@PattyP - there’s a lot of questions! My experiece so far says that the biggest source of errors is traffic, followed by gusty wind. Or maybe vice versa. Minimising these is critical.
Braking is probably the next factor to minimise, but there are some great suggestions from @hvvelo above about how to mitigate these - so I don’t think tests with braking are junk, but care is needed.
So, with the above in mind, the test track is far better for quality testing.
Generally, the advice is to test at race-speed, particularly for clothing, because the best speedsuit at 40kph isn’t the best at 55kph. Unfortunately, if you are riding pursuit or short TTs, this means more power. If you want to ride 100m TT, you can test all day! But definitely do the tests at the same speed.
I think people tend to aim for 4km for each test run as a minimum, so 3 laps would be close enough.
Finally, do your tests as ABCABC or AABBCCAA or something like that, where you set a benchmark (A) and then are testing each helmet (B, C, etc) twice, in rotation. This helps you identify changes that may have come from something else (eg temp or wind gradually increasing over the test).
Finally finally, record everything! Keep a notebook, record trackside air temp, humidity if you can, ground surface temp if you can, wind direction, then the order you test and any issues (eg if you can’t hold position or you notice your head is moving about). Then cross-check with wunderground or your chosen weather app. Sorry if this all seems uber-nerdy, but I have been out to test and wasted a session because I got home and realised I didn’t have enough info.
Thanks!
Yes i read about ABCABC etc…or AABBCCAA… How can i measure the ground temp? And the ground surface is asphalt. Do i need to know more about the surface?
I think allready about to take a notebook in the car to wright down all the things i know.
I will first make a benchmark with my normall helmet (Kask Utopia) and than testing other helmets, i think in the way AABBCC etc so i have less moments to stop.
Can i use the lap button or is it better to make for every test a new garmin file?
Sorry for all the questions 
I have always learned, if you want to learn something you have to ask questions. 
Another one:
How can i calculate the change in speed by 40kmh if i know the change in CdA.
For example i ride 40kmh and the CdA with helmet B is 0.002 lower. What is that in speed difference?
You could play with https://mywindsock.com/
Thank you. I have directly make a premium account.
From what I have seen on slowtwitch, the P-09 also comes up very frequently as a high probability to test well - and is also much less expensive than some of these options. The one other consideration I’ve seen is a lot of people report the Aerohead is hot, and the P-09, which should be used without the front plug, is cool. I’m very heat sensitive, and it’s what I’ve been using with no complaints, but I have no other helmets to test against.
I read also that the Aerohead is relative heavy instead of the Kask…iam a light rider, so i think a heavier helmet wil give more stress in my neck area?
Unfortunately in the Netherlands i cant buy easy a P-09, Giro Advantage 2 or POC Cerebellum with the possibility to bring it back…
Yeah, I actually had it listed when I first typed out my response, but I didn’t know if it was still being manufactured, so I left it off.
I’ve used the Aerohead in full IM and 70.3 races that were hot with no real issue…it isn’t the lightest helmet out there, but a few training rides can get you used to it.
Is there a thread dedicated to aero testing? I’ve never been able to get consistent, repeatable results and am looking for tips.
Joe
Ok here we go into the deep TT journey…
This is my fit after my first bikefit and the second picture is with the Kask Mistral. There is some gap between my back/shoulders and the helmet. This position with my head is sustainable.
Iam happy with al the fair comments, because thats the way we learn!
I know you haven’t tested yet, but the shape of your back with that helmet 
You can probably cancel the orders for the others!!
Does anyone else have issue with whack elevation data with Mywindsock when using Strava segments?
I might get +400m elevation difference on a 10k TT when using Strava url vs segment gpx.
Never looked closely at that. But it wouldn’t surprise me all packages seem to calculate differently. TR doubles the elevation that GC and Strava gives and RWGPS does something in between.
I would add the Lazer Volante helmet as a candidate.
I use it currently after having compared towards POC Tempor as well as the Specialized TT.
Slightly faster then the Specialized TT as well as offering better visibility.
As fast or slightly faster overall compared to POC Tempor. (the Tempor was faster when I go fast and straight but as soon as there is parts with lower speeds or I need to pay attention it looses a lot of time)
Anyone using the Oakley ARO 7? The helmet is seldom seen in the pro field (at least in the retail version) and test results are rare so first hand experience would be highly appreciated.
Additional info: I am not (yet) good in holding position so I am going with a non-tailed helmet.
i don’t think there are any pro teams that still wear the oakley helmets which is why you don’t see anyone using the ARO7, probably a few triathletes that do. I would suggest that it’s noteworthy that when Katusha-Alpecin (remember them) did wear oakley helmets, Tony Martin & Alex Dowsett both would wear different helmets with oakley stickers on. Similar to how MVDP wears a lazer hemet instead of the ABUS one.
in terms of short tailed helmets, i wear the MET codatronica, it seems to work well for me & my head control is terrible
This web based one is fairly easy to use to slide parameters around to see CdA vs. speed relationships:
IIRC, they used the prototype ARO9 helmet, not rebranded helmets. Jan Frodeno had requested a longer tail helmet when Oakley had signed him.
I’m a fan of shorter tail helmet, and until recently, a long-time Oakley fanboi. The ARO7 did nothing for me aesthetically and did not pass my “eyeball windtunnel” test. Following “best practices”, the ARO7 just missed everything, IMO.
I know respected guys on the UK are using the MET Codatronica and seeing good results; definitely one to try, especially if you know you move around a little.
Thanks for the feedback. Aesthetically the helmet was my top choice (beauty is in the eye of the beholder 
) and actually it fits really well. I have to admit that the performance aspect was not part of the equation until now.