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.
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.
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 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 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. 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 to get it evenly compacted in just the right ratio with the carbon. Advancements in compression techniques have increased the consistency of carbon filaments which in turn increases the strength of the structure. 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 thick latex bladders to squeeze the composite just right. All this talk of latex and bladders and squeezing and compaction makes it sound more exciting than it really is.
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 and doesn’t allow a level of integration equal to 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 rooms.
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 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 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 – 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.
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. Our engineers go to our factory to 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.
What we have here is a completed Mojo bottom bracket 'lug'. Laying up this part of a Mojo frame takes about the same time as laying up an entire road frame.
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 or foam shells in the more complicated areas 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. These components are laid up in an ingenious fashion 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.
Here's an entire front triangle being put through final assembly, before heat and pressure is applied.
After the layup is complete, the silicone form is removed and the components are set into a big, heavy steel mold. The large steel mold is heated for about 40 minutes at about 220 degrees Fahrenheit. This causes the excess resin in the prepreg to get pressed out of the structure and then harden. When fully cured the separate components integrate with each other into a single monocoque structure.
Here's a mold for a large Mojo HD.
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 and then again once it enters our warehouse. 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.
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. When we want to show someone just how strong carbon is, we often turn to Brian Lopes, a really good racer who we sponsor. Brian puts his bikes through the paces and that usually convinces people that carbon really is a good material to build a frame with. Here's a video of Brian in August 2010 on one of the most famous downhilll courses in the world, Whistler's A Line. Shortly after he filmed this, Brian won his 5th consecutive Air DH downhill race on this very course, against he best downhillers in the world.
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 10 years without a significant history of problems.
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. Most of the layup of our carbon frames finds the carbon prepreg in 0º, 30°and 45º orientations. The 90º weave you often see as the top layer provides the resistance to impact. So while the common worry is about flying rocks cracking your frame the truth is that of the thousands of Mojos we’ve made over the years, the number that have actually broken this way can be counted on one hand.
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 reanodize the frame. None of this is necessary with a carbon frame.
In the 5 years we’ve been making the carbon fiber Mojo though, we’ve sent very few to be repaired, 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 Mojo owners who have unfortunately crashed their frames are appreciative of our policy, and are pleasantly surprised at how little the replacement front triangle or swingarm costs.