glupton1976 – Member
Can you give me that in English now please?

user error.

people think in rather linear terms when it comes to bikes as structures, but they are anything but linear, I bet the rear wheel suddenly locking threw the OP forwards/over the bars(?) that’s because his body and most of the bike had inertia, the rear wheel being locked had the effect of arresting that rather rapidly.

Wrong.
Oddly enough I’ve never gone over the bars locking up the back wheel.

Oddly enough I run a very short chain on one road bike and if I attempt through stupidity to put it in the big rings front and back it just wont go.
Fact is with a short chain there is no way the mech can touch the cassette, its stretches it [mech] out horizontally away from the cassette, I could take some pictures to show but cant be arzzed
However a to long chain and the jockey wheel can touch the cassette and wow betide you if you go little rings front and back.
Now just suppose the chainstay snapped first, worth considering IMHO

I appreciate on a stand with no momentum behind it it wont go

now go repeat your fact while climbing a hill with momentum

come back with your new answer.

Nope still wont go

dales rider, you are simply wrong.

it is a problem that is prevented by using the B screw on the Shimano Mechs.
if the chainstay broke first the rider would have felt the frame feel odd but nothing catastrophic as described.

But the OP has already stated that the frame did not break first.
Or you could just re-read the title of the thread.

Sancho – Member

dales rider, you are simply wrong.

Ooops am I must pop and tell my bikes that, oh and I guess OP was watching carefully when it failed but hey ho not my bike and not my problem.
As for how the B screw can alter chain length well I’ve a lot to learn, in fact I’ll go put some links back in my mates chain and tell him you cant stop the jockey wheel touching your cassette because your b screw is turned to the max, you’ll just have to put up with it.

I am simply trying to advise that the jockey wheels will contact the cassette when the b screw is not adjusted properly.
It’s why the screw is there.
The B screw doesn’t alter chain length ( you do that by adding/removing links) it alters the position of the mech, which when you run either a short chain or a larger tooth cassette you adjust to keep the distance of the jockey wheel away from the cassette, if it is not adjusted properly then the jockey wheel will come in to contact with the cassette, it may not happen on your bikes, but I have seen it on many occasions.

But as the OP has described how it all happened then he didnt need to be watching what happened, as I said a frame failure would not have caused all the damage that was described, as I have seen frames fail on chain stays seat stays the frame would have felt wobbly, or soft, but not locking up wheels etc.

If you want to pop in to my shop (crosstrax) I can demonstrate to you how the b screw can help avoid this issue.

Sancho you misunderstand yourself, but with your limited experience I forgive you. Unlikeley to pop in your shop for a screwing course but I will pop in one day and maybe teach you something more complicated you may just be able manage.

Hows life in the ivory arm chair.

dales rider, you are simply wrong.

it’s ‘woe betide’, not ‘wow betide’

HTH

Dales rider

I totally understand myself and what Im talking about and have a lot of experience, but I dont see how you can’t see this, when the short chain runs big to big it pulls the mech forward, the b screw would stop this, pulling it forward doesnt pull it away from the cassette, in the final movement it pulls it back up and in to the cassette, hence the OPs damaged frame and countless mechs trashed, hence the fitment of the B screw.
I just dont understand why you dont see that.
But again youre welcome to discuss it and point it out to me in the shop over a biscuit and tea

Maybe you could teach me about the software for the Management of Nuclear Fuel Rods in Magnox Reactors, (one of my old jobs before I started fixing bikes about 10 years ago. it was fairly complicated, but hey ho. not my problem anymore.

mickolas – Member

dales rider, you are simply wrong.

it’s ‘woe betide’, not ‘wow betide’

HTH

Accept that I’m sh1te at spelling, my grammer school education to blame.

I see what you did there

I could but wont turn a post into a personal slagging match, nuff said Sancho and leave it at that.

dont worry Dales rider, Im just an argumentative toss bag that doesnt take what’s said on here personally.

Sancho – The B screw has nothing to do with it. There is no B screw on campag chorus 11.

As to what actually happened, I have no idea. As explained, I shifted up the block, bike skids / transmission locked solid due to rear mech having caught spokes of rear wheel and rotated around so it is pointing upwards.

