“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.