By Jeff Archer
PHOTO: WES STEARNS
In the last few issues, we have examined nearly the full gamut of frame materials, including steel, titanium, carbon fiber, and magnesium. Now we’ll look at the material most often used for a bike’s frame: aluminum.
In the search for framebuilding materials other than steel, aluminum was one of the earliest choices, due to its light weight and corrosion resistance. The 1893 Lu-Mi-Num, produced by the St. Louis Refrigerator and Wooden Gutter Company, was one of the first attempts at building a bicycle from the material. The 1930s and ‘40s saw many different balloon-tired models from the Monark Battery Company. These early manufacturers often left the aluminum raw to show off the material, which also eliminated the paint process.
The 1970s brought lightweight road bikes from manufacturers such as Alan and Klein. Once we entered the 1980s, aluminum’s market share continued to increase to the point where it has now become the most commonly used frame material.
Aluminum is the most common, and third lightest, structural metal on earth. It is approximately one-third the density of steel and half the density of titanium, which makes it an attractive material for building lightweight frames. However, its modulus (stiffness) is on the low side. Since the density number is low, frame designers must increase the tubing diameter to compensate for the low modulus numbers (since, as we learned in previous Vintage Velo articles, stiffness increases to the third power of the diameter). Unlike steel and titanium, aluminum will eventually fail when placed under enough cycles of small loads. Once again, the low density comes to the rescue by allowing the designers to beef up the frame in high-stress areas. Considering this list of attributes, we can see why aluminum is currently the most common frame material.
Another reason for its popularity is the myriad of ways in which aluminum frames can be constructed. The 1930s Monark Silver King frame uses cast aluminum lugs attached to aluminum tubing. Aluminum can also be formed into a monocoque structure, made up of two hand-formed sheets of aluminum welded together, as seen on the frame of the 1995 Mantis Flying V.
TIG (Tungsten Inert Gas) welding is one of the most common framebuilding techniques and was used on the 1988 American Comp Lite. American was famous for their “beauty” or “show” welds. These welded aluminum frames require post-weld treatments to return the material to its original strength. To avoid this extra step, many manufacturers choose to bond the frames together. One example of this would be the 1990 Reflex Limited, which was made from aluminum tubing bonded to aluminum lugs.
Aluminum is often used in conjunction with other materials, such as the bonded aluminum and carbon 1991 Trek 8700, and the bolted-together aluminum and chromoly 1989 Fisher CR-7. The most recent innovation in aluminum frame construction is hydroforming: an aluminum tube is placed in a die and shaped by high-pressure hydraulic fluid, allowing much more complicated shapes.
Most of our example bikes have been lightweight racing bikes or mountain bikes. For racing bikes, aluminum is a very light weight and stiff material for those on a budget. For mountain bikes, aluminum can be more readily manipulated, which makes it a good fit for the odd tubing shapes required for off-road bikes, especially those with full suspension.
For utility and commuter bikes, aluminum is somewhat less common. The large-diameter tubes can result in a less comfortable, harsher ride, and their thin walls are more easily damaged in everyday commuter use. Steel is the easiest frame material for adding multiple braze-ons and accessory mounts, and typically the rider who mounts multiple racks, bags, and fenders isn’t worried about paring the last gram of weight from the frame, so aluminum is less appealing. There is also the eventual failure of aluminum to consider, which can be a bigger issue with a fully loaded frame. And finally, many of the custom frame makers build primarily in steel, which trickles down to the production bikes that emulate them.
These bikes can be seen at the Museum of Mountain Bike Art & Technology in historic downtown Statesville, North Carolina. If you can’t visit in person, check out the collection at www.MOMBAT.org.