In the first of a two-part feature, UK physiotherapist Bianca Broadbent and physiologist Barney Wainwright — who are both bike fitters — examine an individual approach to optimising aerodynamics
t’s well understood that the speed, and
I
that is caused by an object travelling through
performance, in cycle time trialling and the
it. A slender classically tear-dropped object
cycle leg of non-drafting triathlon is
creating less air disruption (small CD) than a
determined by the resistive drag forces
blunt object such as a flat plate (high CD). In the
experienced and the mechanical power output
case of cycling, both the A and CD can be
created by the cyclist. For this reason,
manipulated through positional changes and
aerodynamic optimisation is becoming
changes to equipment such as clothing and
increasingly important within cycling, with
helmets. The predominant aerodynamic
technological advances targeting equipment
resistance in cycling is due to pressure drag
and materials to reduce aerodynamic drag (El
(Crouch et al, 2017), and as a result positional
Helou et al., 2010). The aerodynamic drag
changes focussed on reducing CD can be more
forces can account for up to 90% of the total
effective than changes to A, especially at high
resistive forces in the system depending upon
speeds.
the relative air speed and surface gradient, with rolling resistance and bearing resistance
WIND TUNNELS
comprising the remaining resistive forces to
Wind tunnel measures of aerodynamic drag in
overcome (Martin et al., 1998; Kyle & Burke,
cycling, normally represented as the coefficient of
1984). Of the drag forces generated, the bike
drag area (CDA), are widely accepted as the
alone accounts for approximately 20% with the
reference measurement method (Debraux et al.,
body responsible for the remaining 80%
2011), with modern cycling-specific wind tunnels
(Crouch et al., 2017). Therefore, to maximise
offering the ability to measure pedalling cyclists
performance in most cycling events, attention
over a wide range of speeds and yaw angles to
should be given primarily to reducing the
match the event-specific air speed conditions.
aerodynamic drag caused by the body itself.
Because wind tunnels take direct measures of
The aerodynamic drag forces in cycling are
drag forces with high precision sensors and
determined by the frontal area of the cyclist
provide well controlled environmental conditions,
and bike (A) as well as the drag coefficient
they are highly sensitive to small changes in both
(CD). CD represents the disruption to the air
A and CD, (Defraeye et al., 2010) and therefore,
Photo by Sharosh Rajasekher on Unsplash
TUNNEL VISION
