Wednesday, October 2, 2013

Heat Consequences of Aero Road Helmets

By: Zack Allison, Senior Consultant

While sitting on the couch between races at Gateway Cup, the Think Finance p/b Trek Stores Cycling Team and two Source Endurance coaches jumped into  a discussion on aero road helmets. The weather for this particular race weekend in Saint Louis was an average of around 100F at each of the races. The major question was if the aerodynamic benefit of aero helmets outweighs the power loss from overheating on a hot day.  We decided to try and find studies on the subject of whether aero helmets cause power loss due to overheating. We knew it would be a task to find applicable studies on overheating from a specific type of helmet but we did our best.

Good Aero Position
Bad Aero Position
First we looked at the aerodynamic benefit of aero helmets. The study Benchmark of Aerodynamic Cycling Helmets Using a Refined Wind Tunnel Test Protocol for Helmet Drag Research by Stephanie Sidelko of MIT , compared 10 different aero helmets to a standard road helmet on aerodynamic efficiency. Using different angles of yaw, or different directions of wind, they set out to determine just how aerodynamic these helmets were. At the MIT Wright Brother Wind Tunnel, scientists measured the drag of these 10 aero helmets at 30mph. Another variable added to the study, increasing its relevance, is different angles of how helmets sit on a rider’s head. We all have seen an aero helmet on a rider; a good helmet position has the tip of the helmet touching or very close to the rider’s back and a bad position has the rider’s helmet pointing straight up in the air. The scientists took this into account by moving the helmet on the mannequin’s head to mimic these positions and also comparing these values to the normal road helmet.

The fact that some riders put their heads down and many riders have bad aero helmet positions is a big reason why aero helmets are losing their tails. Many of the newest designs in aero helmets have a chopped tail or no tail at all. Road aero helmets are, in many cases, more aerodynamic than an aero helmet with a bad fit.

The results of the study showed that every aero helmet, in position 1 and 2 on the rider’s head as represented in bar graph form in “figure 6 below”  was more aero dynamic than a standard road helmet. At position 3 on the rider’s head, which is basically with the tail of the helmet pointed straight up, most aero helmets were still faster than a standard road helmet but by less of a margin. It is difficult to transfer these drag numbers into wattages as every rider has different wattage numbers at threshold and different positions. An aero helmet is up to 8% reduction in drag at 30mph so for a rider with a threshold of 425 watts, this comes out to around 30 watts of power under these specific conditions according to this study. Each helmet tested in this study had different drag characteristics based on its shape indicating that, as the aero helmet position changes, there is more drag associated with aero helmets with larger tails. The take home point, however, is that in the correct position, an aero helmet is much more efficient than a standard road helmet.    

"Figure 6"
Now that we know aero helmets are faster than standard road helmets we can try to find out if these helmets are creating an amount of extra heat that is hindering our performance.  For that we have to find a new study. Looking for information on heat dissipation of standard road helmets vs. aero road helmets we found the study AERODYNAMIC EFFICIENCY AND THERMAL COMFORT OF BICYCLE HELMETS to be the most relevant.  “Firoz Alam, Simon Watkins, Aliakbar Akbarzadeh and Aleksandar Subic” This study had nearly the same protocol as the aerodynamic study mentioned above except that this study was conducted in a wind tunnel, used various road helmets and put a heating element on the mannequin’s head. The heating element was heated to 140 degrees Fahrenheit and then, while the wind in the tunnel was blowing at controlled speeds, the sensors on the heating element would read temperatures
telling us which helmet had the best ventilation. This study also compared aerodynamic factors of the helmets it tested using different pitches of air. The pitches of air take into account the 2 things: 1. different helmet positions on the rider’s head, as we have seen in the previous study and 2. the relevance of vent placement on the helmet itself, which we will see later is key in keeping a rider cool with an aero helmet.

The results of this study on how heat is dissipated in different levels of road helmets showed that the more aerodynamic the helmet, the less heat was dissipated. In other words ventilation comes at the price of heat dissipation. The helmets that were most aero in the study were actually multi-use helmets that looked very close to the Air Attack by Giro. One of the helmets with vents un-taped is seen on the right. The vents were taped in the study.

 The results of the study showed that the helmets with the worst aerodynamic performance were the most ventilated and looked close to modern helmets of today’s professional peloton. We don’t know the temperature of the wind tunnel air and the 140 degrees of heat that they started with in the helmet is not really close to temperatures in your helmet while on the starting line of a hot Midwest Criterium but we can use it to see just how much heat we are retaining in our aero road helmets compared to our super light, highly ventilated helmets. The most ventilated helmet at a 0 yaw and 0 degree pitch angle of air, or a straight head wind, went from 140 degrees to 75 degrees. This is a 65 degree difference. The most aero helmet with vents taped is significantly more aero than the most ventilated helmet. The temperature inside this helmet dropped from 140 to 124, only a 16 degree drop in temperature in the wind tunnel.

This tells us that the vents work on helmets to cool your head and that there IS in fact a very real danger of overheating from use of an aero road helmet in hot races. There are other factors in the study that we have to take into account before we try to “estimate” at temperature at which an aero road helmet starts to hinder performance.

   What was interesting is that, using different pitches of air and focusing air on specific vents, the study could tell us what types of vents worked on the helmets they tested and what vents did very little in dissipating heat. You can have a helmet with huge vents that does not dissipate heat very well and you can have a helmet with very small, aero, strategically placed vents that dissipate heat very efficiently. The vents on the Air attack and Specialized Evade aero road helmets, as seen here, seem to be small but could be more effective than some larger vents. It’s important to remember that the temperatures used in the latter study are not skin temperature measurements;  the experiment used a heating element and measured drops in heat to see which of the tested helmets had the best ventilation. Also the vents on the most aero helmet in the study were taped.  I’ve never seen anyone tape the vents on a Giro Air Attack but that could drastically change the ventilation and aerodynamic numbers.

Specialized Evade, notice innovative vents and placement
Another major point of the study AERODYNAMIC EFFICIENCY AND THERMAL COMFORT OF BICYCLE HELMETS is that placements of vents are crucial to how they work. The study used different pitches and angles of air to see effects on temperature. Some helmets performed better at different pitches of air, showing us that vents work in different ways. If a vent works best to dissipate heat at 90 degrees of pitch, or straight down, then it’s likely not very useful as you will never really get that air flow unless you are riding with your head all the way down (not recommended). Some helmets have vents that work best at a level air flow. If an aero helmet can utilize smaller vents to reduce drag by making those vents in ideal places and sizes for ventilation then it will be a superior helmet.

These studies work on a fundamental level for us. We Can compare the findings in these studies to helmets currently on the market, how aero dynamic they are, and how well they dissipate heat. The actual temperature changes would be per individual and per helmet and would vary depending on air density, humidity, wind speed, and direction, color of the helmet, and acclimatization of the rider. Fundamentally what we can take away from this research is that aero road helmets are significantly more aerodynamic than standard road helmets. They WILL save you watts. Aero helmets are also significantly warmer than heavily ventilated road helmets. If your event is already dependent on keeping cool then a performance enhancement made by wearing an aero road helmet will have heat consequences. The Source Endurance team gave a rough estimation of 75-90F degrees for when we would trade an aero road helmet for a more ventilated one. Again this would depend on speed of the event and a dozen other factors on race day.  In most situations, where your event is not blistering hot, an aero road helmet will be more efficient.