Tandem Tubing, Double Butted Perspective. To offset the considerably greater need for tortional, bottom bracket and longitudinal stiffness, tandems produced from single bike frame tubing are not stiff enough to be stable. Frame builders have two options: thicker or larger tubes. While thicker tubes create a proportional increase in stiffness (20% thicker = 20% stiffer), larger diameters produce geometrically better results (20% larger = 73% stiffer).
Because double butted tubing isn't commonly available in large diameters, some tandem builders resort to using "aircraft" tubing. While this sounds high tech, "aircraft" merely indicates a minimal industry standard - besides being too heavy for modern airplanes, "aircraft" tubing is also too heavy for lightweight tandems.
A stock thickness for plain gauge (unbutted) cromoly "aircraft" tubing is l.0mm. In frame building parlance this is called a 10/10/10 tubeten tenths of a millimeter thick at both ends and throughout the middle. In the past, a few imported tandems (including Fuji, Nishiki and Kuwahara) were built with 10/10/10 seamless cromoly.
Seamed tubing is formed by rolling flat strip stock into a tube and then welding the edges together. Although the resulting weld is a full length stress riser, the good news is seamed tubing is cheap. Some tandem builders rely on weaker seamed tubing, the gauge used by most of those is 11/11/11anything thinner in a seamed plain gauge tube would be too risky
Conversely, the expensive multiple drawing (pulling) operations which transform a solid 30' billet of steel or aluminum into more than a mile of thin wall seamless tubing work hardens the alloy and improves the grain strength - making seamless tubes nearly 20% stronger.
Most plain gauge tubing leaves the mill in twenty foot sections. Manufacturers simply chop it to the desired lengths and weld the pieces together. Every double butted tube, on the other hand, is cut to a specific length at the mill. Then, using a process unique to bicycle tubing, each piece of tubing is individually redrawn to a thinner gauge for most of its length, leaving the original thickness only at both ends. Because the tubes' exteriors are unchanged, a frame built with double butted tubing looks identical to a frame built with straight gauge tubing.
Most enthusiasts realize double butted tubing is lighter. However, there are three more advantages to riding a tandem with double butted 8/5/8 tubing: strength, stiffness and comfort.
On a bicycle frame, stress concentrates at the joints. When the joints are at corners, stress is intensified When joints are welded, there is further stress at the edge of the heat affected zones. It's no wonder bicycle frames break at the edge of the weld - this is where three types of stress (joint, corner and heat) come together.
Because a thin walled center section of a double butted tube can absorb stress that would otherwise concentrate at the joints, the thick section of a properly designed butted tube can be thinner than a plain gauge tube doing the same job. This explains how a tandem built from 8/5/8 seamless double butted tubing can dissipate stress better than tandems built from heavier, plain gauge tubing.
A tandem with proper materials and design is simultaneously stiff and comfortable. It has a longer wheel base, greater weight and different geometry. Because vibration and stress travel through a frame tube much the same way electricity runs through a wire, a heavier tube will transmit vibration more effectively. Conversely, a thin section of tubing or wire will limit the transmission. Because an 8/5/8 tube can transmit only half as much shock as an 11/11/11 tube, anyone who rides both bikes over the same stretch of pavement will have no trouble telling which frame uses double butted tubing.
Tandem Tubing, Other Perspectives. More about other tubing types and frame material types will be added as reasonable information is found.