What is this tube “butting” stuff all about? First off, tubes can be either externally butted or internally butted. The latter is much more common and less expensive, so let’s focus on it first. Internally butted tubes are typically of the double butted variety, sometimes of the triple butted variety, and very rarely of the quad butted variety. In all cases, the goal is the same: remove material where it is not needed (providing weight reduction).
Double-butted tubes are thinner in the middle, and thicker at the ends. Two different thickness, hence the word double. Not too complicated. Now for triple-butting. If double is good, a butting threesome must be even better, yes? Apparently not. Quoting from Gear-To-Go Tandems’ website (June, 2010):
What is a “triple butted” tube? While triple butted sounds like a 50% improvement, it’s usually an easier-to-produce variation of a double butted tube where the two thick ends don’t match (i.e. 10/7/8). Because the optimal .3mm differential only exists at one end of the tube, a triple butted tube is typically less expensive (removing the taper-producing mandrel is relatively simple when one end of the tube is only partially butted). While many tubing companies hyped triple-butted tubing in the early ’80s, enlightened consumers have made these tubes rare.
I have heard people use the phrase “triple-butted tubing” with an alternative meaning in mind, so I’m not 100% sure how standard the above definition is…maybe there is no industry standard definition? Interestingly, there is even such a thing as a quad butted tube. Quoting again from Gear-To-Go Tandem’s site:
How about a “quad butted” tube? While double-butted and triple-butted tubes both have two internally-tapered transitions (one at each end of the tube), a quad-butted tube has four internal tapers—one at each end plus a pair in the middle to create a mid-tube reinforcement (i.e. 8/5/8/5/8—read each slash mark as a transitioning taper). Easton has produced double-length quad-butted top tubes for Santana since 1993. The thick center section exists where the front seat-tube passes through the one-piece top tube.
It would seem that quad-butted tubing would be desirable only for rather unique applications (like tandem frame top tubes). So, in summary, it appears that regarding internally butted bike frame tubes, double butted is the sweet spot.
Now to the matter of externally butted tubing. The idea of external butting is the same as internal butting: removing material where it isn’t needed (from the middle of a tube length). You can remove that material from either the inside of the tube–in which case the butting isn’t visible unless you can somehow look inside the tube–or from the outside of the tube. Inside or outside… internal vs. external. Therefore, for a tube that is externally butted, the inside diameter of the tube is constant, whereas the outside diameter will vary along the length of the tube (being largest at the ends of the tube). External butting is expensive and must be done in a way that is “pretty” too, because at least on a bike frame, the end result is visible to the paying customer (since we see the outside of our bike frames’ tubes). As a result of the expense of making externally-butted tubes, you don’t see it much.
Merlin provided the following description to Bikyle.com (source: bikyle.com/MerlinTech.htm) regarding Merlin’s manufacturing process for externally butted tubing:
Merlin’s straight gauge double butted tubing begins as “tube hollows” almost twice the outside diameter of the finished tube and with wall thicknesses of nearly 3/4 of an inch. Through cold working and annealing, the tube hollow is transformed into MTS325. The interior of the tube is supported with a mandrel and rockers work the tube externally, literally pushing material outward to the ends of the tube. It requires numerous steps and annealing to bring the tube to its final configuration. This process creates a tube that is free of surface notching, has a proper grain orientation, and is not embrittled. The butting of MTS325 is external and done by a process, proprietary to Merlin, that creates a butted tube without any surface degradation and with correct grain orientation. Internal butts, designed to work with lugs, are swaged which causes significant alteration of grain orientation. An externally butted tube, as used by Merlin proves to be stronger than an internally butted tube of the same weight.
Impressive stuff. I won’t pretend to understand all the details, but something that really impresses is their starting point tube: “…twice the outside diameter of the finished tube and with wall thicknesses of nearly 3/4 of an inch.” It’s amazing they can start with such an oversize and fat-wall tube and manipulate it down to what we see on Merlin frames.