Catastrophic drivetrain failure

[screenman]

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.

 

[GrumpyGregry]

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.

 

[screenman]

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.

 

[potsy]

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

 

[Rickshaw Phil]

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.

 

[GrumpyGregry]

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.

 

[SportMonkey]

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.

 

[GrumpyGregry]

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

de riguer on steel frames since time immemorial.

 

[GrumpyGregry]

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."

 

[HovR]

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!

 

[screenman]

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.

 

[snailracer]

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.

...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.

...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.

 

[screenman]

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.

 

[snailracer]

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:

...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.

 

[benb]

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.