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Carbon Vs. Aluminum Mountain Bike Frames: The Showdown

There are many bike options out there. But do you want a carbon frame or an aluminum one?
By Pat Donahue ⋅ Senior Review Editor
Tuesday March 26, 2019
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Is Carbon Worth the Upgrade?


There are many options when it comes to upgrades, from less costly upgrades like a new handlebar or seat to more expensive upgrades such as replacing your wheels or switching your drivetrain from 2x10 to 1x11. Most upgrades are born from the idea that less weight is better, and we tend to agree as long as it doesn't come at the cost of performance. There are some major components that you can replace to lighten your rig, but what about the frame? We feel that the majority of bike owners out there are going to do a full upgrade in the form of a completely new bike, but there are riders out there that will replace just a frame. This article will speak to both groups of riders about whether upgrading to a carbon frame or upgrading from an aluminum bike to a new carbon bike is worth the cost.

Weighing things ourselves to get to the bottom of our questions here at the Lab.
Weighing things ourselves to get to the bottom of our questions here at the Lab.

The Myth


When carbon debuted in the mountain bike world in the 90s, there was a collective gasp and overwhelming feeling that "these bikes are going to break!" That may have been true in the beginning, but it is no longer the case. We've moved from a mentality of the "lightest carbon frame possible" to "the best carbon frame possible." With a switch of focus from light to durable and building carbon frames to withstand the abuses of a specific genre, we've seen carbon become nearly ubiquitous in all forms of cycling as the material of the pros. While a carbon frame built for cross-country riding may not hold up as well as one built for downhill, the lightness of the cross-country frame is more important, while the durability of the downhill frame is a more significant asset.

In truth, if we tested the strength of the same frame built from carbon and aluminum, you would see that carbon fiber frames are much stronger per pound in terms of both stress tests and impact tests.

Weight vs Price


One of the most discussed aspects of carbon fiber frames is the weight savings over aluminum frames. It is hard to pin down a hard and fast rule about how much lighter a carbon frame is compared to an aluminum one. For example, a Santa Cruz Bronson carbon frame may boast a 1.5-pound weight saving over the aluminum version while the carbon Pivot Switchblade might be 1.9-pounds lighter than the aluminum Switchblade.

Then comes the elephant in the room. Cost. The aluminum Santa Cruz Bronson frame sells for $1999 while the Carbon CC version sells for $3299. That's a $1300 difference. The Transition Smuggler aluminum frame sells for $1999 while the carbon version sells for $2999, a $1000 difference.

That begs the obvious question, is a 1.5-pound weight saving worth the extra cash? That, of course, is a personal question that depends on your goals and finances. It should be noted that component upgrades save weight as well. Even if you had a chunky aluminum frame, it is easy to save a pound or two with the components on the bike.

There is also the fact that having a slightly heavier bike isn't that big of a deal. Fitness and skill are far more important than bicycle weight.

Feel


For our money, the feel of carbon is superior to aluminum, and may, in fact, be worth the upgrade cost in itself. There are two major factors that we see as a benefit of a carbon frame over an aluminum one: dampening and torsional stiffness.
  • Damping: A carbon frame has a damping effect which lowers the harshness in the ride. Aluminum is a stiffer material, and as such transfers, much more of the trail, including rocks and roots, into your hands and behind. The dampening effect of carbon, while minimal, will take some of the harshness out of the ride by soaking up some of the chatter that you get when riding a rough trail. This effect, however minimal it is, will give your hands and butt an amount of respite not offered by an aluminum frame. When multiplied over hours of riding, the effect multiplies, and we find that we can ride comfortably for a longer period of time on a carbon bike. While dampening is definitely real and it really does smooth out the ride, we feel that there may be a placebo effect of carbon that makes it feel much smoother than it is, and we tend to feel faster because of this. This could also be attributed to a lighter weight as well. Whether these exaggerated benefits is part of the mindset of the slaying on a carbon bike or not, we're 100% confident that we ride faster on a carbon bike. If you don't choose to go the carbon frame route, carbon handlebars and a carbon seat post will give you some dampening and soften the ride and also lighten your bike at the same time.
  • Torsional Stiffness: While dampening can soften the ride, it is by no means at the cost of pedal power. When you push down on a pedal, especially in a standing position, the bike will rock to the side. With a carbon frame built correctly, the torsional stiffness will transfer all that power into your rear wheel. (Carbon is a grained material, meaning that the placement direction of the grain is important.) Although both frame materials flex a bit, carbon frames tend to be torsionally stiffer, preventing your bottom bracket from twisting in relation to the chainstay and down tube, creating a snappier ride. Accelerating on a carbon bike feels much snappier due to both the torsional stiffness and lighter weight, and that snappiness, although minimal, will affect both your performance and psyche, leading to faster times and more confidence.

