Ever wonder, when you look at your bike, what's hidden beneath the paint? Once upon a time, a simple tubing sticker would affirm the pedigree of the steel tubes a builder had used, Reynolds or Columbus perhaps, with the paint concealing the hours of craftsmanship put in.

Today, Trek's carbon fiber bikes share that hidden handiwork with their steel forebears, not that you'd guess. The bikes are conceived with computer-generated designs, fluid-dynamically assessed and finite analyzed, and the resulting shapes appear seamlessly machine-made.

The carbon bike production process at Trek's Waterloo headquarters has changed over the years, from gluing preformed carbon tubes into aluminum lugs in 1988, to a ground-up process that combines multiple molds with multiple carbon materials to create a masterfully engineered end product.

Yet despite the aerospace technology, the bikes are actually built from a sheet of flat carbon fabric, completely by hand. There are no third party tube or lug manufacturers involved here and, somewhat paradoxically, what happens now at Trek is a more hand-built bicycle than it's ever been.

We're not simply talking about pressing carbon shapes into a jelly mold with some epoxy resin, though molds are where the process begins. Custom mold-making for Trek is done by a team of engineers run by Jay Thrane. The mold-making facility is run out of the original Trek red barn in Waterloo, the place where Dick Burke and Bevil Hogg started their company, and where thousands of steel bicycles were made in the 70s. Now in this old barn, molds are made that help make thousands of carbon frames. Each of these molds comes out of a solid lump of aluminum or steel, depending on the application, and is CNC machined on site to suit the desired component shape. Then the hard part starts. As frames are getting into more involved shapes, so the molds are getting more and more complicated. At the beginning, the mold cutting facility was turning out five molds a week. Now, despite more machines and a 24-hour running, they're managing just one or two, such is the complexity of the new designs.

Every surface of the mold that will come into contact with carbon has to be sanded by hand and polished to a mirror finish. The molds are then plated (to aid longevity) and fitted with hinges, springs, and associated hardware. It takes experienced engineers like Bill Frix, previously a welder and a carbon lay-up operator at Trek, a good half day to prepare one mold. Each frame size needs up to six component molds to make a complete frameset. All the parts of these complicated molds have to fit together perfectly, and the preformed inserts (pieces that add structure on the inside) need to be able to slot in without compromising closure, because the molding process will find the faults, as Jay points out. With heat and pressure, the epoxy resin portion of the carbon liquefies, so if you've got the slightest crack you'll get something in it. Basically, when you turn up the heat, resin turns into coffee. So the molds have to be perfect from the start.

Once the molds are shipped the mile or so west to the carbon lab, the magic black stuff can be cut and picked. Engineering aside, the process involved when laying up a carbon frame has a lot in common with dressmaking. In fact, as processes go, it probably relates more to the art of the seamstress than the traditional way of building a steel frame from tube and lugs.

Jim Colegrove, composites manufacturing engineer, explains: “We have some very advanced software. First, we use CAD and make the 3D shape that is the frame. I can split this part into specific regions and then flatten them out into a net shape, a perfect pattern that I can then put back into the mold, and I know will fit exactly into the shape. We call this pattern a flat preform, which is then cut out on our CNC cutting table.”

Preforms are the key to building strength where it’s needed and saving weight where it’s not, as engineers select the right type of material for each shape and application. Carbon specialists Hexcel make all of Trek's Waterloo carbon material, and have done so for nearly 25 years. It's all USA-made carbon fiber from Salt Lake City, Utah, supplied in standard modulus, intermediate modulus, high modulus, or ultra-high modulus. It can be cloth or uni-directional, depending on intended usage.

Former aerospace engineer Jim explains their respective properties.

"We use cloth—that standard checkerboard carbon fabric—in specific high-stress or high-impact areas, because cloth has a unique property. Think of it a little like ripstop nylon: it can be more damage-tolerant. It is also much more conformal in very tight, surface contours. Uni-directional stuff is just as it says—fibers that run in one direction. It is flexible too, in the plane of the fibers, but it makes more complex shapes quite difficult. Each material has strengths and weaknesses, and it takes experience and engineering to get the structures built optimally.

