my anecdotal evidence would be the sweat pouring down my face wearing my giro aerohead during a sprint triathlon where the heat/humidity were pretty normal for that race. For me, just not enough airflow. My lg p-09 without visor just works better for me. Many people obviously have no issue with the aerohead

7-10% of body heat is lost via the head - predominantly the forehead. Now the helmet doesn't cover the entire forehead as you mentioned but, it still covers a good bit. Or it impedes airflow. Even if it affects 4% then that is still somewhat significant on a hot day when you are pedaling hard. I don't have the data that you are seeking however.

Can it be both? I think you're actually asking a good question but I think your premise may not be the one I'd think of first.

If a helmet is hot then we need to understand the trade offs one makes to race in a aerohead vs something probably slower but more ventilated.

Humans are 20-25% efficient while cycling. A slower helmet if you go the same speed will require more watts which means you will generate more internal heat that needs to be shed at some point & may increase sweating. Or you're pedaling a bit longer than previously if you don't generate more watts so you're still generating more heat overall on the bike. Anyway food for thought.

Some people are going to be more sensitive to heat/humidity.

In casual observation it seems the less lean you are, the heavier you are, the more muscle mass, the less fit you are the more you're going to reduce your margin of error for a good race by using a helmet that has minimal air flow.

If you're lean, fit, thin, it's going to be easier to shed heat and/or build up less heat.

anyway some food for thought on a good question

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Brian Stover USAT LII
Accelerate3 Coaching
Insta

I can’t tell you how accurate it was, but I seem to recall a study or some info published about this and the conclusion was that “hot” helmets had very little impact on overall performance. Or were you just wanting to understand the heat without respect to performance? Anyone have that study or info handy??

Here is an extreme experiment but I think it applies. Do 4 one hour turbo sessions and compare results: First without a fan or helmet, second with a fan point straight at you but no helmet, third with a fan and a helmet and then a helmet and no fan. I expect your performances to be better with the fan. I also expect the helmet to have little to no quantifiable impact on performance as long as the fan speed is up pretty high. However I think you will feel the difference with and without the helmet when the fan is on.

Does this mean the 'hotness' of helmet doesn't impact performance? My personal experience is yes and no. For TTs up to 40km I think the more aero the better. However I have noticed a serious drop off in run performance if I feel overheated on the bike. I just seem to run better, especially over the longer distances, when I use a more ventilated helmet in hot conditions. It could be a mental thing. My instinct is that heat stress is a cumulative effect with a delayed reaction so you can get away with it in some races but not in others.

I own two helmets and choose based on temperature. I can feel the difference.

Also, visors can be hot AF. I did a hot hilly race where I had to keep the visor up.

I've never seen any data to support the idea of hot helmets causing other effects.
One study shows the aero helmet increased head temperature, but no effects on body temp or performance.
https://www.ncbi.nlm.nih.gov/pubmed/23539083

A great many anecdotes do suggest some people notice a difference, which means there might be: or that increased head temperatures are causing the noticeable difference.

For myself I can't tell the difference between one helmet and another, typically ride with vents taped shut on my aero helmet. A full head of hair compressed against the skull by the helmet, makes a very effective insulation layer. The few wisps of air coming through the vents don't seem to make a difference.

I can't seem to find it, but I know for me a huge portion is mental. When I can't feel the airflow and start to feel my head get hot/ sweat I get in my head about it. I'll panic about overheating and it starts to affect my ability to output power. Another way I've done this "experimentally" is on the trainer with a fan that can be moved up/ down. Did this via a set of 8 minute repeats at 102% FTP (find these induce the most heat stress to me). I would point the fan directly at my head or torso (fan was small/ directed enough to do so). I found when the fan was at my head RPE went down, despite HR/ power being unchanged. Did this across 4 repeats (2 torso, 2 head) and the results held. Since doing that workout, I've actually been able to ignore the feeling of heat outside more, which I think is a plus.

actually, it seems that your experiment would support the idea that it was 100% mental. Don't get me wrong, I believe an Aerohead "feels" (and likely is) hotter than a regular road helmet. The only question for me is whether that feeling and/or actual increase in heat negatively affects my performance, accounting for the types of additional considerations that desert dude mentioned.

+1

Mentally my Giro AirAttack feels warmer in the winter. Yet, I wear it in the summer too as my entire body feels overheated and not just my head.

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Indoor Triathlete - I thought I was right, until I realized I was wrong.