Rear mech whipped up nto the stay by your cranking it

Creates a sharp point impact into the chainstay

I have seen exactly this happen on an mtb in the past

* edit – was much closer to the rear axel than that though

Nope, I pushed the rear mech up under the frame to test this theory and like you say the impact point would have been much further back if that was the cause.

Also , do any of these carbon repair companies offer any kind of warrenty or guarantee the rest of the frame is ok.

If not i would not recomend a customer to pursue this option either.

I can’t see the big ring resulting in excess chainstay load. Thinking about the leverage, for the same pedal force, the chain force with a 50T ring is only half what it is with a 25T. Big-big is better than small-small for chain (and hence chainstay) load. Quite frightening actually – not a road biker so don’t know what size small chainring is, but if it’s 110 diameter (55mm radius) and you have 175 cranks, and put your full body weight on the pedal, the chain tension is over 3 times your weight.

But, Big-big does put the chain quite diagonal, and so puts a sideways load on the mech. If the chain is a bit short, so the mech is lying almost horizontally forwards, the chainring to mech distance gets even shorter and the diagonal angle is greater – with the chain pulling sideways from the big ring, it wouldn’t surprise me if the mech bent or the jockey wheel gave way. Not quite sure what breaks next but something will.

Sounds to me that the lower limit screw on the derailleur wasn’t adjusted properly, and either the chain came off the back of the cassette, or more likely the derailleur cage caught a spoke under heavy torque.

I agree that it’s more likely the chainstay break happened before the mech break. As trail rat says – and as proven by the OP in his case – I’ve also had a mech hit a chainstay but only about 5″ or so (mech length really) along, not in the middle, and it only chipped the paint. Saying that though, there could have been a weak point and the mech did cause the break. Dunno. Definitely worth speaking to them again though.

Those frame repair jobs are impressive TBH, I’d be happy to use one of them.

I don’t understand the mechanism, but I was on a ride a couple of months ago where the same thing happened when the rider went for big big by accident.

If you think it was rider power that did it then the OP needs to call British Cycling pronto.

Shame none of these pros can output frame snapping torque eh.

Chains are strong, quite possible that as it was shifted to a bigger sprocket it became fully taught, then as mech went round the cassette even more tension was placed on it. Something had to give, in this case it was the stay. How good is carbon in compression…?

looks a weird one.

1st thing i would look at

does the rear mech open to that breaking point at full extension?
if not then how did that break? is the carbon shaped at that break ie slightly curved if so its possible that once the rear mech fully extended and the chain locked up you with your turning/crank motion put enough force into the chain that it twisted the frame and cracked the chain…. highly unlikely as carbon is seriously strong just dont see it fracturing if it hasn’t taken a direct point of impact

2nd
argue with the manufacture that the chain stay snapped causing all your problems AND costing you a new rear mech and wheel! due to the failure!

perhaps your cadence sensor and zipties gave sufficient strength to hold that side together ?

How olds the frame ?

I had a hanger snap after the rear much went into the wheel (my fault, limit screws, big-big).

Not sure if this is relevant but the chain stay took a big hit in roughly the same position as your failure. Fortunately not enough for structural damage but has left a nice battle scar.

A few years ago I had a DeRosa Avant Carbon with ultra thin rear stays, bike was fine for <7mths then the stays broke whilst I was up near Alton on a long ride. I had to get the train home. Upshot is I was about to throw the bike through the LBS window in disgust but they replaced the frame and I got to keep the old one. Mate had the old frame and got the stays repaired, bikes still rolling along nicely. I got shot of the replacement and bought an Orca.
I was surprised just how effective Carbon repairs are.

definitely in the wrong place for the RD to hit so does look like in the process of shifting the chain got too right and the frame failed in compression.

the non-drive side would be okay as the drive side is the one under compression and the wheel would pivot at the axle/drop out on the non-drive side.

your RD would then pivot inwards and hit the wheel spokes.

ultimately if the chain was too short (and you fitted it) then it would be user error.

Chainstays are generally built very stiff so a carbon repair should be quite effective and not change the bikes characteristics much.

I had a spill last year that bent mech hanger. Didn’t even notice until the next day I went for bottom gear and the mech hit the spokes. This brought me to a VERY abrupt stop, annihilating the mech in the process. Not the same situation as yours, but you have to consider the forces going through the chainstay when the transmission locks up are far greater than just the driving force produced by your legs. Added in to the mix is the force required to reduce your speed (and hence momentum) to zero in less than one revolution. This probably far exceeds any ‘designed for’ stress. Most chainstays curve out towards the wheel hub, meaning that this force will produce a bending moment, not just a compression load.

I would say the main question is how the transmission locked in the first place.

My tuppence worth…

2nd
argue with the manufacture that the chain stay snapped causing all your problems AND costing you a new rear mech and wheel! due to the failure!

Which would be fraud, of course…

Is this like a Scott and has no replaceable hanger?

