(electronic music)
- Cycling aerodynamics is
much discussed these days,
but sometimes, one gets the
impression, little understood.
In fact, sometimes it's almost as though
the cycling industry is
using the word "aero"
to try to sell us stuff
that might not even make a difference.
We recently made a video, an Alta video,
where I did a little experiment
to measure the difference
in speed with aero
clip-on bars versus drops
and that reminded me just how
complicated the aero topic is.
And, of course, you may
have seen GCN's ill-fated,
or that might even be
ill-informed, attempts to deploy
aerodynamic testing in
a wind tunnel in Milan
to make our commuter bikes faster.
Anyway, we got lots of
comments from you guys
about that video and about
how it wasn't very scientific,
and we know that you are
always right so we thought,
let's get properly stuck
into the nitty gritty,
or maybe we should call that
the "aery fairy" of aerodynamics.
This is GCN's brief guide to aerodynamics.
We're not claiming to cover every detail.
In fact, there are plenty
of experts to do that
if you care to look them
up in peer review journals.
This is just a vague overview
with a few equations and graphs.
So first, who cares about
aerodynamics anyway?
Well, aerodynamics is a drag, literally,
it is a backwards force on
you whenever you're moving
at a constant speed on the flat,
so not accelerating and not up hill.
Aerodynamic drag takes up
about 65% to 85% of your power.
That's most of your power.
The rest of your power goes
on rolling resistance and friction,
and the variation is due to
differences in drag and friction
between individuals and
the different speeds.
Aerodynamic improvements are sometimes
referred to as "free speed"
because you can go faster
without putting out more power
if you can make yourself more aero
which sounds pretty good to me.
I mean, it's a simple win,
get aero, go faster, win.
But the big question is
always how to get more aero?
Aerodynamics is unfortunately
an annoyingly complicated science.
There are lots of rules
of thumb in cycling
that are actually taken as rules often
for how to get more aero and go faster,
and those generalizations might
often work but not always.
What works for one
person, on one position,
or even one set of weather conditions
might not work for someone else.
That's because the drag is
dependent on local airflow
and the flow fields
around a complex shape,
like a human on a bicycle, are
horribly difficult to compute
even before you add in
the movement of your legs,
your cranks, your wheels, the
only definitive way to test
whether a piece of equipment
or a change in position is faster
is to do either a wind tunnel test
or a well-instrumented field
test which to be honest,
is a bit expensive and quite a
lot of hassle for most of us.
So, let's start with our
equation for drag force,
FD, our old friend drag force.
This is actually what we want to reduce
because it's opposing our
motion, so FD, drag force.
CD is your coefficient of drag,
we'll come back to that later.
A is your frontal area.
Rho is your air density, and
U is a relative wind speed.
Relative being important
because if you've got
a massive head wind then
obviously your relative wind speed
is higher than your speed so it's harder.
Similarly, if you have a
massive tail wind, it's easier.
So, how do we go about
reducing the drag force?
Well, let's start on the
right-hand side of the equation.
We could reduce U because
the faster you ride
the more drag will work
against you, and in fact,
it's worse than that
because drag force increases
with the square of your wind
speed, relative wind speed.
It's a nonlinear
relationship so slowing down
will make your drag force much lower,
but that's kind of not the point
of trying to cycle faster, is it,
slowing down to make
your drag force lower.
The point is, that the faster you go,
the more your drag force matters,
so once you get up to
54 kilometers an hour,
drag force is about 90% of the resistance
which is why getting aero
is especially important
not just for time trialists
but also for sprinters.
Actually, it's important for any cyclist
who's going to win a
race or simply go faster.
It just makes more of a
difference in terms of percentage
of your exertion if you're on
your own and you're going fast
because the faster you go
the more drag you have.
So, we could reduce U but
that's really not the point.
That's kind of what I do now
on my commute when I get tired,
and I just slow down.
Next on the right we
have rho, air density.
