Causes of chain and derailleur both breaking?
 

Hello,

Just got the chain and cassette on my road bike replaced three weeks ago (normal maintenance). Was riding today on a flat road when I started to notice that my chain was skipping -- as in, it was almost like the bike was shifting for me without me doing anything. It was annoying but it only happened a few times so I didn't think much about it. Then out of the blue, there was a cracking/smashing noise and I stopped (didn't crash, thank goodness) and discovered that my chain and derailleur were both broken.

Here's a link to some photos so you can see what it looks like -- it's hard to describe.
http://www.facebook.com/media/set/?s...1&l=ebf4367e4d

So my question is... do you think this was caused by the new chain/cassette? If not, what would cause something like this to happen? I didn't go over any pothole, significant debris, etc., and I'm pretty sure I wasn't shifting when it happened (I'm not 100% sure about that, but like I said, the road was flat, and our group was riding a steady pace).

I want to go back to the bike shop armed with at least a little bit of knowledge. I was riding with a group, and everyone was shocked and said they'd never seen anything like this before.

Thanks for any thoughts...

It's very difficult to render an opinion. Because there was recent work done, I might apply the "last man to touch it" rule and blame the mechanic, but it's hard to do that without knowing more.

Here are a few possible causes assuming you weren't in low gear or shifting when it happened.

A stiff link on the chain snagged in the idler cage, pulling the RD back and breaking the hanger, which is supposed to break under this condition to sabe the dropout and frame. This wouldn't be the mechanics fault, since the stiff link would have happened after he last touched it.

The RD was set a bit high (B-screw adjustment) and the jokey wheel was too close to the cassette. This is a decent likelihood if you were riding on the inner ring and a mid size cassette sprocket. It often produces the type of grinding you described, and can cause the RD to get pushed back and break the hanger. Unfortunately it would be impossible to reconstruct the original settings and prove this either way. However, it's more likely if you replaced the cassette with a larger one.

The hanger was weakened from a prior issue, or the bike was dropped bending the hanger inward a bit. The hanger then broke off without an outside cause causing the problem. This is becoming more common and some of the replaceable hangers are significantly weaker than others. It's one of the drawbacks to breakaway hangers, like the side effects of drugs. Overall they work as they should and protect frames, but sometimes they are the cause of the very problem they're supposed to prevent.

Those are a few possibilities, add that a twig or something else thrown up and snagged on the chain at the RD pulley. It's happened to me on my road bike, but I was lucky and felt it, and was able to backpedal and clear it before it caused damage.

The grinding before the break was a warning, but only you know how long you were riding while ignoring it. It's one reason I so strongly favor silent bikes. Every sound then becomes a cry for attention, from a glass shard embedded in a tire going tick. tick, tick, to the sound of my chain telling me the RD trim is off, to hering a loose headset long before I could feel it.

It looks like his hangar is bent 30 to 40 degrees as well. I'd think foreign object obstruction, if it weren't for the autoshifting he describes. I don't know enough about this to actually even have an opinion. Just adjusting my derailleurs a while back was an adventure in itself!

Just the thought of that chain back there whipping around after breaking, gives me calf-gash shudders!

Probably a tight link. This often happens with a new chain. The tight link is usually the one where the chain was joined. A tight link may fail to unfold and lie flat as it comes off the back of the chainring. When the folder link reaches the rear derailuer it stops suddenly at the tab and inertia twists the deralleur off the frame.
The mechanic should have.checked for a tight link.

Is there any way of discovering/fixing tight links before the chain goes on the bike?

The problem is that it's sort of chicken-and-egg. It's virtually impossible to separate cause from effect. The fact that it happened in 2 stages, first getting erratic, then breaking pretty much (but not conclusively) rules out the twig theory, but there too, it could have bent, messing up the trim, and broken after riding this way a while.

Lesson to the OP, and anyone reading this. If something changes, ie bike suddenly develops a wobble, RD gets noisy, crank sarts ticking, etc...., then something changed, and needs to be attended to now not later, even if it's only to identify what changed and decide to fix it later.

Thanks for your quick responses! This is very helpful.

In answer to a couple questions/points:

I ignored the "skipping" for probably about 10 minutes. I started riding five years ago and am still learning what's a serious warning sign and what's an annoyance, so that's a lesson learned, I guess: Bike starts shifting by itself, beware.

