Originally Posted by
silversx80
Not quite. I think you're mistaken on what, exactly, the definition of a fatigue/endurance limit actually is. Yes, forces will exceed that limit, on a singular-level (so not exactly to exceed the limit). Impact, for example, isn't as much of a cycle (no pun intended) as pedal forces. The limit refers to the repeated stresses the material can endure before you get fatigue failure (fractures in the material). Once you hit that limit, the material has essentially failed and those cracks will continue to propagate until catastrophic failure.
So, the point is, the higher the stress, the lower number of cycles the material can endure at that stress. If the stress reaches yield strength, you'll have plastic deformation of the material. Fatigue limit is not a singular force/stress capability, but rather a curve relating number of cycles to stress levels.
It's not clear from the above what you think I'm mistaken about. Yes, there is a curve relating the number of cycles that can be sustained before failure vs. the stress level reached in each cycle. The idea of an 'endurance limit' is that that curve becomes asymptotically horizontal at some fairly low stress level - i.e. that below that stress level there can be an infinite number of cycles and the material will still never fail. That could be an important consideration in an application where the stresses can be kept at such a well-controlled and minimal level.
But in normal cycling there will be a wide range of forces - some pretty small such as when pedaling along at an even pace on a smooth surface, but also some that are much higher such as when sprinting, or struggling to make it up a short but steep pitch, or hitting a pothole. When considering such a wide range of stress levels it's impractical to design a frame so that all of the stresses encountered will be so small that they are below the endurance limit (if one even exists for the material as fabricated into a frame) while also keeping the weight at a level that's marketable.