Long time ago I came across a brilliant series of articles on bicycle frame materials and design, with an interesting name: “Metallurgy For Cyclists,” written by Scot Nicol, the founder of the Ibis Cycles company.
In my opinion, it’s still the best source of knowledge and information on that topic. I wrote on the same topic in the article called “Bicycle frame materials – explained” (yes, a very “inventive” title 🙂 ), but my article is very strictly structured, and with some boring data that probably only I find useful – wrote it for myself, basically. The series by Scot Nicol is a lot better written, and a lot more fun to read – it’s written for us, the cyclists!
I’m delighted and honored for being allowed to re-publish the series on BikeGremlin, with the main goal to preserve it and share it for anyone interested in this topic. It has zero marketing drivel – just lots of facts, knowledge and good old common sense. So here it is, the full seven-part series of what could be considered the best explanation of bicycle frame materials and designs for the non-engineering cycling enthusiasts. Enjoy. 🙂
Relja Novović – Novi Sad, 2021
Metallurgy For Cyclists
By Scot Nicol
(you can always come back here by clicking on the “– T.O.C. –” links at the end of each article)
- METALLURGY FOR CYCLISTS I: The Basics
- METALLURGY FOR CYCLISTS II: Steel is Real
- METALLURGY FOR CYCLISTS III: Aluminum
- METALLURGY FOR CYCLISTS IV: Titanium
- METALLURGY FOR CYCLISTS V: Carbon Fiber
- METALLURGY FOR CYCLISTS VI: Try Something Exotic
- METALLURGY FOR CYCLISTS VII: The Final Chapter
1. METALLURGY FOR CYCLISTS I: The Basics
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:
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.
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.
2. METALLURGY FOR CYCLISTS II: Steel is Real
“Once giants lived in the earth, Conan. And in the darkness of chaos, they fooled Crom, and they took from him the enigma of steel. Crom was angered, and the earth shook. Fire and Wind struck down these giants … but in their rage, the gods forgot the secret of steel and left it on the battlefield. And we who found it are just men – not gods, not giants, just men. The secret of steel has always carried with it a mystery. You must learn its riddle, Conan. You must learn its discipline. For no one, no one in the world can you trust – not men, not women, not beasts … this you can trust.”
– Conan’s dad, from the film “Conan the Barbarian.”
Bicycle framebuilders have known about the secret of steel for a long time. In fact, steel has been used to build more bicycle frames than any other material. It has also been used about 50 years longer than any other material currently in use. In this second installment of our six-part series on frame materials, you’ll learn something about where steel comes from, and more about its advantages and disadvantages in bicycle-frame fabrication. But first, I’d recommend a re-read of the first installment of the series to familiarize yourself with the terminology.
Steel is made mostly of iron whose atomic symbol is Fe, from the Latin ferrum – and that’s where the term ferrous comes from when we refer to ferrous and non-ferrous materials. As you may have guessed, steel is a ferrous material, and aluminum and titanium are non-ferrous.
Iron is the fourth most abundant element in the earth’s crust, so in the near future we probably won’t be running out of the material that’s used to build steel bikes (chromium and molybdenum are different stories, however). Iron rarely occurs as a chemically pure metal, except in meteorites. On this planet, it’s found in various forms, among them magnetite (Fe3O4), hematite (Fe2O3), siderite (FeCO3), pyrite (FeS2) … and many other forms that end in ‘ites.
How do we get from iron to steel? We add and subtract a couple of ingredients while its molten, and voilà, steel (actually it’s a very involved and evolved process involving exothermic reactions, but we’ll save that for the extended-play version of this article).
Specifically, 4130 steel – an alloy steel – which is commonly known in the bike industry as chrome-moly, contains the following alloying agents: 0.28- to 0.33-percent carbon, 0.4- to 0.6-percent manganese, 0.8- to 1.1-percent cromium, 0.15- to 0.25-percent molybdenum, 0.04-percent phosphorous, 0.04-percent sulfur, and 0.2- to 0.35-percent silicon. The other 95-plus percent is made up of good old-fashioned iron. Now, there are hundreds of kinds of steel, but 4130 finds its way into bike frames because, among other attributes, of its weldability, formability, strength, ductility and toughness. (Many low-buck frames are made with 1020 steel, which is called plain carbon steel, and has significantly lower strength than the chromium-molybdenum steels.)
The numbers that I’m throwing out are designated by the Society of Automotive Engineers and American Iron and Steel Institute: 41XX designates a chromium-molybdenum steel (CrMo), while 10XX designates a plain carbon steel – which, if compared to 41XX steels, has fewer alloying agents, lower strength and lower cost. The first number specifies the type of steel: 1 = plain carbon, 2 = nickel, 3 = nickel chromium, 4 = nickel, chromium and molybdenum, 5 = chromium, etcetera, ad nauseam…. The second number relates to different things with different alloys. In the case of 4130, it defines the percentage of chromium and molybdenum in the alloy. The last two numbers tell you the amount of carbon, expressed as hundredths of a percent. 4130 therefore has 0.3 percent carbon.
From now on, in the bicycle lexicon of this series, I’ll be using 4130 and CrMo interchangeably, even though not all CrMo’s are 4130. CrMo is by far the most common of all the steels used to build high- quality bicycle frames. And I’m making an assumption that the readers of VeloNews who ride steel frames aren’t riding Muffy’s (That’s the generic name for the Murray-Huffy style of bike you can buy at those fine American institutions like Kmart and Wal-mart.) Muffy-grade steel is barely above rebar on the steel “food chain”; rebar is essentially a blend of melted 1956 Chevys, washing machines and shopping carts.
Choosing Steel as a Frame Material
The bicycle-frame designer must take many different factors into account when deciding what material to use for fabrication. Even after looking at all the characteristics, there is no clear choice.
But even so, there are many good reasons to use steel as your material of choice in a bicycle frame. Let’s go over the physical characteristics that were defined last time, and see where steel fits into the scheme of things, as compared to titanium and aluminum.
(Disclaimer: For the sake of simplicity, I will refrain from making comparisons to carbon fiber, metal matrix composites and other materials now. When those materials are covered, comparisons will be drawn to Ti, Al and steel.
