The Truth About Frame Materials, Part II
Triathlete Magazine, August 1999
by Rick Denney
Last time, we talked about the properties of metals used in bike construction. That discussion is important here, so let’s review a bit. We defined stress as the load per unit area of cross section of the material. Strength is the maximum stress that a material can endure, and can be improved by cold-working and heat treating the material. Strain is how much the material flexes under a given stress. Stiffness is inherent in the metal, and increases when a metal shows less strain under a specific stress.
Steel is about twice as stiff as titanium, and three times as stiff as aluminum. The strength of steel varies from about the same as titanium to about twice as high, and aluminum’s strength varies from about half that of titanium to almost as much.
Finally, we talked about fatigue. Fatigue is a failure: a crack that forms when a material is repeatedly stressed above its fatigue limit. Steel and titanium have relatively high fatigue limits, and frames of these materials can avoid fatigue forever if designed with low enough stress. Aluminum has a much lower fatigue limit—too low to do any good. So aluminum frames are designed to reduce stress so that the number of load repetitions will be extremely large—large enough so that the frames will not fail in normal use.
In this article, we’ll talk about how designers work with these materials to achieve the stiff, light, and durable frames that we all want.
As in the last article, we will slaughter some sacred cows. The big one in this article is the myth that all aluminum frames are "harsh." Of course, harsh is not a technical term, which makes it a favorite of advertisers because it has no specific meaning. But there’s no arguing that aluminum frames have a reputation for being very stiff. Two paragraphs up, though, I said that aluminum is only one-third the stiffness of steel. So, what gives?
Let’s review our history, or at least my version of it. In ancient times (the 70’s), aluminum frames did not have the reputation for being stiff. In fact, they were like wet noodles. Then a fellow named Gary Klein determined to prove that aluminum could be stiff in a bike frame.
How did he do it? To make an aluminum tube as stiff and strong as a steel tube of the same diameter, you have to use three times as much aluminum. That means that the aluminum tube’s walls have to be three times as thick as the steel tube. This works fine, except that it ends up being as heavy as steel, if not a little heavier. So, how did Klein make it light as well as strong and stiff? He made the tubes bigger around. The bigger the tube, the stiffer it is, even with the same wall thickness. The bending stiffness of a tube increases by something like the fourth power of its diameter. So, a tube that’s 50% fatter is around five times as stiff, without changing anything else. So, he designed the tube to work around the properties of the material. Aluminum is not stiff, but fat tubes are, even when they’re made of aluminum.
Compared to a one-inch tube (the standard in the old days), a 1-1/8" oversize steel tube is about 1.6 times as stiff. A 1-1/2" aluminum tube is about five times as stiff as the one-inch aluminum tube used on those noodly aluminum bikes in the 70’s. So, if we make the aluminum tube’s walls three times as thick (to make up for the material differences), a 1-1/2" diameter will make it about three times as stiff as a 1-1/8" steel tube. That means we could achieve the same stiffness by thinning the tube’s walls down, and the frame would theoretically be much lighter for the same stiffness.
But we can’t forget strength and fatigue. Aluminum already has poor fatigue resistance, so the designer will thicken the tubes to make the frame strong enough and durable enough. The result is a fat-tubed bike that is stiffer than a standard steel bike when it has adequate strength and durability, but still lighter than the steel bike.
So, now we know why aluminum bikes with very fat tubes, ala Klein and Cannondale, are laterally stiffer than bikes with standard steel frames, as independent tests indicate. Aluminum bikes with tubes that are not so oversized, which includes just about all the others, use thicker walls to achieve the durability and comparable stiffness to that of steel and titanium.
Titanium fits between aluminum and steel in stiffness and weight, but it’s as strong as many steels, and as durable against fatigue. Consequently, a titanium bike with oversized tubes can be as stiff (because of the fat tubes) and as strong (because of the material) as a steel bike, while still being considerably lighter. Typical tubing diameters for titanium bikes lie between steel and aluminum bikes.
Steel bikes with the fattest tubes (e.g. Serotta’s Colorado Concept frames and Eddy Merckx’s MX-Leader) are also among the stiffest designs, and the properties of steel allow these frame builders to shape the tubing to fine-tune the stiffness at different parts of the frame.
So, where does carbon fiber fit into all this? To explain carbon composites, I have to go back and refine one of my definitions for metals. I defined strength as the maximum stress a material can endure. Actually, metals have two strength values that are important in frame design. I said metals are elastic, and they are, up to a point. If you put too much stress on them, though, they will deform permanently. So, at some point they stop being elastic and become plastic. Engineers call this point the yield strength of the material. Yield strength is less than (but roughly proportional to) the ultimate tensile strength that I reported in the last article, which is the stress at which the material breaks when stretched.
Carbon fiber differs from metals in that it never yields. The yield strength is the same as the ultimate tensile strength. Composite frames will not bend—when they reach their ultimate strength, they break. Carbon fibers are independent of each other, and therefore fatigue cracks can’t travel well in carbon composite frames.
So, what is a carbon composite? Carbon fiber strands, either in bundles or in a woven fabric, are laid in a pattern, and then molded into an epoxy plastic. The strength comes from the carbon fibers, but the epoxy is what keeps the fibers oriented properly. And the strength of a carbon frame depends on the orientation of the fibers and the quality of the construction. Unfortunately, their characteristics can only be determined by testing, not by inspection, so the reputation of the builder is really important for the average consumer. But even more important is a meaningful test ride.
The epoxy does more than just hold everything together, though, because it damps high-frequency vibration. Like metal, composite materials are very elastic at low frequencies, such as we see with pedaling and riding over big bumps, but plastic at high frequencies. When you tap a metal frame with your fingernail, you’ll hear a ring. With a composite frame, you’ll hear a thud. It turns out that the ring of metal frames is in roughly the same frequency as the texture of the road, and metal frames ring more than we realize when we ride them on rough pavements. Composite frames are therefore quieter. This may explain why carbon composite frames have a reputation for a smoother ride. We feel the buzz of road texture especially in our hands and elbows, and the plastic in the composite absorbs vibration at those frequencies. This may also explain some of the perceived harshness of aluminum, because aluminum in a fat-tubed bike may actually transmit more road buzz than other designs.
I need to make steaks out of one last sacred cow before we sum up. For bike frames with seat tubes, the frame is made stiff vertically by the double-diamond truss design. So-called vertical compliance does not meaningfully exist. The truss is so inherently stiff that, for all practical purposes, it’s infinitely stiff up and down. (This applies to what we sit on—forks are another story—and I’m also not talking about lateral stiffness.) That means that the transmission of large bumps (not road buzz) to our posteriors has nothing to do with the material or tubing design. It has everything to do with the geometry of the bike, but that’s a subject for another time.
Let’s sum up. Design is more important than material, which is why it is so important for us to test ride bikes before we decide which material to buy. Many myths about materials really have more to do with design than with their inherent properties.
But there’s an even more important point. A bike that doesn’t fit can be made of $10,000 worth of unobtanium, and it won’t be worth as much as an inexpensive steel or aluminum bike that fits. So, a good fit supersedes all considerations of frame materials. And there’s another point, too. I have quality frames of all materials in my, ah, extensive fleet. My ultimate aim is to cause a fatigue failure in just one of them. Why? Because it will mean that I’m strong, and that I ride lots. It’ll mean I’m keeping those cranks turning.