Past and Future of Carbon Fiber

I’m no composites engineer, but I think carbon fiber is an amazing material and a significant innovation for humankind. I think most of you would agree with me. While carbon fibers first saw use in the 1800s (in light bulbs, of all places), its modern application began in the 1960s. Considering how new this technology is, it’s almost unbelievable how quickly carbon fiber has become common in the bicycle industry.

Carbon made its way to bicycle frames in the 1980s, and quickly spread to almost every component and subcomponent – front forks, rims, handlebars, seat posts, and more. It seemed that eventually nothing would remain metal, except for perhaps the chain and any bolts or fasteners that just can’t be practically made from molded carbon.

It struck me, however, that despite its tremendous growth and development, carbon hasn’t taken over the bicycle world entirely. After a huge push in the 90s and 2000s, things leveled out. In fact, some bicycle components have retreated back to metal construction for reasons such as price, durability, ease of ownership, safety, or even the unscientific factor of “it just works”.

In this article series, we’re going to investigate this topic, to find out what components made the cut. Where did carbon fiber survive as either the best choice – or at a minimum – a legitimately viable choice for the average cyclist with an average budget? Where is the juice worth the squeeze? And perhaps more importantly, where is it not? What parts of your bike are better off being made from aluminum, steel, titanium, or even magnesium?

Let’s find out, shall we?

General Pros and Cons of Carbon

Before we can really dive in to specifics, we need to cover some basic information about how carbon fiber behaves. This will help us understand and evaluate why carbon might be a good (or bad) choice. We’ll keep this at a level of geek jargon that’s appropriate for the average cyclist or triathlete, so I apologize in advance to my engineering and chemistry PhDs out there.

The Good

To begin, carbon fiber is lightweight. If I wanted to be more accurate (and nit-picky), I’d say that it can be made a lot of different ways and could possibly end up being heavy – but generally speaking it’s a light material for many bicycle applications. Most people like lightweight bikes rather than heavy bikes, so it’s easy to see why this characteristic is appealing.

The next big bonus for carbon is that its shape can be highly customized. Before the molding process, carbon fiber starts out as a flexible fabric. To that, you add resin (think ‘glue’), heat, and pressure, and out comes a rigid molded object such as a bike frame. If you change the shape of the mold, you change the shape of the finished product. Compared to making a bike frame out of metal, you have MUCH more shape flexibility with carbon. This can lend itself to performance characteristics such as reduced aerodynamic drag.

In addition to shape flexibility, it’s also possible to manipulate other aspects of carbon fiber parts, such as the thickness, stiffness/flexibility, how brittle it is, and so on. For something like a bicycle, changing these things can affect ride quality, durability, and price.

Above image © Scott bikes

Finally, the last big ‘pro’ of carbon is that – at least in theory – it lasts a long time. Alloys such as aluminum and steel have a relatively short fatigue life compared to high-tech carbon fiber. As long as a carbon part doesn’t receive an impact that causes it to crack, it should keep on trucking for many, many miles. Note that we’ll leave an asterisk by this point for now, because there are some real-world factors that affect product longevity beyond the on-paper fatigue life.

The Bad

While it may seem counterintuitive that there are downsides to this miracle material (aside from its tendency towards higher prices) – there are some real-world concerns. We’ll start with the low-hanging fruit. While carbon tends to be light, that is by no means an absolute. Cheaply-made carbon parts – if they’re made by someone who’s at least somewhat ethical – tend to be over-built with excess material to maintain a margin of safety. If you want to make up for a lack of precision in manufacturing, you can make up some safety factor by simply beefing it up. Given this situation, it’s possible for high quality, optimized versions of metal components to weigh less than their carbon counterparts.

One such example is the Thomson Masterpiece aluminum seatpost:

Above image © Thomson

As mentioned previously, carbon has a very long fatigue life. It can be used over and over and over again, retaining good performance and safe operation. That’s all well and good – until you crash. When carbon sustains a very significant impact, it fails by cracking. In comparison, components made from metal will often bend or dent. Of course, there are a lot of special cases, exceptions, and downright strange situations, but I mention this topic because it’s usually safer to ride home on a dented alloy handlebar than a cracked carbon bar. In other words, carbon is great until bad circumstances arise. At a minimum, carbon requires more frequent inspection for damage, and crashes usually necessitate unwrapping handlebar tape to look for cracks.

On a related note, the failure point of a carbon part isn’t often in the carbon itself, but where it is bonded to other materials. Take carbon cranks, for example. You can make most of it out of carbon, but not the mechanical interfaces such as the pedal threads (i.e. where you screw the pedals on). Carbon is a lousy material to use for the threads, so manufacturers bond in an aluminum pedal lug – it’s literally baked into the cake. I can’t tell you how many times I’ve seen failures at these relatively delicate bonding points.

Why? For starters, different materials expand and contract differently with temperature change. Materials have different amounts of flexibility, so when they’re placed under load, the two joined surfaces might bend different amounts. Same goes for carbon bike frames that have an aluminum bottom bracket shell or seat tube liner. Carbon likes to be left alone. When being joined with other materials, great care must be taken during the process.

Finally, carbon fiber doesn’t work very well for surfaces that sustain a lot of rubbing and friction, and direct impact. One example is bicycle chainrings. When you shift gears, the chain gets smashed right up into the side of the neighboring chainring with great force. Carbon tends to fray when it gets damaged, and it just doesn’t stand up well to this type of abuse. As such, we see very few carbon chainrings. Similarly, carbon seatposts can become scratched, marred, or damaged after being tightened over and over.

On the friction front, there is just no getting around the fact that carbon fiber braking surfaces do not provide the smooth, quiet, predictable stopping performance in a variety of weather conditions like a good quality alloy braking surface. One big sign that the industry finally accepted this fact is the push towards disc brakes (which relocates the braking surface to a separate rotor at the center of the wheel). I'll make one exception and note that some of the latest carbon braking surfaces at the highest price points - when used with the best brakes and temperature-specific pads - can perform in the same zip code as aluminum.

In Part 2 of this article, we’ll dive into the specifics of which parts of our bicycles have truly embraced the carbon revolution, and which have largely stayed with aluminum, steel, and other alloys. Stay tuned!