Another broken Ritchey crank...
Ron Ruff
I inspected them right before leaving for Gila, and didn't see
anything unusual... but a mile from the finish on stage 3 the left arm
snapped when I stood up. They had ~ 13,000 miles. No racing and hardly
any sprinting on them either before Gila.
Yes I crashed. It was right after a sharp turn though, so I was going
less than 20mph and wasn't hurt badly... made it in on one leg. John
Verheul loaned me the DA arm off his TT bike so I could continue the
suffering...
Donald Gillies
it. Girder structures apparently do not handle torsional stresses
very well.
jobst....@stanfordalumni.org
They handle neither torsion nor lateral bending, the most common
failure mode for cranks. This is mainly because most cranks are 8x
stronger as levers in the plane of rotation than they are in lateral
bending, the type of loading ignored by most crank designs.
If you look at the failure pictures at:
http://pardo.net/bike/pic/fail-001/FAIL-001.html
You'll see typical lateral failures, like the one to which you refer:
Jobst Brandt
jim beam
> http://www.whitemountainwheels.com/blog/wp-content/uploads/2007/05/RitcheyPro1-.jpg
predictably...
landotter
> If you look at the failure pictures at:
>
> http://pardo.net/bike/pic/fail-001/FAIL-001.html
>
Jobst,
One of the pictures mentions "vanity grooves" in tandem with strap
wear as a cause of failure. Nothing to do with the OP, of course, but
care to enlighten us to what a "vanity groove" is? I'm assuming it's
not a dance track released by a celebrity, but I've been known to be
pretty slow...
thnx,
Max
carl...@comcast.net
Dear Max,
Take a normal rounded crank, roughly oval in cross-section.
Now remove a wide groove of material from the side.
Stamp "Colnago Ernesto" in the groove.
The groove looks different, saves a few grams of weight that don't
matter, lets the manufacturer display a name, and seems to lead to
dramatic failures, so "vanity groove" is a good description.
http://pardo.net/bike/pic/fail-001/crank-fail-013-014.jpg
Cheers,
Carl Fogel
Ted Bennett
It's a relative of drillium. I would propose the name millium, material
which has had grooves milled in its surface.
In an attempt to make cranks lighter, some were milled along their outer
face. The vanity comes in when you contrast the extra speed the
reduction in weight gives you, to the higher risk of breakage. The
amount of risk versus speed is something not easy to settle, but it's
not difficult to envision the result of a broken left crank as you
accelerate hard beside a big truck.
--
Ted Bennett
jobst....@stanfordalumni.org
>> If you look at the failure pictures at:
http://pardo.net/bike/pic/fail-001/FAIL-001.html
> One of the pictures mentions "vanity grooves" in tandem with strap
> wear as a cause of failure. Nothing to do with the OP, of course,
> but care to enlighten us to what a "vanity groove" is? I'm assuming
> it's not a dance track released by a celebrity, but I've been known
> to be pretty slow...
The groove in the middle of the face of the crank reduces the
torsional strength by about 50%. You'll note that Shimano and
Campagnolo ceased machining these groves on their cranks a few years
ago. Other manufacturers didn't catch on and make U-channel shape
cranks that are even weaker in torsion and lateral bending.
The groove has no positive function but people liked its looks.
Therefore "vanity groove".
Jobst Brandt
landotter
D'oh!
That's pretty obvious, now that ya mention it. One of my tri-weekly
bikes has some vintage Suntour cranks circa '78 that have a vestigal
vanity groove. It's only a mm deep and ~7mm wide. Perhaps one of their
engineers figured that this was as much vanity as was sensible to risk.