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Is having a bare head more aerodynamic than an aero road helmet?

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    I didn't really think of much more to add. It's a difficult question to Google for as questions of safety seem to bury anything else. My prediction is that helmets increase the frontal surface area of your head so much that any improved shape would be fairly marginal, thus in an upright position they negatively affect aerodynamics. However, I would not be surprised if aerodynamics is actually improved if in a down-low aero position where your head is silhouetted by your shoulders and frontal area isn't increased much. – Rol Sep 8 '20 at 15:38
  • Do you have hair? And what about facial hair? – Mołot Sep 8 '20 at 15:44
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    Round here, helmets are mandatory, so not wearing one may get you pulled up by the Police. That's not good for your total commute time! – Criggie Sep 8 '20 at 19:53
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    @Rollo edit your post to include your first comment. Not everyone reads all the comments, and that stands as good background for your question - it shows you've done some research. – FreeMan Sep 9 '20 at 13:27
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    @Criggie, there are still countries in the world where helmets are not mandatory for all cycling and some people chose to wear them in commuting while other, same speed, chose not to wear them. – Willeke Sep 9 '20 at 13:33
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I did not do exhaustive literature review, but the first hit, Aerodynamics of ribbed bicycle racing helmets has wind resistance measurements for several different aero helmets with vented helmet and bare head as controls. Bare head had similar air resistance to aero helmets at low yaw angles and the lowest resistance at large angles. The paper's conclusion is that for helmets (or bare head) that do not have huge number of vents, cross sectional area has more effect than the shape of the head or helmet.

This is of course purely academic, since competition outside sanctioned events doesn't count and in official competitions (excluding special cases such as hill climb time trials) you are required to wear a helmet anyway.

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    Actually, hill climb TTs in the UK may permit you to go without a helmet (the race is all uphill, after all). They were debating rescinding this policy late last year, but I'm not sure of the results. But this is a niche event anyway. Also, when the paper said "bare head", did they mean a helmetless head, or a helmetless shaved head? The difference may be material. – Weiwen Ng Sep 8 '20 at 19:20
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    Good points. I'd expect wind resistance doesn't have much effect in hill climb TTs, but the better cooling from not wearing helmet might have. The paper says "bare head" and photos show a mannequin wearing helmet, without hair visible. I'd expect hair bunched under tight fitting cycling cap to be quite close to shaved head. – ojs Sep 8 '20 at 19:27
  • I didn't notice the permalink to paper when I wrote the answer, but I added it now. – ojs Sep 8 '20 at 19:30
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    Interesting. Greg Lemond won the 1989 Tour de France by a handful of seconds, using an aero helmet and (barely functional) aerobars. His nemesis, Fignon, let his long dude hair flap in the wind. He later complained about the bars, but the helmet could as easily have been the difference. Video of their setups: youtu.be/rWyfb3H7LEg – WPNoviceCoder Sep 9 '20 at 19:24
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    @Zloj One has to be bold to join a grand tour anyway. – Vladimir F Sep 10 '20 at 7:54
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According to Fast Fitness Tips, fully streamlined aero helmets are the only helmets that are more aerodynamic than a bald head.

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Is having a bare head more aerodynamic than an aero road helmet?

Try a simple test.

Put a helmet on your head on a very windy day and ride on a road where the wind is at 90 degree angle compared to your direction of motion.

Hear the wind noise in your ears?

Remove the helmet.

Hear the lack of wind noise in your ears?

I don't know if a helmet is more or less aerodynamic when riding in the direction of wind or 180 degrees to the direction of wind.

I do know that a helmet is very, very inaerodynamic if the direction of the wind is not towards or opposite to your direction of motion.

That's what counts. The wind can have any angle, not just in your direction of motion or opposite to your direction of motion.

I bet that in a real circuit where the direction of motion varies, the helmet will on average do more harm than good.

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    Did you try the test? The wind noise comes from air flowing past your ears, and the helmet doesn't really affect it. On the other hand, turning your head by >45 degrees moves the ears away from strongest airflow and reduces the noise. No relation to wind resistance at all. – ojs Sep 8 '20 at 21:16

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