Originally Posted by Fat Boy

Knowing that you are an engineer means we can discuss this somewhat differently that if you weren't.

Vitus frames, which I know only in passing, didn't break due to excessive 'flex'. If they broke, they broke due to excessive stress. Stress relates to load and the amount of material that is made to take that load. Strain is a measure of the deflection, flex in your terminology, produced by the load. Stress and strain are not interchangeable, so you muddy the water when use them as you have.

Easton and Columbus offer large diameter tubing to reduce deflection and that is why higher quality frames are less prone to breaking you say? With respect to what? If we are comparing to a steel frame, then the modulus of elasticity of aluminum is roughly 1/3 that of steel. To produce an aluminum frame with the same deflections for a certain load, then the respective moments of inertia need to be increased to compensate. Keep in mind, this does not mean that we have to use 3 times the material as a steel frame. Aluminum race frames are generally a bit lighter than similar quality steel race frames. Depending on how we've chosen our materials, our aluminum frame may very well operate at higher stress levels (if not in absolute terms, then certainly as a percentage of yield) in localized areas than a steel frame even though it's overall deflection (flex) is less. The deflection is related to the stresses, but as a byproduct. Flex or the lack thereof doesn't keep our frame in one piece or cause failures.

And now we get to the oft-revered endurance limit. I don't know what stress levels steel frames are built to, but from what I've seen, steel bikes are built to see stress levels _higher_ than the materials endurance limit. This is a bike, not a bridge. While an endurance limit is a quality that steel has, it's only of importance if a designer keeps stresses low. If a designer chooses tubing that is hefty enough to keep them that low, then the bike turns out too heavy. Heavy bikes, specifically heavy road bikes, don't sell well. So, designers keep the tubing thin, the stresses a bit higher (but below yield) and they get the opportunity to actually sell the bikes they are designing.

Now this is all statics. This is the easy stuff.

The fun part is when we think about how these big tubes act when we start pumping road vibrations through them. Big diameter, thin-walled AL tubes have high natural frequencies. When road input transmits the right vibrations into the frame, those tubes start to dance around and the infamous 'road buzz' rears it's head. Skinny, dense steel tubes have a lower natural frequency. They get excited at lower frequencies that aren't as objectionable to many riders. Steel doesn't 'damp' anything, it just resonates lower and people assume that the higher stuff is damped. No, it's just not transmitted as efficiently.

Figuring out the particularities of these issues sounds like a hell of a fun geek-fest to me. I'd really enjoy doing it. So far I haven't had any takers.