On Friday, September 7, 2012 5:02:56 PM UTC-7, Leslie wrote: > > AND, there ya go.... Straight from Grant.... Cool! > > (I'm assuming everyone gets the RBW emails?) > > -L >
Grant says: "Talk of frame tubing and stiffness always leads to this: Two tubes of identical diameter and wall thicknesses will be equally stiff regardless of the tensile strength of the tube. Heat-treating doesn’t affect stiffness, and neither does tensile strength. This is what is said. Metallurgists have locked it in. I’ve been deep into bikes for 37 years and I’ve read it a hundred times, written it fifteen, and said it right around eighty, but I don’t believe it anymore. My Pal Jeff is an ironworker, and about ten years he was talking about how rebar comes in different diameters and strengths, and if you were bending a dozen or more lengths of rebar of a given strength and diameter, and then a stronger one of the same diameter sneaked into the pile without you knowing it, it took twice the force to bend it. This was disturbing to hear because it violated what I had locked in and told to others. I’ve thought a lot about it a lot but it didn’t seem scientific enough to repeat, and then about a year ago I read a sciency source saying something like hey, maybe tubes and rods of identical dimensions bend differently after all, and that’s what it took to switch me over. Some metallurgists will squawk at the suggestion that strength affects stiffness, but when the rebar bender with (in some cases) an eighth-grade education and twenty years of experience on thousands of rods picks up a rogue lookalike and says darn, Jeff, I can’t bend this son-of-a-gun, he is doesn’t have a reputation to defend, has nothing to lose one way or the other and doesn’t give a hoot, so you should listen….and try for yourself, and you’ll see it’s the same." Sorry, gotta rant a little: The conclusion Grant reaches here is not correct. Metallurgists do know what they are talking about. His example only proves how easy it is to confuse stiffness with strength. Using the example of *bending* a piece of steel is irrelevant and misleading. We don't bend steel frames by riding them. Tubes used in a bike frame are sized so that they never reach the yield point. Under all normal riding loads they spring back, they do not permanently deform like the rebar Pal Jeff was bending. The stiffness of the two pieces of rebar (the 'regular' and the 'stronger') are the same *up to the point at which they begin to permanently deform*. For the regular rebar, it takes relatively little force to reach that point. Once the metal starts to deform, the force it takes to continue bending it does not go up, and at some point it actually decreases. That makes it feel soft. The stronger bar isn't any stiffer, but its higher yield strength means it can be bent a lot further before it starts to deform. It takes a lot more force to bend that bar far enough to yield. *That does not mean it is stiffer, that means it is stronger! * The coil springs in an auto suspension are made from steel that's roughly as thick as rebar, but they can bend a long way and spring back. That's because they are made from much higher strength steel. Rebar would make a lousy spring, it would compress once and not spring back. Again, that's due to strength, not stiffness. Our steel bike frames are built more like springs than they are rebar. I think Grant has undermined his point, that given reasonable metallurgy (CR-MO steel or an equivalent), and in tubing gauges that will make a bike ride well, ultimate strength is not usually a factor. It can start to matter if the tubes get really thin walled, but Grant doesn't tend to build that way. A plain gauge tube can be perfectly good if the goal is to increase the stiffness of the tube (which a thicker middle section will do) and the thickness at the joint is sufficient for strength. Butting is needed when the tube belly is too thin to make a strong joint. That's all fine and sensible. There's no need to ignore scientific facts to get there. Rant over. Bill Mennuti Stockton, CA -- You received this message because you are subscribed to the Google Groups "RBW Owners Bunch" group. To view this discussion on the web visit https://groups.google.com/d/msg/rbw-owners-bunch/-/7ixIi7pXznUJ. To post to this group, send email to rbw-owners-bunch@googlegroups.com. To unsubscribe from this group, send email to rbw-owners-bunch+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/rbw-owners-bunch?hl=en.