We started with density in the opening article because it is perhaps the easiest property to understand. Unfortunately for steel, it is “density challenged,” to use 1990s vernacular. Weighing in at 0.283 pounds per cubic inch, it’s almost twice as dense as titanium (at 0.160) and pretty near three times the density of aluminum (at 0.098). Clearly, density is a very important property, because light weight is where it’s at with bicycle frames these days, and high density makes it tough to push that weight envelope.Fortunately for steel, there are other important properties to examine.
This is where steel shines, as compared to Ti and Al. Young’s Modulus for steel is approximately 30 million pounds per square inch. The titanium alloy Ti3Al-2V is 15.5 million psi, and 6061 aluminum is approximately 10 million psi. Those ratios (three to two to one) are almost identical to the density ratios between these three materials. That means that the stiffness-to-weight ratios for the three materials are about the same (provided you’re looking at stiffness in tension or compression).
If you really want to know, Young’s Modulus is the ratio of stress-to-strain in the region below the proportional limit on the stress-strain curve. This was briefly described last issue. All you need to know is: the bigger the number, the stiffer the material. Wait a minute, though. How come, if steel is so stiff and Al is not so stiff, that those big-tubed aluminum bikes are so incredibly stiff? Young’s modulus measures the stiffness for all of these materials with the same-size specimen, or section. We can call the measurement section modulus. One of the pieces of the puzzle the bike designer gets to throw in is the size and wall thickness of the tubing used. Then we get to figure the polar-section modulus of the material by the formula: 0.196 (D4-d4)/D). All this formula says is that as a tube’s diameter increases (D), the stiffness increases to the third power of that number (d is the inside diameter). Comparing a one-inch tube and a two-inch tube of equal wall thickness., the fatty is going to be eight times as stiff as the little weenie tube. And the weight will only double. Now does the ride of those Kleins and Cannondales start to make sense?
Another simple illustration of how this works is to compare two tubes of the same weight, and look at the increase in stiffness as you increase the diameter. Take a one-inch steel tube with a wall thickness of 0.049 inches. Compare that to a 1.5-inch tube with a wall thickness of 0.032 inches. They weigh the same, but the 1.5-inch tube is 1.6 times as stiff.
Your next question should be: “Why not increase the diameter of steel tubes like you do with aluminum, so that we get an even lighter bike?” This is where the “beer-can effect” comes into play. As a tube’s diameter-to -wall thickness ratio gets above 60- or 70-to-one, the tube is more likely to suffer failure due to buckling, or “beer canning.” Al and Ti, being lower density materials, allow you to have thicker, buckling-resistant walls.
Once again, this property is an indicator of ductility. Simply, it measures how far a material will stretch before it breaks.While the previous properties – density and stiffness – don’t change significantly with alloy and heat treatment in any given material, elongation is another story. Like strength, elongation is all over the map depending on heat treatment and the nature of the alloy. Elongation is expressed as a percentage.
When tensile testing a material, it’s pulled apart and stretched until it breaks. Marks are made on the specimen, and the distance between them is measured before and after the specimen breaks. The difference is expressed as the percentage elongation. Steels used in bike tubing typically measure elongations of 9 to 15 percent. If the elongation number dips below 10 percent, I consider it a flag to take a closer look at the overall properties of the material.
Risk of brittle frame failure increases as this number decreases. In particular, you need to look into the strengths of the material – toughness and the endurance limit. And within these tests – toughness for example – who’s to say which method would be better: Charpy, Izod, or some other test? Accurately analyzing a material with low elongation requires a lot more information and expertise than I can provide you in this short and sweet synopsis.
Tensile Strength: Ultimate and Yield
There is a huge variation in the measured tensile strength of different steel alloys and different brands. Generic CrMo might have a yield strength of 90 KSI, whereas True Temper OX3 measures out at almost twice as much: 169 KSI. It’s possible for a bike that’s made out of either of these materials to break. We know for a fact that straight gauge American airframe tubing is a very reliable material to build a bike with. But it has a strength of only 90 KSI. Again, maybe we’ll find that the toughness and elongation of this material is fantastic, so we can get by with a lower strength.
If the True Temper OX3 tubing is twice as strong, does that mean you can build a frame with half the wall thickness? Yes. Will it be as strong? No. Will it be as stiff? Heck no. Will is last as long? Doubt it.
The Big Picture
The point here is that there is a lot to consider. If you merely look at a couple of the numbers, you’re not necessarily getting the whole picture. It’s easy for a metallurgist to convince an ad guy about the superiority of one material over another. Look at the two materials mentioned above. Very different strength numbers, identical density, yet you can build a good bike out of either material.
Steel is a wonderfully reliable material for building bikes. It’s safe to say that there’s no more successful material ever used. It’s easy to work with, can be easily welded or brazed, requires simple tools for fabrication, fails in a predictable manner (as opposed to sudden or catastrophic), and is cheap!There have been few challengers to steel’s throne of best material in the last 100 years. For a couple of decades, we have seen aluminum increasingly being used in bikes, and titanium has been used successfully for about 10 years.But it’s 1994 now, and steel is being seriously challenged by an increasing array of promising new materials. To learn more about these, stay tuned….The next installment of this “Heady metal” series will cover aluminum.
3. METALLURGY FOR CYCLIST III: Aluminum
Good morning … afternoon … evening (circle one), class. Today, we are going to study aluminum. What we learn today will be based on the knowledge you’ve already gained during our two previous sessions. Did you all get a chance to review the first lesson – an overview? How about the second, on steel? Good. This one on aluminum marks the halfway point of our six-part series.
Aluminum as a frame material has increased dramatically in popularity over the last decade. In the early 1980s, aluminum bikes were a novelty, only available from a small, select group of high-end manufacturers. Then, in 1982, Cannondale jumped on the scene and began to push the material downmarket. Today, almost every medium-to-large manufacturer has at least one aluminum bike.
Furthermore, there’s plenty of material for them to use – aluminum is the most plentiful metal in the earth’s crust. And except for magnesium and beryllium, it’s also the lightest structural metal. A primary source of aluminum is the ore bauxite, named for the town where it was first discovered – Les-Baux-de-Provence, in France. The ore contains hydrated alumina (Al2O3*2 H2O) with impurities of iron and titanium oxides. Sounds like one-stop shopping for the bike industry’s metal requirements, eh? It’s not really, as we have better sources of titanium and iron ore.
