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Aug 11, 2023

How Carbon Fiber Layup Makes Bike Frames Stiff and Compliant in all the Right Places, with Alchemy

For being the lightest/stiffest/strongest material to build a bike frame out of,

For being the lightest/stiffest/strongest material to build a bike frame out of, I didn't expect the engineers at Alchemy Bikes to analogize carbon fiber to putty. But, compared to steel, aluminum, or titanium, carbon fiber opens a world of opportunities for tuning the stiffness and ride quality on a bike.

"You can really sculpt it," Brad Click, Alchemy's carbon fiber technician told me.

There are of course inherent differences between carbon fiber and metals that contribute to ride quality and aren't interchangeable, but carbon fiber tends to be more "tunable," from the thickness of the tube walls, to the jointing of tubes, and down to the shape of the tube itself.

We read dozens of press releases every year from brands who talk about building compliance, or flex, through part of their frames while bolstering other parts for more stiffness. Why? Some tubes are subjected to more brute force through the trails and need to be as strong as possible, while other tubes can offer a bit of flex to smooth vibration and bumps for the human perched on top. Like many bike components, it's a balance of strength and sensitivity, like The Rock in one of those movies where he's cast as the unlikely caretaker.

To get a better idea of how this balance works, I stopped by Alchemy's new shop in Golden, Colorado to hear how the engineers build the best/lightest/strongest bikes they can with rider comfort and frame performance at the forefront.

Alchemy offers road and gravel bikes with geometry tailored to the rider's body dimensions. How does this happen with a carbon frame? With tube-to-tube construction, top tubes and seat tubes are cut to the right length like a steel tube, and then they are joined together with a carbon overwrap process.

Before that happens though, Click is in the layup room with spools of carbon fiber ready to create the layup schedule — the orientation of the sheets of carbon fiber in the tubing itself.

"Each particular tube or part of the bike has its own layup schedule," Ryan Cannizzaro, founder of Alchemy explains. "Which is just how many [carbon fiber] layers and what directions you’re gonna lay them down in."

For the most part, Alchemy uses uni-directional carbon fiber instead of woven carbon fiber. With uni-directional, the fiber runs in one direction like the grain in a 2×4, though it can be cut so the fiber runs at a different angle. By determining the best layup schedule for each tube, Alchemy fine tunes the ride characteristics, the amount of compliance, and the stiffness in each section of the frame.

By itself, a sheet of carbon fiber can easily be ripped into shreds. "Once they cross each other, you have two fibers fighting against each other," Click says. When Alchemy's carbon crafters build the layup schedule and the sheets start to overlap at different angles, that's when the frame starts to get strong. Put two sheets with opposing fibers together and suddenly it becomes very hard to rip.

Alchemy layers the sheets of carbon fiber together in a tube. One sheet may have the fibers running at 30° and on another sheet the fibers may be at 45°. The engineers determine the best layup schedule for each tube for the desired stiffness, instead of using woven carbon fiber where the ride quality isn't as tunable. On average, there are about six sheets of carbon fiber comprising each tube.

Alchemy only uses tube-to-tube construction on their road and gravel frames which are still made in-house in Golden, Colorado. They also give the frames a closed chamber construction. For example, the inside of the top tube is closed off from the seat tube. This gives the frame better vibration damping. However, on their mountain bike frames, they use open chamber monocoque construction. With suspension and bigger tires, vibration damping isn't as important.

Cannizzaro explains that tube-to-tube construction and the intensive layup schedules are a better fit for its road and gravel bikes and monocoque is a better fit for its full-suspension mountain bike frames.

"On a mountain bike, [tube-to-tube] is not as necessary," he says. "You want that mountain bike to be as stiff and durable and light as possible so the suspension can do all the work, because on a mountain bike, the suspension is what you’re getting the ride characteristics out of, as long as the frame is stiff and durable enough to be able to let that suspension do the full capabilities of what it can do."

Like most carbon fiber frames, Alchemy uses a combination of molds and bladders to create the frame or frame tubing. Carbon fiber sheets are laid in the molds and bladders are inserted through the frame tubes. The heat is cranked up, the molds have downward pressure onto the frame and the bladders inside the tubes are inflated up to 200psi to create pressure from the inside out, sandwiching the frame tubes and making the carbon fiber stronger and more dense. When the resin is mixed into the carbon fibers during this process, the frame hardens.

