Strong, light, cheap, pick two. This famous bike design adage is attributed to Keith Bontrager, mountain bike pioneer and frame builder. The options for bicycle frame materials have widened since the early days of KB. In order from heaviest to lightest material, we can now choose from steel, titanium, aluminum and carbon. Regardless of frame material used, you pay for design and fabrication make any manufactured structure safe. So the cost of bike frames reflects, in part the difficulty of making a safe, reliable frame of that material, and KB's adage still applies. The order for increasing cost of bicycle frames is steel, aluminum, titanium, and carbon fiber.

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In spite of steel's downsides of being relatively heavy, not the strongest and vulnerable to rust, it has some outstanding characteristics as a bicycle frame material. Steel bicycle frames are cheap because iron is easy to obtain and refine, steel alloys are easy to make and steel tubes are easy to machine and join. The strength of steel can be greatly increased with complex alloying, so a steel frame can build up to a complete road bike weighing 21 pounds.

 

Titanium has an atomic weight between that of aluminum and iron. Titanium alloys do not rust, and their ultimate strength is higher than the best steels. A titanium frame can easily wiegh less than the same size steel frame because a smaller amount of a lighter metal is needed (total bike weight ~ 17 pounds). On the downside, titanium is extremely energy intensive to refine and difficult to machine so the frames are much more expensive. Furthermore, titanium is more flexible than steel, so while it works well in the triangulated truss structure of the bike frame, it does not work so well for forks. The cantilevered structure of the fork means the legs must be beefier, both to provide rigid steering control and to provide a greater margin against failure (since fork failures are so injurious to the rider). Even on a steel frame, the fork is disproportionately heavy. The extra metal needed to make the titanium fork rigid enough means it would end up as heavy as a steel fork, at much higher cost.

 

There are two advantages of steel and titanium not shared by aluminum and carbon fiber: fatigue limits and fail-safe failure mode. If you bend a paper clip it does not break, but if you bend it back and forth enough times, it snaps. This is because the repeated stress produce micro-cracks, which eventually work their way through the whole of the rod. Steel and titanium each have a fatigue limit, a stress level below which the metal does not weaken no matter how many times the stress is applied (for steel it is 1/2 the ultimate strength). Over a century of building bikes with steel has taught us the tubing alloys, weights and diameters which keep the stresses below the fatigue limit, so steel frames are immortal for the just-riding-around stresses. If you do over stress a steel or titanium frame, the failure mode is the initiation of a crack, which grows slowly. Cracks are usually detectable, visibly or by ride feel, well before the tube breaks completely. So this fail-safe mode means a breaking steel or titanium frame is less likely to crash the rider. 

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Aluminum's atomic weight is even lower than titanium, and it is much easier to machine and weld than titanium or steel. Unfortunately, aluminum has a no fatigue limit so aluminum bikes and parts inevitably weaken with use no matter how gently used. The original fat-tubed aluminum bikes (Kleins and Cannondales) used twice as much metal as a steel frame to insure they would last as long as a steel frame. Since aluminum has half the atomic weight of iron, the aluminum Cannondales were about the same weight as lightweight steel bikes. Since then, finite element analysis has allowed designers to pare down unneeded weight of aluminum frames while leaving enough metal where the highest stresses are. Modern Aluminum road bikes weigh c.18 pounds. The same issues outlined above apply to aluminum forks so an aluminum fork that is significantly lighter than a steel fork should be regarded with suspicion.  The other concern foe aluminum as a frame and fork martial is that is has a catastrophic failure mode in contrast to the fail-safe mode of steel and titanium. Often a crack in an aluminum part grows so rapidly that it appears to fail (snap!) without warning.

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Carbon is a flyweight material and can be made into fibers with immense tensile strength and virtually infinite endurance. Carbon fiber structures are actually composites of carbon fibers in an epoxy matrix, i.e., a carbon fiber reinforced plastic structure. Metals are isotropic, equally strong in all directions, which is great for unexpected forces, but it means a metal-tube bicycle carries extra weight to be strong enough in the directions to resist normal bicycling forces. Carbon fibers are anisotropic, meaning they are strong only in tension, so the strands must be wrapped in many directions and/or made into fabric to give strength in multiple directions. This means that every square centimeter of a carbon fiber bike can be designed and constructed specifically to deal with the stresses in that part of the frame. A top-of-the-line carbon fiber frame is made from upwards of 300 individual pieces of carbon fiber fabric, each glued into position by hand. The intricacy of design and execution make carbon fiber frames the lightest with the most consistent and tuned ride quality, because they are optimized for the loads imposed by riding.Carbon fiber bikes weighing 15 pounds are now the standard top-of-the-line.

 

Carbon fiber has a few downsides, however. The laborious hand lay-up process jacks up the price of carbon fiber frame higher than any metallic frame. Yet, humans are imperfect, and 300 pieces of fabric offer 300 opportunities to make a mistake: too much epoxy, too little epoxy, voids, inadequate compaction. The anisotropic carbon frame does not handle unexpected forces well. Everything from crashes to small accidental impacts (as could happen bike falling over or someone wrestling their bike out of the next slot in the parking rack) can easily crack, puncture, or shatter a carbon fiber frame or fork. While carbon fiber is immensely strong, the epoxy metric has the short fatigue life and brittleness familiarly seen in plastic. Lastly, unlike metal tubes which show you their damage with visible cracks that grow before they give way, carbon fiber parts have layers glued together, and cracks or delimitation between the layers are invisible from the outside. The possibility of hidden damage and the catastrophic failure mode of carbon structures have led Upcycles Bikes to refrain from selling used carbon fiber composite bicycles.