Monday, 29 August 2016

What The Heck Is "Modulus" Means?

Every bike company liberally salts its literature on frame design with buzzwords mumbo jumbo like high-modulus or even “ultra high-modulus.” But those terms aren’t the bike industry’s to toss around. Carbon fiber is graded by its stiffness, rated in terms of tensile modulus. But what is modulus? Modulus is used to grade different types of carbon fiber, and it refers to tensile modulus or Young’s Modulus, which is the relative stiffness of an isotropic elastic material. So now what the hell is an isotropic elastic material? Think of a carbon tube that has five plies of fiber, each of the plies oriented in a different direction so that all five plies have lines that intersect, much like an asterisk (*). Each of these plies transfer load out away from the center of the asterisk equally, making for a quasi-isotropic formation. Since carbon fiber is elastic in that it can absorb shock, tensile modulus measures how much elasticity carbon fiber has.

This is commonly referred to by engineers as “Modulus of Elasticity” and is a driving force behind the designs of manufacturers like ENVE Composites. Modulus of Elasticity is calculated by dividing stress by strain. Stress is a force applied over a unit area such as PSI or pascals (Pa). Strain is how much a material deforms when stress is applied, and is calculated by how much deformation occurs as compared to the structure’s original dimensions. Imagine when you hit a bump on your bike, how much the frame deflects from and returns to its original dimensions; that’s Modulus of Elasticity.

For carbon fiber, those ratings and the stiffness range each covers are set by the Japan Carbon Fiber Manufacturers Association (JCMA). The carbon is ranked as low, standard, intermediate, high  and ultra high modulus depending on its modulus of elasticity. Here is a quick run-down on the different grades: 

Low Modulus below 200 GPa (Giga Pascal)

Standard Modulus 200-280 GPa 
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).

Intermediate Modulus 280-350 GPa 
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.

High Modulus 350-600 GPa
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.

Ultra High Modulus +600 GPa
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.

Or maybe you satisfied with table?

All modulus of carbon fiber can come from the same basic strand, the difference is like an onion. The outermost layer of a carbon strand has lots of microscopic ridges and valleys, so by peeling off a layer, you get a more refined fiber filament, which is standard modulus. Peel off another, and you get intermediate. One more and you are at the densest, most refined type of carbon fiber, high modulus. The drawback with high modulus fiber is that although it is the lightest form of carbon fiber with the greatest stiffness characteristics, it is far more expensive and brittle than low modulus, can shatter very easily, resulting in a cracked carbon frame.

Most of the carbon fiber used in the cycling industry is standard modulus or intermediate modulus; on more expensive frames, higher grades do come into play. But that doesn’t sound very sexy, so there’s often a bit of grading on the curve as companies slip high-mod and UHM into their copy. Some companies are moving away from massaging the grading system altogether, simply creating their own carbon grading system with company-specific marketing terms like FACT or Advanced Grade.

Therefore, true high modulus carbon fiber is rarely used in bike frames, especially mass-produced frames made overseas. High-modulus fiber is expensive, so bike companies judiciously use relatively small amounts in key areas like the downtube, bottom bracket, and chainstays to resist pedaling forces and make the bike stiffer. But they’re placed in the mold along with standard- and intermediate-modulus and high-strength fiber to create the kind of durability, performance, and ride quality a good carbon frame has. Additionally, because of its brittle nature, high modulus carbon fiber usually needs to be reinforced with a tougher material like boron to resist damage upon impacts, making for an even more expensive bicycle frame.

Okay, now you know what "modulus" terms mean. It's important knowledge if you want to buy a carbon frame/parts but..carbon fiber its not only all about modulus, you still have to know about resin and layup. Stay tune and be a smart cyclist.


Ride On!

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