I have not been following every one of these emails, but my take on tubing .
The first thing is that steel has the almost exactly the same modulus of elasticity no matter what the alloy. What this means is the mild steel o r 4130 CrMo tubes made with the same wall thickness will be of equal stiffness. So the stiffness of the final frame depends on wall thickness
and tubing diameters along with frame geometry. The way to make a tube stiffer is to make the diameter larger, but of course this adds material s o it will add mass. If you then decrease the wall thickness you can make a
stiffer tube with less material. However, good practice dictates a ratio of around 50:1 diameter to wall thickness for a tube. If the tube gets too thin it can fail from a buckle or dent. This problem can be helped by us ing steel with a higher yield, alloy 4130 is very good here. If the wall thickness is very thin then heat treating the material helps. This allo ws the frame to ride over bumps and take the input of a strong rider without failure. All of these points have been gone over in the many years bike frames were made from steel and are worked out mostly. Not much room for
improvement using steel. From the rule of 50:1 stated above one can see that using a less dense material can allow a larger diameter with a thicke r wall to either make a stiffer tube or a tube of the same stillness but lighter. This is why aluminum bike frames can be lighter than steel and still be stiff and strong. Tensile and yield of aluminum is not very hig h but density is so much less you can design around this. Note also that t he specific modulus of aluminum and steel is almost the same. Materials mad e from neat metal of almost all sorts keep nearly the same specific modulus.
Carbon fiber has a much better specific modulus than steel and is very low
density. Nicely designed structures of this material can be very light a nd incredibly strong. This is why for instance aircraft in the past have be en made with 4130 tubing in the 1930's, changing to stressed skin aluminum up
to the presnt, now carbon fiber skin for some surfaces or the whole structure in some cases.
I was the first certified 753 frame builder in the USA and have some comments about the tubing. This is the same alloy as 531 but heat treate d to improve the tensile by 50%, to around 150,000. It has very thin walls , so thin a frame made using 531 would fail without a doubt. The drawback is this heat treating will be compromised if the brazing temperature in highe r than 1200f. Silver braze Easyflow45 (with cadmium) melts at 1150f or so, so this safety range of 50f is a challenge for the best craftsmen. Reynolds
required a finished frame to be sent to England where they cut it apart an d tested the tensile strength of the brazed areas. I can assure you this w as no marketing hype, very few people could pass this test. If the tube was
hotter than this limit the frame would fail. I personally think a proper ly made frame of 753 was the very best steel frame, (for racing). One side note, I was so smitten with this material I talked Hero Tange into taking a set of 753 and seeing if he could duplicate the results. He did, and the
resultant product was Prestige. This is a different alloy and the temperature limit is not so tight, so is easier to build with.
Jim Merz Big Sur
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