U.S. Built Custom Bicycles in Titanium and Titanium-Carbon Mix

Technical Supplement: Steel


The predecessors to today’s steel frames were first produced in the 50’s. Technology has not changed much since then, and the ride of a steel frame is still the standard by which all other bicycle frames are judged. Steel can be described in general terms as lively, solid, and predictable. Steel's durability and tunability make it a favorite of many cyclists.

The steel tubing used by bicycle frame manufacturers typically falls into one of three basic categories: microalloy; heat treated cromoly; or standard 4130.

Microalloys include Columbus Foco™; Reynolds 853™; True Temper HOX Gold™; and Dedacciai Zero™ tube sets. Microalloys are unique in that they become stronger after welding due to air hardening. This means the tube itself gains strength, and its fatigue life enhanced. In addition, the tube fabricator can make the tube thinner and thus lighter. These are the lightest, yet strongest, steel tubes. Generally speaking, the ultimate strength of microalloys falls into the 170-190ksi range.

Heat-treated cromoly tube sets include Columbus Nemo™; Columbus Genius™; Reynolds 725™; True Temper OX 3™; and Dedacciai Zero Tre™. These tubesets are characterized by: high strength and good workability (formability); a wider variety of tube set options; and the ability to be brazed without damaging the tubes. Their ultimate strength typically falls between 150-175ksi.

Standard 4130 representatives are Columbus Brain™; Reynolds 525™; and True Temper OX 2™, among others. This material has good formability, and is used to create thicker, more stout tubes. Thus, optimally, these tubes would be used when building a frame for a larger rider. Ultimate strength generally is in the 120-150ksi range.

Seven's Exclusive Tube Sets

Argen™ and Cirrus™ butted titanium

Seven's Argen™ butted and Cirrus™ Ultra-butted™ tube sets both start with the highest-quality, domestically sourced seamless 3Al-2.5V titanium tubing. Though it comes with certificates from the mill, the tubing's finish, straightness, and concentricity must also pass our in-house inspection before we will use it in a frame.

Seven has developed four proprietary tube butting technologies, which, unlike some tube butting processes, neither modify nor degrade the tube's grain structure. This ensures that each tube has the longest possible service life. And although it's not easy, Seven maintains a wall thickness tolerance of +/- 0.001"—less than the thickness of a human hair. Once we've butted the tubes, we subject them to another round of inspection.

Integrity 325™ straight-gauge titanium

Seven's Integrity 325™ tube set uses the same high quality, domestically sourced 3Al-2.5V tubing as is found in our Argen™ and Cirrus™ tube sets. However, it does not go on to be butted before being used in a frame. Therefore, the wall thickness and diameter remain constant over the length of the tube.

Argen TC™ and Cirrus TC™ titanium and carbon fiber

The Argen TC™ tube set (used in the Elium Race and Odonata) and Cirrus TC™ tube set (used in the Elium SG) employs the same tube butting technologies as does our Argen™ and Cirrus™ tube sets. However, they feature carbon fiber tubes, which are used strategically instead of titanium in certain tubes within the frame. As a result, for example, the Odonata's use of carbon fiber seat tube and seat stays offers two important benefits: weight savings of 10% over our Axiom SG titanium frame, and improved damping characteristics.

We designed the Elium (SG & Race) and Odonata's titanium lugs to optimize joint strength and provide a durable interface between the carbon and titanium. The titanium tubes and lugs are machined to reduce stress risers where the carbon and Ti meet. Furthermore, the key to long life is the epoxy used, as well as the bond area between tube and lug. The consistency of the bonding area between the carbon tube and the titanium lug is critical for optimized bonding strength. Since filament winding offers extremely tight tolerances for all dimensions of the tube, including tube diameter and wall thickness, we're able to maintain the proper adhesion necessary for improved fatigue life and better joint durability.

Origin™ butted steel

Seven works with the world's best tubing suppliers, including Reynolds, Columbus, and True Temper, to gain access to the widest selection of top quality steel tubes available. In general, the goal of each individual tubing manufacturer is to create tubes that are lightweight, reliable and exhibit superior mechanical characteristics. While all of our suppliers manufacture excellent tubes, no single company makes a wide enough variety of sizes to cover all of Seven's requirements. Thus, Seven's Origin™ steel tube set, from which we fabricate all of our steel frames, draws from the best, taking the performance of steel frames to a new level.

For performance that lasts, the Origin™ tube set is composed of proven, dependable, heat-treated cromoly steel and super-strength microalloyed steels such as Thermacrom that have an Ultimate Strength of 140,000 to 200,000 psi and elongation of 8 to 15%.

Each tube manufacturer has its own specialty and philosophy about how a tube set should be made. Building on the strengths of all the manufacturers, an Origin™ tube set consists of tubes selected specifically for a particular application and the unique cyclist. For example, in one frame, Seven might pair an S3™ top tube with an 853™ down tube and seat tube, and an exclusive Origin™ rear triangle. In another, Seven might choose a UltraFoco™ top tube, EOM™ seat tube, and an Versus™ down tube, with chain and seat stays we've created specifically for that mix.

Although the Origin™ tube set is unique in and of itself, its seat and chain stays are indeed its hallmark. Designed purely for function, Origin's™ stays don't require tapering. Before butting was invented, tapering helped to soften a frame's ride. Today, tapering serves little purpose in improving the ride of a high quality frame because of the wide range of thin-walled tubes and varying diameters that are achievable with modern equipment. Indeed, Origin™ chain stays are stiffer for a given weight than any tapered chain stay on the market.

Origin™ chain stays are the stiffest stays in the industry. They are the first—and only—chain stays to fully leverage an oversized, untapered design that offers an unmatched stiffness to weight ratio. Their oversized nature requires that they have accentuated curves, which allow for superior tire, chain ring, crank, and heel clearance.

Similarly, the tube set's seat stays—the lightest available—have undergone an evolution to become the refined functional tubes that they are today. Origin™ seat stays boast maximal vertical compliance without sacrificing torsional rigidity. As a result, the bike's ride characteristics—including traction and tracking—are much improved as the rear wheel maintains contact with the ground over rough terrain, through tight corners, and during acceleration.

More important than any other aspect of an Origin™ tube set is the fact that it is a "tailored" tube set. The tube options, sizes, and wall combinations are virtually limitless, ensuring that it will always meet the demands of the rider. The Origin™ tube set provides advantages that no other tube set, nor frame manufacturer, offers today.