Making Aluminum into Tubing
The actual process that changes the aluminum we find in the earth’s crust into a tube suitable for building a bike or lawn chair is complex, ugly and energy-intensive. It’s appropriate that the most important process for getting from bauxite to aluminum is called the Bayer method, because studying it will give you a headache…. It takes about 9 kilowatts of energy to produce a pound of aluminum – far above what’s required for steel. And although the production of recycled aluminum takes less than 5 percent of that amount of energy, virgin aluminum is needed to make wrought products – those that are rolled, extruded, or drawn.
A number of different alloys are produced using raw aluminum. For bicycle fabrication, the resultant wrought aluminum products commonly use a four-number designation system. An example of this would be the venerable 6061 alloy. (See “Aluminum alloys” for other examples.) Cast aluminum alloys use a three-number tag, a period, then a fourth number. Both wrought and cast alloys use another number that comes at the end: the temper designation. No doubt you’ve seen the T4 or T6 condition listed after some of the alloys: 7075 T6 or 2024 T4, for example. It describes what cold work, heat treatment and aging processes (if any) the material has been subjected to.
The tempering has a huge effect on the mechanical properties of many alloys of aluminum (some alloys are, and some aren’t, heat treatable). When you weld a 6061 downtube to a 6061 head tube on a bicycle frame, the as-welded condition will have lower strength than before it was welded. You then need to solution heat treat, and artificially age the frame, to return it to high strength. This goes for the Duralcan material used in the Specialized M2 bikes, too, as the base alloy is 6061, with about 10 percent aluminum oxide by volume. And although 7005 alloys, like the Easton Varilite, don’t need to be heat treated after welding, they do need to be artificially aged. When you age and heat treat, you’re mucking around with solid solutions; crystalline structures; the saturation of alloying constituents; their subsequent submicroscopic precipitation; and a bunch of other very small, but very significant changes that I’m not going to discuss.
Alloys that aren’t heat-treatable are often strengthened by cold work – also known as strain hardening, or work hardening. Rather than change the structure by recrystalizing it, cold working changes the structure through brute force, such as rolling, drawing, straightening or flattening the material. Examples of this type of alloy are the 5086 and 5083 alloys that currently are seeing some use in bicycle frames.
Note that when you heat treat – which really should be called thermal treatment – there are two different steps. The first is the solution heat treatment, which is usually done between 800 and 1000 degrees Fahrenheit for a number of hours. The aluminum is then quenched – in air or water, depending on the alloy – to room temperature. After that, the aluminum must be precipitation hardened (also known as aging).
The alloying elements that went into solution during the heat treatment will precipitate out over time, increasing the strength of the aluminum. Since the alloying elements are more soluble at elevated temperatures, aging is usually done in an oven (bake at 250 to 350 degrees Fahrenheit, for eight to 36 hours), so that the process happens more quickly. This is the process you hear about called artificial aging.
The first property of aluminum that we’ll examine is the easiest to understand, and happens to be the one that makes aluminum so desirable as a frame material. It’s called density. Aluminum, as you know already, has approximately one-third the density of steel and one-half that of titanium. Since our industry is so weight-saving conscious, aluminum has become a very important player. In fact, the more I learn about materials, the brighter the future looks for aluminum.
Consider that some of the new aluminum composites have strengths close to or matching that of CrMo, with one-third the density. But, as you good students know, we need to look at many things in combination with strength and density, so let’s do it. Even though the modulus numbers for aluminum are low compared to other common framebuilding materials, you are able to build a plenty stiff bike with it, because the low density allows you to build a bike with large-diameter tubes, without a weight penalty.
As you’ll remember from the last installment of this series, build a bike with large-diameter tubes, and the stiffness increases dramatically. And since the density is low, the walls can be thick enough to provide good buckle-resistance along with the stiffness. How stiff a frame rides is a function of its design. Alans and Cannondales are both made of aluminum, but nobody – at least not correctly – ever called an Alan stiff, nor a Cannondale flexible.The first big property challenge for aluminum is elongation. How far will aluminum bend before it breaks? Not nearly as far as titanium, and usually not approaching the limits of steel, either. If you’ve learned anything from this series, though, it’s that you have to look at a combination of factors before making a judgment.
It’s true that low elongation increases the risk of a brittle frame failure, and elongations below about 9 percent should get close scrutiny. But we need to look at strength, toughness, and the endurance limit, too.
What we find is that aluminum (except for a couple of exceptions like the 5086 alloy) doesn’t have an endurance limit. That means that even a minuscule load, if applied enough times, will eventually result in a fatigue failure. Kinda scary, don’t you think? Steel and titanium are fine in this department, aluminum is not. Clearly, there are a lot of aluminum bikes out there. Are they all going to break? No, they’re not.
How do you design around this? I posed the question to “Sir” Charles Teixeira, the Easton engineer who is responsible for the Varilite tubeset (I added the “Sir” part, we’ll call him Chuck). Chuck Teixeira is a smart guy, and he knows materials. When he designs things, he pays attention to a few simple rules: One of them is to put the material where you need it. This is a very simple concept, but one that people seem to easily loose track of. The steel guys figured it out a century ago: butt the tubes.
Well-designed butts can make your frame stronger and lighter. In fact, looking at what tube sizes have worked in steel is an excellent way to determine what properties are required for other materials. This is what Teixeira did in designing the excellent Varilite tubes, which came out in 1990 and were first used for Doug Bradbury’s Manitou bikes. These were some of the first butted aluminum tubes to see wide use in the market.
Trek had been doing a bonded aluminum bike with butted tubing for a few years previous to that, but widespread use didn’t happen until the last couple of years. Klein and Cannondale got on the program a couple of years ago, and the Specialized M2 just got butted this year.
The Varilite tubes have extremely thick walls in the areas of high stress, and they taper down in the areas that handle less stress. In this way, stresses are dispersed in the tube, and the life of the structure is increased. It’s not rocket science, just good design.
Optimizing Aluminum’s Advantages
To optimize the advantages of aluminum, you have to deal with its inherent disadvantages. One of the ways to accomplish this is by designing in a large margin for error. Although there are many different situations, Teixeira said that one rule of thumb he uses is to increase the tube’s static strength by about three times that of the steel bike.
