Gear-obsessed editors choose every product we review. We may earn commission if you buy from a link. How we test gear.

A deeper look at carbon fiber shows there's far than meets the eye

Myth: A carbon frame won’t last as long as a metal one. Reality: As long as you don’t crash hard or take a hammer to the frame, a 
carbon bike can theoretically last forever. In fact, steel and aluminum last only so long before the metal fatigues and can no longer be used safely, but carbon 
remains stable indefinitely.

Myth: After a crash, you should have your frame inspected by a professional. Reality: Most cracks that would compromise your bike’s integrity are easy to 
spot. You can also run a soft cloth over the tubes—snags indicate potential 
problems. If you encounter any issues, or want a second opinion, contact your 
manufacturer or a reputable carbon-repair shop. The resident experts can check 
for damage with an ultrasonic wave reflector or infrared camera.

Myth: Cracked frames can’t be repaired. Reality: Shawn Small, owner of Ruckus Composites, is quick to dispel this fiction. “In many cases, carbon can be repaired more easily than aluminum,” he says. He should know. His shop mends between 1,500 bikes a year. To fix a damaged frame, Ruckus grinds away the damage, layers sheets of prepreg carbon over the void, then repaints the bike.

Myth: Bikes made of carbon are more fragile than aluminum or steel models. Reality: Carbon frames are relatively strong and can withstand a significant 
amount of force, but only if it’s applied in the direction engineers intended—for example, road shock from a pothole. But if that force comes from an unexpected direction—if you crash into a tree or step on the chainstay—the material is 
fragile enough that it would likely rupture.

Myth: The sun can damage your carbon frame. Reality: Actually, it’s safe. Bike makers use UV-resistant paint and/or waxes to protect your frame from potentially harmful rays.

I'm standing in Earl's Cyclery & Fitness in South Burlington, Vermont, looking down a row of carbon-fiber bikes, contemplating the differences between the $1,900 crimson-and-white one in front of me and the $9,000 matte-black beauty at the end of the line.

I've been around bikes long enough to know that high-end components can make a big difference in the bottom line, but that only accounts for so much. What I want to know is: What about the frames?

I lean in close enough that my breath forms condensation on the tubes, but the construction techniques within these bikes clearly lie below the surface. I turn to Joe Drennan, Earl's manager, and ask, "What gives?"

He explains that bike makers use several grades of carbon. "When you spend more, you get a stronger material, so manufacturers can use less, which makes the frame lighter," he offers. That makes sense, but when I press Drennan on how the grades differ, how they affect a bike's feel on the road, and whether one brand's premium carbon bike is like another's, he goes behind the counter and returns with a stack of catalogs. Clearly, I have homework to do.

Back home I flip through the literature and tap into Google. I'm bombarded with information. Carbon manufacturing is so complicated and has so many variables that at first I doubt I'll be able to make sense of it. I don't want a masters-level class in material science, but I want to understand how these frames are designed so I can make an informed decision. But I persist over the next few weeks, eventually picking up the phone and talking to nearly a dozen carbon-fiber engineers from brands like Cervelo, Giant, Specialized, and Trek. Over time, the mysterious material comes into sharper focus.

The most surprising revelation? The carbon used in every single bike—no matter the cost—comes from one of just five manufacturers. As I would learn, those fibers can be tweaked a multitude of ways before they end up in a frame, but every bike starts with more or less the same raw materials. "Building a carbon bike is like putting people in a kitchen, all with the same ingredients," says Jeff Soucek, director of research and development at Felt Bicycles. "Some will be great chefs, and they will make something delicious."

Nearly all the carbon produced by the five manufacturers is destined for the aerospace industry (the total amount of carbon used by all bike makers in a year is less than in three Boeing 787 Dreamliners). The Japan Carbon Fiber Manufacturers Association has categorized five grades of the material, four of which are used in bikes and can help guide your bike purchase. Those grades are defined by, among other things, the carbon's stiffness (also called modulus) and tensile strength (which is tested by pulling the material until it fractures). As you move up the scale, the material generally becomes stiffer and more expensive, but not always stronger (see graphic, next page).

All carbon begins as thin fibers that must first be made into sheets—by weaving, or by aligning the strands in a uniform direction—before they become part of a bike. Then manufacturers add glue-like resin to create a composite material called pre-preg, which can be cut and layered into complex shapes. Then things get really interesting.

