Downhill mountain bikes have frames typically made of steel, aluminum alloys, or carbon fiber polymers. Overall the frame must be durable (have a high fatigue limit), stiff, and have high yield strength (the force needed to deform) and elongation (the amount of deformity a material can take before cracking). Yield strength and elongation determine crash worthiness.
Steel, alloyed with carbon to produce mild steel, is used in cheaper bikes and is very common. Steel tubing, the workhorse of the industry, is cheap to produce and exhibits satisfactory strength and weight characteristics for most applications. Alloying with chromium and molybdenum, producing chromoly, gives steel greater strength, resulting in thinner walls and lighter frames.
For higher performance, aluminum is often selected. This element is lighter than steel, but not as strong and susceptible to fatigue. The addition of other metals, such as copper and silicon, overcome these limitations. Often, the ends are butted, that is, the ends are made of thicker material than the center section. This preserves strength while reducing weight.
Aluminum frames typically come in two varieties, 6061 and 7005. These numbers distinguish the type of processing the metal has undergone and their resulting properties, such as fatigue and strength. 6061 aluminum is used in many common applications such as automotive, boating, and scuba tanks. For better performance, 7005 is used. This is aerospace grade, and is typically 5-10% stronger than 6061. It is often double butted.
Steel, Aluminum, and Titaniam
Where speed is concerned, a carbon fiber polymer may be used. This is a composite material fortified with thin carbon fibers, and is exceptionally strong, light, and corrosion resistant. It also features interesting properties, such as being sturdy in one direction and flexible in another. It may be formed into any desired shape, making fully aerodynamic forms possible. The drawback is this material is often very expensive. It is typically used in racing bikes.
For still higher performance in metal, there is titanium, which has an even greater strength to weight ratio. It is also corrosion resistant, and is often used in jewelry. Though the ninth most abundant element in the Earth’s crust, it is difficult to find in economic quantities. Processing is intricate, causing the price to be high. A new refinement process has recently been developed which could greatly lower the price in the future. Currently a titanium frame retails for about $2000 US.
Watch a titanium frame being fabricated at this plant in Tennessee: