Bell launch ‘fastest’ aero helmet: Star Pro + video

well said. Its also worth remembering that whilst the club scene generally are pro helmet. The CTT, one of the longest standing cycling organisations running time trials, have no rule that makes helmets compulsory. (for adults)
That said they do have a rule that says arms must be covered...

Well I just recently arrived at the start line in a local club ten TT having left my (non aero) helmet plus goggles in the car boot. At seventy six I blame onset of short term memory loss, however I went 29 seconds quicker on a windier evening than the previous week. My moderate time was impressive only to me as I pondered the marginal gains of riding with a bald head versus helmet, wearing a helmet is not something I do as a general rule, but do so in TT's to placate the pro helmet brigade.

FJM1002: The human head seems pretty aerodynamic to me in shape. At least, not significantly worse than road helmets. It's hard to find studies comparing helmets to headforms (most are helmets against helmets). Apparently Kyle Chester did research on this, but I can't find relevant ones online. Sheldon Brown's site has a report on his results though:

http://www.sheldonbrown.com/rinard/aero/aerodynamics.htm

A pure aerodynamics (i.e. pre-helmet-rules, no need to care about padding, very thin) TT helmet gives a 2% improvement over a bald headform. An ANSI approved TT helmet (old Bell Stratos) *increases* the aerodynamic drag by 1.3%, compared to a bald head. This means the human head (bald at least) lies somewhere between an ultra-thin, pre-helmet-rule TT helmet and a post-rule TT helmet. This means the (bald) human head is actually pretty damn aerodynamic.

The problem is the drag equation, the force of the drag F = ½.ρ.v².Cd.A. The padding and foam required to meet crash tests increases A - by a lot. For a road helmet, my back of the envelope calculations suggest upwards of 20%. So, just to break even, the helmet needs to *reduce* the Cd by proportionally the same amount - by 16% (i.e. by 1/1.2). That's a big ask.

Now, hair is very unaerodynamic, and according to the report on Kyle Chester's results, the ANSI approved TT helmet has less drag than a head with hair. However, like I said, that's what a cycling cap is for. Next, road helmets are always going to be worse than pure TT helmets. Even aero road helmets. It's just an inescapable fact of the need to vent heat.

So to summarise:

- It seems the (bald) human head compares quite favourably to TT helmets, even the now "illegal" 'pure' aero helmets. Though, also, there's not much data out there.

- Hair is unaero. A decently fitting cap should fix that and get you most of the way back to the aero performance of a bald head, I would contend. The cycling cap also costs just £3, rather than £200+.

- Venting in helmets is unaero (lots of helmet to helmet comparison studies out there showing this), however for any helmet you'd want to wear for longer than a TT, you tend to want venting..

midschool: I didn't comment on safety aspects, only on aero performance. Note that the manufacturer in this article isn't marketing this helmet for its safety, only on aero performance. Indeed, it's a rare helmet manufacturer who makes any strong claims about safety (ever wonder why?).

A cycling cap is even more aero, at a tiny fraction of the cost. Plus it helps wick sweat out to the moving air where it evaporates faster, and helps keep sweat out of your eyes.

Cycling caps: Cheap watts.