The heart and soul of the bicycle lie within the frame. The frame is the piece that binds all the components of a bicycle together into one cohesive, beautiful, working machine. It's what defines the identity of any particular bike. Without it, a bike is nothing. So it’s no surprise that a lot of what goes into choosing the right bike comes down to choosing the correct frame, and when it comes to frames, material matters.
Most frames will be made of either steel, titanium, aluminum, or carbon fiber. Steel and titanium are classic choices that still find favor in the world of custom and handmade bike building. The vast majority of modern bikes, however, will be made from aluminum or carbon fiber. Some riders are firm believers in one choice over the other. For most though, choosing between the two is a more nuanced problem and requires carefully weighing the benefits and downsides of each. These are the factors to consider when choosing what your next bike will be made off.
Comfort is a complicated topic because it’s hardly objective and largely rooted in what riders perceive. The conventional wisdom in cycling has been that aluminum is stiff and harsh, while carbon is compliant and smooth. This idea, however, is now outdated. It was true 20 years ago when aluminum frames were still unrefined and being built using traditional designs. Aluminum is actually a fairly soft metal, so to make it strong enough for bicycles required the use of thick oversized tubing. This made aluminum frames super stiff, fine for racers who desire maximum power transfer, but too harsh for most everyday riders.
Carbon frames, on the other hand, were unique in that they could be engineered to be stiff in certain directions and flexy in other directions. This means they could have comfort over bumps and rough roads comparable to steel and titanium bikes, while simultaneously being stiff enough to transfer power more efficiently a metal bike. You’ll often hear this sort of magical ride quality being described by marketers as being “laterally stiff and vertically compliant.” Carbon also tends to dampen vibration better than aluminum, simply because of its material properties.
Over the last couple of decades, improved knowledge and investments in better frame building technology have closed the comfort gap between aluminum and carbon significantly. The process of hydroforming now allows manufacturers to vary the shape of aluminum tubing throughout its length to achieve the same laterally stiff and vertically compliant ride characteristics. It can be formed thinner, sometimes as thin as a soda can, in areas where compliance is desired and thicker where stiffness and strength are necessary. All this means that a modern aluminum frame isn’t really less comfortable on regular roads than its carbon counterparts.
With most modern bikes, objectively quantifying the difference in comfort is between two similar bikes is incredibly hard. Bike design has a huge influence over comfort. Obviously, a road race bike is going to feel less comfortable than a touring bike no matter what the frames are made of. More than frame material, the two things that really influence how comfortable your bike feels are your tires and touch points. If your bike feels harsh, running wider tires, lower pressures, or both are likely to make the greatest perceptible difference. Things like more compliant handlebars, seatposts, and thicker bar tape can make noticeable improvements too.
The Bottom Line: Carbon has a slight edge, but if comfort your biggest concern, frame material is often secondary to other more important factors. Look at things like tire clearance, fit, and geometry. Ultimately, no matter the frame material, comfort is something that can always be fine-tuned or improved.
Weight is a primary concern for a large number of riders. Even if you’re not a “weight weenie,” having a lighter bike does improve the experience of riding by making climbing and accelerating easier. It’s possible to build very light and capable bikes out of either aluminum or carbon. Carbon, however, rules supreme in this area. A carbon frame will always be lighter than an aluminum equivalent. When it comes to strength to weight ratio, few materials can come close.
Not all carbon, however, is created equal. Carbon comes in various grades and lower grade carbon has a greater amount of fillers which reduce the cost but adds unnecessary weight. It is entirely possible for a lower grade carbon frame to weigh more than a high-end aluminum frame. In this case, you get what you pay for. For the lightest frames in production, though, carbon is really the only option.
Using high-grade carbon and clever engineering, designers are able to produce carbon bikes that are very strong, while being ludicrously light. Many major manufacturers now regularly produce carbon road bikes that weigh less than 15 lbs, and cross-country mountain bikes weighing around 20 lbs, that are all easily within reach if you have the cash.
