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As you
already may know I like to investigate as much as possible in everything I do,
may be because I am an engineer. So this time what I am going to do is an
aerodynamic test with two types of bicycle helmets which are designed to be
used in cycling and triathlon.
Some months
ago I went to Unibike cycling trade fair and I wrote to entries in my blog, one
of them specifically devoted to two Spanish brands: Spiuk and Catlike. Now that
we have sunny days and a good weather for cycling I have decided to try two Spiuk
new racing helmets: the time trial Aizea and the road Dharma.
This new
entry doesn’t intend to be a general view of the helmets as there are many webs
and magazines that have already done that, you just have to google a bit and
you will find much more information and much better quality pictures of them.
The idea is to write 3 entries, one of them describing my first thoughts and
how I am going to test them, the second one a velodrome test and the third one
an open road test.
What results
do we expect?
As this
picture shows, the Aizea fits lower than the Dharma (4cm) and there is an
obvious difference in the total frontal area of the cyclist: 0.3491 m2
vs. 0.3580 m2, that means a difference of 2.5% in the total area. As
you may also see the Dharma is much more ventilated, with those two issues we
expect that the Dharma is less aerodynamic than the Aizea.
There are
also some details in the next picture, because the sudden change in the back of
the road helmet creates a turbulence zone and a low pressure behind the head
that is great for head cooling but it increases the air resistance. Even more,
the windshield of the Aizea helps to avoid the transition between the forehead
and the helmet reducing the drag.
Therefore
we expect that the Cd (drag coefficient) and the CdA (area x drag coefficient)
are greater in the road helmet than the Aizea. However, to give a general idea
of the benefits of the Dharma helmet in the future I will compare the Dharma
against my old road helmet, with and without the winter protection.
According
to the MIT http://dspace.mit.edu/handle/1721.1/40486 and with the some further analysis by Diego
Calderón y Alejandro Martínez ( https://wallace78tria.wordpress.com/
y http://www.amtriathlon.com/2006/11/cascos-aerodinmicos-john-cobb-es-uno.html
) an aero helmet with its tail perfectly attached has a drag a 25% vs a road
helmet, if the tail is horizontal it is a 40-45% and completely vertical tail
is more or less the same as a road helmet. This means an approximate reduction
of 10-14W in the first case and a 8-12W in the horizontal case at 36 km/h.
Unboxing
the Dharma:
It has 20
ventilation holes that guide the air through the helmet but not hitting the
head; they are also cleverly positioned to avoid direct sunlight over the head.
The rear fitting system is really small and very comfortable but a bit tricky
to use, you must press the wheel against your head to adjust the length of the
straps.
It also
comes with the typical solar shield for MTB, extra cushioning pads, an optional
bug stopper net and an aerodynamic wind shield very useful in winter. This wind
shield is very clever because only covers the entrance holes but not the exit
ones, therefore it can cool your head when you are going uphill but it won’t
chill your head when going downhill.
This helmet
is the one that I used in the Titan Desert 2015 where having a cool head it’s
very important, but avoiding to sunburn my head.
Unboxing
the Aizea:
It is said
that first impressions are the 95% of the sales and with this new aero helmet
Spiuk has improved a lot from the old Kronos. Now is an in-mould helmet instead
of a helmet with a soft aero shell as the old Giro or the BBB. This helmet
covers your ears, has a removable windshield, two tails to choose and you can
choose your inclination by adjusting the straps. The final weight with the most
aerodynamic options is 470 grams, 10 grams lower than is said on the web.
Is a helmet
made to be used with the windshield, the ventilation closed and the long tail
option to increase the aerodynamic benefit. However as in long distance
triathlon is very difficult to ride on a very extreme position some riders will
prefer to choose a bit more comfortable option.
The optional
short tail helps those riders who tend to look downwards while riding, for
example when keeping an eye on the gps or for those who usually suffer from a
sore neck after the swim. What it does is to reduce the frontal area when
looking to the ground but keeping a good aerodynamic surface as it hasn’t many
surface transitions or ventilation openings. Therefore it is still better than
a road helmet.
The front
screen attached with magnets can be easily removed if we want to use our
glasses or if we want to increase the ventilation. However it doesn’t provide a
great vision as having a rounded shape makes it a bit difficult to have a
really true image deforming sometimes what you see. Even though I rather prefer
to use it that not using it as this issue is not too problematic. May be in a
very hot and hilly Ironman like Lanzarote or Enbrunman where you have many
kilometers riding in a very low speed I would prefer not to use the windshield.
Like the
Dharma, the possibility to change the whole helmet inclination by changing the
straps fitting helps to adopt a better helmet position when you ride on the
bars. However if we are riding an UCI time trial legal bike with the bars very
close to the knees we can have a very bent back and we may hit our back,
therefore we can readjust the straps to avoid that issue.
Another
last thing that I think that must be improved is tail replacement system, I
think it isn’t a good idea to be done that way. You feel as you are breaking
the helmet.
Test
description
My
intention is to try both helmets to show a time difference with the same power.
I will also evaluate the temperature evolution against the outside temperature
under the helmet in one of the ventilation channels.