A lot of factors come into play here, so this isn’t an iron- (or aluminum-) clad rule. A basic premise is that the lower the displacement (flexing), the lower the stress, resulting in less chance for fatigue. It’s also good to spread the stresses out to places of lower loading. This is the idea behind butts, lugs and gussets. Spreading the stresses down the tube also allows you to build a bike that has more resilience and a lively feel, rather than an ultimately rigid structure.
Then there’s stress corrosion, another eyebrow raiser. If you mess up that artificial aging, then stress corrosion may come back to haunt you. As you can see, we have a very complicated puzzle in front of us.
What does the future hold? I asked Teixeira this question, and the outlook wasn’t full of fantastic new materials formerly used for Space Shuttle muffler bearings and F-16 dipsticks, as you might think. There will be advances, but the claims made by many of the slick marketers aren’t panning out. It’s still hard to beat good old 6061, when you look at the whole package. It’s the most versatile of all alloys, has excellent toughness for an aluminum, and good elongation, too. Like the point I made last time with high-zoot CrMo versus generic CrMo, we know that you can make a good bike out of either – it’s just that it takes smart design from the tube on up to build a good bike.We’ll learn more about some of the new higher-strength aluminum alloys and associated materials in the exotics part of our series, which will come at the end, after titanium and carbon fiber.
As you may have guessed, the next installment in our Heady Metal series will cover titanium.
4. METALLURGY FOR CYCLIST IV: Titanium
The Titanium Development Association calls titanium “the material of choice,” and there are a lot of people in the bike industry who would agree. This, the fourth part of our metallurgy series, is about that mysterious and expensive metal, titanium. Its reputation within the industry is excellent: light weight, super strength and fatigue life, a magical ride … and a heavy price tag, to boot. So let’s find out what the physical characteristics are that give titanium such an enviable reputation.
Titanium is not as rare as you might guess – it’s actually the fourth most abundant metallic element in the earth, after aluminum, magnesium and iron. In fact, there’s a lot more titanium in the earth’s crust than there is chromium or molybdenum, two of the essential ingredients that accompany the iron used for steel bike tubing.
Density and Other Properties
As we learned last time, density is the giant feather in the property cap for aluminum. This is an area where titanium also shines, and although its density is almost double that of aluminum, it’s only 56 percent as dense as steel.
Our second property is stiffness, or Young’s modulus (E). The titanium that you find used in a majority of bicycle frames has an E of around 15 million pounds per square inch – approximately half that of steel. This means that steel and titanium are roughly comparable when it comes to the stiffness-to-weight ratio. Previously, we learned that the stiffness of a frame depends on design and the properties of the material used. The same goes for titanium – you can provide a flexible or a stiff ride, depending on execution. Because of the relationship between titanium’s high strength, low density and moderate modulus, most fabricators choose tube diameters that provide a supple, shock-absorbing ride. To push titanium down into the realm of the super light, the modulus becomes a problem, because then the frame gets too flexible. In this case, I’m talking about frames that weigh in the neighborhood of two pounds. Building ultra-light frames is not an easy task in any material … including titanium.
Ti’s Real Plus: Elongation and Tensile Strength
So titanium gets two second-place marks as compared to steel and aluminum in the first two properties we examined. But when we look at property No. 3, elongation, titanium is miles ahead of either material. This is the property that tells you how far something will bend before it breaks, a kind of safety factor for framebuilders.
Elongation numbers for titanium are often 20 to 30 percent. For comparison, typical steels can be 10 to 15 percent – the higher strength steels go down as low as 6 percent. Aluminum typically runs in the 6 to 12 percent range. Higher strength aluminums again creep into the low range of single digits, with warning bells ringing loudly. Things without much elongation are said to be brittle. Brittle frame failure is not a good thing.
The tensile strength of titanium is also excellent. The cold-worked-stress-relieved yield strength (see “Touring the Ancotech mill” to find out more on CWSR) of the 3/2.5 alloy (that’s the alloy usually found in bicycle frames) is typically 100-130 KSI or more. This compares favorably with many steels we find in bicycles. Remember, too, this is achieved with fantastic elongation numbers, and at almost half the weight. And we haven’t even talked about fracture toughness and endurance limit yet.
The fatigue strength is another property where titanium performs beautifully (By now, you may be asking: “Is he ever going to say anything bad about titanium?” ). As explained in the previous installments, there is not a definitive measurement of fatigue strength that will tell us how the material will last in a bicycle frame. Bicycles are subjected to forces of varying amounts in a random, cyclic fashion. As long as these loads are kept below a certain level, titanium and steel both have thresholds below which they will never fail. Almost none of the aluminum (including the metal matrix composites), magnesium and beryllium used in bicycle fabrication has a defined endurance limit, so you need to design around it, as was explained last time.
Now for the Bad News…
The negative sides of titanium are several, and they will keep titanium from becoming ubiquitous in the market. First, it’s expensive. Not only is the cost of energy used to extract the metal costly, but the processing requirements are cost intensive as well.
The other problems have to do with fabrication. You’ve certainly heard that titanium is hard to weld and machine. A more accurate statement is that it is different to weld or machine. What you can’t do is cut corners with titanium. Meticulous procedure is essential. Without it, you risk contaminated welds, which can result in catastrophic failure of the weld.
At the recent Cactus Cup race, I came around a corner on the course to find a guy with a titanium bike that had a freshly severed head tube. A quick inspection revealed my suspicion: a contaminated weld. Machining titanium is either a dream or a nightmare, depending on your procedure. If you use the proper speeds and feeds, and the right cutting tools, it will machine beautifully.
If steel is “density challenged” and aluminum is “strength challenged,” then what challenges face titanium? Modulus is the biggie. Even if we start building our bikes out of higher strength titanium like 6/4, the modulus will stay the same. As the walls get thinner and the diameters larger, stiffness goes up and weight goes down – but to enter the next generation of reduced-weight framesets using conventional tubes and methods, the walls will be so thin that buckling will be a problem. There are ways around the buckling, however. Several manufacturers already have titanium bikes that have internally butted, externally butted, formed or swaged tubes, or some combination thereof. Watch for more development in this area as a way to continue exploring the limits of lightweight, strong frame design with adequate stiffness.
