Unbroken spoke, broken rim

Chris Slade said:

Had an interesting mechanical problem today while out on a CTC run. When
changing down the gears to go up a hill, the rear mech managed to foul the
spokes. The derailleur was completely undamaged but a spoke nipple was
pulled straight through the rim leaving a large hole. Never had this happen
before. The moral of the story is check the adjustment of your gears even
if you hardly ever use the lowest gear. The wheel was about two and a half
years old, and had seen better days, so it needed replacing anyway. Bit of
a pain though. I would normally expect to see the spoke snap where it bends
at the hub, I've had quite a few do that in the past with a badly adjusted
wheel.

Are you sure that the failure was caused by the derailleur? Could the
rim failure have caused the derailleur to tangle in the spokes?

I have had a similar failure on a mavic rim and the spoke did become
very loose. See:

http://www.btinternet.com/~john.tomo/KIF_2154.JPG

OTOH possibly the rim was about to fail and the derailleur problem
just helped it on its way.

John T.

John Tomlinson said:

Chris Slade said:

Had an interesting mechanical problem today while out on a CTC run. When
changing down the gears to go up a hill, the rear mech managed to foul the
spokes. The derailleur was completely undamaged but a spoke nipple was
pulled straight through the rim leaving a large hole. Never had this
happen before. The moral of the story is check the adjustment of your
gears even if you hardly ever use the lowest gear. The wheel was about two
and a half years old, and had seen better days, so it needed replacing
anyway. Bit of a pain though. I would normally expect to see the spoke
snap where it bends at the hub, I've had quite a few do that in the past
with a badly adjusted wheel.

Are you sure that the failure was caused by the derailleur? Could the
rim failure have caused the derailleur to tangle in the spokes?

It's possible. I've tried changing down the gears (unloaded) and the
derailleur doesn't touch the spokes, so it sounds plausible. OTOH it might
act differently when under load. I'll have to be careful with my new wheel.

Quoted message said:

I have had a similar failure on a mavic rim and the spoke did become
very loose. See:

http://www.btinternet.com/~john.tomo/KIF_2154.JPG

There are several spoke/rim interfaces that look like that now, and one that
the nipple has completely come through.

Quoted message said:

OTOH possibly the rim was about to fail and the derailleur problem
just helped it on its way.

The rim was very worn where the brakes had worn down too much and scraped
the surface, but the rest of the rim seemed sound. Difficult to tell by
looking though.

--
Chris

"I don't want to dwell on the story too much since, well, the producers of
the movie didn't, so why should I?"