When we identify bikes, we don’t call them by their fork model or their wheels, we identify bikes by the component which make the bike whole, the frame. There is practicality to this philosophy because it is the frame manufacturers who usually package and sell whole bikes. That’s why the difference between frame brands, even if almost all the other components are the same, can often mean differences in overall build quality, warranty support, availability, and especially price.
The qualities of a frame can also easily make or break the performance of an entire bike. One of the chief determinants of the quality of a frame is the material it is made out of. Whether it is ubiquitous aluminum, sleek and sexy carbon fiber, industrial steel or elegant titanium, the material of a frame is an important specification and the differences are striking – not only in appearance but also in performance.
The vast majority of frames are made of aluminum. The material is very light and malleable enough to make into specific geometric shapes that can produce very stiff structures despite the softness of the material. This is exactly why a sub-millimeter thick can of soda can support the weight of a person standing on it vertically. This allows frame designers to shape bicycle structural tubes to resist the stresses produced on certain parts of the bike, allowing them to build them very thin while still being rigid enough to resist huge forces. Less material means less weight, and more importantly to manufacturers, less money spent.
This kind of optimization opens up many very lightweight bicycles to the same Achille’s heel as soda cans. A single dent on a thin load bearing portion of the frame can compromise the integrity of the entire frame. Dents may not be enough damage to cry over, but they can open up an aluminum bike to fail spectacularly in the future should the dented part of the frame be required to resist intense forces.
Another weakness of building for maximum stiffness in an aluminum bike is that stiffness transmits force and aluminum is not a naturally elastic material. The stiffer it is, the less of a natural spring and damping the material has and the harsher the forces are that are transmitted from the ground to the rider’s body. This is especially a problem in hardtails and is actually a huge reason for the popularity of steel and titanium mountain bike frames. Aluminum frames are purposely built very stiff because the material does not naturally stand well to fatigue and the less bending it experiences, the longer a lifespan it will have.
How are aluminum frames made?
The popularity of aluminum is further driven by its ease to machine into complex shapes, its availability, and its recyclability.
Just about all aluminum frames are built from pre-formed tubes and CNC machined parts that are assembled on a jig. A jig is a frame component layout template, usually mounted on a table, where you can assemble all the pieces of a frame like a puzzle which ensures proper angles and proportions before welding. This is important for bikes with different sizes which may look exactly the same but actually have slightly varying tube lengths and shapes. Frames are welded while mounted on the jig and then set aside for heat treatment in an oven and then painting or anodizing.
Can you just fix it?
It’s important to note that the heat treatment process is the reason you can’t just fix a cracked aluminum frame by just welding over the crack. The heat from welding actually compromises the aluminum around the weld, leaving it weaker than if it were just a crack. The whole frame would have to be heat treated again to get some the strength of the material back.
Some sleeker aluminum frames are built using the monocoque technique instead of a jig. Here, two sheets of aluminum are stamped into a mold such that each one is a half of a frame that has been bisected down the middle. They are then welded together, heat treated and painted. This design is used by manufacturers such as Orange Bicycles, Intense, and Foes to achieve shapes that would not be possible to achieve with conventional tubing. If your frame has a weld straight down the middle of your top tube, you have a monocoque frame.
Once an exclusive material, carbon fiber is now the premium material of choice for almost every major mountain bike manufacturer. The widespread availability of this is due to the ramp-up of carbon fiber production facilities in China and Taiwan where cheap labor, lax environmental regulations, and mass production have made it accessible for the consumer market. Almost all carbon fiber building techniques are extremely labor intensive which makes building a carbon frame in Western countries, even ones where labor is relatively cheap, more than twice as expensive to build as one in East Asia.
Almost every major manufacturer out there has their own technobabble or patent pending term for how they build their carbon fiber frames but most of them are just variations of the same method. Carbon fiber construction is actually two components: carbon fiber, which comes in strips of extremely thin sheets of carbon molecules to be laid into a mold and resin, which is then poured into the mold and usually pressurized and forced into the carbon fiber to make a smooth surface and dense structure.
The variety of different carbon fiber building techniques stems from the fact that bikes are hollow, so negative molds still have to have some positive components to lay the carbon fiber over in order to create the hollow spaces in the tubes and preferably remove from inside the frame once it is finished. This is usually done with an inflatable bladder system or foam which can be dissolved after molding.
The manufacturing process of carbon fiber is actually less complex and less prone to human error than using jigs and welding because the build quality is more dependent on precision built molds. It is one of the reasons why prices have come down so much and outsourcing has worked so well. But even though we’re well into the carbon fiber revolution, a frame built out of the stuff is generally $1000-$1500+ more expensive than its aluminum counterpart.
As an engineer, I would say the premium price is worth it though. The material, if used right, is superior in every way to aluminum and even other exotic materials. It can be built lighter, stronger and is a far more flexible material to design around.
