GEAR » WHEELS
f you have done the training and
have perfected your position
on the bike to be the most
aerodynamic it can be while still being
able to get the maximum power that your
training has produced through the pedals
– then you are ready to start looking at
making your bike as fast as possible. The
first place to do this is in the wheels.
If your goal is to start dropping weight
on your bike, rotational weight is worth
more. Grams in the wheels, pedals and
cranks have more value than grams
in the saddle, handlebars or frame. In
fact, if a frame is made lighter at the
expense of rigidity and performance,
you would rather it be heavier. Wheels
have an additional factor in that their
aerodynamics plays a bigger role in the
overall slipperiness of the bike. More than
any other component. Yes, much more
than the frame.
I
IT'S ALL ABOUT THE AERO
There are a number of considerations
when choosing a fast set of wheels. We are
assuming that we are talking about
wheels for a non-drafting cycle
leg in a triathlon. So essentially
a time trial. Straightaway we
acknowledge that aerodynamics
is more important than weight.
If we were doing a draft-legal
event, or a cycling bunch race, we would
favour weight more because we would be
constantly accelerating and decelerating
throughout the race. The weight of the
wheels, and specifically the rims, play an
important role in how easy that will be.
Generally though, deep-rimmed carbon
wheels will not be heavy so weight should
not really be a big concern.
Next we need to decide on the front
and back combination. The most
aerodynamic combination is a disc wheel
at the back and a deep-rimmed wheel on
the front. Firstly we need to dispel some
myths about disc wheels. It is simply
aerodynamics that makes them faster. A
disc wheel does not drive the bike along
or provide momentum as many seem
to think. In the early days of disc wheels
some manufacturers experimented with
weights inside the wheel that were thrown
outwards towards the rim once the wheel
was turning in the hope it would provide
momentum. So once the wheel was up
to speed, it would help to maintain that
speed. This turned out not to be the case,
however, so now all carbon disc wheels
are simply trying to cheat the drag and
turbulence created by revolving spokes.
DISC FACTORS
There are completely flat discs which,
while being a bit more slippery, are more
of a handful in cross winds. A lens-
shaped disc wheel has a slightly bigger
frontal area but is more stable and more
aerodynamic in crosswinds of varying
degrees. In real-world conditions, a disc
is normally never a problem as far as wind
goes though. It is bolted firmly into the
back of the bike and most of our weight
is holding it down. So apart from in
truly extreme conditions, a disc wheel is
always rideable.
One factor to consider is that disc
wheels with solid cores are very, very rigid
and unforgiving. So on rough surfaces
the road vibration is unfiltered up to the
rider and this can contribute towards
overall fatigue, far outweighing the
aerodynamic benefits. On rough roads
it is better to go for a deep rimmed but
than in an enclosed velodrome. Any
turbulence felt on the front wheel affects
the handling and the way the bike feels
and responds to your commands.
Riding a very deep-rimmed front wheel
in extremely windy conditions can be
harrowing. Especially in the aero position
with your elbows doing the steering.
Now front wheels vary in depth from
30mm to 100m. The deeper the rim,
the shorter the spokes and the more
aerodynamic the wheel will be. The deeper
the rim, the more surface area there is for
Njord to get his hands on and pluck you
from the direction you are trying to travel
in. Just like the shape of a disc wheel
affects it’s handling, different rim designs
also make a difference in the stability of
the front wheel. More rounded, aerofoil-
shaped designs handle crosswinds much
better than flatter shapes.
Current wind-tunnel testing also
suggests that there is such a marginal
aerodynamic difference between a 90mm
and a 50mm rim depth, that the shallower
rim should always be the first choice
because the energy cost of fighting
the bike is far less than with the
deeper rim in all but the stillest
wind conditions. Let’s be honest,
when have you ever done a ride
with absolutely no wind at all?
THE MOST AERO COMBINATION
IS A DISC WHEEL AT THE BACK
AND A DEEP RIM ON THE FRONT
spoked wheel, where the spokes will
absorb a lot of the road noise.
On the back we can use a 90-100mm
deep rim without any consequences to the
handling of the bike. There is still the hole
in the middle for the worst crosswinds to
blow through and we have the spokes to
make the ride less harsh. Most time trial
frames nowadays do a great job of hiding
the back wheel from the wind anyway so,
without looking up any data from wind-
tunnel testing, I would wager that there is
a very small aerodynamic disadvantage
between a deep-rimmed back wheel
and a full disc when slotted into a heavily
fairinged time trial frame.
AT THE FRONT
The front wheel is where things become
more tricky. The front wheel is the
vanguard of your assault against the
wind. It is the first thing the air has to
move around and is therefore the most
important aerodynamically. Ideally we
would want a disc on there, but that
would be a disaster anywhere other
IT'S WORTH IT
Before we finish, let’s get back to
some mythbusting. It has always been
theorised that it is not worth buying
aerodynamic wheels unless you are able
to average a certain speed. Firstly, unless
your route is completely pancake-flat,
there are going to be periods of higher
speed descending. So even if your overall
average for the ride is less than 30kph, I
am sure you will spend some time going
over 40kph. Another, connected myth,
is that there is only an aerodynamic
advantage at high speeds. Yes, that is
true if you consider 20kph a high speed.
As soon as we start moving and
having to push the air aside to do that,
paying attention to our aerodynamics
will help us. Wind-tunnel research has
demonstrated that pro cyclists climbing
a mountain like Alpe d’Huez at 20kph still
benefit from aero wheels, and even sitting
on another rider’s wheel, which brings
us to another point – we are still drafting
if we are closer than 10m on a climb. But
that is a topic for another article…
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