Now, lower air density means lower drag,
but it's kind of hard to organize
low air density on demand,
and go into altitude, where
the air is less dense,
is a bit inconvenient for most of us.
Not to mention the
effect that altitude has
on your breathing and your
cardiovascular performance
which might mean that
you actually go slower
despite having lower drag,
but that's why many cycling aero records
have been attempted at altitude
because they have lower
drag from air density,
so that's not gonna help us much.
Now we get to frontal area.
Now this is where everyone thinks about
when they want to get more aero,
making themselves smaller in the wind,
because your drag is directly proportional
to the frontal area, so the
smaller your frontal area
the lower the drag; hence,
we all want to get "slammed."
Now, it's worth noting,
that you can reduce
your frontal area in more ways
than just getting lower
and getting slammed.
So aero bars make your elbows narrower
which reduces your frontal area,
and head position makes a huge difference.
It takes a lot of discipline and strength
to keep your neck cricked
into an uncomfortable position
so that your head is low
and you can still see where you're going.
I mean, actually let's face it,
it takes a lot of discipline
and training to stick
to any of these
super-tucked aero positions
as I'm sure Graeme Obree would testify.
Now to add here, that
the dots in the equation,
they mean "multiplied by,"
so that's like a multiplication symbol.
Now, to the drag coefficient,
let's deal with this last,
this mysterious drag coefficient.
I say mysterious because it
really is very hard to predict.
You can kind of guess whether
it's likely to be good or bad,
but for a complex shape,
like a cyclist on a bike,
it's really at best an estimate,
you have to measure it in a
wind tunnel or on the road.
So, what is this drag coefficient anyway?
Well, in simple terms,
it's just a kind of measure
of how streamlined an object is,
and there are two main contributions
to the drag coefficient,
there's skin friction and form drag.
So skin friction can be approximated
to how rough something is,
and to reduce skin friction,
you want a nice, smooth surface
that is slippery to the air
like the super-aero
materials that are used
for high-end time trial
skin suits or that aero gel
that Lotto-Suodal was recently found
to be using in time trials.
The aero improvements from
shaving your legs, and,
in fact, arms if you dare,
trust me, it's horrible,
come from the reduction in skin friction
of smooth versus hairy skin.
Now form drag, on the other hand,
is about the shape of an object,
and that's kinda tricky
both to know what's best
and then to make it happen
at least as a cyclist on a bike.
So a teardrop shape is
roughly what we're aiming for,
the smoother you can make
your body shape the better,
so if you can fill in the
gaps, close up the front
and rear ends, add a
fairing, if you're allowed,
behind your bum, anything
that makes your shape smoother
and more like a teardrop and
avoids loose flappy parts
that increase turbulence.
That will help.
The really tricky thing is that sometimes
frontal area and coefficient of drag
work against each other.
Not literally, but what
I mean is that sometimes
when you put all this effort
in to getting lower and lower
with a smaller frontal area,
your coefficient of drag
could actually be higher
because your body has formed
a less smooth aerodynamic shape
and then your drag is higher
even though you're actually lower
because your frontal area is lower.
So, you've put all this
effort into getting small,
but you're not getting any faster.
That's really annoying.
So, in conclusion, the frontal area
and the coefficient of drag are the things
you can reduce to go faster for free.
It's worth concentrating on getting
the big things right first,
making yourself small,
but not so small that you
can't pedal, but, for example,
keeping your elbows tucked
in makes a difference
and doesn't cost any energy.
You can use smooth, tight fabrics,
if you feel happy about your body shape
and try to make your
position and your bike
a smooth shape by filling in gaps.
Oh, and work on holding your head low.
It's probably the biggest
factor in both frontal area
and coefficient of drag.
Well, I hope my aerodynamics
lecturer from uni
isn't watching because I might
have got something wrong,
but I hope you find this useful.
Why not check out, Simon
and I doing an experiment
in the wind tunnel in
Milan, clicking down here,
or you can see my clip-on aero
bars versus drops experiment
in Alta Video down here.