Will titanium be considered the material of choice in the future? Its position and reputation as a magical metal probably won’t be seriously challenged for a while. But even so, look for some action from the aluminum fabricators, who are evolving their craft, and whose frames will get stronger, cheaper and lighter, giving the customer an excellent value. The titanium guys won’t stand idly by and just watch this happen, though. Litespeed is already pushing the price envelope to new lows with excellent road and mountain-bike frames in the $1000 range. Although the extremely low-price barrier probably won’t be broken, continuous improvements in tube forming and fabrication techniques will keep titanium’s demand and reputation strong.
In the next issue, the “Heady Metal” series covers a non-metal, carbon fiber.
5. METALLURGY FOR CYCLIST V: Carbon Fiber
If you’ve followed parts one through four of this series on bicycle metallurgy, you’ve learned a lot about the physical characteristics that are important to consider when designing aluminum, titanium or steel bicycle frames. This installment takes a step outside the realm of metallurgy, and looks at the use of carbon-fiber composites in bicycle frame applications.
The Wonderful World of Composites
It’s common to use the terms carbon fiber and composite interchangeably, even though all composites are not carbon fiber. For example, both plywood and concrete are composite materials. The term composite refers to combinations of materials that result in enhanced properties not provided by the materials alone (concrete is a composite of cement, sand, gravel and water; Cheeze Whiz is air, artificial flavors and artificial colors).
In scientific terms, composites are generally acknowledged as those materials in which either particles, short fibers or long fibers are dispersed in a matrix. In the case of the Duralcan metal matrix composite that is found in the Specialized M2, aluminum oxide whiskers are dispersed in a 6061 aluminum matrix; while advanced composites – the types used to build bicycles – have continuous fibers embedded into a matrix (typically epoxy).To qualify as an advanced composite, it is generally thought that the fibers are continuous, greater than 50-percent fibers by volume, and the fiber has mechanical properties superior to fiberglass. Fibers can be carbon, Kevlar (a.k.a. aramid), boron, ceramic, silicon carbide, quartz, polyethylene … and probably others that I’m not aware of.
A Simple Lexicon
Here’s a simplified explanation of how terms will be used. A fiber is a single strand of reinforcing material. A bundle of parallel continuous fibers are bound together with a glue, or matrix. A single layer of this matrix is called a ply, and multiple plies are laid up to form a laminate. The plies can be laid up in various angles to produce different characteristics of the laminate. If you’ve forgotten about the other terms used in this series – like tensile strength and elongation – re-read the first installment of this series to reacquaint yourself with those terms, because they’ll be essential to our discussion.
The Numbers Look Good
If you look at the numbers that carbon fiber can boast, your initial thought might be that it’s crazy to build a bike out of anything else. But you astute students of the School of Bicycle Geekdom already know that numbers are not the only thing to look at – you need to check out the fine print. And get this: With carbon fiber, you need to throw most of what you’ve learned out the window.
Yes, it’s true that the potential for composite frame materials is tremendous. Unfortunately, the results of some composite bicycle-frame projects have been less than satisfactory. There are reasons for the high failure rate that composite frames have endured, but the fault is not that of the material. I know you may find this hard to believe, but sometimes even rocket scientists make mistakes. The situation is similar to what happened with titanium in the 1970s. Teledyne made some frames that failed, not because the material was bad, but because the design was bad, or the execution of the design was bad. Similar things have happened with composites, and the image of the material is not as good as it should be.
The common folly is for the designer to underestimate the complexity of the bicycle frame. Since carbon-fiber structures are not very fault tolerant (unlike metal structures), the design and execution plays an even more important role. And sometimes the fault is not in the design or execution of the structure – the fault may be a big rock coming in contact with the downtube. While the tube might not fail from such a large impact, the repercussions are usually hidden on the inside of the laminate, or within the laminate. Microcracks can then spread through the matrix, decreasing the ability of the fiber to transfer load. Metal tends to do a bit better in these situations – but you can make metal frames that break without warning, too.
It’s All in the Lay-Up
What I’m getting at is the fact that composite materials are very complex … more complex than metals. In addition to the material itself having greater complexity, the structures are not as straightforward as metal structures. As you have learned in this series, the designer of a metal structure has two variables: material choice and geometric configuration (like tube sizes, shapes and thicknesses). Those wacky composite guys not only get those same two variables, they also get to determine how the composite matrix is laid up. Bear in mind that two structures of identical geometric configuration, weight and composite material, but with different lay-up, could yield a completely different result. Not only is it possible for the obvious – like stiffness – to vary, but fracture stresses and failure modes could also vary tremendously. And the failure modes of composite structures are plentiful: exploding laminate, fibers pulling free from a matrix, first-ply-failure, matrix cracking, and delamination. And I thought designing a metal bike was tough….
Another curve ball thrown into the mix is the geometric shape of the frame. Sure you can make a frame with tubes and lugs like Trek or Giant, but you can also lay them in a shape of your own design, like Kestrel or Look. With lugs and tubes, the designer at least has metal frames with which to compare; but with a new shape, a whole new set of equations needs to be developed.
Tensile and Compressive Strength
Let’s take a look at the physical properties that have been examined with aluminum, titanium and steel frames, and see where carbon fits in (or doesn’t fit in). The way strength is measured in the laboratory is by a tensile test. In a tensile test, we use tension to pull a sample apart until it breaks. Imagine we’re pulling on a bundle of carbon fibers, doing a tensile test. It performs very well in a tensile test – actually, it performs extremely well.
But what about the compressive behavior of carbon? Not too good by itself, kind of like a bowl of spaghetti. You need some kind of adhesive to bond the fibers together, and give the material compressive as well as tensile strength. The matrix connects this whole disorganized mess of fibers by transferring the load between the fibers and between the plies. Since the matrix and the fiber combine to make up the composite, we’ll look at them together to give comparative results.
Density and Modulus
At the risk of being accused of comparing apples and oranges, I’m going to give you some guidelines for a generic carbon fiber lay-up. Bear in mind that there are many different ways to look at this, and I’m only making a comparison for the sake of continuity in the series. The density of your laminate is in the neighborhood of 0.056 pounds per square foot, which is about 60 percent of the weight of aluminum, our previous lightweight winner. The modulus of a generic not-very-high-zoot carbon fiber is about 30 to 33 MSI, or about 10 percent higher than that of steel, previously the stiffest of the three materials we’ve looked at. So you can see we’ve got some stiff, light stuff here.
