Dérailleur and Chain broken

[BrynCP]

Set off on a journey today and within 7-8 miles my chain started slipping badly, there was no warning, it wasn't gradual, it just started happening. I unscrewed* the barrel adjuster a bit and tried all the gears at the side of the road bit but it didn't improve (nor get worse) while actual riding.

*My theory was because the cables are new about 400 miles ago, they'd maybe stretched so needed to be tightened up.

Carried on for another 5 miles with the chain slipping still, before catastrophic failure:

• Hanger snapped at the fixing point to the frame
• Dérailleur (105) bent in several places
• Chain off with 1 outer plate snapped off on one side and bent
• Me in the middle of the road

I can't work out what went so terribly wrong:

• No change to limit screws, so the dérailleur shouldn't have found its self in the spokes. I'd heard no rubbing
• Wheel hasn't been taken off for anything
• Chain was approaching, but wasn't yet at the first marker on the chain wear indicator. It was lubricated.
• I unscrewed the barrel adjuster, therefore applying tension to the cable so that should have, if anything, took it away from the spokes?

Did I mess it up playing with the barrel adjuster?

 

[screenman]

Yes.

 

[Yellow Saddle]

No.

This is what happened.

1) Chain was installed poorly or, you made a bad gearchange a while ago.
2) You went for a ride fiddling and all.
3) The sideplate popped open with the open jaw pointing backwards on the slack run of the chain
4) The chain started skipping each time the open jaw hit the cassette or even jockey cage.
5) The jaw opened wider and wider.
6) The jaw was now open so wide it swallowed the derailer cage.
7) The cage bent backwards and started to ride over the cassette.
8) The hanger broke with the force of your pedaling.


Go over the above motions with the bike in front of you. Look for the chain plate and confirm which way it points. It is quite possible that the original join is at fault but it needn't be. A bad shift can do the same.

A worn chain or poorly indexed derailer cannot do what happened to you.

 

[Spiderweb]

A similar thing happened to me while on a guided MTB ride in the Sierra Nevada mountains. We were a mile or two into the ride and had not even reached MTB terrain, I changed gear and the derailleur ended up in the spokes resulting in a sheared hanger. The owner/guide couldn't explain what had happened as the hire bikes are serviced/ checked before each ride. I did wonder if the mech hanger had gotten slightly bent during transportation in the van on the way to the trails although you would have thought I'd have had indexing issues from the get go but there were no such problems.

 

[BrynCP]

The chain had been installed for 1000 miles, so I don't think I installed it wrong.

1) Chain was installed poorly or, you made a bad gearchange a while ago.

I did get to the bottom of a hill, and hadn't down shifted. I then tried to pull away in highest gear and shifted down as doing so, particularly in a rush because a car behind me was getting irritated. Would that really be enough though? Thinking about it, it was within a minute or so of this the skipping began.

Go over the above motions with the bike in front of you. Look for the chain plate and confirm which way it points. It is quite possible that the original join is at fault but it needn't be. A bad shift can do the same.

I split my chain to make a single speed to get me home, I threw the other links in a bin at the road side. I can't remember which way it was pointing.

 

[Yellow Saddle]

The chain had been installed for 1000 miles, so I don't think I installed it wrong.



I did get to the bottom of a hill, and hadn't down shifted. I then tried to pull away in highest gear and shifted down as doing so, particularly in a rush because a car behind me was getting irritated. Would that really be enough though? Thinking about it, it was within a minute or so of this the skipping began.



I split my chain to make a single speed to get me home, I threw the other links in a bin at the road side. I can't remember which way it was pointing.

No problem with the discarded chain. The derailer cage will have evidence of the chain biting it on the ankle. Have a look, it is there just above the tension pulley there will be a mark pointing rearwards.

 

[Tim Hall]

Unless you've got a Rapid Rise derailleur, unscrewing the adjuster tightens the cable (as you correctly say) moving the mech towards the spokes (the opposite of what you said).

 

[Yellow Saddle]

Unless you've got a Rapid Rise derailleur, unscrewing the adjuster tightens the cable (as you correctly say) moving the mech towards the spokes (the opposite of what you said).

Yes, but that doesn't explain the type of incident. At worst (if the limit screws weren't right), the chain would have gone past the first sprocket into the spokes. That stops the wheel, with the derailer unscathed.

 

[BrynCP]

Here is one side of the derailleur (hanger is on the top left of pic)

Here is the other (hanger is in top right)

Looks like the cage above has totally bent around, some other specific areas of damage:

Snap in the cage, and I can't even picture what that at the top used to be (just realised, that's the back fixing for the top jockey wheel - now it's at the bottom!)?

What look to be fresh scratches:

 

[BrynCP]

Unless you've got a Rapid Rise derailleur, unscrewing the adjuster tightens the cable (as you correctly say) moving the mech towards the spokes (the opposite of what you said).

Indeed, I was thinking of the front. I only turned it a bit and checked all the gears before I proceeded, but perhaps the chain did get sucked in. I don't have a guard on the back as the shop took it off when they replaced my wheel as they struggled to get the cassette on with the guard.

Although that wouldn't explain the skipping, as that was supposed to be a remedy to the sudden and persistent skipping. Yellow Saddle's explanation would be more consistent with that aspect.