Words and images by James Wolf
Edited by Connor Smith and Drew Haugen
The first time I rode a bamboo bike, I immediately wondered “where did the vibration go?” It was like they had repaved the road that I ride on every day. But the road hadn’t been repaved, I was just riding a bicycle made from a frame material that significantly smoothed the ride quality.
For me, the real value of riding bamboo over other materials is strictly performance-driven. My team and I are here to build the best bikes in the world, not to make grandiose claims like “we’re saving the planet”.
As cyclists, we become much better at putting out wattage over a longer period of time when we are not being vibrated. Vibration and shock accelerate the rate at which a cyclist fatigues during a ride. So, it would logically follow that by reducing the amount of vibration transferred to the rider, you end up feeling better on the bike for longer and become a better, more efficient motor.
Over the years, I have worked with professional bike racers, aerospace engineers, machinists, naval architects, mechanical engineers, and other bike manufacturers to advance the technology inside each Boo frame.
I’ve also opened my doors to some talented mechanical engineering interns to learn more about the structure of bamboo, structural testing, and engineering to help us build better bike frames. One of these mechanical engineers is Connor Smith from Glasgow, Scotland. Among other things, we were able to better understand what makes a Boo ride so incredibly smooth. I’d like to share some findings of Connor’s research with you today.
The Structure of Bamboo
Looking at a cross section of bamboo you can see small black dots, as well as a lighter-colored filler material. The black dots are vascular bundles of cellulose fibers, running the entire length of the bamboo tube. When the plant is growing, these fibers carry nutrients from the roots to the leaves. The filler material in between the strong and stiff fibers is called lignin, a relatively light and soft material.
The structure of bamboo can be compared to the structure of reinforced concrete–the bamboo fibers being analogous the strong reinforcing steel bars, or re-bar, and the lignin functioning similar to the concrete, which holds the fibers in place and prevents buckling. This natural composite structure creates a very strong, and relatively lightweight, material capable of being used in many engineering applications.
Figure 1 shows a cross-section of our Tam Vong bamboo. It should be noted that the outer layers (located at the top of the segment) have a high concentration of fibers and a low concentration of lignin, leading to high stiffness and density near the surface of the bamboo. The lower part of the segment shows the opposite; a high concentration of lignin and low concentration of fibers, creating low density and low stiffness in these layers.
This composite structure of bamboo, combined with the varying densities of its different layers, means that the bamboo excels at strength and stiffness as well as vibration damping.
Next, I’ll explain the three ways in which bamboo we’ve found bamboo eats vibration.
Bi-Density Frequency Damping
Being a composite of two materials with vastly different densities, it becomes hard for either one to enter a state of harmonic resonance under vibration. The density of one material works to cancel out the frequency at which the other part wants to vibrate. Just as noise- canceling sound technology works, two different waves can act in a complementary fashion to eliminate all sound waves and produce silence.
Gradient Density Frequency Damping
With this gradual change in density of the bamboo’s layers comes a gradual change in natural frequency. Due to this phenomenon, when a range of vibration from the riding surface is transmitted through the frame, different concentric parts of the bamboo tube vibrate at different frequencies, absorbing a larger range of vibrations than a material with a single natural frequency.
Metals are single density materials and therefore only dampen one frequency of vibration. This means all other frequencies go undampened and transfer through the frame to the rider.
The third vibration absorbing property of bamboo is due to the viscoelastic properties of the cellulose and lignin. When a frame made from a stiff material with a dominant elastic response (such as carbon, titanium, steel, or aluminum) is subject to vibration, the energy can only be converted into kinetic energy i.e. flexing of the tubes.
However, when a viscoelastic material like bamboo is subjected to vibration it deforms and “flows” on a microscopic level before elastically returning to its original microscopic formation. As the biopolymer chains are deformed, friction is created between the particles, releasing energy in the form of heat. This form of energy dissipation, called shear strain, is much more effective than pure elastic deformation.
Bamboo is extremely unique, displaying elastic properties in the hugely stiff outer layers and viscoelastic characteristics in its inner layers. This technique, known as “Constrained-Layer Damping”, is a concept frequently employed industrially in extreme vibration applications. Bamboo successfully reproduces these properties without the need for expensive manufacturing and joining methods.
If you examine the progression of the bicycle frame from its conception in the early 1800’s to the present, engineers have spent a great deal of their time developing and trialling different frame materials and technologies to make bikes both stiff for power transfer and compliant for handling and comfort.
While it’s true that we have made many advances in materials science, bamboo has boasted this combination strength and smoothness for millennia. Bamboo has been in constant R&D for over 400 million years, optimizing the material properties which we now use for constructing buildings and bridges, while also displaying highly-efficient vibration damping characteristics as well, similar to many of our most recent aerospace and automotive systems. You don’t need to be an engineer to see that Mother Nature knows what she’s doing.