The frame is the heart and soul of the bicycle. It's the piece that binds all the components of a bicycle together into one cohesive, 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 bike frames will be made of either steel, titanium, aluminum, or carbon fiber. Steel and titanium are the most popular options for custom and handmade bike building. The vast majority of modern bikes, however, will be made from aluminum or carbon fiber. Some riders will prefer one 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 not completely objective and it's largely rooted in what riders can perceive. The conventional wisdom in cycling has been that aluminum is stiff and harsh, while carbon is compliant and smooth. Now, however, this wisdom is a bit outdated. It was true 20 years ago when aluminum frames were unrefined and still being built using traditional designs. Aluminum is actually a fairly soft metal, so in the early days, to make it strong enough for bicycles required the use of thick oversized tubing. This made aluminum frames super stiff, which was fine for racers who desired maximum power transfer, but too harsh for most everyday riders.
Ride quality has long been a claimed benefit of carbon frames. Carbon is unique because it can be engineered to be stiff in certain directions and compliant in other directions. This means a carbon frame can be designed to have comfort over bumps and rough roads comparable to compliant steel and titanium bikes, while simultaneously being stiff enough in key areas to transfer power more efficiently. You’ll often hear this sort of magical ride quality being described by marketers as being “laterally stiff and vertically compliant.” Carbon as a material also tends to dampen vibration better than aluminum, simply because of its material properties.
Over the last couple of decades, improved knowledge and investment in better frame building technology have narrowed the comfort gap between aluminum and carbon significantly. The process of hydroforming aluminum now allows manufacturers to vary the shape of aluminum tubing throughout its length to achieve 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 significantly less comfortable on regular roads than its carbon counterparts.
With most modern bikes, objectively quantifying the difference in comfort 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 touchpoints. If your bike feels harsh, running wider tires, choosing tires with more supple casings, and lowering air pressure will likely 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. Is the bike intended for road racing or endurance riding? 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 many riders. Even if you’re not a “weight weenie,” having a lighter bike can improve the experience of riding by making it easier to climb, accelerate, and maneuver the bike. 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 almost always be lighter than an aluminum equivalent. When it comes to strength to weight ratio, few materials can come close to carbon fiber.
Not all carbon, however, is created equal. Carbon comes in various grades and lower grade carbon has a greater amount of fillers which reduces the cost but adds 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 the best 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 complete, and cross-country mountain bikes weighing in around 20 lbs. These superbikes are all easily within reach if you have the cash.
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. When looking at entry- to mid-level bikes, most riders likely can’t notice a difference of a pound or two. 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 too that the frame only contributes to a portion of the total weight. Components are the other half of the equation. A carbon frame with low-end components can end up weighing the same or more than a nice aluminum frame with high-end components. Wheels, in particular, will 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.
Responsiveness & Stiffness
One of the impressive things about modern bikes is how they are able to be stiff and responsive without feeling 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 derail your bike's 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. With aluminum frames, manufacturers can make a competitive bike using different tube shapes and varying thicknesses to control the ride characteristics. The advantage of carbon is that as a material it simply has far greater potential for tuning. It can be made to achieve the exact characteristics that are desired. Its ride characteristics can be easily controlled by simply changing the carbon layup or direction that carbon fibers are laid. It can be made stiff in one specific direction, and in one specific spot. Engineers can get extremely particular about how a frame rides and maximize its performance.
The benefit over aluminum, however, might be marginal for the end-user. Though carbon is better on paper, the vast majority of modern bikes are stiff and responsive enough to be ridden hard and fast, and any advertised improvements of one frame over another are often incremental and only perceptible by the pickiest or most skilled 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 to be responsive and stiff. The responsiveness and stiffness of carbon frames can be engineered and tuned to a higher level and it can be finely controlled, giving carbon a slight edge over aluminum. Most average riders, however, will be just as fast on either.
