Snapped Chain Stay Roubaix SL2

[WindyRob]

A friend of mine was part of a team crossing the USA. Unfortunately he suffered a heart attack on day 2. he survived and is recovering. The rest of the team continued and one of them started to use his bike. The chain failed and rather than replace it was shortened by two links. Some time after this whilst climbing a hill the chain stay snapped.

The bike was shipped to the UK and returned to the shop, specialized initially agreed to replace the frame under warrantee, a roubaix SL2 7months old and <2000 miles. once they inspected the frame they saw that the bottom bracket casing had failed and now claim that the shortened chain used with big chainring and largest cassette ring caused the frame to crush itself and now they wont replace it.

Has this happened to anyone else or is this just BS to avoid having to replace the bike?

 

[Herzog]

The amount of force required to snap a chainstay (as demonstrated in part by some videos recently posted on this forum) would be massive (unless defective of course), and I would doubt that the shortened chain caused the failure. Moreover, would exprienced riders, as they no doubt were if crossing the US, be using the large chainring and largest cog...I doubt it.

Strange though, Specialized are usually very good on replacing frames.

 

[mickle]

Removing two chain links is an amateurish screw up. Although the use of big-to-big is not recommended, it must be possible to access/ ride in that gear combo because even the most experienced of riders make mistakes sometimes. Especially in an event like the RAAM where logical thinking will be compromised.

In a set-up failure situation such as this something's got to fail. The loads are enormous and the chain is probably the strongest of all the components involved. I bet the bearings are shagged in BB and hub too.

Specialized is right.

 

[black'n'yellow]

If the loads were that enormous, and if the chain was so tight that it caused the frame to fail, then the gear hanger or mech would have failed long before the chain stay or BB gave out. Hangers are specifically designed to be one of the first failure points in order to prevent stuff like this from happening.

 

[raindog]

In fact, if the chain was that short and tight, I doubt you'd even get it onto the biggest sprocket/chainwheel combination, and as black'n'yellow says something would give or jam up long before the frame broke.

 

[martint235]

I don't think the mech and the rear hangar come into it. All the load is being applied between the top of the rear cog and the top of front chain ring. I can't see how the shortened chain would make a difference either though as the distance between these two points is fixed by the chainstay regardless of chain length.. I'm sure someone will come up with a reason this thinking is wrong but it sounds like too much pressure was put through the frame causing a fail. On this basis I'd expect Specialized to replace it.

 

[black'n'yellow]

chain tension is handled by the rear mech - if you run too much cross-over on a chain which is excessively tight to start with, then the chain will try to pull itself 'straight' and the transmission as a whole will fail at the weakest point - which will be the mech. Or the hanger.

 

[raindog]

I've been thinking about this while I was having lunch, and, in fact, if the chain was that bloody tight you wouldn't even be able to turn the cranks - the bike would be unrideable.

 

[martint235]

The rear mech does handle chain tension but I don't think it handles the force put through the chain stay. This force runs along the length of the chain between where it pulls on the chainring and on the rear cog surely?

 

[GrumpyGregry]

If the loads were that enormous, and if the chain was so tight that it caused the frame to fail, then the gear hanger or mech would have failed long before the chain stay or BB gave out. Hangers are specifically designed to be one of the first failure points in order to prevent stuff like this from happening.

The hanger or mech probably should have failed before the stay, but if the BB shell was already going, or the stay already damaged in some way, before the final failure incident, and the forces were transmitted at just the right angle it is entirely possibly the stay went, in effect, before the hanger had time to deform.

 

[RecordAceFromNew]

I don't know if the rear mech or hanger would always break when the chain is straight between large/large. I have just examined a long cage Deore and it seems it should accommodate that.

I think Mickle made a good point. Those chainstays aren't designed to take compressive load I suspect, and while carbon is strong against tension it is non-isotropic and pretty lousy against compressive load unless specially and relatively heavily built like a carbon seatpost. The post of a 230mm carbon seatpost weighs around 150g, but that frame weighs only 1200g while having members around 15 times longer in total than that of the seatpost. As we all know, even such seatposts require VERY careful clamping to avoid damage/failure.

Frankly I believe people who think carbon bike frames are robust and durable are deluded. Carbon frames are only robust under load they are designed for, and they are not designed for random abuse* bikes typically get, clamping by service stands or transportation racks, or loads such as in the present case etc. etc. To me that means carbon frames are NOT robust and durable.

The manufacturer's warnings have them pretty well covered too, I would have thought.

* the evidence of which is plain to see on most alloy or steel commuters on the road

 

[black'n'yellow]

I think Mickle made a good point. Those chainstays aren't designed to take compressive load I suspect,

of course they are - what other type of load is a chainstay designed to take..?

Carbon frames are only robust under load they are designed for,

just like any other frame material then...

 

[RecordAceFromNew]

of course they are - what other type of load is a chainstay designed to take..?

Perhaps you could answer the question yourself - if those stays were cut and you either sit on the saddle or stood on the pedals do you expect the rear hub to move closer or away from the bottom bracket?

just like any other frame material then...

Not exactly. Since steel, alloy and Ti are isotropic, they are strong in directions other than the design load paths too. Also they are more abrasion resistant.

 

[black'n'yellow]

the r/h stay, in particular, experiences a compressive load. Less so the l/h stay. Look it up.

So carbon fibre is not isotropic - big deal. Materials all have their purposes - and they will all break under the right circumstances.

I've broken steel and aluminium frames - but I haven't broken a carbon one yet...touch wood - an anisotropic, naturally occurring material...

 

[GrumpyGregry]

of course they are - what other type of load is a chainstay designed to take..?

A load in tension I would have thought. Not sure anything much is the way of compression is being transmitted but I'm no engineer or physicist.