These ultralight carbon bikes are cool, but it’s unlikely that the average user will be able to extract the full benefit from them. Most riders could probably stand to lose a few pounds around the waist and get the same or even greater performance gains for less money. Average riders likely can’t notice a difference of a pound or two between two bikes. It’s only when you begin reaching the upper echelons of the sport as a highly competitive racer with a finely tuned body, that the pure performance benefits of ultralight carbon can actually give you an edge.
It’s worth noting, also, that the frame only contributes to a portion of the total weight. Components are the other half of the equation. A carbon frame with a low-end build can end up weighing the same or more than a high-end aluminum frame with a high-end build. Wheels, in particular, can make a huge difference in bike weight and how heavy it actually feels when riding.
The Bottom Line: With the right components aluminum frames can still be competitively light, but if you’re looking for the ultimate in lightweight performance, carbon is unbeatable. Lightness isn’t cheap, however, and it will likely only matter most to those who compete at a high level or simply want the best.
One of the impressive things about modern bikes is how they are able to be stiff and responsive without being punishingly harsh. This is the lateral component of “laterally stiff and vertically compliant.” When you push hard on the pedals or dive into a corner, a good frame needs to be laterally stiff to resist the torsional forces that try to rob you of power or precise handling. Modern carbon and aluminum frames are designed with this in mind.
Does one material do this better than another? Technically, it would be carbon that comes out on top in this respect. With aluminum frames, manufacturers use tube shapes and varying thickness to control the ride characteristics. The advantage of carbon is that as a material it simply has far greater potential. It can be made precisely to the exact characteristics that are desired. Its ride characteristics can be easily controlled by simply changing the layup or direction that fibers are laid. It can be made extremely stiff in one specific direction, and in one specific spot. This allows engineers to get extremely particular about how a frame rides and maximize its performance.
The benefit over aluminum, however, can be marginal. Though carbon is better on paper, the vast majority of modern bikes are stiff and responsive enough, and any advertised improvements of one frame over another are often incremental and only perceptible by the pickiest riders. Most riders simply become accustomed to how their bike rides, and that’s fine. If you had the same model bike in carbon and aluminum, you’d likely only be able to tell the difference if you were immediately switching back and forth between them.
The Bottom Line: Both carbon and aluminum frames can be made responsive and stiff. The responsiveness and stiffness of carbon frames can be engineered to a higher level and be finely controlled, giving carbon a slight edge over aluminum. Most riders, however, would be fine with either.
One of the greatest fears many riders have is damaging an expensive carbon frame. Carbon has long been touted as being stronger than steel, and in many circumstances it is. This is why it is in widespread use in the aerospace industry and motorsport. But in the cycling world, carbon hasn’t exactly had a sterling record. There are plenty of anecdotes and accounts of carbon cracking and in the worst cases, completely failing.
This isn’t to say carbon is fragile. Its proven itself at the highest level of the sport, and it wouldn’t be used across the industry if it wasn’t durable. The fact that carbon has essentially become the defacto frame material for high-end enduro and downhill mountain bikes speaks to carbon’s durability in the harshest riding environments.
Often a frame’s durability comes down more to its engineering than its material choice. Carbon frames are designed to work, not fail. When they do it’s often the result of poor design or poor manufacturing. Aluminum is not immune from those faults either. With any frame material, it’s possible to break a frame with sufficient violence or bad luck.
It just happens to be the case that people see aluminum survive catastrophic incidents more often than carbon. This is one reason why aluminum is popular for high consequence disciplines of cycling like amateur crit racing, downhill and freeride mountain biking and any discipline where there is a high likelihood of crashing. Aluminum can get damaged in a crash and more often than not it remains usable. Beyond that, any properly cared for bike frame should last a long time. Carbon fiber hypothetically has an indefinite lifespan. Aluminum does fatigue and has a finite life, but for most riders, this isn’t even a concern because it would take an absurd amount of riding to reach it.