One of the greatest fears many riders have is damaging an expensive carbon frame. Carbon fiber's strength to weight ratio is higher than steel. This is why it enjoys widespread use in the aerospace industry and motorsport. But in the cycling world, carbon hasn’t exactly had a sterling record. Talk to any group of cyclists and you'll likely hear plenty of stories of carbon cracking and in the worst cases, completely failing.
This isn’t to say carbon is fragile. It is the material of choice at the highest level of the sport, and it wouldn’t be used across the industry if it wasn’t durable enough. 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.
However, there are plenty of instances of carbon becoming damaged during unusual destructive events. Carbon frames are designed to work well and be extremely durable under normal riding conditions. When they do fail, it’s often the result of a violent force the frame was never intended to encounter and thus not designed to survive. (E.g., taking an impact strong enough it'd likely damage metal as well, crashing into a car, or falling over on a fence post are events most carbon bike frames aren't designed to encounter regularly.) Despite its strength, carbon fiber is a fairly brittle material and some excessive or focused impacts can cause carbon to crack. If a crack appears, a frame should be retired for safety. Unfortunately, designing a carbon bike frame to be indestructible would make it uselessly heavy.
Aluminum is popular for high consequence cycling disciplines like amateur crit racing, downhill and freeride mountain biking and any discipline where there is a high likelihood of crashing. Aluminum frames are less expensive to replace and it's possible for them to experience violent events like crashes and still be usable. Many riders feel more comfortable crashing an aluminum bike than a carbon one. Aluminum is not immune from damage either, however. Dents in key areas can compromise and aluminum frame. Cracks can appear in welds that can eventually lead to frame failure. With any frame material, it’s possible to break a frame with sufficient violence or bad luck.
Any properly cared for bike frame should last a long time. Carbon fiber hypothetically has an indefinite fatigue life. Aluminum, on the other hand, does fatigue and has a more finite life. But for most riders, this isn’t even a great concern because it would take an absurd amount of riding and time to reach the limit.
No matter your frame material, it's important to keep an eye on your bike to ensure it remains safe to ride. The simplest way to tell if carbon is cracked is to tap on an area that's received damage. Aluminum dents can vary from cosmetic blemishes that are usually shallow and smooth while structural damage usually comes from dents that are deeper or have sharper angles. Ultimately, it’s always best to get your bike inspected by a trusted professional.
The one slight advantage carbon has is that it can sometimes be professionally repaired when cracked. This involves carefully removing an area of damage and then reproducing the layup with new carbon. This should be done by a carbon repair professional like Broken Carbon. When buying used bikes, always be sure to carefully inspect frames for damage, no matter the material. The Pro's Closet professionally inspects all of our bikes for damage and we don't sell anything that doesn't meet our standards.
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 been considered more robust, durable, and cheaper to replace if the worst happens. Carbon should be looked after carefully but it can sometimes be repaired if there's minor damage.
There’s no real contest here. Carbon is more expensive. There’s no way around this because there's more engineering involved, the manufacturing process is more labor-intensive, and it requires specialized molds that further increase the cost. It's possible for aluminum frames to 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.
When considering bikes at an equal price point, an aluminum bike will generally have a nicer component spec than a carbon bike. The components factor 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 damaging an expensive carbon frame. This is another reason why aluminum frames are still popular among road racers and mountain bikers. There is a general piece of advice about 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 bike manufacturing has gotten good enough to produce bikes that look a lot like their carbon counterparts. Hydroforming allows manufacturers to form aluminum frames into smooth aerodynamic shapes. Seamless welds can add to that smooth appearance, making some new aluminum frames almost indistinguishable from carbon at a distance.
For some, 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, workhorse sort of look. For some mountain bikers, thick beefy welds being on display can make a bike feel tough and ready for gnarly riding. For some, the look of either raw aluminum or raw carbon can be enticing.
Then there’s the situation where carbon is often seen as a status symbol. Some may bristle at the very thought of riding "lowly" aluminum. Others may see carbon as too tryhard. Ultimately, a good looking bike could be made out of anything if it’s done right. It's up to the rider, and if you're happy, that's all that matters.
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.