# Catastrophic drivetrain failure



## benb (17 Dec 2012)

So on the way home, with no warning, my rear wheel locked up and I came juddering to a halt.

It was on a 40 limit dual carriageway, so I was pretty lucky I think - the car behind me was some way off.
Somehow I managed to unclip and stay upright, and hobbled over to the side of the road.

My rear mech was in the spokes.

I can only assume that the chain partially broke, jammed, and dragged the mech up into the spokes.

In retrospect, there had been a faint clicking once a chain rotation for about 5 minutes.
Is there anything I could have done? The chain is fairly new - it really shouldn't have failed like this.

Lucky I haven't got round to selling my hybrid yet.


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## Matthew_T (17 Dec 2012)

That is catastrophic! You must have legs like a machine to have bent the mech that far out of place! It looks like you hit a landmine!


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## Drago (17 Dec 2012)

Holy Cameron! Never seen anything like that before. Glad you survived uscathed.


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## Andrew_Culture (17 Dec 2012)

Hell's Teeth!


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## Cheshire Celt (17 Dec 2012)

Bit of muck off on that and it will be good to go lol


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## benb (17 Dec 2012)

Thanks all.

Will take it to the LBS on Wednesday. Hopefully not too expensive to fix.


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## Matthew_T (17 Dec 2012)

benb said:


> Will take it to the LBS on Wednesday. Hopefully not too expensive to fix.


I think the LBS will leave his job when he see's that mess.


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## GrumpyGregry (18 Dec 2012)

Matthew_T said:


> That is catastrophic! You must have legs like a machine to have bent the mech that far out of place! It looks like you hit a landmine!


Ben's legs are special.


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## screenman (18 Dec 2012)

Those little plastic discs that go behind the cassette do work, even if they do not look cool.

Sad about the bike, certainly not a good time of year for spending more than needed. Glad that you are OK though.


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## craven2354 (18 Dec 2012)

The same thing happened to me when I was half a mile short of my first 100 mile ride :/ I don't know what caused it the guy that fixed it couldn't work it out either 

I ended up on the floor though I was half way up a hill and the bike just stopped and I went sideways has


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## Andrew_Culture (18 Dec 2012)

I did the same on my MTB by doing a bunny hop to try and seduce my wife.

It didn't work.

It _really _didn't work when she saw the repair bill.


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## screenman (18 Dec 2012)

Normally it occurs when the stop screw is not adjusted properly, that combined with the flexing of the rear triangle. there may of course be other reasons.


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## AndyRM (18 Dec 2012)

Chain looks a bit slack...

I did the same thing a few months ago, giving it some to carry a bit of momentum up a hill. Very annoying!


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## Globalti (18 Dec 2012)

Same thing happened to Gti Junior when his plastic-bodied cheap Campy derailleur snapped and wandered into the spokes. He had been hearing a clicking noise for a few minutes before and he ignored it, then bang. It was fairly catastrophic.


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## ianrauk (18 Dec 2012)

Ouch.
Same thing happened to me on a SMRbtH from Brighton.
Caused by my saddle pack bursting open and shedding my Montane jacket into the gears.
I had just turned off a dual carraige way doing about 30mph when it happened.
Jacket wrapped around the cassette and tore the deraiileur in half and twisted the chain.
Not a happy bunny.

Tried to single speed it but no good.
Luckily there was an Evans close by who took the bike in there and then. Supplied and fitted a new derailleur, chain and cable within half an hour.


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## Scoosh (18 Dec 2012)

The only time my rear wheel totally locked up was when I had been doing something to the freewheel (I can't now remember what nor why ) and, 50metres into a 150km audax, going up a hill to some traffic lights, the rear wheel seized up solid. Safe enough but no Completed that day ... and my riding buddy didn't notice I wasnae there for another 20 mins ... 

Oh - and the other time was very recently, when I had been adjusting the boom length on my 'bent and hadn't lengthened the chain sufficiently and the rear derailleur needed adjusting ... and the derailleur ended up amongst the spokes ... once again, in a safe place and at slow speed and no long-term damage nor short-term cost 

My luck should be due to run out soon, though ...


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## snailracer (18 Dec 2012)

screenman said:


> Those little plastic discs that go behind the cassette do work, even if they do not look cool...


In this case, the plastic "dork disc" probably would not have helped. Note that the chain is not jammed between the largest cog and the spokes, which is the only situation where the plastic disc would have come into play.