Its stiffness and strength characteristics are also tunable depending on what kind of carbon fiber you lay and where meaning it can be built to be stronger in some areas but more compliant and lighter in others. But as a mountain biker, I have to say that there are many other components where a $1000 upgrade would go a lot further in terms of performance.
Despite this, a carbon frame is still definitely an upgrade over an aluminum frame of the same design and should be something to look out for if you can afford it.
Although this isn’t a material you would expect to be exotic, it still holds that regard in the mountain biking community. “Steal is real”, is its motto and it perfectly encapsulates what is loved about bike frames built out of the material. It has very different characteristics from aluminum. Steel is much harder and stiffer than aluminum.
Oddly enough, these traits are more apparent in the built aluminum frames. This is because geometry and tube design tend to play a larger role in the stiffness of a structure and aluminum, being softer and more malleable, can be shaped into more complex geometrical tubes that can maximize the stiffness of a frame. Steel can’t be shaped easily because it is too strong to be easily shaped.
Another key difference between steel and aluminum is, despite its strength, elastic, meaning it can deform and return to its original shape, to a certain degree, infinitely without any negative impact on the structure. Aluminum, on the other hand, has a much smaller window for this which most forces will easily overwhelm, making it have a finite lifespan. Consequently, steel frames are built out of much simpler tube shapes than aluminum and have smaller diameter tubes.
They can also be built to take advantage of steel’s elasticity and allow the frame to be more compliant than aluminum frames without risking its long-term durability. These characteristics also lead to steel bikes generally being tougher than aluminum frames. Their thicker construction and more elastic material make it far more resistant to direct impact and fatigue compared to aluminum.
This amounts to steel frames offering a little bit of compliance that takes the edge off of small impacts on the trail. It is the reason why a steel hardtail has a place in many mountain biker’s dream bike stables. The longevity of steel also adds to its charm. Steel bikes are less likely to crack than aluminum, but when they do, they far easier to repair with a weld.
Steel frames are not in the mainstream because there are unavoidable compromises that are involved with building frames out of steel. A steel frame will never be as light as an aluminum frame with the same specifications. Iron is a much heavier element than aluminum and there’s no way around that. Steel is also very difficult, and therefore more expensive, to CNC machine. This means that the tricky parts like the head tube, bottom bracket, and dropouts, are all far more expensive to fabricate on a steel frame relative to an aluminum frame.
The quintessential exotic material. Titanium’s weight and strength are about halfway between that of aluminum and steel but it generally has more in common with steel. It is not as strong as steel but it still isn’t ductile enough to be easily shaped into complex geometric shapes.
It is a rare and exclusive material though, so most titanium frames come with exceptional workmanship. The finish of titanium is also absolutely beautiful. Titanium is very resistant to corrosion, which along with its distinctive raw finish, makes it downright criminal to paint over the metal.
The same things that keep steel from being a mainstream production material also hinder titanium. On top of that, titanium is also a far more limited and expensive raw material. Although material sourcing and production in the east have brought down the price to be almost competitive with carbon fiber frames, titanium is still too difficult to work with to compete with aluminum and carbon fiber as a mainstream frame material.
What’s the best material for a mountain biking frame?
I’m not going to beat around the bush and say, “each material has its own strengths and weaknesses etc.”, the best material for a mountain bike frame is carbon fiber. Its flexibility to be molded into any shape and its properties tuned in almost all manners, whether they be flex, stiffness, impact resistance or weight, gives it the highest potential among all the materials to be built into a bike frame with the least compromises.
But where engineers have a lot of freedom to get things right, they also have had a lot of freedom to get things wrong. Many carbon fiber horror stories with carbon bikes can be attributed to bikes with some attributes, often lightweight, prioritized over others leading to ugly failures. I would avoid cheap carbon frames and components that aren’t from reputable manufacturers. If a quality carbon frame is out of your price point, you will still be very happy, and much safer, on a quality aluminum frame.
Limiting yourself to these two options will, at some point down the line, lead to the temptation for something different, something unique, something with character. For a lot of us experienced mountain bikers, this is where the steel and the titanium frames start gracing our google searches and credit cards. I will be the first to say that that’s okay. This won’t be a problem for most new riders but it is a fair warning that somewhere down the line, when mountain biking becomes less about utility and more about passion, many mountain bikers start looking for a “bike for life” that will last decades. That is what titanium and steel are perfect for.
There frames out there made from other materials, from scandium to bamboo, but the four materials above are what you’re going to see the most on high-performance bicycles. Though the material a frame is made of is a key factor in how the bike behaves, it should not take precedence over build quality and component selection. If you come across a choice where you have to choose between an aluminum bike with a better spec or a carbon bike with a worse spec, I recommend that you compromise on the material and take the better spec. It is easier to upgrade your frame in the future than upgrade bit and a piece of your build.