Air resistance can be divided into:
Frictional resistance.
This resistance occurs when layers of air pass each other at different
speeds and thus influence each other. The air immediately surrounding the
cyclist moves past the environmental air, which results in resistance.
Shape resistance.
This is the most important kind of resistance. The air in front of the
cyclist is pressed together, but behind the cyclist the air is more or less
sucked away. This leads to a difference of pressure in front of the cyclist and
behind him, which in turn leads to an opposing force. The extent of resistance
is determined by the size of the frontal surface which is perpendicular to the
direction of movement and the shape of the body, also referred to as
streamline. This is the measurement that indicates to which extent the air is
enabled to glide gradually past the cyclist and his bicycle. Wind tunnel
experiments have shown that the cyclist is responsible for 75% of the air
resistance, and the bicycle for 25%. Some researchers assert that a
streamlining of the bicycle is only meaningful at speeds of more than 56
km/hour.
It is obvious that a good aerodynamic position on the bicycle depends on many
factors (such as speed) and can differ from individual to individual. Certainly
for riding a time trial or the world hour record, an individual assessment of
the position on the bicycle is of vital importance. As a rule, a horizontal
position of the torso is the most advantageous position when it comes to
matters of air resistance. This implies that the upper part of the hip and the
acromion must be in a horizontal line. At a deviation of only 10 degrees
upwards, the speed decreases with an average of appr. 1 km/hour, or 2.5% (Van
Ingen Schenau 1985).
The resistance experienced by a cyclist consists of three components:
Transmission resistance.
This is the resistance that is caused by the mechanical parts of the
bicycle which convert power into speed. These mechanical parts include the
chain, chain wheels, sprocket wheels and ball bearings. Well-greased ball
bearings and a well-oiled chain only yield a minimum of resistance. Between 3%
and 5% of the capacity of the cyclist is used up by transmission resistance.
The transmission resistance remains constant when the speed increases.
Roll resistance.
This is the resistance which arises as a result of deformation of the
tires. Deformation of tires occurs as a result of the weight of both the
cyclist and the bicycle and as a result of irregularities in the road surface.
This deformation costs energy which does not return into the system. Linearly
the roll resistance increases slightly when the speed increases. At a speed of
44 km/hour on an even road and wind still conditions, the roll resistance of an
average person amounts to 12% of the power yielded.
Air resistance.
This is the most important type of resistance that a cyclist has to overcome
because air resistance increases as to square of the velocity. In the
above-mentioned example the air resistance amounts to 88%. The diagram, below,
assumes a cyclist weighing 75 kilograms, a body surface of 1.8 square meters,
0.45 square meters of frontal surface in a standing-up position and a bicycle
weighing 9 kilograms. The formula for calculating the air resistance is:
W = p/2.Cw.A.V2
P = airdensity
Cw= drag coefficient
A = frontalarea
V = speed