Prior to switching to aluminum (in 1993) the Sovereign was built from Columbus "Cyclex" (a slightly fortified cromoly) in 9/6/9 gauges and "oversize" diameters. "Oversize" is shorthand for frame tubing too large for conventional lugs. "9/6/9" is bike-speak for double-butted tubes where the wall thickness is .9mm at both extreme ends and .6mm through the rest of the tube. Most double butted tubesets, including the Ishiwata sets Santana used prior to 1984, are slightly heavier: 10/7/10.
For economic reasons the Arriva switched to 9/6/9 Tange from 9/6/9 Columbus in 1989.
Contrary to the popular myth held by many who ride, sell, build and even write about bikes, no brand of steel is stiffer than another. All alloys of steel, including those produced by Columbus, Reynolds, Tange and even Huffy, are virtually equal in terms of weight and stiffness. The only way to affect the stiffness of a steel tube is to change its dimensions.
Our 1990 Noventa, built with Columbus' revolutionary NivaCrom alloy, was Santana's first frame with larger-still "SuperSize" tubing diameters. Because ultra-strong "tool steels" are too brittle to be drawn into tubes, until NivaCrom appeared in the late '80s, cromoly (or Reynolds 531; a virtual-c romoly) was the strongest steel used for bicycle construction.
NivaCrom is an entirely new alloy---a tool steel utilizing small amounts of Niobium and Vanadium to provide the ductility necessary for drawing, butting and tapering. Because NivaCrom is fully 30-40% stronger than cromoly, NivaCrom can be safely drawn to a 7/4/7 gauge.
Since a NivaCrom is neither stiffer nor lighter than cromoly (or even mild-steel), why bother? The answer is that an alloy's strength determines the amount of material needed to prevent failure. In the case of bicycle frames, a stronger alloy allows thinner gauges. A frame built with 9/6/9 mild steel would be both stiff and light---unfortunately, it would break in a few weeks.
The stiffness of a steel tube is mostly a function of its diameter. To double a tube's stiffness you have a choice: you make the tube-walls twice as thick (the weight also doubles), OR you can use the same gauge and increase the diameter by 18% (which increases weight by only 18%). While NivaCro m is not a panacea, a stronger alloy can be drawn thinner, and if you then re-invest a portion of the weight savings in larger diameters, a frame built from NivaCrom tubing can be stronger AND stiffer AND lighter AND more comfortable.
The NivaCrom tubed Noventa was (and is) a breakthrough design in tandems. The performance difference between it previous models was undeniable. Unfortunately, butted NivaCrom tubing is twice as expensive as butted Cromoly (which makes a NivaCrom tandem $600 more expensive than a double-butted cr omoly tandem and $1000 more expensive than plain gauge tandems). While we think NivaCrom is worth the expense, in seven years no magazine has yet agreed to even test one!
After NivaCrom spoiled us, we shopped around for something nearly as good as NivaCrom, but less expensive. The winning proposal came from Tange; a SuperSize version of their best seamless cromoly drawn to 8/5/8. Tange's "SuperSize 8/5/8" tubing became available on the 1993 Arriva, the same year the Sovereign switched to aluminum. A SuperSize 8/5/8 tubeset produces a lighter, stiffer, stronger and more comfortable frame than oversize 9/6/9---experienced tandem riders will have no trouble feeling the difference. Because thin tubing requires additional drawing operations, an 8/5/8 tube is considerably more expensive than a 9/6/9 tube (and possibly thrice as expensive as the typical 10/7/10 tube found in production single bikes). Any butted tubeset is more expensive than the 11/11/11/plain gauge "aircraft" tubing used by a majority of custom tandem builders.
If Santana has used Tange Prestige tubesets since 1993, why doesn't Santana use the word "Prestige" in its catalogs or advertising? The tubing Santana obtains from Tange is a special version that bypasses the final heat treatment process. While heat treating improves the strength somewhat, it als o decreases a frame's long-term durability by making the tubing brittle (which is why the tubing is heat-treated after being drawn and butted). Since we expect our tandem frames to be used for dozens of years, Santana will not build frames from heat-treated steel. Even though our tubeset is the identical alloy drawn to the same 8/5/8 gauge, a "Prestige" label would be confusing.
When the 1993 Arriva proved nearly as impressive as the Noventa, we followed up by giving the Visa a "SuperSize 8/5/8" tubeset in 1994 (albeit with slightly smaller chainstays, downtube and bottom tube).
The other development for 1994 was the world's first double-butted and ovalized cromoly bottom tubes---which saved another half-pound. Current versions of both of these steel frames are now lighter than a same-sized aluminum Cannondale.
For 1997 the Arriva gets a larger-still bottom tube and a very exotic double-butted AND externally tapered downtube (a giant version of Ben Serotta's "Colorado-Concept" downtube).
All of Santana's tubesets are seamless. Your '94 Trek tandem has oversize 10/7/10 True Temper tubing. True Temper does not offer seamless steel.
Are all double butted tubes the same? No. The difficult part of manufacturing double-butted tubing is producing the matching pair of steep internal tapers. While a 10/9/10 tube, for instance, would meet the accepted definition of "double butted," (i.e. it is thicker at both ends) it would offer 1/3 the weight savings and do a lousy job of transferring stress away from the ends of the tubes (where frames break). For steel tubing a .3mm differential is considered perfect. The highest quality cromoly tubesets are drawn to 10/7/10, 9/6/9, or 8/5/8. Because it is stronger, NivaCrom can be drawn to 7/4/7.
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.
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. Prior to this, tandems with a double-length top tube used a plain gauge tube---a popular time and money saving (easy-to-jig) compromise that LOOKS high-tech and adds a full half pound to the weight of a frame.
Easton also draws quad-butted seatstays for Santana; thicker at both ends and the where the cantilever boss is attached.
Finally, what is Aer-met? Aer-met, like NivaCrom, is a proprietary steel alloy originally developed for the aerospace industry. Whereas NivaCrom's alloying ingredients produce a tool-steel with incredible ductility, the goal of aer-met's alloying agents is unparalleled strength (more than twice as strong as cromoly). Because an aer-met billet is too hard to be drawn, aer-met tubes are formed by bending up the two edges of a strip of aer-met until they meet each other, and then welding them closed. Exceptionally brittle, Aer-met cannot be butted, tapered or ovalized---and can only be used for a bicycle's main frame. Because the thinnest Aer-met tubes are .5mm thick (in effect 5/5/5), a length of this ultra-light tubing is actually slightly heavier than a same diameter 7/4/7 NivaCrom tube.