Technical Supplement: Tube Manipulation
A tube that is double butted, such as those found in Seven's Argen™ tube set, has a thicker wall at its ends and is thinner in its midsection. The joints of a frame are its most highly stressed areas, and in a typical titanium or steel frame, possess lower mechanical properties—i.e., strength—than the unheated portions of the tubes. Thus, most failures occur at the frame's joints. Butting efficiently strengthens the heat-affected zone at the joints without adding significant weight.
Seven's ultra-butted Cirrus™ tube set takes butting to a whole new level. It's similar to a double-butted tube in that the wall thickness at the ends of the tube is greater; however, the wall thickness will vary extensively over the tube length according to the complex stresses the tube will undergo at various points. Seven's Cirrus™ tube set makes possible the lightest bikes available.
Tube Butting Processes
Traditionally, tubing has been butted internally. Using an internal mandrel, material is displaced from the center of the tube to make the tube thinner in that area. The mandrel must then be drawn out of the tube, past the thick sections, so the tube must be limited in its thickness differentials. Internally butted tubing has some advantages, such as its usefulness in lugged construction. However, internally butted tubes typically are limited to a 40 percent thickness differential to allow the mandrel to be removed. Therefore, there are fewer tube sizes from which to choose, and the variation in thickness may not be ideal. Externally butted tubes suffer from no such differential limitations.
In addition, the properties of an internally butted titanium tube are affected by the excessive manipulation that occurs when using a mandrel. Although cold working can increase strength, too much cold working after the tube mill's final annealing and stress relieving cycles can detrimentally change the texture of the tubing. The tubing's CSR (Contractile Strain Ratio) increases—beyond its optimal rate—resulting in poor fatigue characteristics.
External butting is a superior method for tube reinforcement, and, generally speaking, produces a lighter tube with equal strength, or a stronger tube with lighter weight, than an internally butted one. There are two key advantages to externally butted titanium tubing—one metallurgical, the other, mechanical.
Metallurgically, drawing titanium tubing at the mill determines its crystallographic texture, or grain orientation. And grain orientation affects yield strength, ductility, and fatigue strength. The mill controls the grain texture with a lengthy process of rocking and heat treatments. The control of these steps creates an optimized fatigue life and ductility. The tube leaves the mill with the best possible combination of properties. Therefore, when titanium tubing is cold worked by tapering or internal butting—whether by the mill or by secondary vendors—the tube's properties will be compromised.
Mechanically, in pure engineering terms, an externally butted tube is a more efficient use of material. For a given tube's weight, an externally butted tube will be stiffer; for a given stiffness, the tube will be lighter. In addition, internal butting can hide scratches or notching due to mandrel movement. Surface scratches create stress risers that can lead to premature failure. Externally butted titanium avoids these problems.
Using proprietary processes that do not affect the tubing's grain structure or internal surface, Seven creates externally butted titanium tubing that maintains its fatigue strength and ductility. These processes allow for every possible permutation of tube diameter and wall thickness, in addition to an optimum strength-to-weight ratio, and create no surface defects or scratches.
Odd tube shapes (ovals, so-called geometrically enhanced tubes, squares, six-sided tubes, etc.) are becoming more and more common these days. At first glance, these shapes appear to enhance the performance of the bike. Unfortunately,
in most cases, performance is actually compromised.
Stiffness: A round tube is the optimal shape to achieve a balance of all bending and torsional stresses a bike undergoes. Odd shaped tubes always compromise torsional stiffness, and bending stiffness is always compromised in at least one plane. Overall, round tubes are stiffer for a given weight. Although oval tubes may be stiffer in the major plane, they are more flexible in the minor axis. In addition, oval tubes suffer most from a lack of torsional stiffness.
Grain Structure (titanium only): Grain structure is what provides the strength and toughness of a material. The mill draws the tube in a specific way so that the resulting grain structure is optimized for strength and fatigue endurance. Thus, if the shape of the tube is changed, the tube's grain structure is also changed. As a result, the tubes' strength and durability are compromised.
Notch Sensitivity (titanium only): Most shaped tubes are formed by a mandrel. However, a mandrel causes internal scoring. Internal score marks are stress risers that can potentially lead to failure. So while round tubes may not be the latest fashion, they are reliable, durable, and result in bikes with optimize weight to performance that last for the long haul.
To be clear, there are occasions when a shaped tube is useful. However, its important to look at the underlying purpose of a shaped tube, since some aspect of performance is almost always compromised as a result using a non-round tube. The information below provides strengths and weaknesses for the three main categories of shaped tubes.