What makes an aero helmet aero? Is its long tail or some other factor? Do aero helmets offer a performance advantage over standard road helmets? According to most experts, after getting aerobars and establishing a solid aerodynamic position, the next biggest gain a rider can make is by purchasing an aero helmet. Depending upon the study cited, an aero helmet will provide an approximately one minute savings over 40km; of course, the raw time savings will be greater for slower riders.
John Cobb, aero guru and bicycle designer, has spend a good amount of time in wind tunnels testing all sorts of factors that impact on cycling aerodynamics. Joe Friel summarizes Cobb's findings as follows:
Any aero helmet was ‘faster’ than any road helmet when in the aero position. But here’s the one that blew me away: Aero helmets are more aero when the tail is sticking up in the air (face looking down) than when the tail of the helmet is against the back. I’ve always believed just the opposite as it seems logical. I even wrote a blog on this last year.
This is a critical point, as most riders assume that the purpose of an aero helmet's long, sweeping tail is to smooth airflow transition over the back. Cobb himself notes in his original article that
During these early tests we discovered that the original long-tail helmets that were designed for the '84 Olympics really didn't work too well with aero bars. These helmets were designed for use when riding cowhorn bars with very slack seat positions.In other words, the original design of aero helmets arose prior to the widespread use of aerobars and aggressive steep seat angle bike positions. Cobb continues on by observing:
During these years I kept noticing that the pointy helmets were always faster when the riders would drop their head down and have the tails sticking up in the air. I believed this was because of the way that using aerobars made the air go over the shoulders, but the helmet companies never put much stock in that theory. They were diverting most of their R&D money to the safety side of things, with which I have no quarrel. And they have developed much lighter, cooler and prettier helmets. But they are also much slower helmets.Cobb's testing seems to contradict popular wisdom: If this point of an extended tail on an aero helmet is to manage airflow over the back, then why would sticking the tail of the helmet vertically into the airstream make negligible impact on aerodynamic performance? Clearly, something was amiss.
In later in his article, Friel offers a compelling hypothesis:
The reason why they are more aero when the helmet tail is pointing up has nothing to do with the tail of the helmet; it has to do with the air vents on the front. When the tail of the helmet is against the back and the rider is looking ahead the front air vents create a lot of turbulence which increases drag. When looking down so that the tail is raised the air flows around the helmet more smoothly since the vents aren’t exposed to the wind. So if you tape over the air vents the helmet creates much less drag and you go faster.
What Friel is saying--and what is suggested by Cobb--is that the benefits derived from an aero helmet arise not so much from its shape, but rather from the fact that aero helmets offer a smooth and solid aspect to the wind. The reason why aero helmets were faster with their tails up than when their tails rested flat on their backs is because tilting the tail of the helmet up takes the front vents on the helmet out of play. Accordingly, a smooth, ventless, and relatively round helmet should test faster than an aero helmet with a long tail and lots of vents. (NB: It was later theorized that the long tail of an aero helmet does have a practical aero benefit for riders who place water bottles behind their seats--the tail-up position directs airflow a bit higher over the back, which takes that messy airflow of seat back mounts out of play.)
Note how raising the tail of the helmet down removes the front vents from play.
My own field and aero testing confirms the observations above--that helmet vents and aerodynamics don't work well together and that smooth helmets are better than helmet surfaces with many discontinuities:
When in a good aero position, the results of a full season of testing yielded the following data, arranged from fastest to slowest:
The take aways:
Once you get rid of the vents in a helmet and
make it as low profile as possible, they all are pretty good. Some might be a
bit better for you position, but I don't think that any aero helmet will be dreadful. Those
who extend their neck and drop their head low have a lot more flexibility in
helmet choice than those who sit with their heads high above their shoulders
when in the aero position. If your aero helmet has vents, consider taping them over.