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proof that rock walls are stronger than blade spokes........(7 posts)

proof that rock walls are stronger than blade spokes........JohnG
Aug 16, 2002 6:41 AM
OK, I was whining recently about the busted spoke on my Campy Neutron rear wheel.

Here's a pic that clearly shows damage from an altercation with a rock wall (Tuna Canyon to PCH). There's a slight indentation on the leading edge of the spoke. This 'hit' occurred about 6K miles ago. Hey it could have been worse........ the other side of the road was a shear drop-off. ;-)

JohnG
i'm still not convinced (nm)czardonic
Aug 16, 2002 8:18 AM
Yes, are you sure the spoke is broken?(nm)Sintesi
Aug 16, 2002 9:14 AM
I just checked..... and it still looks broken to me.JohnG
Aug 16, 2002 9:33 AM
Here's the same pic with some editing. The 'dent' can be seen as a faint line down the front right edge of the spoke. The dent extends about 1 spoke width (long dimension) down the spoke (maybe 2-3mm).

FWIW, I tried bending the long section of busted spoke and it is VERY-VERY stiff. WAY stiffer than any other spoke I've worked with. That might explain the brittle nature of the failure. ???? Beuller-Beuller???

JohnG
Let's just agree to disagree. (nm)czardonic
Aug 16, 2002 9:53 AM
re: proof that rock walls are stronger than blade spokes........Jofa
Aug 16, 2002 11:53 AM
That is an ordinary fatigue fracture, the kind that is common to all failed spokes except those that snapped during building, which are characteristically 'necked out' at the failure. The cause is a microscopic crack or surface feature, or an impurity in the metal, which is exacerbated by cyclic unloading of the spoke (riding). Eventually the crack is so complete that only a small portion of the surface areas of the two spoke parts is still connected, And this portion eventally fails plastically (this is probably the shinier part of the failure, at the back of the picture).

The crack was probably there before the wheel was built (if an impurity, it obviously was), but in any case, an impact of the kind you describe is highly unlikely to have caused it: it will instead plastically deform the spoke into a dent, as we (well, you: the photo is too lo-res for the rest of us) can see. The proximity of dent to failure is coincidental.

Jofa
Looks like it could be a grain boundary fractureKerry
Aug 16, 2002 5:04 PM
See how the center of the spoke failed in a different fashion than the circumfrence? It could be that this spoke had a too-coarse grain structure in the center, which would make it significantly weaker. If this is the case, this is what one might call a defective spoke.