The Materials Science of Bike Frames, Part 1

I came across a thread at mtbr.com on the relative merits of different mountain bike frame materials. The discussion went round and round with a lot of anecdotal evidence and "I knew a guy who broke his [aluminum, carbon fiber] frame..." stories. I worked in the field of destructive materials testing at the Center of Excellence for Advanced Materials (CEAM) when I was a mechanical engineering student at UCSD, and later at an aerospace company where almost everything I designed was made out of carbon fiber and aluminum, so I thought I'd share my understanding of the basics of materials science pertaining to the aluminum/carbon fiber debate, and throw some steel in there for fun.

First off, materials like aluminum, steel and titanium are very well understood, and are created from specific mixtures of metals (alloys) that yield consistent, predictable results in terms of strength. There are primarily three different measures of a material's strength: tensile, crush and fatigue strength.

  • Tensile strength is tested by measuring how much pressure a material can withstand when being pulled apart (i.e. in tension) before it permanently deforms (yields) or ultimately breaks.

  • Crush (compressive) strength is tested by measuring how much pressure a material can withstand before it deforms (yields) or ultimately breaks when being crushed.

  • Fatigue strength is tested by bending a material back and forth repeatedly until it fails (breaks).

Aluminum has superior tensile strength and crush strength to steel, with tensile strength about 55% that of titanium. What aluminum lacks in tensile strength, it makes up for in stiffness. Nobody wants to ride a noodle bike, no matter how strong it is. An aluminum bike favors well in terms of stiffness when compared to a steel or titanium bike of the same weight. Aluminum tubes can also have thicker sidewalls than steel or titanium tubes of the same weight, so it favors well in terms of impact resistance when compared to these materials. 

Overall, aluminum has excellent tensile strength, crush strength and stiffness compared to other metals, making it the material of choice for high performance bike frames and components. Titanium bikes can be made stronger than aluminum bikes, but the material is prohibitively expensive, and the prices are fairly steady, since titanium has been in production for awhile. Steel bikes will never be as stiff as aluminum bikes pound for pound, but a lightweight steel bike has a certain springiness to it that is appreciated by many people, and weight can be added strategically to provide stiffness in critical frame locations.

The critical shortcoming of aluminum is its finite fatigue life. What this means is that aluminum bikes ALWAYS eventually break. ALWAYS. It's just a question of how many fatigue cycles the aluminum will survive. The beefier the frame, the longer it will last. Frame design, assembly and weld quality will also affect the frame's life, but there is no such thing as an aluminum frame that will last forever - if it gets ridden. An aluminum frame subjected to frequent use, heavy loads, jumps and crashes may last a few years, while an aluminum frame ridden carefully on smooth roads may last for fifty years.

Lower fatigue loads relative to the strength of the frame will increase the number of fatigue cycles to make a frame last long enough for the majority of riders, but given enough use, every aluminum frame will eventually fail. This has been demonstrated over and over. It's taught in all the engineering textbooks. Aluminum's eventual failure when subjected to cyclical loads is not a question of "if," but a question of "when."

Some materials, such as steel, have infinite fatigue life, which means that they will never fail unless the load exceeds a certain level, which causes "plastic deformation" (i.e. bending). For example, steel truck springs won't fail or ever need to be replaced unless the truck is overloaded or the springs severely rust or get other stress risers in them. The weight at which the springs are considered "overloaded" is well above the loads the springs are subjected to under normal use, so you will never have to replace the springs in your truck. The springs will outlast the truck, as long as you don't go way over the rated hauling capacity, or take jumps in it.

Similarly, a heavy steel bike frame (i.e. strong enough that you can't bend it) will last forever, if you keep it from rusting. A little rust is OK, as long as it's only on the surface (the bike frame to the right is over 70 years old and still reliable). A lightweight steel bike frame is another story. In order to make a steel bike frame under 5 pounds (aluminum hardtail frames are commonly under 4 pounds), you must reduce the amount of material to the point at which it will not withstand as heavy a load, so it may bend, or eventually crack.

That said, you can always weld a broken steel frame, and the mended section will be almost as strong, or stronger than before, depending on how the repair is performed. The addition of a small gusset - a strip of reinforcing steel - welded to reinforce the cracked area, will mend a cracked steel frame stronger than it was before it cracked, with only a modest addition of weight. I've had a gusset added below the seat post clamp on the seat tube of my steel Bontrager Race Light, which got cracked after I bent a cheap seatpost inside of it. The same frame had also been repaired when a car sideswiped me and bent the seat stays in about 3 inches. The whole rear triangle was cut out and a new one was welded in. Seat tube gussetWith the additional seat tube gusset (see left), the bike is stronger than new, despite having taken a beating.

Aluminum frames, once broken, are not so easily repaired. You can weld them, but the area that cracked will have been severely weakened and will likely fail again. Aluminum frames are also subjected to a heat treating process after being welded in the factory, which increases their strength. Repair welds are not heat-treated, so they will never be as strong as the original on an aluminum bike, and the heat of the repair weld may further weaken the surrounding material. Most bike shops will tell you to throw away your aluminum frame when it gets a crack.

Back in the day, nearly all bikes were made out of steel. Advances in metallurgy, welding and heat treating have made aluminum more popular than steel nowadays for the vast majority of intermediate to high-end bikes, while steel frames are rarely found on anything but cheap, low-end bikes. Despite its shortcomings when it comes to fatigue life, aluminum's strength to weight ratio has made it the favored material for bike frames. Most people don't ride their bikes hard enough to crack an aluminum frame in three years, so the finite fatigue life of aluminum is something the industry has accepted, and riders have chosen light weight over longevity.

Part 2: Carbon Fiber >>