When we throw the epoxy into the mix, things start to get interesting. A well-made laminate will have 62- to 65-percent fibers by volume. The Rule of Mixtures says that the modulus is proportional to the percentage of fiber in the matrix, since virtually all of the resulting mechanical properties come from the fiber. In other words, the matrix transfers the load to the fibers. So if we start with 30 MSI modulus, with only 65 percent of the matrix contributing, we end up at about two thirds of that, or 18 to 21 MSI for our modulus. Still not too shabby: density one third of titanium, and modulus about 25 percent higher.
This modulus measurement is only in the zero-degree direction though (that’s the direction parallel to the fiber in the ply), and as we know, bicycles get varying stresses applied from varying directions. That matrix does a good job of holding together those fibers, so they don’t buckle under the combined loading. Let’s rotate the ply so that the modulus is measured perpendicular to the lay-up of the fibers. Now our modulus reads a pathetic 1.5 MSI or so, essentially giving us the modulus reading of the epoxy. Yuk! What’s worse, the modulus drops off precipitously between zero and 30 degrees, giving low results almost all the way to 90 degrees. This matters because bicycle tubes (or structures) are subjected to torsional loads as well as longitudinal ones. What’s the answer? Add layers of plies that are at different angles (often 45 degrees) to the initial zero-degree layer. The result is an overall modulus of approximately 10 to 14 MSI, still not too shabby. Again, these are generic numbers for the sake of a simplistic comparison.
What is extremely cool about the ability to lay-up a laminate, is that you can dictate the exact characteristics you want your tube or structure to have. Stiff in torsion, soft in bending. Soft in both, stiff in both. You determine the characteristics – the material doesn’t dictate them. This phenomenon is called anisotropy, and you just can’t do it with metal.
Now for the bad news: carbon’s weak link is elongation. Elongation is your safety net, but with carbon it’s low, low, low. Depending on lay-up, it’s possible to get some elongation out of carbon. For example, there is a scissoring of layers in the 45-degree plies, but in general we’re dealing with a material that doesn’t have an overabundance of ductility. Composite designs are not meant to permanently bend. And when they fail, they fail all at once, so designers build in a big safety net. This is similar to what the aluminum designers do, in order to overcome the low elongation of that material.Most manufacturers are very secretive about their lay-ups, so getting good info isn’t always easy. Reading through the Trek technical manual yields numbers for the specific modulus of that company’s lay-up, which measures the modulus divided by the density. Backing these numbers out yields an 8 MSI modulus for the Trek OCLV lay-up.
The strength of the advanced composites is very high. The zero-degree strength for even a standard carbon unitape (the building block of the laminate) is 300 KSI or better. Looking at the big picture, the strength of the laminate still ends up way above 100 KSI, and this is at very low density. Trek’s specific strength numbers yield actual ultimate values of about 115 KSI. Take a look at the 8 MSI modulus and 115 KSI strength that Trek claims for its laminate, and compare to aluminum. The carbon has about 20 percent lower stiffness, but is 40 percent lighter, and the strength is roughly double, while still being 40 percent lighter. Very impressive numbers.
A Brilliant Future
What’s the future of advanced composites? Their reputation is definitely on the rise. These days, most of the hideously ugly carbon projects have gone away, and there are several very successful carbon production lines happening. The two biggest players at this point are most likely Trek with its OCLV bikes, and Giant, which markets its bikes under several different brand names as well as its own. My guess, after polling a few people in the industry, is that there are probably two to three times as many carbon-fiber bikes sold in the world today as there are titanium bikes. Surprising perhaps, when you consider all the hype that titanium has received. But when you look at how inexpensive a frame from Trek or Giant is, this starts to make sense.
The future of composite bikes will likely parallel my prediction for aluminum rigs – that the material advances will be a lot less significant than our process and execution of making these promising materials work to their best advantage.
Steve Levin, the engineering manager of Schwinn, gave Scot Nicol considerable input for this article. Thanks, Steve.
Next time: the whopping subject of exotic materials.
6. METALLURGY FOR CYCLIST VI: Try Something Exotic
You probably thought that with this installment — part VI of our six-part series on bicycle metallurgy — we’d be done with the subject. You were wrong, and you should know me better than that by now. I’ve added a seventh part, because there is too much remaining to discuss. And because I’m having too much fun doing it…
This time we’ll start to talk about exotics: those materials that we didn’t include in our coverage of aluminum, steel, titanium and carbon-fiber composites. The final episode of our series will have more on these exotics, plus a wrap-up and maybe even a quiz: I’ll present you with some materials that have fantastic “numbers,” and you can try to determine what they are, and why they would stink as bicycle materials.
The previous installment covered the subject of carbon-fiber composites (it’s not a metal, but we explained that last time). In order not to confuse things, I didn’t distinguish between thermoset and thermoplastic composites. The carbon-fiber composite bikes that you’ve seen on the trail for the last few years are of the thermoset variety. Thermoplastic composites are newcomers. The differences in fabrication techniques are analogous to those between making bread and making chocolate.
Making a structure out of thermoset composite is is like making bread. You mix up some ingredients (flour, water, yeast); put them in a mold (bread pan); apply heat (oven); and a chemical reaction occurs (yeast rising). Voilà: Wonderbread. It’s a one-way process. You can’t re-melt the bread and start over.
Thermoplastic construction is more like molding chocolate. You mix ingredients, then heat them up until they melt. You then mold the material into the shape you want. Make a mistake? No problem. Re-melt and re-mold. Although the number of times you can do this before it degrades varies with the material you’re using, it’s a simple process and apparently doesn’t even smell much. This emission-free reform/recycle capability has led some people to call thermoplastic a green material.
A very important attribute of thermoplastics is that they have much better impact resistance than thermosets. On the other hand, it’s hard to bond anything to a thermoplastic. Given the number of bits that you need to attach to the average bicycle frame, this is be a formidable hurdle. Perhaps because of this, the only application of thermoplastics to cycling that I am aware of to date is the Yeti bike, a collaboration between Kaiser Aerospace and Penske.