With those
data I will help you to choose the best option to race and the advantages and
disadvantages of them. I will also try to test the temperature evolution and
the aerodynamic drag in the Dharma wearing or not the windshield.
I will do
two different tests, one in an open velodrome and other in a road to simulate race
conditions. At the velodrome I won’t use a gps for tracking the speed because
its precision is very low under this type of track, therefore I will use the
wheel circumference and a speedometer. However
in the other test, with 8.5km in each way of the road using the wheel I will.
My tools
will be my time trial bike with a powertap wheel to measure the power directly
in the hub, because if I use another device I won’t be able to see the chain
power losses. My computer will be a Garmin Edge 1000 and for temperature
control I will use a kit made by Arduino with some temperature gauges with a
precision of 0.1ºC. One of them will be placed under the horizontal tube of the
bike and the other one under the helmet.
My
calibration process will be:
- - For
the Powertap I will check once the torque precision by putting a loose weight
in the crank with 20 kg, I expect to have a torque of 34.3 Nm because of the length
of the crank. Then I will do a “set to
zero” before data recording.
- - To
calibrate the speed in the velodrome I will do a mark on the ground and then
travel as straight as possible for 5 complete wheel turns, then I will measure
the length and divide it by 5. I will do
it at least two times to check the precision.
-
- To
calibrate the temperature I will use the Garmin Edge 1000 to check the
temperature in a room and in a fridge.
Model and simplifications
The test
will be valid for me in riding on my time trial bike non UCI standard (the seat
tube angle is 80º and UCI allows a maximum of 72º). However this doesn’t mean
that the results aren’t valid, this means that other exact values will be
obtained for another rider.
As it is
very difficult to measure the power at a constant speed I will try to do it in
a small range of speeds, therefore I will have to discard some tests that the
speed varies a lot.
Note: if
you would like to see more information with the equations related to the forces
and power involved in bike riding you can check: http://lustaufzukunft.de/pivit/aero/formulas.html
The
equations that evaluate the power related to speed in a bike are:
Power in
the hub = Prolling + Paero + Pslope + Pacceleration
Power = V x
F
V =
velocity = speed
F = force
or drag
As I am
going to test it at a mostly constant speed, the initial speed is going to be
the same as the final speed and the time interval used to do the test will be
big enough to reduce the total acceleration and braking I can assume that the
Power lost in acceleration is 0. Even more, as the test will always be done in
a circular track there won’t be any height variation, so I can assume that the
power lost in changing the potential energy will be also 0. This is my new
power equation:
Power in
the hub = Prolling + Paero
As I am not
going to have a puncture in my test, vary my weight, change my position, have
significant changes in air density (constant temperature, humidity and
pressure), I can assume that:
Power in
the hub = A x V + B x Vwind2 x V
Where A is
a constant = m x crr x g = (total mass: rider + bike + equipment) x rolling
coefficient x gravity acceleration
Where B is
a constant = 0.5 x rho x Cd x Area ; rho is the air density, Cd is the drag
resistance coefficient and Area is the frontal area opposed to the movement. Cd
x A is usually known as CdA directly.
Where Vwind
= wind speed perceived by the rider = Vreal + V = real wind speed in the riding
axis + bike speed related to the ground.
Even more,
if the wind is lower than 4 km/h at 1.5m over the ground we can assume that the
wind is zero with an error lower than a 0.5% and if it is lower than 2 km/h the
error is less than a 0.1%. For the velodrome test we can directly assume that
the wind speed will be 0.
Therefore,
for the velodrome we can have this equation:
Power in
the hub = Avelodrome x V + B x V3.
Where
Avelodrome is related to a rolling coefficient at the velodrome.
With all
these equations we can now test the Dharma and the Aizea and obtain the
differences in both of them. It is important to note that the A coefficient
will be mostly constant when swapping helmets and the B coefficient is the one
that is going to change. Knowing those two coefficients will allow me to know
the Crr (depends also on the riding surface) and the CdA of both helmets.
Why are so
important these curves?
The power
to speed curve allows me to know the benefits in using one or the other helmet.
Knowing these curves will help me to obtain the power that I need to ride at an
exact speed.
This means
that we can know how fast we can ride and therefore we can predict our
finishing times on many races. Basically we can compare the time reduction
thanks to using one helmet.
To help it
to be a bit clearer I will calculate the predicted times for half ironman,
ironman and Olympic distance triathlons.
Test
method: mainlines
The idea is
to do some series of turns in the velodrome, which will last at least 3 minutes
per speed. I won’t change the gear
through the test or the position, I will try to maintain a constant cadence and
I will start and end at the same speed. To avoid changes in the CdA I will ride at all
the speeds without stopping or drinking water, because if I stop it will be difficult
to achieve the exact same riding position.
I will
check with an anemometer the wind speed in three parts of the road to correct
the equations. If we have a wind over 4 km/h in the center of the velodrome I
will have to cancel the test because I will have many parts of the track with changing
winds, resulting in a wrong testing procedure.
At least I
will do the test for each helmet at 3 different speeds, 30, 35 and 38 km/h, as
the power changes with the power of 3. All the tests will be run the same day
to avoid changes in weather conditions.
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