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About Carbon

Carbon fiber is the very best material for bicycle frames. It has the best stiffness-to-weight ratio and best strength-to-weight ratio of any material, with infinite fatigue life and shock-damping capability 10-15 times that of metals. But it's also important to realize that not all carbon is the same. There are different grades of carbon, just as there are different grades of metals-it can vary in strength and density, which has obvious ramifications when it comes to bicycle design.

We use two different grades of carbon fiber, 700k and 800k. The "k" is short for ksi, which denotes the material's tensile strength in thousands of pounds per square inch. So 700k carbon can resist a tension load of 700,000 pounds per square inch of material. And 800k is even stronger, taking an 800,000-pound load. In other words, this stuff is amazingly strong. (Just for comparison: chrome-moly steel falls into the 63-97 ksi range, 3Al/2.5V titanium is 73-90 ksi, 6Al/4V titanium is 128-138 ksi, 6061-T6 aluminum is 40/45 ksi, and 7075-T6 aluminum is 73-83 ksi.)

While strength is a vital material property, the one that's even more significant is stiffness, since a stiff frame that resists pedaling forces is a more efficient one. And the stiffer the base material, the less of it you'll need, which makes for a lighter bike.

Carbon's stiffness trumps other frame materials, hands down. Our 700k material has a stiffness (modulus) of 33.4 million pounds/square inch, and the 800k carbon fiber we use is 42.7msi. (Chrome-moly steel, the stiffest metal used in bicycle frames, comes in at 29.7 msi, 3Al/2.5V titanium is 14.5 msi, 6Al/4V titanium is 16.5 msi, while 6061-T6 and 7075-T6 aluminum are 10.0 and 10.4 msi, respectively.) And since 800k carbon is about 30% stiffer than the 700k carbon, we can use about 20% less material, which reduces structural weight by a proportionate amount while retaining the same stiffness and strength. But 800k carries with it a higher raw materials cost, plus it requires a separate engineering analysis-you can't just substitute 800k carbon for 700k and call it a day (well, you could, but the part would then be structurally overbuilt, weigh more than required and wouldn't possess a finely tuned ride-in other words, it wouldn't be a Kestrel).

Some of the other guys designate their materials using gsm, which refers to material density in grams per square meter of carbon cloth. But density has nothing to do with strength. A sheet of 110gsm is composed of fewer carbon strands than a 120gsm sheet, and it's implied that 110gsm is lighter. But you could end up having to employ more 110 gsm carbon to reach the same strength in the finished product. Ultimately, every frame (or component) is built to satisfy a series of stiffness and strength requirements, and be as light as possible-which makes actual engineering properties the most sensible way of sorting materials from one another, in our opinion.

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Kestrel Bicycles | 5300 Soquel Avenue, Suite 101 | Santa Cruz, CA 95062 USA
phone: 831-464-9079 | fax: 831-464-9069 | email us