Snapped Chain Stay Roubaix SL2

[the snail]

Let's then assume he tries shifting into 50/25 - what happens..?

The derailleur cage rotates so it's in line with the chain, the chain would squeeze the jockey wheels, but that force is perpendicular to the chain tension. Then the weakest part of the structure fails, which by the look of it was the chainstay, which broke where it bends outwards under the bending moment due to the chainring being outside of the stay, which is oval and weakest in that direction.

 

[mickle]

Ok, picture this: Imagine if you will, a bike with telescopic stays. We fit the chain in the normal way - through the mech - and then extend the adjustable stays to take up all the slack, until the chain is taught (not tight, but taught) around the largest sprocket and largest chain ring. The rear mech and its cage plate is stretched forwards, the lower jockey atop the chain and the upper jockey beneath. This is within the design parameters of the mech, any mech, if it's fitted in line with the manufacturer's recommendations pertaining to capacity/tooth difference etc.

Now then. Using whatever means at our disposal we now extend the telescopic stays - lets say - 5mm. The chain is now under massive tension, the stays under massive compression. The chain however has not changed its shape. It was taught before and now it's super tight. But it has not moved relative to the mech. So it can't break the mech or bend the hanger. Glad we got that cleared up.

And it all still 'works', that is, the pedals will turn the crank which will drive the chain around and propel the wheel. It'll feel nasty, heavy, draggy and probably make a godawful grunching noise, but it'll turn. And unless the rider changes gear it'll keep on turning until soemething fails.

My money is on the stay failing first if it's a lightweight frame. The folk in the warranty dept of Specialized are right.

 

[oldfatfool]

Ok, picture this: Imagine if you will, a bike with telescopic stays. We fit the chain in the normal way - through the mech - and then extend the adjustable stays to take up all the slack, until the chain is taught (not tight, but taught) around the largest sprocket and largest chain ring. The rear mech and its cage plate is stretched forwards, the lower jockey atop the chain and the upper jockey beneath. This is within the design parameters of the mech, any mech, if it's fitted in line with the manufacturer's recommendations pertaining to capacity/tooth difference etc.

Now then. Using whatever means at our disposal we now extend the telescopic stays - lets say - 5mm. The chain is now under massive tension, the stays under massive compression. The chain however has not changed its shape. It was taught before and now it's super tight. But it has not moved relative to the mech. So it can't break the mech or bend the hanger. Glad we got that cleared up.

And it all still 'works', that is, the pedals will turn the crank which will drive the chain around and propel the wheel. It'll feel nasty, heavy, draggy and probably make a godawful grunching noise, but it'll turn. And unless the rider changes gear it'll keep on turning until soemething fails.

My money is on the stay failing first if it's a lightweight frame. The folk in the warranty dept of Specialized are right.

Fair enough but taking your example would it have been possible to select that gear with the chainstay already extended. In order for the gear to be selected the chain as to go even taughter to ride over the teeth and once the cog is engaged properly the chain effectively slackens.

Wether this frame as received some external damage to cause a pre existing condition we don't know but I would maintain that if the gear is selectable and rideable then the force exerted should be within the tolerance of the frame. While ever there is any play in the rear mech then there is no more force applied to the frame regardless of chain length.

The most force that can be exerted is the weight of the rider, so are we saying that if you are daft enough or inadvertantly stop in big chain little cog and then try to set off you will snap a chainstay??

 

[mickle]

Fair enough but taking your example would it have been possible to select that gear with the chainstay already extended. In order for the gear to be selected the chain as to go even taughter to ride over the teeth and once the cog is engaged properly the chain effectively slackens.

Wether this frame as received some external damage to cause a pre existing condition we don't know but I would maintain that if the gear is selectable and rideable then the force exerted should be within the tolerance of the frame. While ever there is any play in the rear mech then there is no more force applied to the frame regardless of chain length.

The most force that can be exerted is the weight of the rider, so are we saying that if you are daft enough or inadvertantly stop in big chain little cog and then try to set off you will snap a chainstay??

The gear is selectable, even if the chain is too short, thanks to modern shift ramps. Any mechanic will tell you of occasions where a too short chain has been installed on a bike - I've witnessed it sevearal times myself - where it's still just possible to engage big to big. It'll go in to that gear relatively easily, but it becomes a real challenge to get it out of that gear. Really massive forces come into play. You have to virtually dismantle the bike to get it free. Now, if it's possible to engage this gear in a workstand, using only hand power, you can imagine how much easier it is to do it whilst pedalling - up a hill, stomping on the pedals.

I disagree that the greatest force is rider weight, but anyway, we're not talking about these forces in isolation - the compression forces of a too tight chain are in addition to normal pedaling and rider weight loads.