What likely happened here is chainsuck - the low-tension run of chain nearest to the ground failed to release from the chainwheel and got dragged around, effectively shortening the chain and bending the derailleur so it got caught up in the spokes of the rear wheel. An extremely large plastic disc may have helped here (as used to be standard on some Decathlon bikes IIRC) but most discs tend to be the smaller types that only protect against bad limit screw adjustment on the rear mech.


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## benb (18 Dec 2012)

snailracer said:


> In this case, the plastic "dork disc" probably would not have helped. Note that the chain is not jammed between the largest cog and the spokes, which is the only situation where the plastic disc would have come into play.
> 
> What likely happened here is chainsuck - the low-tension run of chain nearest to the ground failed to release from the chainwheel and got dragged around, effectively shortening the chain and bending the derailleur so it got caught up in the spokes of the rear wheel. An extremely large plastic disc may have helped here (as used to be standard on some Decathlon bikes IIRC) but most discs tend to be the smaller types that only protect against bad limit screw adjustment on the rear mech.


 
Yep; I wasn't in the lowest (largest cog) gear, so it wasn't the case that the chain slipped off the largest cog into the cogs.
Still can't work out whether I could have done anything to prevent it - I'm reasonably good at keeping the chain clean and lubricated.


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## snailracer (18 Dec 2012)

benb said:


> Yep; I wasn't in the lowest (largest cog) gear, so it wasn't the case that the chain slipped off the largest cog into the cogs.
> Still can't work out whether I could have done anything to prevent it - I'm reasonably good at keeping the chain clean and lubricated.


http://www.fagan.co.za/Bikes/Csuck/


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## Moodyman (18 Dec 2012)

Regular maintenance is a good way to spot potential problems but you say the chain was recently installed so you would not be expecting this.

I think the click per chain rotation should have been a clue. I hate creaks / noises and will stop to check things. I once had the click you describe and spotted a broken chain link plate on one side. I rode in a very low gear and spun in low cadence to minimise torque for the remaining mile into work. I then removed the offending link at lunchtime for the homeward commute.


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## benb (18 Dec 2012)

Moodyman said:


> Regular maintenance is a good way to spot potential problems but you say the chain was recently installed so you would not be expecting this.
> 
> I think the click per chain rotation should have been a clue. I hate creaks / noises and will stop to check things. I once had the click you describe and spotted a broken chain link plate on one side. I rode in a very low gear and spun in low cadence to minimise torque for the remaining mile into work. I then removed the offending link at lunchtime for the homeward commute.


 
That's probably what it was. You just don't expect a click to suddenly deteriorate into something like this!


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## Herzog (18 Dec 2012)

benb said:


> That's probably what it was. You just don't expect a click to suddenly deteriorate into something like this!


 
You're right. It would have been nice if the click had turned into a bonk, and then failed.


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## Pale Rider (18 Dec 2012)

ianrauk said:


> Ouch.
> Same thing happened to me on a SMRbtH from Brighton.
> Caused by my saddle pack bursting open and shedding my Montane jacket into the gears.
> I had just turned off a dual carraige way doing about 30mph when it happened.
> ...


 
Luckily the gears didn't shred your credit card as well.


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## GrumpyGregry (18 Dec 2012)

Out of interest ben what brand was that chain?

I'm hoping it isn't a connex/wippermann


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## benb (18 Dec 2012)

GregCollins said:


> Out of interest ben what brand was that chain?
> 
> I'm hoping it isn't a connex/wippermann


 
KMC I think.


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## SportMonkey (18 Dec 2012)

I've had this happen before changing gears mid range. You probably took a knock and weakened the hanger an age ago and it's been slowly dying. You'll need at least a new cage too.


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## potsy (18 Dec 2012)

SportMonkey said:


> I've had this happen before changing gears mid range. You probably took a knock and weakened the hanger an age ago and it's been slowly dying. You'll need at least a new cage too.


Seem to remember Ben coming off a while ago, is this the same bike?

When mine went on our ride it wasn't for a few miles after the off iirc


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## SportMonkey (18 Dec 2012)

potsy said:


> Seem to remember Ben coming off a while ago, is this the same bike?
> 
> When mine went on our ride it wasn't for a few miles after the off iirc


 
I forgot about that, it didn't even look like your bike was damaged, then pop.


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## screenman (18 Dec 2012)

The plastic disc stops your mech from getting mixed up with the spokes and causing the sort of damage you see here.


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## Hip Priest (18 Dec 2012)

At least you're in one piece! My catastrophic drivetrain failure led to a broken bike, a broken helmet and cracked ribs.


