viernes, 22 de mayo de 2015

Racing Helmets: Spiuk road vs. aero (I)

Go to index

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  and with the some further analysis by Diego Calderón y Alejandro Martínez ( y ) 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:

When you open the box that contains the Dharma with a textile bag you have no doubt that its look is astonishing. It is even lighter than it claims (220 grams), with a small descending tail that helps the cooling effect.

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:

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.

Don't forget that everytime that you click on the advert / banner o the top of the page Google Adds will pay me a small amount of money to help me to maintain this website (less than 0.1 USD).

No hay comentarios:

Publicar un comentario