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Workin' for the Man
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Discussion Starter · #1 ·
Several framebuilders offer a choice of straight gauge, double and/or triple butted tubes. The price of a straight gauge frame is usually significantly less than the other options.

Is a weight penalty the only difference between straight gauge and butted (assuming the frames are otherwise identical)?
 

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Steaming piles of opinion
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RotatingShifts said:
Several framebuilders offer a choice of straight gauge, double and/or triple butted tubes. The price of a straight gauge frame is usually significantly less than the other options.

Is a weight penalty the only difference between straight gauge and butted (assuming the frames are otherwise identical)?
Practically speaking, yes. Stiffness is far more correlated to tube diameter than thickness, and vibration damping differences are better described as differences than as one being superior to another.
 
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When my wife had Tom Kellogg build her custom Ti Spectrum he said weight was the only difference. The bike would ride and handle exactly the same. She went with butted.
 

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Workin' for the Man
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Discussion Starter · #4 ·
That was fast! Thanks for the timely responses!
 

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corning my own beef
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Coincidentally, Leonard Zinn addressed this topic just recently when answering a technical question sent in by a reader of VeloNews.

Here is the text of the Q&A:

"Dear Lennard,
Where do you stand on going double butted or not with titanium frames? I find it interesting that Kent Eriksen goes straight tube solely, but recognize lots of smart and accomplished builders make use of double butted 3/2 and 6/4.

Does it make more sense for certain applications and less for others or other riders? I'm 6-foot-2 and 190 pounds (at least in winter I'm that heavy).

Steve
"


"Dear Steve,
I think it’s useful to explain how double butting came into being. It was created in the age of steel lugged bikes as a way to make the bike lighter without making the tubes so thin in the heat-affected zone that they would be damaged during brazing, and providing more thickness where the stresses are highest, namely at the joints. The ends were thicker than the center sections on all of the three main tubes except the top of the seat tube, since that area would be reamed anyway and you can’t have two diameters when you’re trying to fit a seatpost inside it.

Double butting is also used on aluminum bicycle tubes as well, for the same reasons, except that in this case, the tubes are welded, not brazed.
Welding is the process of melting the surfaces of the two parts to be joined to each other, usually along with some filler (in rod or wire form) of the same material.

By contrast, brazing (and soldering) is the process of melting a metal with a lower melting point onto the joint between parts made of a higher melting point metal, thus adhering them together.

Aluminum and magnesium are welded with an electric arc of alternating current (AC), rather than with a direct current (DC) arc (which is used to weld steel and titanium). With AC welding, the current moving toward the surface heats and welds, and when the current direction switches (usually 60 times/second), it pulls the oxide scale from the aluminum surface. Aluminum and magnesium oxidize so quickly that without this frequent scale removal, they could not be welded. Welding consumes some length of the tubes, partially explaining the large welds on aluminum and magnesium joints relative to TIG-welded steel and titanium joints. Due to this consumption of material, thicker tubes near the weld is a benefit.

On welded steel bikes, since the frame is not heat-treated afterward as it is with many aluminum alloys, the steel is weakened near the joint due to the high heat of welding – far higher than what is used in brazing or silver soldering – removing much of the temper the steel gained during drawing, butting and heat treating. A thicker tube near the joints bolsters the tube that’s been weakened at its highest-stress point; so again, butting makes sense, because a steel tube would weigh a lot if it were the thickness required for a strong joint throughout its length.

A butted tube, however, is less stiff than a straight-gauge (continuous thickness) one. Grab a yardstick (or meter stick) at the ends and flex it. Notice that the majority of the bowing happens in the center of its length, not at the ends. That’s what happens in a tube as well.

So, with a titanium tube, you can cut some weight by butting it, but you introduce more flex to a tube that’s already more flexible than a steel one of similar dimensions. Titanium welds, like steel welds, are small, and a good welder can weld titanium tubes that are just as thin at the ends as steel tubes. But the lower density of titanium allows more of it to be used and still be lighter than steel. So the necessity for butting is reduced due to the lower density, and a gain in stiffness is consequently achieved by using straight-gauge tubing. That’s why you see such a difference of opinion about using butted titanium tubes or not among Ti builders, whereas all steel builders will always choose a butted tube if it’s available. And for a rider of your height and weight, the stiffness of straight-gauge tubes would be beneficial.

Lennard"
 
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