This page explains the aerodynamics of a rotating wheel.
Read on to find out
- why an aero wheel offers greatest benefits in
tailwind situations, and
- why you will benefit most from aero wheels when drafting
someone or riding in a pack.
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Front wheel in motion
Picture a front wheel in motion: Lift your bike and
let the wheel rotate at 40 kph – the bike itself
remains stationary (we will get to the moving bike in
a second). Let’s have a closer look at the physics
involved. The top part of the wheel moves forward at
40 kph, while the bottom part moves backward at the
same speed – we say its forward velocity is minus
40 kph. The hub does not move forward at all. |
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The real-life situation
at 40 kph
After this short prelude let’s hop onto our
imaginary bike and accelerate to 40 kph. Now we have
to add the forward motion of the bike itself: The bike
moves at 40 kph and the wheel rotates at 40 kph. What
does that add up to? Very simple: With 40 kph on your
speedometer the hub moves forward at exactly that speed.
The top part of your wheel, however, cuts the air at
80 kph – the bike’s velocity PLUS the rotationary
speed. Similarly, to get the speed at the bottom of
the wheel we have to add forward speed of the bike and
forward motion of the contact patch of the tire: 40
+ minus 40 = 0. |
Summing up: The top part of a bicycle’s wheel
always moves forward at twice the speed of the entire bike,
while the bottom part doesn’t move at all. These are
the speeds with which the wheel cuts through the air.
Note that the drag a wheel creates increases dramatically
with increased speed: As speed is squared in the equation,
doubled speed means four times as much wind resistance. Now
what are the consequences?
Conclusions
1. Most of the drag is created at the top part of
the wheel. By reducing the length of the spokes (using deep-dish
rims) we reduce the maximum speed with which the spokes cut
through the air, thus drastically reducing drag. That’s
why we only use deep-dish rims.
2. Riding in the peleton (little headwind, high
rotationary speed of the wheels) requires aerodynamic spoke
profiles. That’s why you will find only aerodynamic,
lens-shaped spokes in Lightweight Wheels.
Example: Peleton riding at 50
kph
Imagine yourself in a tightly packed peleton during an ordinary
Tour de France stage, leisurely cruising along at 50
kph. Let’s say the air in the peleton moves
forward at 20 kph. Thus our speed in relation
to the air around us is only 30 kph. However,
our wheels still rotate at 50 kph, letting the spokes at the
top cut through the air with 80 kph (2*50-20). That’s
exactly the speed relative to the air the top part of the
wheel would see at 40 kph without the draft!
As drag is proportional to the square of the speed, the
wheel creates an enormous drag – unless it has been
designed to be very aerodynamic. The drag of the rider or
the rest of the bike is comparatively small, as they are well
shielded from the wind, moving only at 30 kph relative to
the air around them.
Conclusion: The drag created by the wheels is especially
high when drafting since they rotate faster in fast pack-riding.
The drag created by other components is only of minor importance.
The importance of aerodynamic wheels increases dramatically.
Thus, aerodynamic wheels deliver their greatest advantage
where speeds are high. That is the case in time trials, but
also when the peleton is close together on flat sections of
the race.
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