|Photo credit: Mombat, 1991 Mountain bike action|
It's been a while since I've cracked a materials textbook, but we get questions about these things every once in a while. Here are some of the basics for alloys you're going to see in the bike world. I'm going to keep it as non-engineer-y as possible, so don't worry about it when I say things like weight instead of mass.
Let's start with properties. Some important properties for materials on your bike are strength, weight, corrosion resistance, cost, and manufacturability.
Within the strength discussion there is yield strength, fatigue, elasticity, and impact resistance. Yield strength describes how much stress you can put on a material before it permanently bends. This is important for a single large impact, like a drop on a mountain bike. Fatigue is important if you do that drop 10,000 times. Little forces create little bits of damage that build up over time.
Elasticity as a property is pretty straight forward so I'll just talk about the marketing behind it. Skinny tires are out. Tires are getting bigger. Comfort is the new speed. I appreciate that this style of riding is trending more, but I think words like compliance are silly. Stiff carbon frames are too uncomfortable, so we need to make them more flexible. We need it to sound intentional. We'll call it compliance!
Impact resistance is especially important in a touring bike. A high end frame may be strong to ride, but if the tubes are thin, they'll be prone to denting. There's more discussion on this sort of stuff in an older blog post.
Corrosion is important depending on where you live. Salt from water or roads is nasty stuff for your bike. Rust on steel and pitting in aluminum are pretty common forms of corrosion for bikes. Most components are going to have some form of corrosion resistance. Paint, coatings, or material properties are the most common means of protection. Best to keep an eye on corrosion, as it can lead to failure if it's bad enough.
Not much to talk about in terms of cost: some materials cost more than others. Make sure you're spending your money for the right reasons.
Manufacturability is less important to the customer, but just know that some materials are easier to work with than others. Some are easier to machine (cut into a shape), some are easier to weld. For example, stainless steel and titanium are more difficult to weld than cromoly. So even if the material itself is stronger, there's a chance (if the welder is inexperienced) that the connections can be weaker.
On to materials! Steel is the most important alloy of cycling. I'm not here to discuss the differences between different annealing/hardening methods or the differences between Reynolds and Columbus tubing, but I'll discuss basic properties. Steel is easy to weld and easy to braze. It's the most elastic of the materials discussed in this post, so it'll stretch more before it breaks.
4130 Chromoly is a useful steel in the bike world. It's strong, light, and can be worked into all sorts of shapes. As you may have guessed, this is what our frames are made out of.
Aluminum is a light and rigid, but brittle. It's good for weight savings but usually not the strongest option. It's also easy to machine, but harder to weld. Aluminum is also fairly corrosion resistant, although over time, pitting is possible. Because of its brittle nature, failure is more catastrophic than steel (meaning it will break all at once instead of bending like steel). Additionally, aluminum frames are thinner than steel frames, so they're easier to dent. There are four different types of aluminum alloys common to cycling: 2000 series, 6000 series, 7000 series, and scandium.
- 2000 series aluminum is easy to machine, but hard to weld. Copper is the main alloying metal. It's not all that common in cycling.
- 6000 series aluminum is probably the most common, easy to weld, generally good all around. Magnesium and silicon are the main alloying metals. Most of our aluminum components are 6061, (stems, crank arms, etc.).
- 7000 series aluminum is generally the strongest, but can be expensive and brittle. Zinc is the primary alloying element. Our chainrings are made out of 7075 aluminum.
- You'll occasionally see scandium frames in the bike world, which refers to scandium aluminum alloy. It's light and strong, but it's prone to failure so is less common these days.
Titanium is an exciting material in the cycling world. It's stronger than steel and has natural corrosion resistance, though it has a few drawbacks. It's expensive and tends to be more brittle than steel There are two different titanium alloys commonly used in the cycling world, 3Al-2.5V and 6Al-4V. The numbers refer to the ratios of aluminum and vanadium in the alloy. 3/2.5 is standard as far as titanium goes; 6/4 is the more exotic of the two, lighter and stronger than 3/2.5, but harder to work with. The points discussed in manufacturability apply here. It's generally used sparingly in places where strength and weight savings are more important. One exception is the Litespeed Tellico. It's famous for being Litespeed's first entirely 6/4 frame. And yes, I love old Litespeeds.
Material knowledge is important for a cyclist, but ultimately you should trust the manufacturer's judgement in material selection. Chances are they have the knowledge, experience, and testing capability to decide what's best for their frame designs.