Subject: Frames "going soft"
From: Jobst Brandt
Date: April 20, 1998
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 separates.
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.
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