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## benb (18 Dec 2012)

potsy said:


> Seem to remember Ben coming off a while ago, is this the same bike?
> 
> When mine went on our ride it wasn't for a few miles after the off iirc



No, I've never come off on this bike, only the hybrid once and the MTB several times.

I did bash the mech with my heel quite hard a few months ago, which needed tweaking back into alignment, so it could have been that. Or it could have been a chain problem.


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## Cyclopathic (19 Dec 2012)

Drago said:


> *Holy Cameron*! Never seen anything like that before. Glad you survived uscathed.


Hahahahahahahahahahahahahahaha........Hahahahahahahahahahahahahaha.


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## Cyclopathic (19 Dec 2012)

Andrew_Culture said:


> I did the same on my MTB by doing a bunny hop to try and seduce my wife.
> 
> It didn't work.
> 
> It _really _didn't work when she saw the repair bill.


But...but...I did it for youuuuu.


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## benb (19 Dec 2012)

It's not good. The mech mounted directly to the frame, and the part of the frame aft of the dropouts is badly bent - so much that it is touching the cogs on the cassette. The LBS doesn't have the facilities to deal with this, so I'm going to take it to a framebuilder in Croydon. The bike was £1,100 new, which was 18 months ago, so I'm thinking if it costs more than £400 to repair, it's not worth it.


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## Andrew_Culture (19 Dec 2012)

Cyclopathic said:


> But...but...I did it for youuuuu.


 
A friend asked why I was still feeling the need to seduce my wife when we've been married for years, this friend is single...


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## Andrew_Culture (19 Dec 2012)

benb said:


> The bike was £1,100 new, which was 18 months ago, so I'm thinking if it costs more than £400 to repair, it's not worth it.


 

Have you got insurance?


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## benb (19 Dec 2012)

Andrew_Culture said:


> Have you got insurance?


 
Not specific to the bike, but I'll check what's covered with my home insurance.


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## Andrew_Culture (19 Dec 2012)

benb said:


> Not specific to the bike, but I'll check what's covered with my home insurance.


 
Great, this is meant in a brotherly way rather than a patronising know it all type way!


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## benb (19 Dec 2012)

Andrew_Culture said:


> Great, this is meant in a brotherly way rather than a patronising know it all type way!


 
Thanks Dad.


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## Andrew_Culture (19 Dec 2012)

benb said:


> Thanks Dad.


 
You're welcome son, now stay the hell out of my booze!


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## Andrew_P (19 Dec 2012)

Seriously no separate rear hanger?


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## Tim Hall (19 Dec 2012)

benb said:


> It's not good. The mech mounted directly to the frame, and the part of the frame aft of the dropouts is badly bent - so much that it is touching the cogs on the cassette. The LBS doesn't have the facilities to deal with this, so I'm going to take it to a framebuilder in Croydon. The bike was £1,100 new, which was 18 months ago, so I'm thinking if it costs more than £400 to repair, it's not worth it.


 
Steel frame? Should be fairly straightforward I would have thought.


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## screenman (19 Dec 2012)

Beano1, why do you think it might not have worked? Normally what happens in cases like this is the mech gets caught in a spoke and gets carried around by the momentum of the wheel. 

There are some cases where chain problems can mess a mech up. but rarely as violently as in the picture shown.


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## benb (19 Dec 2012)

Well, Chas Roberts in Croydon has given me a ballpark figure of under £200, but he'd have to see it to be sure. So that's cheaper than I was expecting.


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## snailracer (19 Dec 2012)

screenman said:


> ...*why do you think it might not have worked*? Normally what happens in cases like this is the mech gets caught in a spoke and gets carried around by the momentum of the wheel...


Because the disc would literally need to be a foot in diameter to prevent the mech getting caught in the spokes.

Most discs are barely larger than the largest cog, they are only good for preventing the spokes getting scratched if the chain falls into the gap.


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## screenman (19 Dec 2012)

Look at the shape and see which part of the mech hits the wheel first. They do not need to be large at all.

See if you can find a picture of damage like the OP had that had a plastic thingy on at the time, as I could not do so.

Look at the picture closely you will see he was on the largest sprocket at the time of incident and that the mech is wrapped up in the spokes, yes a long way from the sprocket but the momentum of the wheel would have caused it to travel that way.


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## GrumpyGregry (19 Dec 2012)

Tim Hall said:


> Steel frame? Should be fairly straightforward I would have thought.


@benb is it aluminum? If not, Tim has an adjustable spanner and I have the brute force. All Chas Roberts will use is a fancy bolt and breaker bar to do the dirty deed.


