The Materials Science of Bike Frames, Part 2

<< Part 1: Fatigue Life, Steel and Aluminum

The battle between steel and aluminum was fought and won for all but the "steel is real" faithful, but the battle between aluminum and carbon fiber is just beginning.

Carbon fiber bike frames are not made from a specific "recipe" like 4130 chromoly or 6061-T6 aluminum (6061 is the alloy, T6 is the heat-treating process), but rather a wide variety of fibers, epoxies, layups and manufacturing processes, so carbon fiber bikes can vary widely in terms of their performance and strength characteristics. The carbon fiber and epoxy resins used in the aluminum-lugged Treks with straight carbon fiber tubes made in the nineties is not the same material or manufacturing process used in bikes today.

The way carbon fiber performs depends a lot on how the "weave" of fibers is laid out in relation to the directional forces the bike frame is subjected to. The fibers are only strong in axial tension (i.e. when stretched/pulled), so they must be laid up in a criss-cross pattern to resist tension from all directions, then suspended in an epoxy resin matrix to keep the fibers in place and provide compression strength.

In other words, the fiber provides most of the strength, while the epoxy resin keeps it all together. Most carbon fiber frames made today have infinite fatigue life, meaning they will never wear out or fail under normal loads. They will never rust or corrode either, but the strength of the epoxy that holds it all together could in theory be adversely affected by exposure to solvents or other factors that lead to chemical deterioration over time.

The chart below was created with data from a number of sources to demonstrate just how strong carbon fiber epoxy composite is compared to other materials - in tension. This does not apply to compression (i.e. crushing), where nearly all of the strength comes from the epoxy that holds the carbon fiber strands together.

Tensile Strength

Chips, dings, dents, welds and chemical deterioration/corrosion weaken all frames, either by introducing stress risers or otherwise changing the frame's structural composition. All frames are subject to these things, and frame builders address these issues by adding gussets to welds or "armor" to composite frames.

Carbon fiber bike frame manufacturing is a very labor intensive process, with virtually all of the layup work done by hand, while the vast majority of steel and aluminum bikes these days are produced with welding robots on an assembly line. Aluminum is plentiful and the aluminum alloys used in bike frames are relatively inexpensive to produce. Carbon is plentiful, but carbon fiber is currently very expensive to produce. This may not always be the case.

So, which material is better? There is no question that carbon fiber epoxy composites are more than 20 times stronger than aluminum or steel, and over 10 times stronger than titanium when subjected to tensile loads. The fiber layup and epoxy composition determine a bike's strength under compressive loads, which can vary widely, and I can't locate any comparison data on crush tests. Lastly, carbon fiber has infinite fatigue life, so it will not fail due to normal wear and tear.

Before you dismiss strength under tensile loads as being unimportant relative to crush strength, consider that your entire bike is suspended by tensile loads, on counter-tensioned spokes. A single spoke would buckle under very low compressive load, but if you lace 36 of them pulling against each other in your rear wheel, they'll withstand the load of a 200 pound person landing off a jump.

Similarly, the strands of carbon fiber in an epoxy matrix are oriented in many directions to support each other. While some strands make be subjected to buckling loads under compression, they are supported by orthogonal carbon fiber strands undergoing tensile loads. The criss-cross directional layup of the carbon fiber strands within the epoxy matrix, plus advances in the durability of the epoxies used in carbon fiber composites are what give carbon fiber its crush strength [need data to compare crush strength among various materials].

Just as aluminum was initially only found in high end bikes, its manufacturing costs came down and then it worked its way into the mid-range bikes, so too will carbon fiber bikes become more prevalent. Will aluminum bikes become obsolete? No. Aluminum, steel and carbon fiber bikes all have advantages when it comes to cost, longevity and performance. Aluminum wins on cost and performance. Steel wins on cost and longevity. With modern materials and manufacturing techniques, carbon fiber has the potential to win on longevity and performance. 

 "Strong. Light. Cheap. Pick Two."
                                          -Keith Bontrager

Only time will answer the question of carbon fiber's longevity outside the test lab, in real-world use, but its durability looks excellent with the current crop of carbon bikes. Trends indicate that the manufacturing costs of carbon fiber will continue to drop. Keith Bontrager's above statement remains one of the most accurate and best stated laws of bikedom. If carbon fiber proves to be strong under real-world use over time, and manufacturing costs come down, Bontrager's Law will have to be reassessed.

<< Part 1: Fatigue Life, Steel and Aluminum



Wikipedia - Tensile strength
Wikipedia - Compressive strength
Wikipedia - Fatigue limit
Wikipedia - Stress concentration
ASM - Material Data Sheet, Aluminum 6061-T6
Carbon Fiber Tube Shop - Properties of Carbon Fiber Tubes