Our heritage is drawn from Formula One, the world leader when it comes to cheating air resistance in the search for more speed. On two wheels as on four, aerodynamic drag is the single most important resistance affecting performance, with roughly 80-90% of resistance affecting a rider being aerodynamic drag*. 80% of this is due to the rider – body position and clothing/equipment – but 20% is the bike so the importance of aerodynamics is significant.
*The remaining 10-20% of resistance affecting the rider is split between rolling resistance, drivetrain friction and kinetic energy (changing speed).
In real terms, if you ride on a flat road at 10mph (16km/h) aerodynamic drag is more than half of what slows you down; at 20mph (32km/h) that figure rises until it’s almost all of what you slows you down – around the 80-90% mark. Even when attacking a climb, aerodynamics will have an impact: on a 10% climb, a fifth of your effort is being expended to overcome aerodynamic drag; only on climbs steeper than 5% does weight become more of an issue than drag.
Aerodynamics is a sub-field of fluid and gas dynamics and is the study of the interaction of airflow and a solid object – how air flows around a body that’s moving through it, such as a bicycle. At first, the air is undisturbed but as the bike passes through it, the air has to part left and right to allow the bike to travel through it. The air then presses on the surface of the bike’s tubing and creates pressure; the rearward facing portion of this is known as pressure drag.
As air has viscosity, the air molecules that come into contact with the surface of the tube will tend to stick to it: as the tube moves through the air, still more air molecules will flow past those stuck to the tube’s surface in layers on parallel paths with no turbulence. This is the boundary layer which, at first, flows smoothly over the surface in laminar flow. This is the most desirable airflow as the air passes relatively undisturbed over the tube. However, it’s not entirely without drag: the passing air across the air molecules stuck to the tube’s surface creates a shearing force: friction drag.
Sadly in many situations, laminar flow can’t be maintained perfectly across the entire surface of the object. So the air molecules can no longer flow smoothly: instead, they mix and move on turbulent paths; if the tube has acute changes in shape (as a round tube does) then, as moving air has inertia, it is no longer able to follow the shape of the tube and separates from it, creating low-pressure regions and more drag.
If the rear of the tube isn’t profiled to bring the airflow back smoothly together behind the tube, then a stalled flow occurs: this is a region of recirculating air that has low pressure, in which the air spirals and flows in the reverse direction – very much like an eddy that forms downstream of a rock in the flow of a river. A stalled flow creates the most aerodynamic drag.
Although we can’t control the air’s density (other than moving to a higher or lower altitude) or the speed the rider is travelling at, we can control the shape of the tubing and the frontal area.
The best aero tubing profile is one that controls a sustained laminar flow around the tube, guiding the airflow back together again smoothly without any stalled flow. The impact? A true aero shape is roughly 23 times more aerodynamically efficient than a traditional round tube.
And the frontal area? Quite simply, drag increases for larger things: so minimising the frontal area of any design will minimise drag.