I think it was the rear mech being caught in the spokes of the rear wheel and being rotated around that caused massive twisting of the rear triangle due to the chain suddenly shortening that basically put a huge sideways load on the drive side chainstay causing it to snap.

I checked again today, by putting the chain on big / big and the mech was completely stretched out to breaking point. I still don’t get how it then came into contact with the spokes though and got jambed. That was what caused the damage

Maybe you could teach me about the software for the Management of Nuclear Fuel Rods in Magnox Reactors,

Sorry 😳 My time at grammar school made me do it.

If the b tension was set wrongly then the top jockey wheel would have rubbed on the casette every time the big rear sprocket was engaged whether the front was in the big or small chainring , oops just read that no b tension screw involved 😳 .
Personally I don’t think you would know what happened first with this sort of accident , and you may or may not be able to get some idea by examining the damage afterwards but I doubt you would be able to say conclusively in what order things happened .

I still don’t get how it then came into contact with the spokes though and got jambed. That was what caused the damage

The mech was trying to pull straight but it couldn’t, so the cage twisted, caught the spokes, and was pulled around.

I suggest someone re reads there campag instruction manual.
And finds where the b tension screw is located on the cage of the mech.
And not knowing about it probably helps explain the situation as campag are more sensitive to chain length as the b screw is working on spring tension and doesn’t have an absolute stop like shimano

Like here?

Like the associated text on page 25 …

“Derailleur adjustments must be performed by skilled personnel; a badly adjusted derailleur can result in an accident, personal injury, or death.”

so how’s this for a theory:

setup out due to cage adjustment screw or chain length. mech reluctant to shift as upper jockey wheel is forced to try and occupy same space as large sprocket, as dales rider suggests. in this case though, the jockey wheel cracks under the stress and locks the drivechain. this then pulls on the cage (through rider effort) and twists it into the spokes. carnage ensues as previously described.

?

Once again, nothing to do with B screw, and this is where I put my money.

setup out due to chain length. mech reluctant to shift as lower jockey wheel is forced to try and occupy same space as large sprocket, In this case though, the lower jockey wheel collapsed under the stress and locks the drivechain. this then pulls on the cage (through rider effort) and twists it into the spokes. carnage ensues as previously described.

The jockey wheels on the new chorus are the ones with a cut out and are not solid, this enabled it to collapse.

I stand corrected regarding the B screw and thanks for pointing out my mistake, but this was not the problem. The chain could only just be connected together on big/big without running it through the rear mech, so then add in trying to run it over 2 jockey wheels and it’s game over.

So…… Not a frame manufacturing fault then……?

I stand corrected regarding the B screw and thanks for pointing out my mistake, but this was not the problem. The chain could only just be connected together on big/big without running it through the rear mech, so then add in trying to run it over 2 jockey wheels and it’s game over.

In which case surely you’d have issues lower down the block.

You’re supporting a theory that says the jockey wheel hit the largest sprocket, but then saying it’s nothing to do with the b-screw you didn’t adjust because you didn’t know about, but which would cause the exact problem you are suggesting 😕

Either way it certainly seems you need to suck it up, RTFM next time and learn from your expensive mistake.

Well you have my sympath and thanks, thats another bike company offmy list at least they could have offered a frame at cost. Reading their warranty its fairly shoddy dont they expect their bikes to last ?

Interesting…

I’d say you’ve got a chain of events (No pun) that ultimately lead to the frame damage here…

Excessively shortened chain leads to mech failure; collapse of top jockey wheel, bent cage? Possible deflection of the (non-replacable) mech hanger? and in turn the damaged mech enters the rear wheel.

Mech wrenched round and jams wheel, sudden rather massive loading of rear triangle snaps Drive side stay (other damage also possible)…

So most likely root cause? – Not adding a couple more links to the chain…

TBH I tend to have as many links as I can on my road bike without losing all chain tension in the small/small position, more links to “share the Wear” so to speak. A highly tensioned drivetrain isn’t realy as much of an advantage on a road bike as it is on an MTB…

Not to crow though, the OP has just knackered a pricey frame, so it’s a rather uncpomfortable lesson, hope he can replace/fix the frame and keep riding…

Not to crow though, the OP has just knackered a pricey frame, so it’s a rather uncpomfortable lesson, hope he can replace/fix the frame and keep riding…

Yes but…. This has been round the default ‘Wah wah, it’s not my fault’ loop. It’s only the persistence here that’s uncovered more of what happened (re: the B screw).