Durability


The myth that carbon is fragile again rears it's ugly head, but it is just a myth. While both frame materials are susceptible to catastrophic failure, carbon fiber frames tend to be stronger per pound than their aluminum equivalents. We see a lot of pictures of broken carbon frames pasted into forums to try and show how fragile carbon is, but it's just not the case. We don't know if these people have ties to aluminum investments or if they hold on to bygone beliefs about carbon, but what we do know is that carbon frames are stronger and lighter than aluminum, simple as that.

Aluminum is great because it's cheap, fairly light and fairly stiff. The problem is that with stiffness comes stress fracturing. Carbon has the dampening effect that soaks up repeated abuse and allows it to spring right back into place, while aluminum just eats each impact. After thousands of miles, these impacts add up to many microscopic cracks in the aluminum, and over time stress failure cracks will appear, typically at the wades, the weakest points. Fatigue cracks will lessen the strength of the frame and will also reduce its stiffness, eventually causing failure. While this certainly won't occur overnight, you should be aware of it.

Carbon, on the other hand, besides being better over time, has a much higher tolerance to frame bending. If you were to run your bike straight into a wall or rock or case a double where the impact is directed straight through the frame, the impact on the front wheel would push through the fork and into the head tube. The force on the head tube is translated as if you were trying to fold the front wheel back towards the bottom bracket, bending the down tube where it connects to the head tube. On a carbon bike, the catastrophic failure point is much higher than an aluminum bike due to the flex qualities of the carbon material. Just as one impact of this nature could break an aluminum frame and not a carbon frame, repeated stresses in this manner would cause an aluminum frame to break sooner as well.

Carbon is also said to splinter on impact while aluminum will survive the same impact. While both frames have their breaking points, typically carbon will "bounce" off a rock due to those same dampening qualities that we addressed earlier, while an aluminum frame will tend to absorb more of that impact, sometimes in the form of a dent. There isn't that much information out there on this regarding specific impact forces, and we weren't allowed to take an axe to the top tubes of any of the bikes, but we don't feel that carbon is any less susceptible to impact breakage than aluminum and possibly more resistant. Regardless, carbon is actually repairable while aluminum, not so much.

That being said, we did manage to break a carbon frame during our test period, so we know that it is not invincible. The force of a big crash can damage any bike. Would an aluminum bike have survived when this carbon frame splintered? We don't know for sure. Would we resort to only buying aluminum frames because of this? Nope. We are still fans of carbon.

Broken top tube of the Carbon Trek Slash 9.8
Broken top tube of the Carbon Trek Slash 9.8

Not all carbon is not created equal, so be judicial in your purchase decision, especially if the price for that carbon bike is lower than you'd expect. Manufacturing practices go a long way in determining the quality of your ride, regardless of material, and we recommend bikes built by reputable companies with solid warranties.

Lifespan


As technology changes so quickly in the modern era of mountain biking, we don't know how much lifespan will affect your choice, because either material will give you a solid 7-10 years of life, but if you really want to buy a bike that is going to last, carbon is the way to go. Impacts and abuse aside, carbon doesn't fatigue over time and your frame will be as good in 20 years as it was the day you bought it, although heckling will probably increase, as might your belly.

Other Materials


There are other materials out there that have stood the test of time, namely steel and titanium. We've seen both disappear for various reasons, steel being much too heavy and titanium for being too expensive and still heavier than carbon. There are uses for both, often times custom built frames for riders of unusual size (ROUS's). Titanium, for instance, is extremely compliant and stiff and when building a bike for a very tall rider you can't simply make a frame bigger, it has to be designed for that rider or the forces of the rider will negatively impact the frame. Titanium is very stiff and building a bike for a rider that is much taller or heavier will allow the builder to keep the same design as a smaller bike without compromise.

Conclusion


There are many choices out there, but we are all firm believers in the power of carbon. While it may lighten your wallet, it will also lighten your ride. We think that the cost difference is negligible compared to the performance boost and weight savings. It's not just a matter of lighter, it's a matter of stronger and better ride characteristics and we think if you have the means to afford a carbon bike, do it. Check out our Best Trail Mountain BIkes to see what carbon bikes we fell in love with. If you can't afford the jump, don't be disillusioned, there are many bikes we love riding with aluminum frames, check out The Best Mountain Bikes Under $2500 review to see what we rate best.


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