Take a step back from your mountain bike. Now take a look at the very center of your bike. Assuming you own the most common frame design around, you’ll notice that your bike is made up of a bunch of tubes that are welded or bonded together to form two triangles. (Some materials—particularly carbon fiber—can be constructed into a frame without using tubes.) This double triangle design is called a diamond frame. The head tube, top tube, down tube and seat tube make up the mountain bike’s main “triangle,” while the seat tube, chain stays and seat stays form the rear triangle.
Many different frame options exist these days, including steel, aluminum, titanium and carbon fiber. Not all of these materials are created equal. But because your frame is the backbone of your mountain bike, it is important to know the difference between them. Here is an attempt to define the most common frame materials available to you.
Just as the diamond frame is the most common frame design, steel tubing is the most popular bike frame material. Steel can, and usually is, butted—meaning that the walls are thinner in the center than the ends of the tubing. Thicker walls typically appear at the ends because this is where the tubing is stressed the most, and is also where the tube is welded or brazed to other frame tubes.
When speaking of bicycle frames, two types of steel exist: high-tensile steel and chromoly (chrome molybdenum). High-tensile steel is known for being strong and long-lasting, but not quite as light as chromoly steel. In general, steel is the least expensive metal.
Aluminum is a lighter-weight material that was the first-ever alternative to the steel bicycle frame. Though it is one-third the density of steel, you’ll notice that aluminum tubes can be larger in diameter than steel tubes. This is because the material is also one-third the rigidity and one-third the strength of steel. Aluminum is widely used on mountain bikes today, such as this one, and offers a lighter, stiffer and efficient ride. It is a pretty affordable lightweight option.
Boasting one of the highest strength to weight ratios of any material, titanium is lighter than steel but equally as tough. Because of the welding difficulty (titanium is known to react aggressively to oxygen) and cost of extracting the raw material, it is also typically an expensive material. Titanium can flex while maintaining its shape so well that it is also used as a shock absorber on some bikes. You typically see titanium frames on higher-end mountain bikes.
Carbon Fiber Frame
Tough and exceptionally lightweight, carbon fiber is made up of a bunch of knitted carbon fibers that are attached together with glue. This non-metallic material is also resistant to corrosion and can be molded into any desired shape. Because of its lower impact resistance, carbon fiber is likely to damage if crashed. This material is increasingly popular, but notably expensive.
What's Right For You?
There are several factors to take into consideration before choosing a material that is right for you. Your weight, how long you plan on owning your bike and your bank account are all important things to consider before deciding on a frame material.
As far as weight goes, mountain bikers who may be leaning towards the “Clydesdale” category should choose a higher strength frame material. Though this may add a bit of weight to your frame, you’ll be happy in the end with a bike that can flex without breaking.
Another important factor to consider when deciding on a bike frame material is how long you plan on owning the bike and where you will be riding it. Live in Southeast Alaska where a constant mist greets you each morning? Consider an aluminum frame over steel, as aluminum won't rust as fast.
Not looking to remortgage your home to pay for your new bike? Steel, while heavy, is the least expensive metal out there. Titanium is the most expensive. Aluminum and carbon fiber are increasingly becoming more affordable.