Only Four of these Carbon Frames were made using custom steel lugs and carbon tubes

In an effort to help you cut through the smoke and mirrors and preponderance of gobbledygook, we have decided to prepare for you a no-nonsense primer on carbon technology. We’ve noticed that there is a lot of hype about certain products, and a lot of claims of superiority in one way or another. In our opinion, there are a lot of great carbon fiber road bikes out there. We’ve all got geometry pretty well figured out. Most of the carbon bikes you can buy ride well. In other words, not a lot separates all of us, and we’re not going to trash talk anyone. Not in public, anyway.

Why are we doing this hardcore soft sell? We feel that telling our customers the truth is what’s most important. So, without further chest-beating, here’s what we know about carbon fiber.

Ibis Carbon Fiber Bike Circa 1988

The word “composite” literally means – “made of several parts”. In a carbon fiber composite structure the parts are the: the reinforcing fiber – generally we’re talking about carbon, but it could be glass or Kevlar, and the resin. Carbon composites derive virtually all of their strength from the carbon filaments within the composite, but those filaments are nothing without the resin that binds them together in a matrix. A major factor in high quality carbon composite bicycles becoming a reality has been the advancements in the manufacture of both carbon filaments and resins over the past decade. In terms of carbon filaments, the tipping point has been the ability of manufacturers to produce stronger filaments. The tensile strength of the filament used in most high end frames is rated at 500 ksi (thousand pounds per square inch) or better though some manufacturers still use filament that is 10% weaker. Fiber filaments are also rated by their modulus – stiffness– and can be referred to as either being low, intermediate, or high modulus fiber, or by a measurement of the tensile modulus of the material expressed in msi (million pounds per square inch) or ksi. The strength and stiffness of carbon filament do not always correlate with each other. As a result, the design of a composite structure has to balance these two attributes in order to optimize the performance and durability of the finished product..

Like we said though, the filaments are nothing without the resin. And advancements in low viscosity resins have enabled reduction in the volume of resin used in a given composite and consequently increased the concentration of carbon filaments increasing the strength of the structure. The resin needs to be able to flow through the filaments and coat them evenly – a process called “wet-out”. With filaments as fine and tightly compacted as are found in modern composites it’s easy to imagine how little microscopic gaps between the filaments could be left dry. Those voids can lead to fiber separation and failure, and like the bits of pasta on your plate that don’t get covered by the sauce, nobody wants that.

Bike manufacturers usually start with carbon composites in one of two forms – prepreg or tubing. Carbon composite tubing is basically pre-cooked in a generic form that allows the manufacturer to cut the tube to size and glue it into lugs that can be made a variety of materials – including: carbon composite, steel, titanium, and aluminum. This method of construction can provide more leeway as far as building with custom frame geometry. However, lugged carbon construction doesn’t allow the builder nearly as much choice of tube shapes and sizes as a monocoque, which can result in extra weight and material and concentrates stress at the weakest points of the frame – the bonded joints.

Prepreg, the other main way of using composites, is short for “pre-impregnated”, and it refers to a sheet of filaments pre-impregnated with uncured resin. The prepreg is adhered to tack sheets like the backing on shelving paper so that it can be more easily handled. Properly handled sheets are stored in freezers to keep the resin from curing prematurely, and in production the sheets are cut and laid up in climate controlled clean rooms by people wearing bunny suits to keep them from becoming contaminated.

To give bike frames their structural strength manufacturers employ a variety of unidirectional carbon fiber prepreg sheets. Each sheet is designated by the fiber orientation as being either a 0°, a plus 45°, a minus 45°, and/or a plus or minus 22.5°. Each orientation bestows a different mechanical attribute to the structure. 0° sheets develop build strength along the length of the structure. Plus and minus 45° sheets resist twisting, and the 22.5°’s fend off crushing loads. Together they determine the strength and stiffness of our little mechanical structure.

Another word you’ll here often when researching composite bicycle frames is. Monocoque - meaning “a structure in which the shell bears most of the stress”- composite frames are molded using layers of prepreg in a very specific sequence and orientation. In practical terms, this means that large components of the frame (like the front triangle) are formed as a single integral piece. If properly designed and built, this unified structure distributes dynamic stress over a wider portion of the moncoque – instead of creating areas of stress concentration at the joints as in traditional frame manufacturing. Monocoques also allow the Ibisians more creativity in the forms they can design. Which - on the whole - delivers you a lighter, stiffer, and more stylie (in our humble opinion) ride.

The sequence of prepreg layers is called “the lay-up schedule”, and is determined by a variety of methods. Lots of folks talk about how they employ FEA - finite element analysis - to develop their schedule, but what they don’t tell you is that FEA is only as good as the person who sets up the analysis. And the more complex the form or the material, the more difficult it is for FEA to give you useful information. Composite materials and the interesting forms it allows are very complex. We use FEA to develop the basic lay-up schedule, but then the artistry comes in. Engineers at our factory build up sample frames and test them over and over until they find just the right ‘recipe’ that mixes the appropriate amounts of strength and stiffness at the lightest total weight.

Now we come to the cooking. A critical aspect of composite manufacturing is the skill of the workers actually laying up the prepreg according to the lay-up schedule and the quality controls built into the manufacturing process. This is to insure that every frame meets the strength, stiffness and weight goals for that design.

Sections of the frame are layed up by hand around silicone forms according to the schedule. This process must be followed precisely to ensure the desired final result. Controls are integrated into the process to ensure that the sequence is followed and that just right amount and type of material is used. After the lay up is complete, the silicone form is removed and the components are set into a big, heavy steel mold. These components are laid up in an ingenious fashion (someone else figured this out, not us) so that the fibers from the various components overlap and become one unified structure when cured. Air bladders are inserted into the completed layup, the mold is closed and then inflated to roughly 150 psi. The large steel mold is heated for about 40 minutes at about 220 degrees Fahrenheit. This causes the resin in the prepreg to “wet-out” and then harden. When fully cured the separate components integrate with each other into a single monocoque structure.

Once the cured frame is removed from the mold it goes through many hours of hand finishing to give it a smooth surface, ready for paint or clear coat. But that’s not the end; every Ibis frame is tested for strength and stiffness in several ways and must meet our specifications before it leaves the factory. To our knowledge this type of testing is unique in the bicycle industry and insures the quality we’re after.

As you can tell, the process is a lengthy one, and only a few frames can be made per day. That keeps our production capacity fairly small. This is okay with us, as we want to keep our priorities straight (Ride More, Work Less) and get out on our bikes now and again.