. The irony is, the low confidence I have in my own mechanical skills - and the resultant fear of severely damaging something - is what prompted me to just swing by the bike shop in the first place! Sigh...