Note that carbon and aluminum frames fail in slightly different ways. If carbon receives a focused impact with enough force, it will usually crack in the impacted spot. Cracks can be both visible and invisible. The simplest way to tell if carbon is cracked is to tap the area. It’s possible to crack an aluminum frame also, but aluminum will usually dent first. These dents can vary from cosmetic blemishes to structural damage that can end the life of a frame. These can be tough to diagnose and it’s always best to get an opinion from a trusted professional.
The one slight advantage carbon has is that it can sometimes be professionally repaired when cracked. This involves layering new carbon on top of the damaged area, which can affect the looks, weight, or feel of a frame, but saves it from the junkyard. Ultimately, riders don’t need to worry too much unless they are buying used. When buying used bikes, always be sure to carefully inspect frames for damage, no matter the material. This is an instance where having a trusted professional inspect the bike can put your mind at ease about the durability of a potential purchase.
The Bottom Line: Modern frames, in general, are fairly durable and any frame that’s properly cared for can last a long time. Bad luck and crashes can happen to anyone and in these cases, aluminum has historically shown itself to be more robust and durable. Carbon can be repaired if there's minor damage.
There’s no real contest here. Carbon is more expensive. There’s no way around this because the raw material costs more, the manufacturing process is more labor intensive, and it requires specialized molds and machinery that further increase cost. Aluminum frames can mostly be built by machine, but laying up carbon still requires many steps done by hand. The high price of carbon can be offset slightly by using lower grade carbon, which means there is more filler material. This, however, increases weight, slightly degrading one of carbon’s greatest benefits over aluminum.
In general, at an equal price point, an aluminum bike will have a nicer component build than a carbon bike. The build factors greatly into how well a bike performs. As a consumer, there’s a balancing act between paying for frame quality and component quality. A carbon frame with worse components might cost the same as an aluminum frame with better components. Picking and choosing where your money gets spent is just part of the fun.
The lower cost of aluminum frames also makes them great options for cycling disciplines where you might not want to risk an expensive carbon frame. This is another reason why aluminum frames are still popular among amateur crit racers and mountain bikers. There is a general piece of advice about the spending money in the cycling world: if you can’t afford to replace it, don’t buy it.
The Bottom Line: Aluminum is cheaper. Whether or not that makes it better or worse can be a matter of taste or perception. Good bikes are made at every price point. The lower cost of aluminum frames may allow some riders to use higher-end components.
The introduction of carbon fiber opened up new possibilities in bicycle design. The smooth curves and swooping shapes of today’s bikes would have been unimaginable in the era of steel and titanium. As a modern rider, it’s hard to deny that most bikes today look darn good. What’s impressive is that aluminum bikes have gotten good enough to look a lot like their carbon counterparts. Hydroforming allows manufacturers to form aluminum frames into similar swooping and smooth shapes. Seamless welds can add to the smooth appearance, making many new aluminum frames almost indistinguishable from a carbon at a distance.
Welds can also be a mark of beauty. Perfect, clean, and even welds are representative of excellent craftsmanship. They can give a bike a utilitarian, blue-collar, workhorse sort of feeling. For some mountain bikers, thick beefy welds being on display can represent the rough and tumble intentions of a particular bike. For some, the look of raw aluminum or raw carbon can be enticing.
Then there’s the situation where carbon is often seen as a symbol of the upper echelon of the sport. Some may bristle at the very thought of riding aluminum. But unless you’re a pro, it’s all rooted in vanity. A good looking bike could be made out of anything if it’s done right.
The Bottom Line: Beauty is in the eye of the beholder. Ride what you like and be happy!
Based on this info, ask yourself questions when deciding on a new bike. What's your budget? Are you an elite rider or are you more casual? How much does weight matter? Are you planning to crash a lot? Some riders simply don’t need carbon. Some can’t stand the thought of riding aluminum.
Remember, though, that the most important part of a bike, even more so than the frame, is the rider. Pick the frame that you think can make you the happiest rider possible. That’s the best way to ride.
Bruce is a writer, a rider, and not much else. He loves getting his bikes dirty, trying new tech, and riding tough trails that make him suffer for hours at a time.