Carbon fibre is cycling’s miracle material. Although primarily desired for its lightness, the true value of carbon fibre transcends merely savings grams.
Beyond the ability to construct a robust and light frame monocoque (by using less overall material for similar strength yield), the true magic of carbon fibre, is in its structural properties.
With its intricate fibre and resin structure, carbon fibre has properties that are absent in metal and make the composite material superior for cycling frame construction.
Extraordinary ride quality
Although carbon fibre is very labour intensive to work with, it allows for more intricate shapes than conventional metal tubes and welding profiles. The combination of these organic shapes, instead of the sharper joint profiles of a welded metal frame, present an opportunity to engineer bike frames with exceptional ride quality.
Bicycles without the benefit of elastomer- or air-sprung suspension must rely on the inherent properties of frame material, to provide ride comfort. Carbon fibre has a huge advantage over metal, in this respect.
In both design and construction, a nearly infinite diversity of fibre orientations are possible within unified composite structures. The advantage of this is that different areas of the frame can have specific material and mechanical properties.
The stiffness factor
With carbon fibre you can have lateral stiffness in relation to the head tube and bottom bracket, facilitating optimal power transfer and efficiency between a rider’s contact points: hands on handlebar and feet on pedals. In the same frame, that lateral stiffness can exist without resulting in reduced vertical compliance, which is the material property that provides gravel road ride comfort.
Metal bikes have a single stiffness profile for the entire frame. You can either have great power transfer or plush ride quality. Not both.
With composite materials, a design and engineering team can experiment. By altering the direction and layering profile of fibres, there can be an enormous influence on how vibration is absorbed, without creating a frame that robs energy through undesirable flex, when a rider is applying peak power.
Riding a rigid bike on gravel, the frequency of small-bump juddering is intense and can become fatiguing for a rider. A gravel bike frame constructed with clever carbon fibre design principles can insulate the rider from this terrain inducted vibration fatigue.
Unidirectional carbon fibre has a superior blend of stiffness and vibration absorption to any comparable metal tube set. Despite the price premium, these characteristics make carbon fibre a no-brainer as far as the performance vs pliancy stakes go, particularly when it comes to gravel bikes.
Quality vs durability
A final thought must regard the correlation between ride quality and durability. Carbon fibre is complex to understand and considered engineering analysis, design and quality control inputs, yield the calibre of gravel bike frame you desire. This usually comes at an associated premium price point.
Frames which ride with superior vibration abortion and the greatest comfort coefficient, don't have these attributes in isolation of quality. A great carbon design, which has been sloppily built, will have weaker ride quality, due to the abundance of resin or contaminants in its construction.
Build quality is a crucial factor if carbon-fibre gravel bikes are to perform as they should, in the real world, beyond an engineer's CAD screen.
As with most things in cycling, you get what you pay for.