Gear Face-Off: Aero Helmet Vs. Aero Wheels

Michelle Valenti/Active.com

Dear VeloNews Training Center,

I have a big time trial coming in a few months. I don't have a huge budget for new equipment, but I'm considering investing in either an aero helmet or aero wheels. Which will help me the most? -Todd

Time trial equipment is certainly expensive, but can afford the rider a quantifiable improvement in race performance. Prioritizing equipment can be a daunting task, especially given the marketing hype surrounding aerodynamics. To help in the decision making, let's look at the scientific studies that have been performed in wind tunnels and real world settings.

Many studies have illustrated the primary resistance that a cyclist must overcome, especially at high speeds, is wind resistance. (Grappe et al., 1997; Kyle and Burke, 1984) Moreover, these same studies have pointed out that the body accounts for the majority of the aerodynamic drag, usually about 70 percent. If the body is the primary source of aerodynamic drag, then making changes to the body position can cause substantial changes in drag (Broker, 2003; Garcia-Lopez et al. 2008; Juekendrup and Martin, 2001).

These same studies have also pointed out that optimizing aerodynamic drag does not necessarily result in optimized metabolic cost and respiratory capacity. So there is a balance between aerodynamics and power generation. All of this leads to the conclusion that the most important use of your money is actually getting positioned properly on your bike.

Since you asked an equipment question, let's delve into that a little more. Wheels and helmets are two distinct ways to change your aerodynamic picture. The body represents the majority of the drag (70 percent) the bike must then be the rest (30 percent).

Aero wheels can reduce the drag of the bike by 49 grams of force (Greenwell et al., 1995). An aero helmet can deliver much more of an aerodynamic advantage. A properly positioned aero helmet affects the drag coming from the body.

The 2007 Sidelko study and the 2008 Chabroux study each showed large variations in drag reduction across all tested yaw angles, in three different helmet positions. A recent paper from MIT (Sidelko, 2007) shows even the worst performing helmet reduced drag by 113 gmf! The best performer reduced drag by 175 gmf.

Interestingly, increasing yaw angles tended to produce even better savings. These kinds of studies are actually making manufacturers look at improving their non-time trial helmets, to see what advantages can be made.

Positioning of the helmet is critical. The 2007 Sidelko study and the 2008 Chabroux study, both showed large variations in drag reduction across all tested yaw angles, in three different helmet positions. Indeed, when the rider is looking down at the front wheel, and the tail of the helmet is straight up (think shark's fin) half of the helmets tested were actually worse than a typical road helmet. This stresses the importance of being properly positioned on your bike.

There are other considerations with helmets, mainly heat transfer (cooling). Many Ironman Kona racers use a standard road helmet due to the hot conditions. As we all know, the metabolic cost of dehydration and over-heating will far outweigh any aerodynamic gain.

Looking at the published literature regarding both helmets and wheels, either can contribute to reducing your aerodynamic drag in a time trial or triathlon. Clearly it seems the best equipment for your dollars comes from an aero helmet, however the best use of your resources is to be properly positioned on your bike.

-Sean

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