In a more complex piece of the frame like the bottom bracket, Alchemy uses silicone for applied pressure instead of a bladder. Once heated, the silicone will expand and fill the nooks of the bottom bracket to apply outward pressure in the mold.

Alchemy gave us a general idea of what goes into their layup schedules for their carbon fiber road and gravel bikes and how a bike frame is designed to balance stiffness and compliance. Generally speaking, a bike frame is going to be stiffer at the bottom half, where ground forces meet the bike, and more compliant at the top half, where the rider sits. This obviously matters more for rigid frames (road, gravel, hardtails) than full-suspension bikes where more voluminous tires and suspension handle much of the forces translated through the wheels. When it comes to full-suspension frames, Alchemy caters to stiffness and durability.

"You want more impact resistance in the mountain bike," Matt Maczuzak, head of product development at Alchemy said. "You’re gonna come across rock strikes, and you have different forces [than a road bike]. Mountain bike layup is more geared toward ultimate stiffness because you have your suspension and your tires and the wheels that are providing the comfort in the bike. You basically want to isolate the frame to make it as stiff as possible so the suspension can do what it's supposed to. If your frame is flexy and compliant, then you’re messing up your kinematics."

Top tubes are sort of like the bread in a peanut butter and jelly sandwich. They aren't the most exciting elements, but they hold it together. The top tube has the simplest of tasks.

"Each piece gets built a little differently depending on what it does," says Brad. "The simplest, base level is probably the top tube. It's one tube, cut to fit. It's very simple." The top tube, mostly fighting deflection, and not seeing as much stress as other tubes, gets laid up with the fibers in a straighter direction.

Since the seat stays connect to the seat tube where the rider sits, seat stays can be built with some flex. Alchemy builds in some torsional resistance to fight twisting forces in the seat stay, but compliance is the name of the game here. They can go lighter on carbon or even build a bow or bend in the tube to give it more vertical flex.

The seat stays may be lighter and more flexy or thinner than other tubing, but that doesn't mean that it's necessarily the weak link in a frame. Cannizzaro says "This piece is as strong as it has to be for what it's doing."

Seat tubes are usually built with straight plies. Like the top tube, the seat tube is meant to fight deflection but it's also built to have some compliance for added comfort.

In terms of ride quality, the down tube is all about resisting the twist. Traversing a trail the downtube could be deflecting rock hits, resisting the torsional forces of pedaling the bike, and aside from the bottom bracket or dropouts, is tasked with absorbing much of the impacts from drops and jumps.

Chain stays are built heavier and use more 0° carbon fiber sheets are added for stiffness and durability. Maczuzak says that the chainstays are typically heavier than the top tube because they’re built to handle much more force.

On these pieces, where the fork mounts, or the rear wheel mounts to, the pieces maybe a solid quarter-inch of carbon fiber. Not only do they need to be stiff, they need to be durable.

The bottom bracket resists impact forces on landings and twisting from the rider pedaling. Riding up and over sketchy boulders? The BB should be able to take a hit from a sneaky shark fin. Even if some fibers are dug out, there should be enough material to compensate, though it's always worth a thorough inspection.

The dropouts on Alchemy's carbon frames are a quarter-inch thick too for the same reason. These spots on the frame, with so many variables, are made to last in the event an axle goes wayward or the cassette somehow makes contact with the inside of the frame. Alchemy uses woven carbon fiber in two spots on their frames: the water bottle bosses and the chainstays for pure strength.

Lastly, Alchemy notes that the shock mounts must be as stiff as the above components because the last thing anyone wants is energy that should be going exclusively to the shock being sent elsewhere. That could cause a wallowy-feeling suspension or interfere with the frame's performance.

It should be apparent now that Alchemy takes pride in their carbon fiber work. Because of their handy work in the layup process, the frame comes out of the mold with some noticeable grooves or un-smooth parts on the frame where some strips of fiber are raised over others.

Cannizzaro says that some brands will add extra carbon fiber, knowing they’ll sand it down before paint. Instead of sanding down carbon to smooth it or diminishing the aesthetic of the fiber, Alchemy adds a thick coat of clear paint across the frame and then sands it down so the layup still shows underneath the paint.

Most people choose a glossy paint instead of a matte because they want to see the craftsmanship underneath the color. With up to 40 hours of labor on each frame, it seems like the right choice.