Carbon bikes can contain up to 500 pieces of pre-preg, assembled in 40 or more layers—often combining different grades. Some pieces are as long as a down tube, while others are no bigger than a postage stamp. By using stiff carbon in some places and more forgiving strands elsewhere, engineers can tailor a bike's feel. That's how a company like Specialized can tune a Roubaix to be comfortable on long rides and make a similarly priced Tarmac ultrastiff. As a bike's structure and tube shapes become more complex, it typically requires higher grades of carbon to hit a manufacturers' strength, weight, and stiffness goals. That can significantly add to price.

I will never fully understand all the nuances of carbon manufacturing, but the basics now make enough sense that I feel comfortable walking into a shop and selecting a bike. I return to Earl's and hone in on two models that each cost about $3,500. They're made mostly from midlevel carbon, and when Drennan tells me they both should offer a forgiving ride, I understand why that's the case. Only one question remains: How do I know which one will offer a more pleasing ride?

That's easy, Drennan tells me:

"You still have to ride the bikes."

Tensile strength of 2,500 Mpa or higher

Relatively strong and stiff, this is the least expensive form of carbon fiber and is found almost exclusively in entry-level frames.

Used In: Full tubes, tube junctions, high-stress areas around the head tube, lower down tube, and chainstays (even on some high-end bikes).

Tensile strength of 3,500 Mpa or higher

The strongest of all carbons, it's found primarily on premium frames.

Used In: High-strain areas like flexing seatstays, and in strength-critical regions, like the top tube, down tube, and parts of the head tube.

Tensile strength of 2,500 Mpa or higher

This carbon is on average 62 percent stiffer than standard modulus, but it's more brittle so engineers use it sparingly. A high-end bike might contain 25 percent high-modulus fibers.

Used In: Areas that require extra lateral rigidity, like a down tube, seat tube, or chainstay.

Tensile strength of 2,500 Mpa or higher

The stiffest of carbon types, it is also brittle and very expensive. It's used selectively in top-of-the-line bikes, often with stronger intermediate-modulus carbon—even then, it comprises only about 15 percent of the material.

Used In: Low-impact zones, like the center of the top tube.

Modulus: Stiffness, or how well a material resists stretching.

GPA: Gigapascals. Modulus is measured in Gpa.

Tensile Strength: A representation of how much force a fiber can take before failing.

MPA: Megapascals. Tensile strength is measured in Mpa.

*These Japan Carbon Fiber Manufacturers Association standards are guidelines only—bike manufacturers and other makers of carbon goods can label their products however they choose.

CARBON FIBER There are two precursors to carbon: rock-like pitch and polyacrylonitrile (or PAN), which is a fishing-line like filament favored by bike makers. To make fibers, manufacturers bake the PAN; heating it longer creates a purer, lighter, stiffer product. Premium carbon threads can be as thin as 7 microns, about one-seventeenth the width of a human hair.

RESIN The glue-like resin epoxy holds carbon strands in place by filling the gaps between them so they can provide structural integrity. Manufacturers can mix in additives (microscopic rubber balls, for instance) to improve certain performance traits, such as increasing a bike's ability to stay intact during a crash.

PRE-PREG These are the thin, pliable sheets of fiber impregnated with resin that, with only a few exceptions, are used to manufacture carbon bikes. Each composite sheet has between 3,000 and 24,000 strands of carbon per eighth of an inch. In structural carbon—the type of pre-preg that gives bike tubes their strength and rigidity—the fibers are laid unidirectionally, which makes them stronger. The interlaced weaves you see on many frames are mostly cosmetic, but offer some protection against impacts.

LAYUP Manufacturing processes vary greatly between companies (and even by model), but most composite bikes are made by arranging small strips (right) of carbon around a solid core inside a mold (far right). Heating the mold liquefies the resin, and pressure forms the pre-preg into the shape of the frame.

Pieces of the Puzzle For engineers, building a carbon frame is like putting together an intricate 3-D jigsaw puzzle. Bikes can have hundreds of pieces, assembled in 40 or more layers. To get it right, designers rely on computer software called Finite Element Analysis (FEA), which helps them determine where they should use each grade of carbon and how best to orient the pieces for the ideal mix of strength, stiffness, and compliance. Without leaving their desks, engineers can go through hundreds of variations an hour until they strike the right balance. A high-performance bike like Specialized's Venge, shown, requires about 400 individual pieces of carbon. Each color represents a different thickness of the carbon tubes.