Quoted message said:On 14 May 2005 23:25:38 -0700, "SDK" <[email hidden]>
Quoted message said:Here ist das Link:
http://damonrinard.com/EFBe/frame_fatigue_test.htm
Trek OCLV carbon and Cannondale aluminum frames survived the test
unbroken. Numerous titanium frames broke at bends in tubing (s-shaped
stays be damned) and places where braze-ons or screw holes had been put
in the frame. What I take from this, and from the anecdotal experiences
of people I talk to (who also confirm that Connondales and OCLVs just
don't die easily), is: 1. these two mass market companies seem to have
learned something over the years of making tons of frames (perhaps from
having the greatest absolute number of frame failures to learn from!),
and 2. in general, frames made from other materials have to cut corners
on durability to come near carbon on weight (Cannondale aluminum frames
being an obvious exception!).
--Shayana Kadidal
Dear Shayana,
I'm not arguing, just pointing out where anyone interested
can get a critique of the EFBe frame test:
http://hea-www.harvard.edu/~fine/opinions/frame-test.html
And a brief critique of part of the critique.
The initial objection in the above paper is that the applied force was
too large (1200 - 1300 N) and the number of cycles too small (100000 -
200000), both of which are valid if the test were intended to represent
the stress encountered during a typical pedal stroke. But is a fatigue
failure likely to occur from a large accumulation of such typical pedal
strokes or from a smaller number of the more energetic strokes such as
when starting from a stop, when sprinting, or when encountering a
particularly steep part of a climb? If the failure is more likely to be
from these more vigorous stresses that only happen during a small
fraction of the total pedal strokes, then the test looks quite reasonable.
Let's assume for now that we have a well-made aluminum frame which is
designed to last for at least 200,000 miles of pedaling in an 80" gear
with a steady level of pedal stroke. That means it must withstand 5 x
10^7 stress cycles.
Looking at a fatigue curve for Al, such as that here:
http://naca.larc.nasa.gov/reports/1942/naca-tn-865/index.cgi?page0009.gif
we see that that number of cycles can be attained as long as the stress
remains below about 26000 lbs/sq.in. But note that if there are even
100,000 cycles with a stress of 42000 lbs/sq.in. the frame would fail -
this represents a stress level about 60% higher than we had above for
the typical pedal stroke.
This means that if in every 500 pedal strokes we have even *one* where
we apply a force that's 60% greater than our typical effort then the
eventual fatigue failure will be the result of these relatively unusual
peak efforts rather the overall accumulation of the much larger number
of pedal strokes done at the typical effort level.
So then the question is what kind of distribution of forces we might see
in typical cycling. I'm not a racer, so most of my riding is done at a
rather modest level of force on the pedals. But at times I push hrder -
at least twice as hard as when just cruising along, and I'd estimate
that I have at least 10 of these 'extra hard' pedal strokes in a
typical 20 mile ride. So for my riding, the criteria of at least 1 in
500 strokes that are 60% higher force than average is satisfied and I'd
expect any eventual fatigue failure of my frame to be associated with
these relatively infrequent but more energetic pedal strokes.
It would be interesting to see the distribution of peak pedal forces for
some racing cyclists to see if this would also be true for them. How
much harder are they pushing on the pedals during a contested sprint for
the line than when they're riding along in the peloton? If it's at
least 60% harder, then I think the frame test used an appropriate force
(i.e. the peak force that a typical rider might be able to exert) and
number of cycles.