OP says:
“I have been thinking about this a bit more now and I am struggling to understand how the rear wheel locking as a result of the rear mech getting caught in the rear wheel / on the cassette would cause such failure in the chain stay.”

when you lock wheel under braking: firstly you have generally unloaded it by transferring weight onto the front, hence force required to overcome friction is reduced; secondly and more relevantly, the reaction forces in the frame are distributed differently. on a rim braked bike, the force is applied at the edge of the wheel, has a greater torque arm and less force is required to balance the torque produced at the contact patch. with a disc brake, the force counteracting this torque must be higher as it is applied through a smaller torque arm (the radius of the disc) but it is spread through the chainstay (as a bending moment) and the seatstay (as a compression force). also these parts are generally somewhat reinforced or at least braced on a disc braked bike.

when the transmission locks up,the force is produced by tension in the lower chain run, not the upper portion. a straight chainstay would see this force as more of a compression force but a curved stay will see a greater bending moment. the seatstay is unable to provide much support here as it too is mostly under bending, with any additional component force being in tension not compression.

guess that makes fourpence now

The frame was bought new last April. So still in warranty.

I am calling the shop again as I am struggling to understand their reluctance to discuss this or send the frame back to the manufacturer for inspection.

When the wheel locked up the back tyre skidded so that would have reduced the amount of torque the rear wheel would have applied through the chain.

i think youll find the warrenty details will include the clause “must be built by a qualified person”

“i think youll find the warrenty details will include the clause “must be built by a qualified person”” – I was never aware of that.

That was the first question the Manager asked. So even if it was not your fault and you did have a manufacturing defect then you are buggered unless you paid the shop to build the bike?

I doubt many frame buyers are aware of that little chestnut.

How heavy are you Flanagaj?

The fact still remains that the chainstay snapped because of you running the chain too short. This isn’t the manufacturer’s fault, as frames aren’t designed to be misused.

IMO, you just need to suck it up.

“Warranty applies to original owner only and the bicycle must be fully assembled by an authorized Argon 18 dealer.” – You were right. What a load of b!llocks that is.

I can’t remember a ‘you do realise that walking out of the door with that frame and building it yourself will result in the frame having no warranty’ conversation.

Very poor

Well try provin it wasnt cack handed spannering that broke it.

He has identified that there was a fundimental flaw in the build that has put force ( compression) into a frame of a magnitude it wasnt designed to handle

Your now trying to prove that the lack of compression strength is a design flaw- thats like me having it run over in a race and trying to claim.

I weigh 72kg

Its in most frames smallprint but i will tell you its rarely exercised unless a grave error was made thats blatently being tried on.

So not a sumo then. Just guilty of running the chain too short.

Why did you run the chain too short?

Trail rat – show me the proof and I will stop my my bitching, until you can I think you need to have an open mind that it could have been either my fault or a manufacturing defect.

ps – Are you the manager guy at Tri Uk?

He is not the manager at Tri UK, but he is a mechanical engineer (one of the few one here).

yes thats me caught red handed.

just looking at it again ….

rotational moment from your spokes ripping the mech inwards could cause that mode of failure.

you could try your luck with the distributor if your having no joy with the dealer but i wouldnt hold my breath.

the skeptical cats will have on their skeptical caps.

Given the warranty condition I don’t think I have a hope in hell of getting some sort of compromise agreement. Even if it was a replacement frame at cost that would be something.

Spoke to the shop and they told me it’s my own fault for having the chain too short and that you should be able to use any gear combination on the bike

I don’t agree with this as there’s certain gear combos everyone should stay away from, biggest at the front with biggest sprocket at the back, and smallest at the front with smallest sprocket at the back.

email manufacturer direct. State unwillingness of shop to deal with you regarding this.

State what happened and see what they say? If no go then pay to get it repaired.

They can’t totally void warranty just because it wasn’t built by a particular person. Trading standards and being fit for purpose and all that.

Rich

Edric 64 – Member

Steel is real, you might have a dent but not a fubbarred frame made from chocolate

Not so I know of a very recent case of a Steel failure, wont say more at the moment but all materials are capable of failing with unfavorable outcomes.

you were going uphill and pedalling hard. this puts most of your weight on the rear. to lock the wheel up (overcoming your pedalling force too, lets not forget) and cause a skid (at least in the dry) requires a tremendous force. try doing it by braking….

carbon frames (and hydroformed aluminium) are very light because they allow the designer to remove material where it is not needed to cope with ‘designed for’ stresses. that is why many designs are inferior to a traditional steel frame when it comes to coping with forces outside those of typical (or even extreme) riding.

michaelmcc: using those gears is considered poor form as it increases wear/mech rub – not for fear of instant catastrophic failure.

as others have said, if the drivechain locked up because the chain length was too short or the b tension screw adjustment was wrong, then thats just one to chalk up to experience.

I don’t agree with this as there’s certain gear combos everyone should stay away from, biggest at the front with biggest sprocket at the back, and smallest at the front with smallest sprocket at the back.

You stay away from them because of the poor chain line in these gears, which might accelerate drivetrain wear, but won’t do serious damage with a chain of the correct length.

A chain that is too short can break the frame when you try to shift into a bigger gear.

I can see now why the shop are not interested in doing anything. The Argon warranty only applies if the bike is built by an Argon dealer.

They should have also made that clear when I purchased said frame. At least then I could have maybe tried to claim through warranty

Micheal mcc – unless he has a triple then big front big back is on a double is not a no no – even more so if he is on a compact

Thts why front shifters have the trim feature.

They can void the warrenty if it was built wrong – which is why the qualified person clause is there.