Aero Deep Dive in The Specialized Win Tunnel – Ask a Cycling a Coach 188



@BenTheStig Specialized have some videos to address this: Aero at slow speeds: and on “time saved”


I’m totally going to spreading this nugget of wisdom around to my competition.


Have you used the enve recommendations?
I have one shipping to me soon, and for my weight it’s gonna be about 65psi, that’s really low.
Specially with clincher, tubeless I would be on board 100%

I’m more of the 92 front and 87 rear, drop couple psi down for 4 corner crits stuff.

Wonder if the recommendation was more for aero or rolling resistance? I’m not sure if going that low would be good for the RR…


Haha yeah. Go slower if you want to get your money’s worth out of that disk wheel.


Agreed that a more useful calculation is the reduction of CdA so we can calculate in BBS, but that is also going to be variable rider to rider.

I think that’s part of what they were getting at in that part of the podcast where they talk about “absolutes.” The Watts, the CdA, time saved at x speed is all going to be different. Where the measurements are useful is comparing savings between changes. I.e. this helmet saved me 10 watts but that speed hit saved me 30. It’s a constant measurement at a constant speed. If we understand that we won’t save that exact number ourselves, we can still know that swapping to a better helmet is still faster. The key comes to testing ourselves even in basic field testing to know what our individual CdA is so we can run calculations through BBS.


Awesome, that pretty much confirms for me that as my own mechanic, external cable routing is not a big deal… but if there is one place to bother with internal routing, it is probably the handlebars.


@Superdave do you guys recommend not wearing anything under the Evade II helmet? I’ve always worn a cycling hat to keep bees/wasps from getting stuck in my helmet but I might stop doing that now.


This is a damn rabbit hole.


Whilst I don’t disagree, I just want to make it clear that for a given reduction in Cd the percentage increase in speed will be the same, no matter what speed you were doing in the first place.

Here’s a spreadsheet showing the changes between two CdA values at different speeds:

From left to right I’ve calculated the time for a 40k TT at the given speed, the power required at CdA1, the reduced power required with CdA2, the number of watts saved, the percentage of power saved (which is always constant and equal to the reduction in CdA) and then the relative saving in comparison to the top line.

The second part of the table calculates speeds and times based on the original power output and the relative changes.

Rolling resistance and drivetrain losses have been ignored but the above table matches the Gribble Calculator.

For these calculations to be correct CdA must remain constant across all speeds, which I believe is correct for the speeds we are talking about here. Is that a good assumption?

Ultimately, I think the bike industry use these absolute reductions at a given speed because it’s factually correct but muddies the waters enough to encourage customers to think they’ll get bigger benefits that will be the case for them.

As a wider point, unfortunately there is a bigger problem within the cycling industry with a generally unscientific media willing to publish manufacturer’s claims blindly. Obviously it’s prohibitively expensive to do many of the required tests, but even if it wasn’t I would doubt that there are many with the expertise wanting to work for a bike magazine. The possible exception to this is the German ‘Tour’ magazine who do a lot of testing.

Fortunately there appear to be a number of independent testers out there who are doing it for fun - and maybe putting a few noses out of joint at the same time.



Great analysis. :+1:t3::+1:t3: Thanks for doing this!

I’ll use an example from your table to try illustrate why it’s difficult for the average consumer to interpret marketing claims about aero savings.

Let’s say a skin suit and aero helmet save you the 35.9 watts at 45kph in your table - and that’s the marketing claim - “save 36 watts with our aero helmet and skin suit”.

For a rider who rides at a slower pace, say at 35kph, they will not save 36 watts on their baseline 169 watts. They save only 16.9 watts, so the marketing claim for them is very misleading.

As your analysis shows however, the time savings for all riders on a 40k TT is the same in percentage terms (ignoring rolling resistance like you point out, which will change the answer, but let’s ignore that for now for simplicity) - so why not just use a metric like % time saved in a 40k TT as the marketing claim? It’s more comparable for riders across all speeds.

Once rolling resistance, and drivetrain losses are taken into account, the % reduction in TT time will be less as speed drops (I think), so this is not a perfect metric either as it will vary based on rider speed… leading back to my original suggestion of using reduction in CdA as the best metric to talk about aero savings, as this metric is independent of rider speed.

So what I really want to know about the skin suit and aero helmet is the reduction in CdA, as I’d like to do a similar analysis to you to estimate time savings for a race like Leadville on a MTB where average speed will be in the mid-high teens of kph.


When I get time I’ll do the calculation taking into account the additional losses from the drivetrain and rolling resistance.



Because Cd is a relatively abstract and very hard to measure parameter.

Power at a speed or time delta over distance are fairly direct and measurable metrics for most people. And they go up when better as opposed to down. Which is not nothing.


If all consumers were engineers marketing and advertising would be a very different game :grin:


Cd is too abstract a concept for most consumers
Watts savings are misleading
Time saved in % is the best measure.


I simply cannot believe that shaving my legs will increase my FTP by 20 watts. Is there a link to any other evidence for this that someone can point me to?


That’s not how it works. Shaving your legs will let you maintain the same speed (in this case 45kph, if memory serves) at a power 20 Watts less than if you hadn’t shaved. FTP is your physical capability to maintain power.



If shaving could increase my FTP I’d be bald as a coot :grin::joy:


Yes, I’ve used it and it kind of scared me, but it felt great! I’m not sure if it’s the best crr or if it’s the best combo of comfort/grip/crr.


We totally didn’t want to get it in terms of absolutes. I think Chris even said a few times “it depends” and I pressed him to give a “general” answer.

Before the podcast I asked Chris to give numbers in terms of 250 FTP riders since that’s about the top of our bell curve. I’m not sure if he did that but we were trying to. If he jumps on the forum we can ask him. Maybe @Superdave could let us know if we should reduce the watts given in the podcast by a certain percent for 250 watt riders or not.

And of course that’s assuming crr and cda of a 250 watt rider.


The Spec guys said the original hairy vs shaved test was done c.1980’s(?) in an industrial wind tunnel using a mannequin leg with “fur” glued onto it. It wasn’t conducted with any type of rigour but the results stuck since it was basically the only test available…until now.