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## screenman (19 Dec 2012)

GC have you tried bending what maybe cast iron cold? It has a tendency to snap. I would say that a new rear drop out and paint job may be the correct procedure.


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## potsy (19 Dec 2012)

Is it usual for bikes to have no rear mech hanger?


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## Rickshaw Phil (19 Dec 2012)

potsy said:


> Is it usual for bikes to have no rear mech hanger?


Yes, if they are steel frames the hanger is generally an integral part of the frame.


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## GrumpyGregry (19 Dec 2012)

screenman said:


> GC have you tried bending what maybe cast iron cold? It has a tendency to snap. I would say that a new rear drop out and paint job may be the correct procedure.


cast steel surely? 

I can't say when faced with a bent steel mech hanger that I've ever interrogated the part as to its origins and the manufacturing techniques involved. And I've stared a few down in my time, bent hangars are an occupational hazard off road, and ime sometimes on it. Never snapped one yet, even managed to straighten an aluminium one once. But they've all been on mass produced bikes not boutique jobs.


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## SportMonkey (19 Dec 2012)

benb said:


> It's not good. The mech mounted directly to the frame, and the part of the frame aft of the dropouts is badly bent - so much that it is touching the cogs on the cassette. The LBS doesn't have the facilities to deal with this, so I'm going to take it to a framebuilder in Croydon. The bike was £1,100 new, which was 18 months ago, so I'm thinking if it costs more than £400 to repair, it's not worth it.


 
I'd talk to the manufacturer. Built in hangers are just silly.

If it is steel though a black smith could fix it for you pretty quickly. Your other option is buying a cheap frame and moving everything over.


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## GrumpyGregry (19 Dec 2012)

potsy said:


> Is it usual for bikes to have no rear mech hanger?


de riguer on steel frames since time immemorial.


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## GrumpyGregry (19 Dec 2012)

Sheldon says "When a derailer hanger is bent, it is generally necessary to remove the derailer to straighten it. Bike shops have a special tool, a bar with a sliding gauge on it, that screws into the hole in the hanger in place of the derailer. This tool provides lots of leverage to straighten the hanger with, and has a gauge to judge when it is parallel to the rear wheel. A rough, on-road repair is sometimes possible by removing the derailer and bending the tab with an adjustable wrench."


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## HovR (19 Dec 2012)

screenman said:


> The plastic disc stops your mech from getting mixed up with the spokes and causing the sort of damage you see here.


 
Those discs are only really any good for the top jockey of a short cage derailleur, and even then many of the discs which come stock on bikes are too small for that.

If a derailleur becomes even slightly bent (common occurrence after a fall in a high gear) then the derailleur cage will often be bent inwards enough that it touches the spokes first, which only a disc half the size of the wheel would prevent!


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## screenman (20 Dec 2012)

HovR, see the picture I posted earlier and you will see a small disc protecting the top jockey wheel area from making contact with the spokes, please tell me how the bottom jockey wheel part of the cage would make contact first.

The dishing on the wheel might give you a clue as to why this is extremely unlikely to happen.


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## snailracer (20 Dec 2012)

screenman said:


> Look at the shape and see which part of the mech hits the wheel first. They do not need to be large at all...


But the thing is, the whole rear mech can get bent out-of-vertical in a full-on chainsuck scenario - in which case the lower jockey can catch in the spokes first. A thin plastic disc is not rigid enough to resist the top jockey being pushed towards the spokes. Chainsuck is strong enough to bend rear mechs, so that plastic disc would present little resistance.


screenman said:


> ...Look at the picture closely you will see he was on the largest sprocket at the time of incident....


That is not my interpretation of the picture and, according to the OP, he was not on the largest sprocket at the time.


screenman said:


> ...The dishing on the wheel might give you a clue as to why this is extremely unlikely to happen.


The cog side of the wheel is much less dished than the non-drive side.


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## screenman (20 Dec 2012)

Go back to the OP's picture it shows chain on largest sprocket at the bottom, so this is likely to be where it was mech started to wrap around.

For a mech to be bent enough for the bottom to touch would have put the way out of line and into the wheel if a disc was not there.

For sure the cog side is less dished, the 3 bikes we have in the upstairs store all show a space of about 13 millimetre, between where the top jockey sits in relation to the spokes and where the bottom one sits.


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## snailracer (20 Dec 2012)

screenman said:


> Go back to the OP's picture it shows chain on largest sprocket at the bottom, so this is likely to be where it was mech started to wrap around...