Imagine a metal with half the density of aluminum, strength better than 6061, and elongation around 10 to 11 percent. I’m describing a magnesium alloy, currently being tested by Easton, who say it looks promising. Although the alloy has a low modulus, in the range of 6MSI, that really shouldn’t be an insurmountable problem — as we’ve seen, it’s easy to build a stiff frame from aluminum, despite its low modulus.
However, that leaves the issue of corrosion. Leave a magnesium part out in the rain and it will disappear faster than just about anything except unpainted steel. This problem may be overcome through proper surface treatment, either painting or anodization. Surface treatment might however cancel one of the side benefits of magnesium — its usefulness as a firelighter. If you need to start a fire for some reason, you can just scrape some flakes off your dropouts. The material’s reactivity means that they’ll easily burn. For a mini-Hindenburg effect, just add water! By the way, titanium does the same thing, but it’s a little harder to get it started.
I’m sure you’ve heard of aluminum MMCs, or metal matrix composites. In fact, we’ve already briefly discussed the Duralcan MMC tubing which Specialized has been using in its M2 bikes for years. The Duralcan material is an alloy of aluminum (for bike-industry purposes, manufacturers use either 6061 or 7005 base material) combined with a ceramic-aluminum oxide (Al2O3). Duralcan has patented a process in which the Al2O3 is added while the aluminum is molten and in a vacuum.
The benefits of the process are apparently numerous, but for we tight-wad bikies, the big advantage is that it’s cheap. If aluminum oxide sounds familiar to you, it might be because you’ve sanded something with Al2O3 sandpaper in the past. If so, you’ve used essentially the same stuff that goes in these tubes: 600 grit aluminum oxide. Different percentages of Al2O3 yield different results. The M2 bikes have about 10 percent Al2O3 (by weight) in their mix. Which means they’re 90% aluminum. Changing the volume fraction of the ceramic allows you to adjust the mechanical properties. Add more Al2O3 and stiffness goes up, but elongation and fracture toughness suffer. With a 10-percent mix, the material has about 8-percent higher yield strength, and 20-percent greater stiffness, than the base alloy. The trade-off is that the elongation will be reduced, but the claimed value of 10% is within acceptable limits.
Aluminum bikes are stiff enough already, you say. True, but as you also know, this stiffness is a function of design. Suppose you are designing the rear end of a bike. Since you’ll want plenty of clearance for mud, heels, tires and chainrings, smaller stiffer tubes may be the way to go. For the same reason, you can use increased modulus to reduce the size of your main frame tubes so they don’t resemble giant sausages. Being able to change the modulus of the different tubes means that you can put stiffness exactly where you want it. Small, incremental advances like these continue to drive the evolution of the bicycle frame.
Heat treatments for MMCs are virtually to those for 6061 alloys. There’s even a 7005 version if you don’t want to heat treat, although at the time of writing it hasn’t seen much commerical use. The strength numbers don’t really change over a standard 7005, but you can get those increases in modulus mentioned previously.
I’ve refrained from getting into the huge subject of MMCs that can’t be welded, but exhibit excellent mechanical properties. These are viable for bicycle use, but the designer is required to use lugs, or some other method of tube joining.
Overall, when you look at the aluminum oxide MMCs, there’s no earth-shattering news — just some minor enhancements to the mechanical properties, and some drawbacks. What matters most, as I keep pointing out, is intelligent design of the entire structure. Had enough yet? For the final installment, I’ll take a look at three other aluminum MMCs, do an overview of beryllium and AerMet 100, throw in a mystery metal, and provide that long-awaited wrap-up.
7. METALLURGY FOR CYCLIST VII: The Final Chapter
The end is nigh. This is the seventh and final part of our six-part series on metallurgy as applied to bicycles. This really is the final installment, I promise. I will use it to finish off my discussion of exotic materials, and then give you a sample of mystery material on which to chew. Cerebrally, that is…
I can see the ad guys go crazy with this one: “Cure for manic depression! Try the new lithium bike! Feeling psychotic? Lick the top tube!” That’s right, lithium, the drug used to treat manic-depressive psychosis, can also be used to enhance the mechanical properties of aluminum alloys, producing new materials with amazing figures for strength and stiffness.
Although no-one is proposing to make a bike frame out of pure lithium, it’s worth spending a moment to check out its properties. Lithium is the lightest of all metals, number three on the periodic chart, and far less dense even than beryllium. However, for those who have to work with it in its metallic form, it seems more likely to cause manic depression than to cure it. Lithium is a pain to work with. It’s unstable, and it loves oxygen. Minute amounts of the stuff can contaminate an entire processing facility.
This cussedness is carried over into the lithium-aluminum alloys. Heat treatment is critical… and easy to screw up. If you heat for too long, or too hot — even by a small amount — the lithium will oxidize, leaving you with a soft, almost pure aluminum. Since the alloys contain only about one or two percent lithium, it doesn’t take much to make it all go away.
OK, let’s say that you’ve bought up a batch of heat-treated, aged and work-hardened T-8 lithium-aluminum alloy. It was difficult to produce and so it’s ruinously expensive, but the numbers look great. What now?
If you want to cast a hockey stick or baseball bat, fine and dandy. If you want to get it extruded into tubes so that you can weld it into a bike, it won’t be in a T-8 condition anymore. When you look at more realistic conditions, like T-6, the strength numbers then come back down to earth.
It’s not surprising that, although lithium-aluminum has been around for many years, not much of the stuff has made it into bike tubing — or many commercial applications at all, for that matter. The first question for erstwhile lithium framebuilders is, “Can you get hold of it?” Only if the answer is “Yes” can you go on to the second, which is, “Can you manufacture it?” I’m still waiting for two consecutive “Yes” answers.
Other materials that get thrown into the MMC vat include boron carbide (B4C) and silicon carbide (Si4C). When you add these materials to aluminum, you get some excellent theoretical enhancements. But processing them is not without problems.
Silicon carbide is quite reactive and can break down in the weld zone, reacting with the base metal to form aluminum carbide. Aluminum carbide is weak in strength and reactive — so much so, in fact, that it dissolves in water. Not a good thing for a weld to do. For obvious reasons, silicon carbide hasn’t seen much use in bicycle applications, despite its tempting mechanical properties.
