“Composite materials (or composites for short) are engineered materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct on a macroscopic level within the finished structure.”
Picture 1: first natural composite production line
People often think of composite materials to be something from the recent years but in reality they have been around for a very long time. Natural fiber composites have been used even longer. The pyramids built by the of Ancient Egyptians feature clay bricks and blocks reinforced with straw. The picture below could easily be considered as the very first professional production line for composite components reinforced with natural fibers.
Over the years, with the discovery and developments of “cheaper” metals, composites have been more and more neglected. However, with the developments of glass fiber and the discovery of carbon fibers, composite technology has gradually moved back into the picture.
Picture 2: relative importance of materials
Over the last decade, the bicycle industry has seen a rapid increase of composite materials: frames, handlebars, seatposts and so on; every single component could and has been made in this exotic black material. The industry has become saturated with the terms “extremely lightweight” and “extreme stiffness” but the full potential of composite materials is yet to be discovered.
Picture 3: flax fields
Museeuw Bikes is the first bicycle company in the world to build their bicycle frames with a hybrid weave, consisting of carbon fibers interwoven with natural flax fibers.
When looking at the current offer of bicycle frames, almost everything in the top range is produced using carbon-fiber reinforced composites. From an engineering standpoint these are the most obvious solution when designing for low weight and high strength and stiffness. High strength and stiffness however have some very important drawbacks:
In order to compensate for these drawbacks designers have to work around the exceptional properties of the material to give the bicycle some “vertical compliance” for comfort or flexure of the frame for better handling and cornering. Basically, designers are already making their structure less stiff with an extremely stiff material. Confusing? Let’s look at some data…(pictures 5 and 6)
There is a clear distinction between metals and composites both in weight and stiffness (expressed through Young’s Modulus or E-modulus).
These properties, however, are not taking the application into consideration. When using the material in actual structures designed for optimal weight and stiffness different parameters come into play; material efficiencies! (picture 6)
These parameters are obtained by analyzing the deformation of structures and show the potential difference when constructing a tube or shell/monocoque with the same structural properties.
If you then look at the difference between metals and composites, there is no misunderstanding of the engineering benefits of composites.
But if these figures show that a carbon fiber bicycle only has to weigh 50% of an aluminum frame, why are these not yet made? The reason for this is the behavior of the riders: frames have to withstand many other loads than only the theoretical bicycling loads (eg. side impact).
Imagine a carbon fiber frame with the same weight as an aluminum frame and from these numbers it can be seen that the increase of theoretical stiffness is spectacular, but probably not enjoyable for the rider.
Museeuw Bikes is therefore working with a different blend of materials to come up with the optimal compromise these features: mixing natural fibers with stiff carbon fibers to get the optimal power transfer without compromising for comfort.
Picture 4: Stiffness (E-modulus) of materials
Picture 5: density of materials
Picture 6: material efficiencies (beam is tube to tube; Shell is monocoque)
When looking at the shock absorbing properties of flax fiber composites, you need to look at a microscopic level inside the flax fiber. Museeuw Bikes hybrid weaves are made out of bast fiber bundles and loosened elementary fibers combined with regular carbon fibers.
Picture 7: flax fiber build-up
The biggest advantage comes from a property that most engineers actually try to avoid: Natural Variation. The properties of the individual fibers range from 60 to 80 GPa.
It is this variation that creates a very significant mechanism under loading. Due to small local variations inside the material part of the shock will be absorbed before it is being transferred throughout the rest of the structure. Compare it with a large cake being passed along a large table of hungry people, all taking a small slice.
In the end the cake will be completely gone.
A carbon fiber structure is simply the same large table with people who want to pass along the cake as fast as possible without eating even a small crumble. And the truly amazing thing is that this mechanism has nothing to do with the frame being more flexible; it is the material itself that absorbs the energy and not the structure.