 

[oldfatfool]

The gear is selectable, even if the chain is too short, thanks to modern shift ramps. Any mechanic will tell you of occasions where a too short chain has been installed on a bike - I've witnessed it sevearal times myself - where it's still just possible to engage big to big. It'll go in to that gear relatively easily, but it becomes a real challenge to get it out of that gear. Really massive forces come into play. You have to virtually dismantle the bike to get it free. Now, if it's possible to engage this gear in a workstand, using only hand power, you can imagine how much easier it is to do it whilst pedalling - up a hill, stomping on the pedals.

The challenge getting out surely comes from the fact that the only force being applied is by the return spring in the mech as opposed to the force of pulling on the cable?

I disagree that the greatest force is rider weight, but anyway, we're not talking about these forces in isolation - the compression forces of a too tight chain are in addition to normal pedaling and rider weight loads.

Where else does any load/ force come from, there is only the riders weight on the pedals in the system?

 

[black'n'yellow]

The derailleur cage rotates so it's in line with the chain, the chain would squeeze the jockey wheels, but that force is perpendicular to the chain tension. Then the weakest part of the structure fails, which by the look of it was the chainstay, which broke where it bends outwards under the bending moment due to the chainring being outside of the stay, which is oval and weakest in that direction.

yes, but my point is that the weakest part of the ensemble should have been the RD or the hanger - not the chainstay. Evidently, the chainstay has failed first for some reason, but my original observation was that under normal circumstances, one of the other two bits should have been the first to go...

 

[black'n'yellow]

Now then. Using whatever means at our disposal we now extend the telescopic stays - lets say - 5mm. The chain is now under massive tension, the stays under massive compression. The chain however has not changed its shape. It was taught before and now it's super tight. But it has not moved relative to the mech. So it can't break the mech or bend the hanger. Glad we got that cleared up.

Even at full extension, the chain does not follow a straight line through the mech. The guy has taken 2" out of his chain - that's 1" more than most RDs can typically tolerate, so normal convention goes out the window. That's why I'm a bit puzzled by your somewhat 'infallible' attitude to all this...

 

[Rohloff_Brompton_Rider]

That's a remarkable piece of conjecture - a good entry for the Turner prize on bicycle-related creative writing..

So, all that happened as you say - and the hanger didn't fail and the mech didn't snap first..?? I'm particularly interested in why the wheel bearings would 'seize' simply because of some rapid downshifting..??

jeez really? erm the pressure from the quick upshift coming to a dead stop which will put lateral force onto the bearings (i.e., twist the axle).

the mech is pulled straight and tight in the direction it is designed to go in,


ok so many people have tried to help you out and explain it in SIMPLE terms but your not having it. go get a fancy carbon bike, yours if you have one, make the chain short on purpose and then go do the same as op. it's only a rear mech hanger, what can happen?.....oh wait.

why not take me up on my tassels?

when mocking someone's experience at least provide something more substantial than a smiley dude!!

 

[black'n'yellow]

jeez really? erm the pressure from the quick upshift coming to a dead stop which will put lateral force onto the bearings (i.e., twist the axle).

the mech is pulled straight and tight in the direction it is designed to go in,


ok so many people have tried to help you out and explain it in SIMPLE terms but your not having it. go get a fancy carbon bike, yours if you have one, make the chain short on purpose and then go do the same as op. it's only a rear mech hanger, what can happen?.....oh wait.

why not take me up on my tassels?

when mocking someone's experience at least provide something more substantial than a smiley dude!!

er, you know what - if you're happy with that, then that's fine. It will be interesting to learn if it really did happen the way you say it did, and if the bike in question really does have all that 'seized' stuff on it. Maybe the OP forgot to mention it. I'm not going to bet on it though...

Anyway, who am I to question your 'engineering experience' - especially since I don't actually know what engineering experience you have...other than the fact that you say you have some...

 

[Rohloff_Brompton_Rider]

meeooowww!!!

 

[GrumpyGregry]

meeooowww!!!

 

[zexel]

brompton is right, and this has never happened to me before, but I agree with Mickle too!

I've had a very similar experience with a too short a chain for big/big combination, luckily I wasn't going very fast plus I was on an aluminium bike, basically as bromptonfb has explained above,(luckily just) the rear wheel locked up. More momentum and a carbon frame that would be the kind of damage to be expected.

 

[mickle]

Even at full extension, the chain does not follow a straight line through the mech. The guy has taken 2" out of his chain - that's 1" more than most RDs can typically tolerate, so normal convention goes out the window. That's why I'm a bit puzzled by your somewhat 'infallible' attitude to all this...

I just happen to know what I'm talking about.

 

[black'n'yellow]

I just happen to know what I'm talking about.

yep - so you keep saying. Not seen much evidence of that though...

 

[Rohloff_Brompton_Rider]

don't feed the troll!