Catastrophic Rear Mech Failure

Oxo

Afternoon all,

Just had a nasty incident on my bike, and wondered if anyone could offer any theories as to what's happened? I was cycling up a reasonably steep hill (out of saddle) and just before a 1:4 section, sat down to engage 1st gear (inner chainring, biggest sprocket). Next thing I knew I was struggling to stay upright as my back wheel locked up accompanied with the sound of things breaking

When I inspected the damage, the mech had turned past 90 degrees and was jammed behind the seat stay.


What I can't work out is whether the mech cage failed, and sent it into the rear wheel, or whether it overshifted into the rear wheel and then destroyed itself. There appears to be no damage to the wheel spokes. The part in question is a Campag Centaur 10 Carbon medium rear Mech (2010) with about 300 miles from new... Any therories gratefully received as I'm a bit wary of replacing like-for-like if there could be a problem with the actual mech.

Thanks

Ox

thel33ter

I'd say it went into the spokes, especially taking into account the gear you were in. This sheered the carbon, which put more pressure on the lower mech body, which subsequently snapped as well.

Seems more likely than it simply exploding.

Monty Dog

I doubt it was simply due to the rear mech 'exploding' but due to poorly adjusted gears or gear hanger alignment. Has the frame ever been dropped onto it's side which could have bent the rear mech hanger? I've had this is a CX race where a twig jammed the rear mech and it just wrapped itself around the cassette - cue new gear hanger, rear mech and chain. Get the rear mech hanger alignment checked before you rebuild.

Oxo

Thanks chaps. I guess it's not an issue with the actual design, although I would imagine it's unlikely anyway tbh. The bike has never been dropped or knocked, being an almost new build so poor adjustment must be the cause as you say. Everything felt and sounded fine up to this point with no skipped or missed gears though. Ironically, I was going to order a spare hanger from Kinesis this week anyway. Guess I need to decide whether to go for Veloce (£70) or Chorus (£150+ ) as the Centaur carbon range seems to have been dropped for 2012 (only available in horrible anodised black alloy now by the look of it)

rake

likely it went into the spokes. is the largest sprocket too big, limit screw out,chain too short, bent hanger or its possible the carbon broke.

flasher

As others have said, sounds like it went into the spokes first, interesting that there (appears) to be no damage to the rear wheel!

FWIW. I'd get a new hanger, just because!

g00se

Chain suck? Especially if you were pushing hard?

Possible senario: For some reason (wear, dirt, bottom bracket flex etc), the chain did not disengage properly from the bottom of the chainring and the chain was drawn up the back of the chainring towards the front mech. This pulled on the rear derailleur until there was no more capacity in the cage to deal with the 'shortening' chain and so it was pulled apart.

I've had chain such on a brand new setup (all components less than 100 miles). In the end, I re-setup the front mech from scratch and soaked the chain in degreaser and re-lubed with a light oil. The factory lube turned the chain black with debris sticking to it - it seemed more sticky grease than oil - so I put it down to the muck on the chain. It hasn't happened since.

Twostage

I would have thought that if you'd over shifted then the chain would have come off and got trapped between the cassette and spokes. If the DR made contact with the spokes it would have to have been bent inwards by a greater angle than the angle of the spokes as they go from the hub to the rim.
Suppose I'm trying to say "dunno".

rake

the top jockey can snag the spokes even if the bottom doesnt.

g00se

Just re-read the original post. Sorry - thought you said you had shifted the front, not the rear - probably not chain suck then.

Oxo

Twostage wrote:

I would have thought that if you'd over shifted then the chain would have come off and got trapped between the cassette and spokes. If the DR made contact with the spokes it would have to have been bent inwards by a greater angle than the angle of the spokes as they go from the hub to the rim.
Suppose I'm trying to say "dunno".

Yep, this is what put doubt in my mind too when doing the post-mortem earlier. It really does look like the rear mech just shattered and then ended up being dragged round - I would have expected to have lost a few spokes at the very least if the mech went into the wheel on an "over-shift - but then again I've not experienced anything like this before. Just to reiterate, the whole lot is virtually brand new, professionally put together, recently serviced, spotlessly clean :oops: and there's not been a hint of trouble with shifting before this. I'll take the bike into my LBS for their take on it tomorrow and get the wheels & frame checked over too.

Thanks for your input everyone, much appreciated.

Cheers

Ox

g00se

Maybe a stick or something got caught up in the spokes or chain and wiped it out?

ddraver

Oxo wrote:

Twostage wrote:

I would have thought that if you'd over shifted then the chain would have come off and got trapped between the cassette and spokes. If the DR made contact with the spokes it would have to have been bent inwards by a greater angle than the angle of the spokes as they go from the hub to the rim.
Suppose I'm trying to say "dunno".

Yep, this is what put doubt in my mind too when doing the post-mortem earlier. It really does look like the rear mech just shattered and then ended up being dragged round - I would have expected to have lost a few spokes at the very least if the mech went into the wheel on an "over-shift - but then again I've not experienced anything like this before. Just to reiterate, the whole lot is virtually brand new, professionally put together, recently serviced, spotlessly clean :oops: and there's not been a hint of trouble with shifting before this. I'll take the bike into my LBS for their take on it tomorrow and get the wheels & frame checked over too.

Thanks for your input everyone, much appreciated.

Cheers

Ox

Maybe the cables weren't properly bedded in and as they did so over the ride, it threw the alignment out. I would nt worry about it, I ve done it a few times on the MTB where things knocking the mech/falling off/getting something jammed are more common.

It sucks but it's just bad luck and a hit on the wallet...

tomhammersley

I had this happen a few times on one particular bike last year. I entertained various theories, but on closer inspection, it simply turned out to be due to the mech hitting the spokes.

I note you're running campag - IIRC that has a feature whereby the shifter slightly overshifts the mech before it clicks and lets the chain settle on the sprocket. Perhaps with a slightly badly adjusted limit screw, this could have been the problem?

Especially as it would "look" ok when engaging the mech on the largest sprocket and checking alignment from the back.

keef66

viewtopic.php?f=40004&t=12838524

You could sell your jockey wheels to Godders1