Bike hasn't been dropped, but it does spend a fair amount of time on a bike rack, driving to and from group rides... I'm very careful with it, but who knows, it's possible it could have been bumped at some point.

When they replaced the cassette, it was with one that was the same size, not larger.

So a couple of you mentioned a stiff link in a chain as a possible cause. How does this happen, and how can I prevent it? I'm good about keeping the chain lubed, but should I be inspecting each link when I apply lube to make sure they all move smoothly? Or are there other ways to prevent stiff links? What do I do if I find one?

Thanks again for your help! Really appreciate it.

Oh, and is a stiff link something that's fairly obvious for someone who's not trained to see it?

stiff links are easy to identify. You watch the lower jockey wheel on the rear derailleur and slowly and gently back pedal the crank while holding the bike up.

When the stiff link reaches the jockey wheels, generally you will then see the chain start to catch and the cage will jerk forward.

Get used to your bike, and know how it sounds and feels. You don't have to be an expert in bikes to adopt a simple rule. The bike alays sounds and feels the same, so anytime, anything changes you want to find out what and why. Once you've identified what changed you have to make a decision about what it might mean, and that may not be complicated, but you'll at least have taken the time to consider it.

As for stiff links, they have a number of causes and there's no real preventative except keeping your chain lubed and rust free. Stiff links announce themselves very uniquely and so are easy to identify while riding. They'll skip or make a pronounced noise going through the RD roughly once every 2-1/2 crank revolutions, not synchronized with either the pedal motion, or wheel motion, but seemingly randomly except it it isn't, and you'll notice the pattern after a minute or two. Links rarely ever stiffen while riding, except for mountain biking in the mud, and so are usually first spotted when starting out after the bikes been sitting overnight or longer.

I had this happen when my chain broke (or to be more accurate, my chain's masterlink unplugged). The break occurred on the lower section of the chain, so the spring-loaded derailleur cage whipped the chain back. It coiled around the derailleur, and my own pedal stroke pulled the derailleur up and around. The RD was ripped in half, similarly to yours.

So my contribution to the forensics is that your skipping chain might've been a link in the process of breaking. If you have chain skipping occurring once with each pass of the chain, it's wise to STOP, LOOK, and LISTEN. Sight down the chain lengthwise from the rear and backpedal, it's relatively easy to spot twisted or half-broken links from an end-on view. Watch at the pulleys for a link that hops or doesn't swivel properly.

+1 had that happen with a FDER getting turned into a pretzel.

How many speeds and what brand chain is on your bike. Also, how many miles did you put on the bike after the chain and cassette were changed?

A new chain won't have stuck links, but an incorrect insertion of a pin (oversize or otherwise) to install the chain can create one. Best way to check is to hold a rag over the chain and spin the crank. Put some downwards pressure on the chain with your hand like a second derailleur so that the chain has to flex and you'll have no problems feeling out any stuck links.

A stuck link can cause chain hopping or skipping symptoms very similar to a worn cassette, and those symptoms will show up in the smallest cogs.

Thanks, everyone! Since you were all kind enough to reply, I wanted to give an update. I talked a bit more to the people I was riding with, and the guy behind me actually saw the chain snap, catch in the derailleur, and then everything went to hell. So that made me feel like the chain breaking was what caused the problem, rather than debris, etc.

I'm not very mechanically inclined and couldn't see anything wrong with the chain, but when I took it back to the shop that had replaced my chain and cassette three weeks ago, they identified a pin in one of the chain links that was faulty or improperly installed. I believe it was sticking out. (??)

I'm happy to say the shop is standing behind their work/parts and repairing all the damage caused by the faulty chain (which is pretty extensive... in addition to RD and chain being shot, the cassette and several spokes are bent).

I'm not happy it happened, and in fact I'm a bit freaked out about it (what if it had happened while going fast down a steep hill?), but I think it shows good customer service that they're making it right. I guess now it's just a matter of "getting back on the horse" and trusting the repairs.

Lesson learned: Next time my bike does anything strange, I'm stopping to check it out (although, again, I wasn't able to spot the bad link/pin with my untrained eye, so not sure I would have been able to see anything was wrong if I had stopped).

I have also had this happen with a new chain (<200 km) while climbing a steep hill out of the saddle. Chain snapped, wrapped around RD, pedal stroke yanked the derailleur and bent the entire dropout (non-replaceable hanger). Unsure of cause, but it didn't break at the master link. Everything is fixed now, including the derailleur (new pulleys and inner cage).