mschwett 

trigger warning : discussion of marginal gains to follow

i have two helmets - a giro syntax and a specialized s-works evade. the evade is newer and lighter, but is much larger when viewed from the top. it fits much looser as well, requiring more tightening of the clicker. when i measure my head size, i'm right in the overlap between the M and L but the M felt too tight, like i could barely get it far enough down. riding, though, the largeness of the evade is noticeably annoying, and i keep moving it up so that my sightline when i'm head down-ish is clear.

the evade is supposedly a much more aero helmet... but i'm wondering if it's actually better or worse for the helmet to be larger than necessary. obviously the drag of the head/helmet is less with smaller frontal area and the same cD, but wouldn't the general principle of streamlining suggest that a huge helmet with a smoothly curved frontal surface and long trailing edges actually be better, since otherwise that same air would be hitting the much-less-aero shoulders, chest, and back of the rider?

any interesting recommendations on lightweight and aero helmets that offer more or different size increments than the evade, which is S-M-L? they list M as 55-59cm, and L as 58-62. my badly done head measurement was around 58, but as noted the M didn't seem to fit. the syntax M is also 55-59, with the L 59-32, but it does seem to have a more round shape.

beng1

A very well known pro-racer and cycling coach, who has reviewed much scientific research on cycling and who has also done time testing cycling in a wind-tunnel, said that different cyclists will need different helmets to achieve their lowest drag, due to different body types and riding position and abilities etc..

So you will probably not find a good answer to your question unless you pay a pro coach/fitter for time with you in a wind-tunnel or with some other testing method.

If you are unable to do that then just Google info about helmets until you find articles reviewing the aero-drag of different helmets and take the best shot you can. I got around all this by mostly riding without a helmet at all, and when I do I either use a Tony Hawk Skateboarding helmet, or a scuffed Specialized helmet I pulled out of a trash-dumpster that just happened to be my size. Done and done..............

RChung

I feel your pain. There's skin friction drag and form drag, and while a larger helmet (in cross section) has more form drag, it *can* have lower friction drag. There are interactions between your shoulders, back, and the helmet, and also the position of your helmet.

The bottom line is that if you really absolutely have to know (and not everyone does) you have to test.

If only there were some way to do those tests on your own.

mschwett 

i attempted some A B B A testing on the cycle track (of tire pressure) once, and the results were just … scattered. holding average power within a watt or two i got results that were 3-5% different from run to run, indicating that either 1) wind is a fickle, inconvenient beast (it’s always windy here), 2) i’m crap at holding a consistent position, or 3) both.

RChung

Holding constant power (or constant speed) is pretty hard and finding a wind-sheltered traffic-free place to do it increases the pain in the rear factor. If you can find a wind-sheltered place that's safe, there are protocols you can use that don't require flat ground or constant power, so that's a benefit. Plus, the precision is often better.

mschwett 

i can hold constant power pretty well. but i have never found a wind sheltered place to ride. if i did i’d ride there a lot 😂😂

the cycle track is an oval, of course, but the wind does not seem to cancel out - on windy days i’m slower for the same power, period, with little gain in the crosswind directions and slightly less increase downwind than the slowing upwind.

RChung

Yeah, wind doesn't cancel out easily on an oval. Which track is this? Anyway, this is why sometimes I use a straight out and back, and I test early in the morning. The straight out and back can sometimes make it easier to spot the wind effect. I sometimes think Dogpatch or the east side of Potrero Hill early in the morning might work, or maybe JFK Drive in the Park on a foggy morning. But there no reason for me to test in SF: I can find places in and around Berkeley that work.

mschwett 

the cycle track is at the polo field in golden gate park. it’s rotated about twenty degrees counterclockwise, and since the wind is typically slightly north of due west it’s often at almost 45 degrees to the long sides. there’s also a berm and the irregular trees, so pretty difficult to cancel out.

i could see going north and back south on illinois street when the prevailing wind is from the west being very still. and no need to stop typically, but not guaranteed. washington boulevard has some rollers that would allow variable speeds without braking, few cars, and feel like relatively little wind but i’ve noticed in my data that i’m quite a lot slower when there’s a reported headwind, so it’s obviously a much bigger factor than it feels like. i have hundreds of segment attempts on most of those roads and the variation relative to power is really significant.

PeteHski

I would take the pragmatic approach and focus more on helmet fit/function than aero efficiency - at least within the current range of high-end aero helmets.

As an experienced F1 motorsport engineer, I can also confidently say that speculating/guessing on marginal aero gains/losses is a total waste of time. You either measure them objectively or you don't. Your question about the aero gain/loss with helmet size was a 50/50 bet.

RChung

Yeah, I've thought about the Polo Field before. Here's the thing: if all you're interested in is the relative difference between the two helmets, not the actual true difference in CdA, you typically don't have to cancel out the wind as long as the wind is pretty even (gusty winds are typically a no go). Do a few laps, varying your speed and power widely, swap helmets, do a few more laps varying speed and power widely. If the winds are steady, the amplitude of the VE profiles will be consistent even if inaccurate. If the amplitudes are consistent, the overall slope of the VE profiles will tell you whether the helmets differ. You don't have to (and shouldn't) hold your speed/power constant. Vary your speed/power widely, while maintaining your position: I accelerate slowly from just above walking speed up to near my max while holding position, then soft-pedal coast back down to just above walking speed. Lather, rinse, repeat. Even if you don't normally use aerobars, they can help minimize upper body movement and help lock you in to a repeatable position. You don't have to do ABBA testing.

Calsun

My priorities are that it be a MIPS helmet and that their is good airflow for cooling my head and a good fit in terms of the straps when adjusted. Helmet comfort when riding is most important and I like that the newer helmets have better forward visibility when riding down in the drops. My older helmet needed to be trimmed back with a blade to see without scrunching my neck to avoid debris on the road.

mschwett 

i’ve found both of the ones i have to cool well enough, even though one is much less ventilated. MIPS for sure. the sizing is a mystery, they’re both specified for the same head size.

ussprinceton

I want an HJC Furion

GhostRider62

My Gen 1 Evade is "good" on my upright when on the TT bars but lousy on my reclined recumbent (18 degrees).

Just do Chung testing. The differences are enough to matter and also not that hard to test for. The helmet is worth more than an aero front wheel on my recumbent but less than that on my upright.

ussprinceton

Jrasero

I used a Bell Z20 Aero. One of the cheaper aero helmets. Seen them as cheap as $110