The Truth About Frame Materials, Part I
Triathlete Magazine, July 1999
by Rick Denney
Have you heard that aluminum is harsh? Or that steel is real? What's up with titanium? And, how can carbon soak up bumps and be stiff, too? What is stiffness, really? Have a care for your sacred cows, because we are going to slaughter a few. Once we’re done, though, hopefully you’ll understand why design is very much more important than material. But it’s too much to cover all at once. So, in this article, we will talk about the properties of the three metals commonly used in bike frames. In the next article we will build on this knowledge and talk about how designers work with these material properties to achieve what we all want: A frame that weighs nothing, transfers power infinitely efficiently, and rides like a ’62 Cadillac.
But first, we have to discuss a few terms. Hang with me here, because we can’t really understand bikes without them. The first is elasticity. Most people think of it as the ability to stretch. But that’s not really it. Elasticity is more the tendency to return to its original shape. And an elastic material stores energy when we flex it, and returns that energy efficiently when we release it. Think of the steel springs on your car. They are so elastic that we have to slow them down with shock absorbers. All metals used in bike frames are highly elastic up to the point where they stay bent.
Stress is one of the most important words in any discussion of materials. We define it as the force per unit area applied to the material. That per unit area part is very important. We take the total force, and divide it by the area of the material—the square inches. So, the units for stress are pounds per square inch, or psi. If a material has an ultimate strength of 50,000 psi, then that means a square bar one inch on a side (one square inch in area) will break when stretched with a 50,000-pound load. The total force on an object is the load.
The maximum stress that a material can endure we call strength.
Stress only describes the force on a material; it says nothing at all about how the material responds to that stress. That’s what we call strain. Strain is how much a material flexes under a given stress.
Now for the final term. As objects are loaded, they are stressed. The stress results in strain. Materials that strain less for a given stress are stiffer than those that strain more. For those unafraid of math, stiffness is what you get when you divide stress by strain. This value is called the modulus of elasticity, which is just a fancy term to describe the inherent stiffness of a material.
Now that we’ve defined some terms, let’s talk bikes.
Of the three common metals used in bicycle frames, steel is the strongest, followed by titanium, and then by aluminum. 7000-series aluminum has a strength of around 55,000 psi. The common 3/2.5 titanium alloy breaks at a stress of around 110,000 psi, which is about the same as that most traditional of all materials, mang-moly steel (aka Reynolds 531). The newer steel alloys are much stronger. Reynolds reports that their 853 tubing material has an ultimate strength of 190,000 psi.
But wait a minute! 531 and 853 are both steel, right? Why is one much stronger than the other? Part of the reason is in the alloy. And sometimes, materials are made stronger by heating them in a particular way. But mostly it’s because the material is worked. Here’s the idea: You heat the material just hot enough to soften it up, but not so hot that it all runs together, and you squeeze it. This forces the molecules of the metal to be oriented in grains, like wood, so that it is much stronger than if it was just melted and then allowed to cool (which, for you tech-heads, is called annealing).
Stiffness varies the same way. Steel is the stiffest material, with a modulus of elasticity about twice that of titanium and three times that of aluminum. Here’s an important point: The alloy and the working affects the strength, but not the stiffness. 853 may be nearly twice as strong as 531, but they both have almost exactly the same stiffness.
The problem with steel, if it has a problem, is that it’s too strong. It’s so strong that the walls of the tubes can be very thin and the tube will still be as strong as other materials. But it’s impractical to make the tubes that thin, because they are hard to braze or weld and they dent too easily. And aluminum is relatively weak, so we have to overcome that problem with design. Titanium is right in the middle, and that’s why some say that it is the ideal material.
Let’s slaughter one of those sacred cows. Steel frames do not go soft. Okay, I said it. So what is metal fatigue? It is a crack resulting from repeated stress cycles above a critical point that we call the fatigue limit. The crack might get started at a stress peak, like a weld or a deep scratch. Stress is extremely high at the tips of cracks, and so the crack grows, just like in a windshield. Eventually, the crack has nearly crossed all the way through the tube, and what’s left isn’t strong enough, and it breaks. This may seem sudden, but the good news is that a fatigue crack causes so much popping and creaking that a cracked frame will usually give you lots of warning.
Fatigue, however, has no effect on stiffness. Strength degrades, but not stiffness. So, a crack-free steel frame that’s 20 years old is as stiff as the day it was made.
While we’re at it, let’s get rid of another of those revered bovines. Aluminum is subject to fatigue in different ways than steel and titanium, but good aluminum frames are designed such that they will not fail from fatigue in your lifetime. Aluminum has its fatigue-prone reputation because it has no fatigue limit. Both steel and titanium have a fatigue limit, and if stress remains below that limit they will never fatigue, but aluminum will eventually fatigue if you stress it enough times. The lower the stress in each cycle, the more cycles it takes to cause fatigue. Therefore, aluminum bike makers select designs based on minimizing stress. The lightest aluminum bikes (a Felt, for example) can’t be stressed as highly, and so should probably be avoided by we clydesdales. Actually, bike makers that use steel and titanium have to worry about fatigue, too, to make sure that the stress stays below the fatigue limit.
Construction quality counts. As welds cool, the metal tries to twist. The frame resists this twist, and you end up with parts of the joint fighting other parts of the joint. Mild heat treatment allows these twists to relax in aluminum frames without ruining the strength-enhancing grain structure. With steel and titanium, it’s important to weld properly to avoid overheated or porous welds that can fatigue.
So, how do you avoid fatigue problems? First and foremost, buy from a reputable builder. Avoid ultralight designs, unless you too are ultralight. Also, inspect your bikes frequently for cracks, no matter what they are made of. Use your ears as much as your eyes, because fatigue cracks are hairline-thin, and they usually make a racket before you can see them easily. Important places to look are where the down tube meets the head tube, at the shifter bosses, and around the bottom bracket.
We’ve talked about material properties and fatigue, and we’ve dispelled a few myths. But we haven’t yet talked about how frame designers work with these properties, and we haven’t talked about how these properties affect the ride. Finally we haven’t talked about carbon fiber. But we will, so don’t go away.