Are the vents in your helmet keeping you as cool as they could? Perhaps not.
That's the conclusion of a study into helmet ventilation being reported by the Bikebiz website. The study, 'Heat transfer variations of bicycle helmets', published in the Journal of Sports Sciences, found that the ability of helmets to cool the head varied by as much as 30 percent, and that more and larger vents aren't better if they are not designed correctly.
The study involved mounting 24 helmets on a thermal headform in a climate-regulated wind tunnel. The paper's authors write, "The influence of design details such as channel length and vent placement was studied, as well as the impact of hair." They found, "a negative role of some vents for forced convection," meaning that some helmet makers are installing vents that are actually counter-productive.
Get a hair-cut, ya hippie!
A far bigger problem, though, is hair. Adding hair to the headform, "reduces the heat transfer by approximately a factor of 8 in the scalp region and up to one-third for the rest of the head for a selection of the best ventilated helmets."
Some helmet makers do emphasise that they perform research with methods similar to this paper's. A common claim is that a properly shaped helmet back can influence cooling by 'sucking' air through the vents and internal channels. The study agrees, saying, "the rear of the helmet can be important for overall ventilation performance."
The paper concludes, "intuitively important factors such as vent cross-section or exposed scalp surface are often limited in their impact by other parameters… The wide variation in ventilation performance… serves to emphasize the lack of systematic understanding of the principles behind bicycle helmet ventilation.
"A key result of the present study is the negative role of at least some of the vents in the cooling power of several helmets, suggesting that helmet structure could in many cases be better optimized for protection and ventilation simultaneously."