tron@fluke.UUCP (Peter Barbee) (10/11/85)
I think there is a critical point that is not being mentioned in this tubing discussion. Virtually all steels have the same modulus of elasticity. What this means is that they all exhibit the same stiffness (assuming they have the same cross section). What makes the cro-mo alloys superior is a considerably higher yield strength. Yield strength is defined as that stress where 2% permanent offset (or strain) occurs. Because the cro-mos are stronger they can be made with thinner cross sections and not permanently deform. Notice that since their walls are thinner a typical cro-o frame will be less stiff than a carbon steel frame. Double butting the joints helps make up for this, in fact, it is easily possible to make the frame more stiff (than typical carbon steel) and weigh less through judicious reinforcment (double butting). What this all means is; look more closely at how the frame is constructed than which cro-mo is used. How long are the double butts? Is the frame geometry that which suits your needs? The length of frame members plays a *very* large role in stiffness, if we model a member as a beam with two pinned ends under a distributed flexural load the equation for maximum displacement is; 5 * w * l * l * l * l / ( 384 * E * I ), where l is the length of the beam, E is modulus of elasticity (constant for steel) and I is moment of inertia (related to cross-section), and w is the load (pounds per inch). This means length enters the equation as a third power and material stiffness (the product EI) is linear. Notice that I have not gotten into column theory, it is complex but length plays an even greater role in stiffness. Hoping this helps, Peter B