American Custom Bicycles in Steel, Titanium and Ti/carbon mix

Technical Supplement: Surface Treatments

Anodizing

A small number of manufactures offer anodizing as a way of decorating the surface of titanium frames (or components). Basically, the process creates a dense, colorful film of titanium oxide, which adheres to the frame's surface. Ironically, this is the same type of oxidation that any skilled titanium welder will prevent at all costs, because it will cause the weld joint to become extremely brittle. (This is why Seven's expert welders bathe both the inside and outside of a titanium frame's weld joints in an inert gas, thereby protecting the molten metal from the high levels of nitrogen and oxygen that naturally occur in the environment.)

The titanium oxide film created by anodizing is, thus, extremely brittle, so the frame's surface cannot flex as it would during normal use. Cracks form through the anodized shell, which will eventually propagate into the tube wall, ultimately causing frame failure.

Seven strongly advises against anodizing titanium. Hence, we will void the lifetime warranty on any Seven frame that has been anodized.

Shot Peening

Shot peening is a process of firing thousands of small spherical "shot" at a surface. It literally peens the surface of the material, reworking it to put it under compressive stress.

In theory, the failure point for most structures that undergo fatigue cycling, such as a bicycle frame, is its outermost surface. The natural tension of the surface can lead to crack induced failure. Correctly applied, shot peening creates localized compression of the tubing surface, thereby reducing the likelihood of crack initiation.

However, it is extremely difficult to properly shot peen an intricate, welded, small diameter tubular structure, such as a bike frame. The difficulty arises from the trajectory of the shot. To be effective as a means of increasing fatigue life, the shot's trajectory is best applied at approximately a 90-degree angle to the material's surface. Unfortunately, a frame's tube diameter is small enough that much of the shot is hitting the surface at 60 degrees or even less. At this angle, the shot is skimming the surface, instead of peening it. This skimming does not benefit the surface structure.

The main drawback to shot peening as a titanium surface treatment is in the durability and maintenance of the finish. If it becomes scratched or worn-looking—as is likely from normal use—it can't easily be restored. By contrast, the satin finish applied to Seven titanium frames can be maintained simply with the use of a little ScotchBrite™ (Very fine AA) and a light application of a wax-based furniture polish.

Polishing

The highly polished finish offered on some frames is created using an abrasive process that smoothes the surface by removing material. It's difficult to control the amount of material that is removed—particularly around the frame's joints. This material reduction will make the joints weaker, which could lead to frame failure.

A polished finish is also more difficult to care for long-term than a satin finish. A small scratch on a satin finish can easily be removed using Scotchbrite™, so its elegance is easy to maintain for the life of the frame.