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 bikes out there. And most of the carbon bikes you can buy ride well. In other words, we’re not going to trash talk anyone. Not in public, anyway.
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 matrix material, such as epoxy 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. 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 700 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 standard, intermediate, or high modulus fiber, or by a measurement of the tensile modulus of the material expressed in Msi (million pounds per square inch). The strength and stiffness of carbon filament do not always correlate with each other. Unfortunately fiber higher than intermediate modulus tend to get weaker as they get stiffer. 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 the key to that resin doing its job is to get it evenly compacted in just the right ratio with the carbon. With shapes as complicated as are found in modern composite structures it’s easy to imagine how some areas might not get fully compacted. Those areas of poor compaction can lead to fiber separation and failure so we’ve got a variety compression techniques and are constantly improving them to chase those areas out. Depending on the area and compaction need we use foam shells, carbon shells, nylon bladders, and pressure intensifiers to squeeze the composite just right. All of which makes it sound more exciting than it really is.
Most frame manufacturers (including Ibis) use “prepreg” as our carbon raw material. Prepreg refers to a continuous sheet of filaments “pre-impregnated” with uncured resin. The prepreg is adhered to a carrier sheet, or backing 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 layed up in climate controlled rooms.
To give bike frames their structural strength manufacturers employ a variety of unidirectional carbon fiber prepreg sheets, or “plies”. Each ply is designated by the fiber orientation as being either a 0°, a plus 45°, a minus 45°, and/or a plus or minus 30°. Each orientation bestows a different mechanical attribute to the structure. 0° sheets build strength and stiffness along the length of the structure. Plus and minus 30° sheets resist twisting, and the 45°’s fend off crushing loads. Together they determine the strength and stiffness characteristics of our little mechanical structure. There can be hundreds of individual sheets of prepreg in a single frame, each one a unique shape that goes into a predetermined location in the layup.
Another word you’ll hear 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 monocoque – The easiest way to avoid stress concentration at the joints is to not have joint at all! 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.
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 ensure that every frame meets the strength, stiffness and weight goals for that design.
The prepreg layers are placed by hand around silicone forms or foam mandrels according to the layup schedule. The foam is enclosed by an inflatable bladder prior to layup. 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. These components are layed up in an ingenious fashion so that the fibers from the various components overlap and become one unified structure when cured. The mold is closed and then inflated to roughly 150 psi.
After the layup is complete, the uncured assembly is set into a big, heavy steel mold. The mold is closed and heated for about 40-60 minutes at about 250 degrees Fahrenheit. The combination of heat and pressure first causes the resin in the prepreg to flow, compacting the laminate and fusing the plies together as the excess resin gets pressed out of the structure. As the temperature increases, the epoxy hardens by way of a non-reversible chemical reaction. When fully cured the separate prepreg components integrate with each other into a single monocoque structure.
Here's a mold for a large Mojo HD (this one is a few years old).
Once the cured frame is removed from the mold it goes through many hours of machining, bonding of small fittings, and 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 and then again once it enters our warehouse. To our knowledge this type of testing is unique in the bicycle industry and ensures the quality we insist on.
As you can tell, the process is a lengthy one, and only a few frames can be made per day, with individual and expensive molds required for each frame size. 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.
By the way, we’re not new to this, way back in the day we made some carbon frames, 1988 to be exact. Here they are. They were light and stiff for their day. They were expensive though because we used an extremely high cost fiber to get the mechanical properties we desired. Our scanners weren't that good then, but here are a couple of visuals for you.
Carbon fiber has both phenomenal strength and superior fatigue resistance when compared to other commonly used frame materials. We test our frames in the harshest environment a bike is ever likely to see, the Enduro World Series. Our team is the #1 rated team in the world and races exclusively on our carbon fiber frames.
And as it is with other materials, a crash can wreak havoc on your nice carbon frame.
How much do you have to worry about the durability of carbon fiber after a crash? As you might imagine, depends on the crash.
First of all, carbon fiber mountain bikes are not new phenomena. Trek and Giant have had carbon fiber mountain bikes in the field for more than 20 years without a significant history of problems. And Kestrel’s MX-Z came out in 1988.
If you crash any bike hard enough, you’re going to need to repair it or replace it. Before we talk about repairing carbon bikes though, we’ll tell you a little bit about what we do to the frames so that maybe you won’t need to get it repaired. On our bikes, the areas that are most prone to damage get extra material that otherwise wouldn’t be needed.
Let’s say you run out of talent in a big way, and crush some fiber along with your own bones. The good news is carbon can be repaired. You might not believe this, but often it is easier and less expensive to repair than Aluminum, Ti or Steel. An impact that severely dents an aluminum tube might need a tube replaced. Aluminum bikes are heat treated, so in addition to removing and replacing the old tube (if it can be removed), you also need to heat treat, realign and repaint or re-anodize the frame. None of this is necessary with a carbon frame.
In the 13 years we’ve been making molded carbon fiber frames though, we’ve actually repaired very few, due to our very generous no-fault crash replacement warranty. If you should crash your frame, we offer replacements at a very attractive price. In fact it’s usually cheaper than getting it repaired so we find that Ibis owners who have unfortunately damaged their frames are appreciative of our policy, and are pleasantly surprised at how little a replacement front triangle or swingarm costs.