Is it this picture? If so, I can't see any part of the chain on the largest sprocket:








screenman said:


> ...For a mech to be bent enough for the bottom to touch would have put the way out of line and into the wheel if a disc was not there.
> 
> For sure the cog side is less dished, the 3 bikes we have in the upstairs store all show a space of about 13 millimetre, between where the top jockey sits in relation to the spokes and where the bottom one sits.


The force going through the chain is larger than the force of the rider's foot on the pedal, so, IMO, it's plausible that a rear mech can be bent severely out-of-vertical, and a flimsy plastic disc won't stop it.


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## benb (20 Dec 2012)

screenman said:


> Look at the shape and see which part of the mech hits the wheel first. They do not need to be large at all.
> 
> See if you can find a picture of damage like the OP had that had a plastic thingy on at the time, as I could not do so.
> 
> Look at the picture closely you will see he was on the largest sprocket at the time of incident and that the mech is wrapped up in the spokes, yes a long way from the sprocket but the momentum of the wheel would have caused it to travel that way.


 
Pretty sure I wasn't in the largest sprocket. I had a close look, and it looks very much as though one of the flat plates on the chain came off its pin, which caught on the jockey wheel enclosure. The momentum of the bike then bent the frame pushing the mech into the spokes.


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## benb (20 Dec 2012)

GregCollins said:


> @benb is it aluminum? If not, Tim has an adjustable spanner and I have the brute force. All Chas Roberts will use is a fancy bolt and breaker bar to do the dirty deed.


 
It's steel. I think I'll take it to the framebuilders. It won't be until the new year though.


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## snailracer (20 Dec 2012)

GregCollins said:


> Sheldon says "When a derailer hanger is bent, it is generally necessary to remove the derailer to straighten it. Bike shops have a *special tool*, a bar with a sliding gauge on it, that screws into the hole in the hanger in place of the derailer. This tool provides lots of leverage to straighten the hanger with, and has a gauge to judge when it is parallel to the rear wheel...."


From Park Tool:


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## mickle (20 Dec 2012)

benb said:


> It's steel. I think I'll take it to the framebuilders. It won't be until the new year though.


 
It _really_ doesn't need a framebuilder - just a competent LBS workshop. I've got a dropout alignment tool languishing in my garage if you'd like to borrow it.

Worst case scenario - the hanger separates from the dropout and you'll need to employ a frame builder to tidy up the edge of the dropout and braze a new hanger tab on to it. But there's a few bends back and forth left in it before that happens


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## benb (20 Dec 2012)

mickle said:


> It _really_ doesn't need a framebuilder - just a competent LBS workshop. I've got a dropout alignment tool languishing in my garage if you'd like to borrow it.
> 
> Worst case scenario - the hanger separates from the dropout and you'll need to employ a frame builder to tidy up the edge of the dropout and braze a new hanger tab on to it. But there's a few bends back and forth left in it before that happens


 
Maybe I'll take it to Pearsons - they know their stuff.


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## fossyant (20 Dec 2012)

The dropout is steel. Take an adjustable spanner to it and straighten it carefully. Done it myself to a road bike after a crash. Don't get ripped off !

Steel frames don't use replaceable mech hangers as you can bend them back carefully. That's the property of steel. Same can't be said for aluminium.


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## GrumpyGregry (20 Dec 2012)

benb said:


> Maybe I'll take it to Pearsons - they know their stuff.


That is true. They also know how to make money from people. I realise the quoted ballpark of £200 is probably sight unseen and they are catering for the worst possible starting point, and it may included painting, but it seems like one heck of a lot of money to me for 15 minutes of sweat whilst they set about beating your bike with a metal bar.

The last time I saw an LBS do the job, with the proper tool and everything, they refused payment.


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## GrumpyGregry (20 Dec 2012)

Beano1 said:


> If you want this not to happen to you:
> 
> Clean your drive train
> 
> ...


 
and even then be prepared for it to happen to you. Because it will.


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## mickle (20 Dec 2012)

Re-align hanger and fit new mech and chain: about £20 in our workshop according to the mechanic. Plus t'cost of t'parts. I know it's only York and not That London but still - quite a bit less than £200.


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## snailracer (20 Dec 2012)

mickle said:


> Re-align hanger and fit new mech and chain: about £20 in our workshop according to the mechanic. Plus t'cost of t'parts. I know it's only York and not That London but still - quite a bit less than £200.


One could even buy the Park Tool derailleur tool for a mere £60.


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## screenman (20 Dec 2012)

http://fagan.co.za/Bikes/Csuck/

Beano1 could you explain to us how chain suck bends a mech, I have seen plenty pulled straight but never bent.

Do you not think that once the loose plate hit the cage this would have stopped your pedaling motion, which in turn would have kicked the freewheel into play.