For example, Hex-MC is a unique material of shorter, chopped fibers. They are thrown down onto a sheet in a very randomized pattern that would simulate a lay-up. We can then mold that into really complex shapes very effectively, because we don’t have long continuous fibers. But it doesn’t have quite the strength or stiffness that uni-directional or cloth does. For contrast, look the bottom bracket. It sees a lot of torsion and bending, because of the head tube load and the load that the rider puts in, so it needs both high stiffness and high strength. So we add small strips of high or ultra-high modulus material in those specific areas to help us out.”

Looking at a carbon frame, it’s so easy to think it’s made like a plastic model airplane, but it’s a complicated business. A Madone road frame has around 180 preforms, or individual pieces of carbon sheet, which can be layered up to increase strength where needed. A Session downhill bike will have 238 preforms, with each being between two and 12 plies of carbon material (either uni-directional, cloth, or Hex-MC). That's a complex cutting list. Carbon is a wonderful material, but it takes good engineering to do right. Without the right expertise you end up with structures that are either heavy or not structurally sound.

To add to this complexity, preforms generally grow in size as the frame size increases, and they may need additional material to allow for the loads that larger riders apply to the bikes. But even the most beefed-up parts of a frame are still only in the region of 1.5mm wall thickness.

The true artisans of the Trek carbon prototyping facility are Kelly Stone and Sue Moe, who have 50 years experience between them in molding carbon fiber. The material is rather like a sheet of toffee, in that it's sticky to touch and bendable, and gets softer when warmed.

As Kelly explains, she only has to get the sheets of carbon in her experienced hands to gauge whether it is fit for purpose.

"You can definitely tell the difference between the types of material and what's not right, and whether there's enough resin or too much. The engineers always tell us the lay-ups to use, the ingredients for each test, but after evaluation, we can make extra pieces for here and there and then test them."

Kelly and Sue know the process inside out: the cooling times, the ideal temperatures, how far you can push the material. Trek has always recognized that engineers need to get their hands dirty too. No matter what the computer says, the hands-on judgment is the true test, and Jim’s happy to get his gloves on and show us how it’s done. He might be slower than Kelly and Sue, but Mr. Plaid, as they affectionately call him, can turn his hand to most things. They can give experienced feedback to Jim and his engineers as to what will and won't work during the lay-up. A lot of this isn't just science—experience is everything at Trek. They have produced and tested so many frames that they have a huge head start in frame development, all that data.

Mr Plaid spreads mold release around a mold and places a preform into the cavity. He talks us through the next steps. Depending on the shape, bladders are added and the whole lot is closed up and placed into the presses. These literally squash the fibers and set the material into shape, at the same time removing excess resin.

The new Session downhill mountain bike race frame has 40 individual preforms in one rocker arm alone. In the same way that a dressmaker might use the bias of a fabric to create just the right fit or texture, carbon is laid into the mold to create the strongest (and lightest) results. Just laying up a mold for a single piece like a swing arm takes around 10 minutes, so the idea that mold-produced carbon is faster and easier to produce than a CNC-machined aluminum part isn't even close. Once all the bits are cleaned up and placed to cool, the next stage of the process can begin.

In the case of a Madone, assembling the molded components into a road frame is remarkably quick. Epoxy glue is used to bond the individual stays, bottom bracket, and front triangle pieces with a proprietary Step Joint design that creates joints of the same thickness as the contiguous tube, so there's no added weight or difference in ride quality caused by the joint. The whole lot is then loaded into a jig to be baked hard in an oven. Once set, the frame can be checked for tracking alignment and sent off to the next stage: adding the finish and the paint—and concealing all that handiwork and technology, in a market that demands more for less as Jim explains.

“I often get asked: why does Trek continue to build frames here when the entire industry has moved offshore, including, to be honest, a good percentage of Trek frames? Why do we still have this factory? And my answer is always the same. You can’t build different products, better products, when you don’t completely understand the science. And the only way to understand the way frames and carbon structures really work is to build them yourself. Having our engineers cutting molds, laying in carbon, seeing their structures come to life is crucial to pushing designs forward. It is really expensive to build things here in this factory, but the products are better because of it. All of our products are better, because we know how things should be built, can be built. And that's because we do it ourselves. We can’t wait for someone else to move the meter and show us how to do it. We are going to lead. That’s been the Trek way since I started here in 1990, and it’s the reason I come to work.”