Boron carbide is the material used in Boralyn. Other boron carbide-enhanced aluminum alloys are undergoing testing. You may not see them until 1995, but there’s a good chance that they’ll be out there. Pacific Metal Craft is producing an alloy they call B4C, and if that company’s claims are true, this is a promising alloy. By putting 15 percent boron carbide in a 6013 base alloy, PMC claims yield numbers of 52-56 KSI, ultimate in the 65-72 KSI range, with a modulus of 14-15 MSI — high for an aluminum material — and an elongation of 4.5-6%. The base 6013 is a high-strength alloy with good fracture toughness for an aluminum — though the boron carbide B4C tends to diminish that — and it is supposed to be easy to work. But keep in mind that not all boron carbides are created equal, and neither are all base alloys. We have yet to hear the final word on this material.
You may find this hard to believe, but there is a metal out there that is significantly more expensive than titanium. It’s called beryllium. Beryllium has about two-thirds the density of aluminum, so it certainly fits into the category of non-density-challenged metals.
In fact, low density is only one of beryllium’s amazing mechanical properties. It also boasts an amazing specific strength (strength over density) and specific stiffness (modulus over density). In fact, its specific stiffness is the highest of any metal on the face of the earth… or within it, for that matter.
An extruded beryllium tube has 40 KSI ultimate strength and a 44 MSI modulus. This gives the stuff the most phenomenal specific stiffness numbers, many times higher than any other metal. For instance, the modulus of steel is only about 30 MSI, and its density is nearly five times higher.
OK, now the bad news. First, the metal has a horrendous elongation number, about 2% in the longitudinal direction, and 0.2% in the transverse. (An interesting note that appeared next to the elongation number in one mechanical engineering handbook I found read: “Ductility values in practice will be found in general to be much lower, and essentially zero in the transverse direction.” Ouch!)
But the real clincher is beryllium’s rarity. Its concentration in the earth’s crust is approximately 6 ppm. Since no rich deposits exist, it costs mega — about 200 times more than aluminum.
Fortunately, there is an alternative to extruded beryllium. It’s called cross-rolled sheet. The $25,000 showpiece beryllium bike that Brush-Wellman (a vertically integrated beryllium company) made for American Bicycle Manufacturing a couple of years ago was fabricated from sheet-rolled tubes, taking advantage of the material’s higher elongation number (>10%) and higher ultimate and tensile strength. To make the bike, the sheet was rolled into tubes and welded together.
One last piece of bad news. Beryllium wins out over all metals on the toxicity issue. In fact, it can kill you. Inhalation of dust particles or vapors containing beryllium may cause berylliosis, an inflammation of the lungs.
A pure beryllium bike is not commercially feasible, but Brush-Wellman has created an aluminum-beryllium alloy which shows some promise. Trademarked as AlBeMet, the material is already being sold commercially in other markets — for computer disk drives, for instance — and Beyond Fabrications of San José, California, has seatposts and handlebars made out of it. Frames are on the way, according to a spokesman for Brush-Wellman. Altogether, the company has four alloys of AlBeMet, varying from 30 to 62% beryllium in the mix, with the following claimed mechanical properties:
Brush claims that these alloys are weldable. It’s interesting to note that strength is quite low for the versions with less than 50 percent beryllium.
AerMet 100 is a promising material that’s been raising eyebrows all around the industry. This new ferrous alloy (steel) was patented in 1992 by Ray Hemphill and Dave Wert of the Carpenter Technology Corporation, and several framebuilders, including Kellogg, Davidson, Curve and Arrow, are presently fiddling around with it.
Check out AerMet 100’s amazing numbers. The density, at 0.285 pounds per cubic inch, is virtually identical to cro-moly steel. Indeed, if you look at the make-up of AerMet, you’ll find a large percentage of nickel and cobalt, which have slightly higher density than iron. But where this material really blows away the other weldable steel alloys (and all other bicycle fabricating materials) is in strength. Our December 13, 1993 VeloNews tubing test included samples provided by Carpenter and showed that AerMet 100 had a yield strength more than 261 KSI, and ultimate strength over 300 KSI. Humm baby!
Combine that with good ductility — the same test revealed 10% elongation — and a 28 MSI modulus, comparable with that of other steels. Carpenter claims that “the alloy is designed for components requiring high strength, high fracture-toughness, and exceptional resistance to stress corrosion cracking and fatigue.”
So what’s wrong with this picture? Nothing, but there are a few clouds on the horizon. As you might imagine, AerMet is expensive, but it’s still only about a half to two-thirds the price of titanium. Also, it’s not yet available tapered or butted, though work is ongoing — and, meanwhile, you can always braze it to 4130.
The real problem for a modern framebuilder is that AerMet 100 is density challenged in the same way that steel is. It’s going to take some work to get those frame weights down to the two-pound level, which I consider the next weight milestone in frame fabrication, without suffering beer-can effects. On the other hand, it does look like AerMet will make it easy to build an extremely durable three-pound frame.
A Mystery Metal…
Gary Helfrich recently told me about a wonderful new material with some unreal mechanical properties. His claims:
- Density: 0.084 KSI, 15 percent less than aluminum (wow!)
- Yield strength: 510 KSI, 12 times that of aluminum, double that of AerMet 100 (triple wow!)
- Modulus: 18 MSI, 80 percent higher than aluminum (wow again!)
- Strength-to-weight ratio 14 times that of aluminum (wow to the fourth!)
If you use 1.25 x 0.030-inch top and seat tubes and a 1.375 x 0.033-inch down tube — to mimic the sweet ride of titanium — you get a frame weight of 1.3 pounds!
What’s wrong with this picture? Here comes that pesky elongation again, rearing its ugly head. Although Helfrich’s material has elongation as good as a carbon-fiber filament, it’s still barely above zippo percent.
The material? Monocrystalline silicon, the material used to make the chips in the computer on which this document was typed.
Don’t be Fooled
Did you get all excited about the Helfrich material? Or did you learn enough from these lectures to look twice at makers’ claims? Two simple thoughts to conclude the series. When assessing materials for use in bicycle design:
I – look at the whole picture, and
II – put the material where you need it.
Scot Nicol, Ibis Cycles