As someone who works with metal most of my time, I would advise against having a go yourself and bending cold.


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## mickle (20 Dec 2012)

screenman said:


> As someone who works with metal most of my time, I would advise against having a go yourself and bending cold.


 
So all those derailleur hanger re-alignment tools in the hands of professional bicycle mechanics up and down the country? Designed and manufacturered by many reputable bicycle tool companies all over the world? According to you we shouldn't be using them? Srsly?


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## screenman (20 Dec 2012)

*" professional bicycle mechanics" I believe that you just wrote this.*


From Sheldon.
Spoke Protector
A plastic or sheet-metal disc that fits between the cluster and the right-side spokes of a rear wheel. This is intended to prevent the derailer or chain from getting caught in the spokes, possibly causing very extensive/expensive damage/destruction to the wheel, the derailer, and the frame.
A spoke protector is not a necessity on a bike that is well treated, because the derailer can't go into the spokes if it's properly adjusted and if it is not bent. Bicycles which are subjected to rough handling, however, are prone to getting the rear derailer bashed in, and in such a case, the spoke protector can prevent very serious damage.

OK so you take the DIY method, make sure whatever you do that the hanger is straight through all planes.


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## benb (20 Dec 2012)

GregCollins said:


> That is true. They also know how to make money from people. I realise the quoted ballpark of £200 is probably sight unseen and they are catering for the worst possible starting point, and it may included painting, but it seems like one heck of a lot of money to me for 15 minutes of sweat whilst they set about beating your bike with a metal bar.
> 
> The last time I saw an LBS do the job, with the proper tool and everything, they refused payment.


 
He did say it may be as low as £50. It depends. If the frame is unfixable and he has to cut away part of the chainstay and seatstay and weld new sections in place, then ... £££

I think I'll take it to him and see what the actual quote is, and then decide.


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## benb (20 Dec 2012)

I was pedalling with some force (albeit on the flat), so I think when the loose chain link caught on the jockey wheel cage, it had enough force to bend it into the spokes. Then the momentum of the wheel pulled it further in and round, bent the frame and then broke the rear mech off completely. At least, that's what I think.

I appreciate the attempts to save me money, but I'd honestly rather get a professional to take a look with something as fundamental to the structure of the bike as this.


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## screenman (20 Dec 2012)

http://www.pinkbike.com/news/technical-tuesday-straightening-derailleur-hanger-2010.html

The reason I suggested that it might not bend back cold is the amount of bend it has taken, a tweak here and there but that is way more.

benb, I think you are making the right choice, please let us know how you get on as I will happily accept being wrong as it would cost you less.


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## snailracer (20 Dec 2012)

screenman said:


> http://fagan.co.za/Bikes/Csuck/
> 
> Beano1 could you explain to us how chain suck bends a mech, I have seen plenty pulled straight but never bent.
> 
> Do you not think that once the loose plate hit the cage this would have stopped your pedaling motion, which in turn would have kicked the freewheel into play...









From a rear view, you can see that a triangle is formed by three points:
(1) the upper jockey
(2) the point where the chain rides over the top of the cog
(3) the rear mech hanger.

Because points (2) and (3) are pretty much immovable, if there is massive overtension in the chain, the length of chain between (1) and (2) shortens. As (1), (2) and (3) are 3 points of a triangle, the shortening results in the jockeys moving towards the wheel.

If the mech hanger (3) could somehow be located _inboard_ of the cogs, the upper jockey would actually move away from the wheel and not crash into it, but that is not how bikes are designed.

Chain overtension can be caused by chainsuck or jammed jockey, but that does not alter the forces acting on the triangle of points (1), (2) and (3).



screenman said:


> ...As someone who works with metal most of my time, I would advise against having a go yourself and bending cold.


Yeah, risky.


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## mickle (20 Dec 2012)

snailracer said:


> From a rear view, you can see that a triangle is formed by three points:
> (1) the upper jockey
> (2) the point where the chain rides over the top of the cog
> (3) the rear mech hanger.
> ...


 
Eh? You're wrong. Chainsuck can bend a ring, put a hole in a chainstay and mangle a chain. Chainsuck cannot destroy a rear mech.


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## GrumpyGregry (20 Dec 2012)

That hanger has got a bend in it in the cock all league of bends. It is going to get cold set, just like the frame would have been when it came off the jig. What is 'cold setting'? Bending stuff, with brute force and big levers. I've seen it done up close at a frame builders. It ain't pretty. It is effective.

