By Scot Nicol
What is the best material to use in building a bicycle frame - steel, aluminum, titanium or carbon fiber? What about something even more exotic? While this certainly isn't as important a topic as who will replace Shannon on "Beverly Hills: 90210," it is fodder for lengthy debates among bike junkies (like myself).
The six-part series we're about to start will examine metallurgy as it applies to bicycles. If we do our job right, you will be educated about all the popular materials currently used in bicycle-frame construction, and we'll take a look at what you can expect for the future.
What I also hope to do is give you a "BS" filter for the clever and often misleading ads that our industry uses to prey on the underinformed. It really doesn't matter that boralyn was used for tank armor, or that rocket scientists designed your bike. You don't even have to wear a white lab coat to design a good bike. Sound engineering and an intimate knowledge of the biomechanical interface between bike and rider are the only prerequisites.
To begin, you have to understand that the traditional bicycle frame is a highly evolved mechanical structure - highly evolved as in 100 years of tinkering. Attempts are constantly made to improve on its design, but most do little improving. Just designing a better frame may look like a simple problem, but it's not. Small improvements are made with materials and engineering advances, but improving by leaps and bounds doesn't happen - unless you believe the ads.
Because the science of bike design is so complex, I won't be able to cover everything that's involved. Instead, I'll stick to the most important ingredients in the mix, and you won't be finding out about body-center cubic versus face-center cubic phases, or about grain boundaries or persistent slip planes. But you'll still get plenty of pertinent information to think about.
Understanding materials' properties is essential to understanding these materials. Unfortunately, terminology related to properties is tossed around at random - this bike is stiff; that bike has a better stiffness-to-size-of-decal-on-the-downtube ratio; this other bike is fortified with 11 essential vitamins and minerals - you've heard the jargon.
In this first installment, I'll define the real terminology for you, both in the technical sense and according to what it means as related to a bicycle. For the subsequent five parts of this series, steel, titanium, aluminum, carbon fiber and "other" will be examined, in that order. You'll draw on the wonderful knowledge learned in this introduction to enlighten you down the road apiece.
Let's Get Right Into It
What material properties are important in choosing bicycle frame material? First, there are three types of material properties:
Density and corrosion resistance are important, for obvious reasons. You won't have much use for information on magnetic permeability and irradiation resistance. And all of the mechanical properties are very important. But what do all of these terms mean, and why are they important? I'm coming to that....
We'll start with an easy one. This is how much a material weighs for a given volume. For example, 6061 aluminum weighs 0.098 pounds per cubic inch. 4130 steel weighs 0.283 lb./in3, and 3/2.5 titanium is 0.160 lb./in3. This is an important and easy relationship to remember: Titanium is about half the density of steel, aluminum is about one-third the density of steel. Use that as a guideline, then start to look at other properties, like strength and stiffness. So you ask, why doesn't an aluminum frame weigh one third that of a steel frame? Read this series and you'll know the answer.
The measurement for stiffness is called modulus of elasticity, or Young's modulus. This, like density, is reasonably easy to understand. If you're "in the know," you'll refer to modulus rather than stiffness in your conversations with friends. Consider: "Like, dude, the pot metal on that Huffy is way stiff," versus, "I postulate, but do not conclude unequivocally, that the modulus of the Sandspeed material is adequate for its intended application." See how much smarter modulus makes you sound?
Young's modulus doesn't change with different alloys or heat treatments of the same metal. A heat-treated Prestige tube isn't stiffer than a seamed 1020 steel tube of the same dimensions. 6061 aluminum tubes with the same diameter and wall thickness are all equally stiff. But when you start using lithium or aluminum oxide, the modulus changes - although the same material won't change stiffness with a change in heat treatment. Can anyone name an exception to this rule?
I know that this sounds like an exciting property, but it's not. Elongation measures how far a material will stretch before it breaks. It's a measure of the material's ductility. What's ductility? It's the ability of a material to deform plastically without fracturing. What's plastic deformation? It's when a material deforms when a load is applied, and remains deformed after the load is released (i.e. "it bends"). Taffy has lots of ductility. Glass is not very ductile, and it has no elongation. Breaking like a piece of glass is not an acceptable failure mode for bikes. What you want is a material that will bend before it breaks. Yes, elongation is a very important property to evaluate when you're looking at materials, and I'll examine elongation with each material analyzed.
This is another extremely important property. "The more strength the better" is a good rule of thumb, but only if you keep close tabs on other properties at the same time. It's called tensile strength because the test used to determine the bending and breaking point of the specimen is done by pulling the sample apart (applying tension).
Now, bikes don't normally fail because tension loads are too high, so it can seem like a stupid test. But, fortunately, the test also happens to be a pretty good indicator of how the material is going to behave - tensile test results are used to indicate strength, ductility, stiffness, and proper parameters for heat treatment or processing. Besides, the compressive strength of metals tends to closely follow tensile strength.
To perform a tensile test, you grab each end of a specimen of a known cross-sectional area, and start yanking. As stress (force per unit area) increases, so does strain (a change in dimension due to stress). Plotting this stress and strain relationship will give you a curve called the load-extension curve. From this, you can determine some of the qualities mentioned above, as well as where the yield and ultimate strengths are. Yield is where you permanently stretch the material; and ultimate is the peak load it will take, usually very close to the point where it fractures.
Guess what? This is another important property to consider but, once again, not by itself. Fatigue failure occurs by applying cyclic stress of a maximum value less than the static tensile strength of the material ... until your specimen fails. This can be a cool test, because the alternating stress mimics vibrations and impacts that happen when you ride your bicycle down the long and winding road.
The fatigue strength itself is a measure of the stress at which a material fails after a specific number of cycles. What's tough though, is designing the proper test. Again, a bicycle is a complex puzzle to consider. There is no standard test for fatigue. Another kink is that fatigue tests are done by cyclic loading of similar stress, whereas the loads you apply to your bicycle parts are uniform.
Ferrous alloys (a.k.a. steel) and titanium have a threshold below which a repeating load may be applied an infinite number of times without causing failure. This is called the fatigue limit, or endurance limit. Aluminum and magnesium don't exhibit an endurance limit, meaning that even with a miniscule load, they will eventually fail after enough load cycles.
This is the ability of a metal to absorb energy and deform plastically before fracturing. A tough metal is more ductile and deforms rather than fracturing in a brittle manner - particularly in the presence of stress raisers such as cracks and notches. Since a very important requirement of bicycle tubes is their ability to deform and give warning of impending failure, toughness is an important property to measure. All things considered, toughness is a dense and complex property to analyze. There are many different ways to measure, some apply to bicycle applications, some don't. Unless toughness is an issue with a certain property, I'll leave it alone. If it is an issue, as in the case of carbon fiber, you'll hear about it.
The Search for Perfection
To answer the question asked at the outset of this article, none of the materials described happen to be the perfect material to use - all have their advantages and disadvantages. Comparing and designing frames out of different materials is difficult because failure modes are so different. And welding, bonding, brazing, machining and finishing these materials are all accomplished differently. But the hardest part is wading through the bs from the marketing guys. Keep reading this series, though, and you'll know just enough to get yourself into trouble.