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8e.5 Frames "going soft"


This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.

8e.5 Frames "going soft"

From: jobst.brandt@stanfordalumni.org (Jobst Brandt)
Date: Mon, 20 Apr 1998 15:31:32 PDT

> I have read accounts of "frames going dead" in cycling literature in
> the past. If you have information that debunks this, I'd like to
> know about it. The explanations I have read claim that the flexing
> of a metal causes it to heat up and harden, making it more brittle.
> Eventually it will break under stress. In fact, I read recently
> that aluminum frames are coming out with warning stickers stating
> "this frame will break someday". I have also read that this happens
> to titanium and steel.

It was in print, therefore it is true! Also known, is that a freshly
washed and polished car runs better. Just the idea that the car is
admirably clean makes this concept appear true for many drivers. The
same psychosomatic mechanism is at work when a bicycle racer thinks it
is time for a new frame. I even suspect that some frame builders
assisted in spreading this idea to improve frame sales.

Metal fatigue and failure occur, but they do not change the elastic
response of the metal. Steel (and of course aluminum and other common
metals) have been metallurgically characterized over more than a
century to a precise understanding. None of this research has shown
the possibility of perceptible change in elastic response from any
stresses to which a bicycle frame might be subjected.

You mention brittleness. Brittleness describes the failure mode of a
material and is not a perceptible unless the material breaks.
Hardness is also not perceptible unless you exceed the elastic limit
and permanently bend the frame, exposing the metal's yield point, the
point at which it no longer rebounds. If not, it springs back
unchanged as do most ceramics such as a dish, or a glass that is
dropped without breaking. If it breaks, it does not bend and none of
the shards show any distortion. It either breaks or it doesn't.
That's brittleness personified.

What escapes the believers of material change is that neither
"softening" or "hardening" effects the elastic modulus of the metal.
A coat hanger and a highspeed steel drill of the same diameter have
the same elastic bending stiffness. For small bending deflections,
both are equally stiff, although the hardened steel can bend farther
than the soft steel and still spring back unchanged. The stress at
which it permanently deforms is the measure of "hardness" of the
metal, not its elasticity.

Classically, when bicycle parts or frames fail, the rider usually
notices nothing before hand. This is true for most thick cross
section parts and often even frame tubes frames. The reason for this,
is that to permit any perceptible change in deflection, all the added
elasticity must come from a crack that has practically no volume. So
the crack would need to open substantially to, by itself, allow
perceptible motion. Since this is not possible without complete
failure, the crack grows in length, but not width, until the remaining
cross section can no longer support the load, at which time it

> If these ideas have been widely disproven, I'd appreciate knowing
> how. I've read all six parts of the FAQ and did not see it mentioned.

The reason this was not in the FAQ may be that the whole subject is so
preposterous to engineers, metallurgists, and physicists, that they,
the people who might explain it, are generally not inclined to bother
discussing whether "the moon is made of green cheese" or not.

> PS. If what you're objecting to is the use of the word "dead" as
> opposed to brittle and inflexible, I'll grant you that.

The objection is that you present something for which there is no iota
of scientific evidence, nor any even slightly credible explanation, as
though it were fact. It is as though bicyclists have a different
natural world, where the technical laws are entirely different from
all other machinery, and the most perceptive technical insights come
from the strongest bicycle racers. "After all who knows more about
bicycles, you or the world champion?" is a common retort.

Jobst Brandt <jobst.brandt@stanfordalumni.org>


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