Originally Posted by
Andrew R Stewart
Not correct. Al has about 1/3 the stiffness of steel. But since it also has a limited fatigue life and will propagate a crack quickly the structure (bike part) needs to be designed stiffer then other materials require, to avoid the liability of failure. This is why Al framed bikes have a stiff ride, they're designed to have this. The exceptions are the bonded AL frames of yesteryear. They used traditional tube diameters, relatively thick walls and without any welds tended to be less brittle. But they also didn't have the lower weight of modern welded AL frames. Andy.
ummmm, i was giving an answer that was easily understandable, not getting into the characteristics of materials before manufacture. Aluminum is actually stiffer, per pound of material, and yes, the designers take that into account. thicker tubes... YIELD STRENGTH is what you are terming as BENDABLE. Aluminum has less MEMORY, and is not very springy, compared to steels and CF.
properly annealing can, and does, remove most of the brittleness around welds.
Hydro-forming, forging, etc., the aluminum creates an improved grain structure that lends strength to the shape.... and carefully engineered shapes also create more strength per structural member. Eliminating sharp junctions increases durability, too.... as does larger tube diameters in ANY material.... once a tube wall is thinned tho, they dent/deform more easily, which then creates a STRESS RISER in the form....
compression or tension loading in design must also be considered... materials yield more quickly under tension.... the downtube/head tube, or the chain stay/BB junction typically fails first in a jumping accident, eh?
and then, there is the selection of the correct alloying elements, and adjusting their tiny, tiny percentages...