I can accept folk thinking I'm talking tosh. I'll leave Tim Hall to be offended for himself. But Mickle does this s*** for a living. If he says its ok, it's ok and you can bet money on it.


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## snailracer (20 Dec 2012)

mickle said:


> Eh? You're wrong. Chainsuck can bend a ring, put a hole in a chainstay and mangle a chain. Chainsuck cannot destroy a rear mech.


OK, do you think that having too short a chain can damage a rear mech?


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## screenman (20 Dec 2012)

GC, Once metal bends beyond a certain point it will not go back into place safely.

Now try this get a piece of metal and bend it once fine, now bend it back you have weakened it.

Your advice should carry a safety warning.


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## mickle (20 Dec 2012)

snailracer said:


> OK, do you think that having too short a chain can damage a rear mech?


We've been here before. 'Too short a chain' can wreck a frame, a hub, chainring, sprocket or chain. It cannot damage a mech.


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## mickle (20 Dec 2012)

To be fair, a very badly bent hanger is more likely to break during straightening than a not very badly bent hanger. And a lot depends on the quality of the steel used in the drop-out/hanger and its condition after welding/brazing. But, as anyone who has tried to break a length of steel by bending it back and for'ards will attest, you can be there all day before it yields.


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## HovR (20 Dec 2012)

screenman said:


> please tell me how the bottom jockey wheel part of the cage would make contact first.


 


HovR said:


> If a derailleur becomes even slightly bent (common occurrence after a fall in a high gear) then the derailleur cage will often be bent inwards enough that it touches the spokes first, which only a disc half the size of the wheel would prevent!


 


snailracer said:


> But the thing is, the whole rear mech can get bent out-of-vertical in a full-on chainsuck scenario - in which case the lower jockey can catch in the spokes first. .


 
Furthermore, if I am not mistaken, that is the lower section of the derailleur cage tangled up within the spokes in OP's picture, not the upper section.


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## MisterStan (20 Dec 2012)

This happened to me in February;






The guy at my LBS reckoned that the hanger had taken a knock at some stage and that as i went onto the big cog, the rear mech got pulled into the wheel.


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## screenman (20 Dec 2012)

Mickle, it depends on what type of steel. I have some 6mm bars here made of a steel that you can bend fine once, try and straighten it hey presto it snaps. Not got a clue what it is called but is great for making the dent tools I sell.

HovR, I would say you are correct, it is also the position it ends up in when the top jockey wheel gets caught in the spokes and the rotation of the wheel takes it around, now reverse the process and see what happens.


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## Tim Hall (20 Dec 2012)

Am I right in thinking the hanger thread (M10 x 1) is the same as Shimano rear axle? So a step up from using a shaved gorillla (© Greg Collins) with an adjustable spanner could be to screw a bit of axle into the hanger hole and then apply the same shaved gorilla.


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## snailracer (20 Dec 2012)

mickle said:


> We've been here before. 'Too short a chain' can wreck a frame, a hub, chainring, sprocket or chain. It cannot damage a mech.


 
Well I have known it to happen: makeshift shortened chain, accidental shift into big-big resulting in derailleur rubbing against/scuffing spokes (but not enough to entangle - luckily). I see no particular difference between chain overtension due to short chain vs chainsuck. I am not saying chainsuck is definitely what happened to the OP, it's just a possibility.

http://sheldonbrown.com/derailer-adjustment.html
_"If the chain is too short, it will be at risk for jamming and possibly ruining the rear derailer if you accidentally shift into the large-large combination"_


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## benb (20 Dec 2012)

screenman said:


> Mickle, it depends on what type of steel. ...


 
Reynolds 725 I think.


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## benb (20 Dec 2012)

snailracer said:


> Well I have known it to happen: makeshift shortened chain, accidental shift into big-big resulting in derailleur rubbing against/scuffing spokes (but not enough to entangle - luckily). I see no particular difference between chain overtension due to short chain vs chainsuck. I am not saying chainsuck is definitely what happened to the OP, it's just a possibility.
> 
> http://sheldonbrown.com/derailer-adjustment.html
> _"If the chain is too short, it will be at risk for jamming and possibly ruining the rear derailer if you accidentally shift into the large-large combination"_


 
I am 99% sure that it was a link coming off its pin on one side, and that jamming up solid against the jockey wheel cage. Certainly there was a link jammed there when we took the whole thing off.


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## snailracer (20 Dec 2012)

benb said:


> I am 99% sure that it was a link coming off its pin on one side, and that jamming up solid against the jockey wheel cage. Certainly there was a link jammed there when we took the whole thing off.


