Catastrophic drivetrain failure

[snailracer]

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).

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

Yeah, risky.

 

[mickle]

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).

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.

 

[GrumpyGregry]

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.

 

[snailracer]

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?

 

[screenman]

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.

 

[mickle]

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.

 

[mickle]

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.

 

[HovR]

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

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!

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.

 

[MisterStan]

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.

 

[screenman]

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.

 

[Tim Hall]

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.

 

[snailracer]

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"

 

[benb]

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

Reynolds 725 I think.

 

[benb]

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.

 

[snailracer]

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.