These excellent articles appeared in VeloNews in the early 90s, and they give away their age with obscure references to beryllium alloys and Beverley Hills 90210 plotlines. After some thought, we decided to republish without updating. The stuff on steel, alumin[i]um and titanium is classic, and Scot's intelligent skepticism towards the wonder materials of the last decade is a pretty good attitude to adopt when confronted with their modern counterparts.
Metallurgy for Cyclists I: The Basics
by Scot Nicol
What is the best material to use in building a bicycle frame -- steel, aluminum, titanium or carbon fiber? What about something 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.
In this six-part series we'll examine metallurgy as it applies to bicycles. If I do my job right, you will be educated about all the popular materials currently used in bicycle-frame construction, and you'll have some understanding of what to expect in the future.
I also hope to give you a bullshit filter for the clever and often misleading ads that the bike 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 ride. You don't 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 must understand that the traditional bicycle frame is a highly evolved mechanical structure -- 'highly evolved' in this case meaning 'product of 100 years of tinkering.' Attempts are constantly made to take its evolution still further, but most fail. Small incremental improvements are made through advances in materials and engineering. Improvements by leaps and bounds simply don't happen. Unless you believe the ads, that is...
Because the science of bike design is so complex, I won't be able to cover everything. You won't be finding out about body-center cubic versus face-center cubic phases, grain boundaries or persistent slip planes. I'll stick to the most important stuff. Don't worry, you'll still get plenty of pertinent information to think about.
To understand the behavior of a material, you have to understand its properties. Unfortunately, the bike industry tends to toss around property-related terminology 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 all heard the jargon.
In this first installment, I'll provide you with the real terminology -- both in the strict technical sense and in its application to bicycle design. The other five parts of the series will cover steel, titanium, aluminum, carbon fiber and "other" materials, in that order.
Let's Get Right To It
The proprties of physical materials can be classified into three main groups:
- Physical -- Density, color, electrical conductivity, magnetic permeability, and thermal expansion
- Mechanical -- Elongation, fatigue limit, hardness, stiffness, shear strength, tensile strength, and toughness
- Chemical -- Reactivity, corrosion resistance, electrochemical potential, irradiation resistance, resistance to acids, resistance to alkalis, and solubility
Which are important in choosing materials from which to build a bike? Well, you probably won't have much use for information on magnetic permeability and irradiation resistance. Corrosion resistance is important, for obvious reasons. But what do all the other physical and mechanical terms mean, and why do they matter so much? I'm coming to that.
We'll start with an easy one. Density describes the weight of a given volume of a material. 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 those figures as a guideline, then start to look at other properties, like strength and stiffness. For instance, you might ask, 'Why doesn't my aluminum frame weigh one third as much as my friend's steel frame?' Read on...
Stiffness is described by a quantity called modulus of elasticity, or Young's Modulus. This quantity, like density, is reasonably easy to understand. If you're in the know, you'll refer to 'modulus' 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 the word makes you sound?
Young's Modulus is a property of the base metal: it doesn't change in most alloys. A Prestige tube isn't stiffer than a seamed 1020 steel tube of the same dimensions, and 6061 aluminum tubes with the same diameter and wall thickness are all equally stiff. Nor do heat treatments generally change things. (Can anyone name an exception to this rule?) But when you start alloying your base metal with lithium or aluminum oxide, the modulus changes.
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 under load, and remains deformed after the load is released. In other words, it bends.
Taffy has lots of ductility, and therefore lots of elongation. Glass is not very ductile, and so it has no elongation at all. What you want in bike building is a material that will bend before it breaks. Shattering like a piece of glass is not an acceptable failure mode in a bicycle. All in all, elongation is a very important property to evaluate, and I'll discuss elongation as it manifests in each of our subject materials.
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. Engineers talk about tensile strength because it's determined by applying tension to a specimen.
Now, this might seem like a stupid test. Bikes don't normally fail because tension loads are too high. But the compressive strength of a metal tends to follow its tensile strength quite closely. As a result, tensile strength is a pretty good indicator of how the material will behave in use.
To perform a tensile strength test, you grab the ends of a specimen of known cross-section, and start yanking. As stress (force per unit area) increases, so does strain (a change in dimension due to stress). Plotting the relationship of these two quantities gives you the load-extension curve. It will also provide you with the yield and ultimate strengths, yield strength being the point at which the material stretches permanently, and ultimate strength being the peak load it will bear. The latter is usually very close to the point of fracture.
Fatigue strength is a more complex phenomenon than tensile strength. You measure it by subjecting a specimen to cyclical stresses until it fails. The maximum cyclical stress must be less than the material's static tensile strength. The fatigue strength number describes the stress required to break the specimen after a specific number of cycles.
Fatigue strength tests can be cool, because they mimic the vibrations and impacts that buffet a bicycle on its journey down the long and winding road. However, a bike is a complex system to model, and it can be difficult to work out a test that adequately reflects real life conditions.
Fatigue strength is closely related to another performance measure called fatigue limit or endurance limit. This is an important consideration in assessing the likely lifespan of a bicycle. Steel and titanium exhibit a fatigue limit, a threshold below which a repeating load may be applied an infinite number of times without causing failure. Aluminum and magnesium don't exhibit such a limit, meaning that they will eventually fail under any repeating load, even a minimal one.
A metal's toughness is a measure of its ability to absorb impact. A tough metal deforms before it fractures, even in the presence of stress raisers such as cracks and notches. Since you want your frame to give you warning of any impending failure, toughness is an important factor in choosing frame materials. However, since it's a tricky property to analyze, and can be measured in many different ways, I'll exclude it from these discussions wherever possible.
Search for Perfection
In this first article of the series, we've touched upon many materials. None of them is perfect. All have advantages and disadvantages. And, even when you have understood the properties of all these different materials, you'll still have to take into account the impact of different welding, bonding, brazing, machining and finishing processes on the final product.
But the hardest part of all is wading through the bullshit from the marketing guys. Keep reading this series, though, and you'll know just enough to get yourself into trouble. Next up: steel.