RE: [CR] frames go soft


Example: Framebuilders

From: Mark Bulgier <mark@bulgier.net>
To: classicrendezvous@bikelist.org
Subject: RE: [CR] frames go soft
Date: Mon, 20 May 2002 00:06:06 -0700


Stevan wrote:
> If you take a stay and crush giant dents in both sides,
> you compromise the grain of the steel and the structure
> of the tube.

Certainly, but the damage is done in that instant the stay is crushed. It will remain the same from then on no matter how much it is ridden, until it cracks of course.
> I had a Bianchi road bike (old one) that was used on the
> track for a long time and I could tell that the right
> chainstay was a noodle. The big dents and the fixed gear
> work out did it in.

I'd say the big dents yes, the fixed gear work, no.
> The other factor is overcooking the tubes during brazing. I
> hope there's no engineer out there that will argue that an
> over cooked frame will last just as long as one that is
> brazed properly at a lower temperature.

You're right that overheating weakens the steel, but it doesn't make it less stiff. When you weaken steel or most other metals, the stiffness is unchanged.
> If so, please go ahead and give the technical explanation.

The material property we think of as stiffness is called Young's Modulus, or the modulus of elasticity. It is so close to a constant that we may safely, for most purposes, refer to it as a constant for all steels. It changes only a small amount with composition; alloy steel, mild steel, stainless steel - all so close that it's usually not worth talking about the difference. And it changes not one bit due to heat treating or cold working. As far as metallurgists know, it has never changed measurably in any steel sample ever over the life of the part in service. (If there are exceptions to this I apologize, they never made it into the undergraduate level materials science textbooks I studied).
> Chrome Moly is an aircraft material and the industry has
> standards for how long these things last.

Indeed, but that is to avoid fatigue failures in use. The metal is as stiff as new, right up the point that it begins to crack. If a part breaks from fatigue, we know the fatigue had been accumulating from day one, but didn't change the response to loads (flex) until the crack opened. This has been verified experimentally and innumerable times in the field. If fatigue could be detected before a crack began, by using stiffness tests, this would be a wonderful thing. But unfortunately the stiffness doesn't reduce, so we are left with replacing parts on a schedule and/or inspecting for cracks as the main ways to prevent fatigue failures in use.
> I've seen a number of frames that were so badly mitered that
> the lug, that cheesy soft piece of stamped and welded sheet
> metal made out of the structural equivalent of peanut butter,
> was acting as a structural part of the joint. Those frames
> will not behave the same as one with good miters.
> Including longevity.

Quite true, but the stiffness will be low on the day it leaves the factory and will not get worse.
> Some frames, do go soft.

Well, on one side we have all the metallurgists saying "it can't happen", and on the other side we have the riders who say they have felt it - but not one measurement to back that up. If you could show it's true, it would really shake up the field of materials science. I'm not saying that can't happen, but I'd say, on something as fundamental as this, on such a well-understood material as steel, the metallurgists are where the smart money bets.

I hope no one takes this as an attack on their integrity or gullibility. I believed the lore I was taught as a bike shop mechanic, until I was taught the lore of the metallurgists in engineering school. Now I take their word as a matter of faith - since I can't do the experiments myself, I trust that they have.

Mark Bulgier
Seattle, Wa
USA