I’d try for an ‘at cost’ damage replacement and failing that, a repair. At least when reassembling, the OP won’t make the same mistake again….

After 20 years of building my own bikes I have indeed learnt a valuable lesson. Always in the past ran the shortest chain so as to have as much tension in it for MTB. I changed the rear mech / cassette / chain early this year and I just used the old chain as reference for length.

You do live and learn, but that is of little consolation as I cannot afford a frame at cost, so will most likely have to go without until I can replace it.

I still don’t understand why people keep talking about the friggin b screw! The upper jocket wheel is fine and never contacted the cassette, the short chain caused the rear mech to deform under load and it must have twisted into the spokes of the rear wheel. It was that which caused the huge forces and snapped the frame.

I don’t think the B screw would have made any difference

I feel for you! Get in touch with that carbon place I linked to way back – they may make you feel better about things. Hope it works out ok.

Im am pretty certaiin that if you were in big / big then the mech would be getting pulled away from the spokes as the big ring is on the outside.

By the by, the frame can be repaired, alternatively, a cost replacemennt good will gesture could be an option.
a bit of humble pie to the manufacturer/shop would get you a long way in my view.

Thanks fellas.

bit of humble pie to the manufacturer/shop would get you a long way in my view.

I have started this process already and am going to see them on Saturday

Failing that I will attempt to get it repaired. My only concern is just how much the rear triangle deformed for the stay to snap and what that may have done to the rest of the triangle.

Im am pretty certaiin that if you were in big / big then the mech would be getting pulled away from the spokes as the big ring is on the outside.

I was thinking that, but I wondered if some other part of the mech could end up nearer the spokes.

how ever once the mech has broken off its mounting the next location for it is in the spokes.

Or… I had a chain snap on my road bike at about 20mph. The chain got caught up in moving wheel, ripped the rear mech right round bending it inwards…

I still don’t understand why people keep talking about the friggin b screw!

Quite – it’s a complete red herring. Even if the top jockey wheel does touch the cassette, it’s no big deal as everything’s moving in the same direction. You’ll just get a bit of noise.

One thing that seems to have been overlooked it that a chain that is too short can create huge forces just as a result of the rider pedalling. When you shift into a bigger gear, the chain moves over one tooth at a time, meaning that the required chain length increases progressively through half a turn of the rear wheel, giving you a large mechanical advantage.

A very rough calculation: say the gap between the two sprockes is two teeth (e.g. 23 to 25). The required chain length grows by 1″ through half a turn of the rear wheel. To do this, your pedal will move (17″ pedal, 53T gear): 17 * pi * 23/53 = 25″, so you effectively have a mechanical advantage of 25:1. For an 80 kg rider putting all his weight on one foot with the pedal horizontal, that’s a force equivalent to 2 tonnes: stuff is going to break.

I think a lot depends on the exact length of the chain. You might be lucky and it’ll just refuse to shift, but if there’s just enough slack that the chain engages properly with a tooth on the next sprocket then you may have a problem.

that’s a gross overstatement of the mechanical advantage but I agree with the principle. assuming an effective radius of about 95mm for 50t ring, my crankarm gives me about 35:19 advantage at the chain on the chainring. this is stepped up at the cassette by the gear ratio (in this case 50:25 so double) giving 70:19 mechanical advantage. for 80kg rider this gives approx force of 295kg equivalent. something will break.

the b tension screw is not a total red herring as failure to adjust it properly could easily cause the upper jockey wheel or mech cage to collide sideways during a shift onto a larger cassette sprocket. this would happen regardless of the chainring being used, however.

I’m with pdw on the cause of the stay snappage.

hang on… I take it back… the advantage is not stepped up by the gear ratio. the maximum tension I can put into my chain is my weight times the crankarm length divided by the effective radius of tha chainring. hence about 148kgf for an 80 kg rider. maybe more if you count the additional force from pulling on the handlebar.

but wouldn’t that mean that we could all break our chainstays simply by trying to pedal with the rear brake locked on, or by stalling on a really steep hill? I’m confused, but I’m coming back to thinking that it has more to do with the sudden reduction of momentum caused by the drivetrain locking up