I agree, that seems the most likely scenario, especially as you mentioned the per-chain-length clicking.
My triangles explanation explains why the mech bends towards the spokes, rather than away.


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## Psycolist (21 Dec 2012)

I've suffered a series of chain breaks recently (all preceeded with that tell tale clicking) and have been lucky enough to escape without any damage, except to my pride, but about 5 years or so ago, on a particularly windy day, I had a plastic bag blown into my drive train. The resulting damage was exactly that seen by 'benb'. I've not heard of this happening without some off bike intervention by a piece of debris or animal (surley you have all had a squirrel bounce off y'spokes at one time or another) but from posts in this thread it seems to be a very common event. You live and y'learn. !


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## Pale Rider (22 Dec 2012)

benb said:


> It's not good. The mech mounted directly to the frame, and the part of the frame aft of the dropouts is badly bent - so much that it is touching the cogs on the cassette. The LBS doesn't have the facilities to deal with this, so I'm going to take it to a framebuilder in Croydon. The bike was £1,100 new, which was 18 months ago, so I'm thinking if it costs more than £400 to repair, it's not worth it.


 

I wonder if the twisting force against the base of the nut has bent the axle.


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## mickle (22 Dec 2012)

benb said:


> Reynolds 725 I think.


That'll be the tubing, not the lugs.


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## mickle (22 Dec 2012)

Pale Rider said:


> I wonder if the twisting force against the base of the nut has bent the axle.


No.


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## mickle (22 Dec 2012)

snailracer said:


> Well I have known it to happen: makeshift shortened chain, accidental shift into big-big resulting in derailleur rubbing against/scuffing spokes (but not enough to entangle - luckily). I see no particular difference between chain overtension due to short chain vs chainsuck. I am not saying chainsuck is definitely what happened to the OP, it's just a possibility.
> 
> http://sheldonbrown.com/derailer-adjustment.html
> _"If the chain is too short, it will be at risk for jamming and possibly ruining the rear derailer if you accidentally shift into the large-large combination"_


Sheldon shmeldon.


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## GrumpyGregry (22 Dec 2012)

screenman said:


> GC, Once metal bends beyond a certain point it will not go back into place safely.
> 
> Now try this get a piece of metal and bend it once fine, now bend it back you have weakened it.
> 
> Your advice should carry a safety warning.


What metal?
What degree of bend?
Is that metal used to make drop outs on 725 tubed frames?
Is this particular drop out bent to that degree?


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## Pale Rider (22 Dec 2012)

mickle said:


> No.


 
Mickle shmickle.


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## GrumpyGregry (22 Dec 2012)

Tim Hall said:


> Am I right in thinking the hanger thread (M10 x 1) is the same as Shimano rear axle? So a step up from using a shaved gorillla (© Greg Collins) with an adjustable spanner could be to screw a bit of axle into the hanger hole and then apply the same shaved gorilla.


Funny you should say that. I've got an old axle or two and was going to give it a try on a beater SS frame (which has a bent hanger) with a view to carrying the axle on next years night rides as I find the complexity of an adjustable spanner hard to master at 04:00 with no sleep. But then shaved gorilla's like yours truly are easily confused.

I shall report back if it ever stops raining (my workshop is the great outdoors).


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## GrumpyGregry (22 Dec 2012)

benb said:


> Reynolds 725 I think.


any chance of a shot of the drop out from behind and or below?


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## mrandmrspoves (22 Dec 2012)

Similar problem happened to me a few months ago when the Deore rear mech snapped at the hinge and collapsed into the wheel on my 631Dalesman. Also had a bent hanger....the problem with the big spanner approach is that even a small degree of inaccuracy will lead to major indexing issues. I purchased a hanger allignment tool (cyclo) for less than £30 and sorted it myself. Easy job with the right tool...


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## mrandmrspoves (22 Dec 2012)

Like Mickle's offer - happy to lend the tool, though postage wouldn't be cheap as it's a heavy bit of kit. You screw one end of the tool into the gear hanger and then line the sliding gauge up so it just touches the wheel rim at 12 o'clock, then rotate the tool to 6 o'clock, 3 o'clock and 9 o'clock. ... this then ensures that the hanger is correct in both vertical and horizontal plains.


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## benb (23 Jan 2013)

Well, picked it up today from Chas Roberts frame builders. They had to heat the frame to straighten it, drill out the ruined thread from the mech hanger, and put an insert in with a new thread.

Total price £70.

Now it's at Pearsons for some new spokes, new rear mech, new chain and general check. That might be another £70-£100

Can't wait to get her back.


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