Aluminum

Aluminum is far weaker than either steel or titanium. Aluminum frames are made with fatter tubes because these sizes are needed to achieve the required strength, and these larger diametres are proportionally stiffer than those typically used for steel or titanium frames. It has to be this way, as smaller tubes would break. Two aluminum alloys predominate bicycle manufacturing, 6061 and 7005. The numbers indicate the alloy, with 6061 using silicon and magnesium and 7005 using zinc. Both alloys are far stronger than pure aluminum, and their strength is further increased by heat treatment. The alloys differ in other properties which affect both manufacturing procedures and finishing requirements. For the purposes of bicycle production, these materials are equally strong, but 6000 series aluminum requires heat treatment after the frame is welded whereas 7005 does not. This is important because proper heat treatment also affects corrosion resistance, and it is easier to control heat treatment of tubes than complete frames. 

Theoretically, 6061 T6 should be superior to 7005 T6 in corrosion resistance, but in practice I have always found 7005 T6 far more resistant to heavy surface oxidization and pitting. My most extreme test was to bury unpainted complete bikes in my backyard for six months. My 7005 prototype looked fine when I dug it up, and with a bit of polish it was easily returned to its original mirror smooth finish. The 6061 prototype was heavily pitted and completely white, and it was not possible to return it to its original finish. Aluminum is severely weakened by corrosion, particularly when it is subject to repeated loading at significant fraction of its yield strength. So I use 7005 in my aluminum frames.

Titanium

3Al/2.5V titanium is the best bicycle frame material, but its advantages are generally not well understood. Many experienced riders think titanium is a "soft" metal, or that it is more flexible than aluminum or steel. In fact, the elasticity of titanium is very similar to both aluminum and steel. Equal loads create virtually equal deflections. But titanium has a higher elastic deformation limit and its resistance to metal fatigue within that limit is much higher than any type of steel or aluminum. The elastic limit of metal is reached when a sample will not return to its shape after loading. A paper clip that returns to its orignal shape after being squeezed has been loaded within its elastic limit; if it looses its original shape, it has gone beyond this load and the change in shape is permanent. If the paper clip were made from titanium, you could squeeze or stretch it far more before it would permanently change shape. And you could bend it back and forth far longer without breaking.

One consequence of this is both titanium and steel frames can be better optimized for particular "stiffness" requirements by using a variety of tube diametres that cannot be used with aluminum. This type of change creates very subtle differences in ride quality.

The titanium bicycle survived my back yard burial test without any detectable changes. Brush off the dirt and it looked like new. The aluminum parts attached to it were a different story. It seems that these components received some degree of cathodic protection by the other frames which the incredibly stable titanium frame did not offer. This means that the other frames suffered galvanic corrosion from the attached parts.

Material Comparison

After working through several designs, each different in subtle ways, I found a geometry that worked. I wanted to know how different materials would affect the design. Although I understood that they should not be significantly different from an engineering perspective, I guess I had swallowed some of the mythology broadcast by the big bike companies and their pet magazines. I had frames of identical geometry made in steel, aluminum, titanium and metal “matrix,” and used, as far as possible, identical tubing diametres. The frames obviously looked different and steel frame was approximately two pounds heavier than the lightest titanium frame. 

I built the bikes with identical components, and spent months riding and comparing them. The process was interesting, and there were very small differences between each bike. I could not match titanium tubing sizes to the other materials, and the tubes were smaller, and the ride quality seemed slightly more compliant. I judged the other frames identical in this respect, the remaining differences being what might be produced by slight variations in tire pressure, which was probably the cause. 

Lynn Kastan at Kastan Manufacturing made the aluminum and "matrix" frames for me. He  went on to propose my test to "Mountain Bike Action,"  and made three aluminum frames for a cover article, predictably called some kind of frame "shoot-out". Equally predictably, the MBA boys found all sorts of differences between these frames that simply do not exist, making yet another contribution to the material mythology that surrounds bicycle frames.