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ASK RBA: ASK RC: BUTTED TITANIUM TUBES AND THEIR EFFECT ON FRAME STIFFNESS
Richard Cunningham
January 18, 2010
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(Photo courtesy of Lynskey)
I'm in the beginning process of learning about custom frames and material technology. Where do you stand on going double butted or not with titanium frames? As a non-frame builder, it would seem one would lose some structural integrity by doing so. Does it make more sense for certain applications and less for others? What about for different sized riders? I'm 6'2" 190-pounds.
– Steve
I would recommend that you take the butted tube option if it is made available to you, especially because you are an XXL cyclist. Titanium is not as lightweight of a material as its mystique represents it to be. It rests about half way between steel and aluminum, so it only takes a little extra titanium to push the weight of a Ti frame out of the lightweight category. The strength-to-weight of titanium is exceptional – but only if your frame builder budgets the thickness and diameter of every part to the minimum requirements for your build and riding style. Spanning the distances required for a tall-rider’s frame takes a lot of tubing and this is exactly where a butted tube can be most beneficial.
The diameter of a tube gives the frame its stiffness and the thickness adds durability and strength. When the diameter is correct, the necessary thickness of a titanium tube that will support all the loads of a bicycle frame is less than a business card, but the fact that it must be welded at the joints requires a little extra material at the ends. Welding adds stiffness, because the extra filler metal builds up at the joint, and the heat affected area concentrates stress unevenly at the junction. Both situations can lead to stress failures in thin-wall tube junctions. A butted tube allows the builder an extra margin of error at the critical joints because the thicker-wall tube more closely matches the strength of the weld and also helps to distribute loads more evenly throughout the joint area. A butted-tube frame can be made lighter weight, without sacrificing stiffness, because it retains the same diameter while using less material in the midsection. A “plain-gauge” tube (with a constant wall thickness), is always a compromise because the welded joint determines that the wall thickness of the entire tube will be greater than it needs to be—so it will always be heavier in theory, than a butted-tube frame..
The possible exception to this rule is a monocoque “tube” that is tapered larger on the ends and slimmer in profile in the center (Lynskey does this). The dramatic changes in the profile of the monocoque tube optimize the strength-to-weight ratio of a plain-gauge structure.
The more subtle benefits of a butted-tube frame are that the thinner midsection allows the entire tube to flex when stressed; giving the frame a longer fatigue life. Another plus is that the frame reacts as a whole unit when laterally flexed in a sprint or when cornering aggressively – which feels more consistent to the rider.
Tip: The weight and strength advantages of a butted tube are minimal on extra small frames, because the thin sections are minimized, and the stresses (assuming a smaller rider) don’t require as much thickness at the frame junctions. Long tubes benefit most from butting – the larger the frame size, the more you need it.
Contact Richard Cunningham for questions or comments, or just to talk bikes at: askRC@roadbikeaction.com
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