Windtunnel
A few years ago we spend some time in the windtunnel with riders of the Rabobank Professional Cycling team. We went to a low speed wind tunnel in The Netherlands (high speed wind tunnels are used for planes and rockets). In the time available we analyzed everything from positions of riders to forks, bikes, wheels and bottle cages. We also compared different types of helmets and the conclusions were basically: vented helmets were less aerodynamic than any helmet that is not vented. However we also learnt that aero helmets were not always the most aerodynamic option, even though to the human eye they look very aerodynamic. Sometimes the looks were deceiving. Whether a helmet is aero also depends very much on the rider’s style (whether there are lots of head movements) and how the helmet is positioned on the head and how it is positioned relative to the back of the rider. Overall though, the aero helmets appeared more aerodynamic than the vented helmets.   Time savings varied from 2 seconds for a 40km time trial to 60 seconds. It must be noted that these savings are likely to be as high as savings get because most of us will ride at significantly lower speeds in a 40km time trial then these pro cyclists.

How it works
In terms of frontal area the two helmets are actually quite similar.  However, sometimes the frontal area is actually less significant than reducing the parasitic drag behind a moving body. In other words it is more effective to try to get the rear tapered so the air can return to laminar flow than it would be to try to minimize frontal area. So if you're going to put effort into improving the aerodynamics of something like a helmet, adding a fairing is probably the best way to go. Without it, the air forms a pocket of low pressure behind you that literally pulls you back. This is the same pocket of air that allows a wheelsucker to suck! The tail on an aero helmet allows the air to become laminar more quickly. In a windtunnel this can be visualized by using smoke. What you try to avoid is turbulence and what you try to create is fine lines of smoke.  This is why the vents are such a problem! They form little forward facing parachutes, but that they create turbulence around the helmet which disrupts the laminar flow around the helmet and allows that pocket of low pressure to form in the area directly behind the head.

How much difference does it make?
As mentioned before the magnitude of the advantage of an aero helmet depends on the speed you are travelling at. At 50km per hour (30mph), which is a speed that is often used in windtunnels, the advantages can be substantial. However, when the speed is reduced and wind resistance becomes quadratically less important, the benefit of the aero helmet decreases.  This is important because an aero helmet also has disadvantages.

Disadvantage of going aero
Because they don’t allow much air cooling (if any) they can become very hot. Research has convincingly shown that heating up the head has negative effects on performance. Because the head is one of the most important places for heat dissipation, the body core temperature will also be higher and this can also limit performance. It has been demonstrated in scientific studies that when the body temperature approaches 39.5  degrees, the body will start to function less in order to prevent overheating. Performance drops can be significant. If the body fails to regulate the temperature, the results may be heat stroke which, if ignored, can even become life treatening. It is difficult to say who will be more at risk of overheating on the bike: the pro triathlete who rides over 40km/h or the age grouper at 30km/h. Traveling at greater speed will require greater power outputs and substantially more heat production, but it also allows a bit more cooling.

The danger of overheating
No matter what level of athlete you are, on a hot and humid day with little head cooling , body temperatures can rise and this could have a huge impact on performance. So which helmet shall I wear in Hawaii? It may depend on the temperatures on the day. I will pack both helmets in my bike case. If it is a really hot day, I will definitely go for the normal helmet. If for some reasons the temperatures are lower than normal, I will use the aero helmet. I will take both helmets and train with both of them in the week before. Hopefully that will help me with the decision. Based opn expected temperatures and temperatures in most previous year I will choose my Giro Ionos over my LAS crono time trial helmet. If in doubt I will use the normal helmet as the chances of losing a lot of time are greater than the chances of gaining a few minutes. After all, riding with a normal helmet worked quite well for Chrissie Wellington in 2007! She was the only girl in the pro women’s field without  a fancy aero helmet but she set the fastest time on the bike which formed the basis for her impressive win!