Aero testing....confused by result

Hi all,

After some recent position changes (see video below for new position) I wanted to do some aero testing.
I got in touch with a guy in UK who is a bit of an expert in aero testing using the Chung Method and aerolab on Golden Cheetah.

Basically ride several times around a loop without changing position or touching brakes, ideally with V low wind and no cars.

I did this tonight, analysed the data (99% sure accurately) and got a CdA of around 0.31…which doesn’t sound right for a TT position so anyone have any ideas?

I’ve checked bike and no issues with brakes rubbing etc.

All I can think of is my power meter is over reading- it’s a 4iiii single sided, so maybe I don’t pedal evenly or inconsistently…or I’m as aero as a brick wall!

Any ideas appreciated!

A bit like virtual power, I’d largely ignore the “actual CdA number”, and focus on changes. Knowing a single number is relatively worthless. What may be useful, are the deltas between any potential changes in setup that result in different CdA.

Maybe the odd value is a sign of an issue in your method or analysis, but if it is “correct” and repeatable (probably the most important consideration), you may be able to leverage it for analysis of different setups.


I know what you mean and yes consistancy to compare setup A vs B is more important…but that CdA is so far off then must be issue somewhere surely. Few v serious TT racers I know who have done a lot of aero testing all looked at videos and estimated in region of 0.23-0.25. I reckoned nearer 0.27…but over 0.3 is crazy!

I must be missing something here, but how often do we have the wind blowing head-on, as opposed to every other possible angle ?

What I am getting at is unless one is planning on racing inside a covered velodrome, the figures, produced by “aero tests” don’t need to be all that precise.

Yeah, that CdA seems very high…your position is very good, especially when you turtle your head. There is definitely some low hanging fruit in your kit / equipment (sleeved suit, etc) but those changes wouldn’t account for the .3+ number (I don’t think).

I’d run the test a few more times and compare the data…I have never done a Chung Method test, but from what I have read, it can be challenging to get right the first few times.

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The effective angle of the wind is the sum of two vectors: how the wind is actually moving and how you approach / move through it. If you’re moving at 30+km/h, then the biggest component is that self-made headwind.


Do you have any power data and speed from a race? You could use Best Bike Split as a sanity check or people on this forum could probably tell you if you are in the ballpark.

Maybe that loop you used is just pretty slow, bad road surface, hilly, etc? Have you tried it on a road bike (or sitting up on th TT bike) to see if that also gives a high cdA?

Hi Robert, if you have the *.fit file from your loop, we can double check that CDA figure using a dynamic power calculation incorporating DarkSky weather data (no charge). Just email

Hi @Alex_Fastfitnesstips, that would be great, thank you. I will send it over later today.

I did 3 laps in a sat up position on base bar of TT bike and CdA was clearly higher- about 0.37 if I remember correctly.

Makes me think method etc is right and I know aero testing takes some practice…but would expect to at least get ball.park accuracy.

Can only think my PM is over reading (due to fault or strong left leg as single sided).


For context - I have a dual-sided pedal PM and my L:R ratio is consistently 46:54. So in my case, a single-sided PM would underestimate my power by 8%. That doesn’t matter in the slightest if you’re using that power to set training intensity and measure improvement (as long as the 46:54 stays fairly constant). It would of course completely screw up an attempt to get a realistic CdA estimate. Not great for Zwift racing either…

And yes, as said above, your position looks pretty decent, especially when you’re turtling from 24 seconds onwards, so you’re almost certainly not an aero brick. Both road surface and climate conditions (not just wind but also temperature, pressure, humidity) can have a pretty big factor on speed. So likelihood is that some combo of those factors and PM accuracy is throwing off the estimate. But it doesn’t mean it’s not still useful for testing out whether position and equipment changes improve or worsen that number.

Couldn’t agree more. The number I get from BBS, Golden Cheetah and Virtual Wind Tunnel are all converging to 0.23.

I was on the other side of the fence. My bike is a low end model with a 60mm front wheel and covers in the back and I thought my GC Cheung number was too good to be true.

Skin suit and an aero helmet have a huge impact. I tested that and it is unbelievable how stupid some people is by paying an incredible amount of money for a super bike without investing in what does have the biggest speed to dollar return.
The same can be said about tyres and tubes.

Hi Rob, I sent you some analysis. hope it helps!

I agree 0.31 seems high to me for that position

Hi like you I have also been doing some testing, with quite mixed results, and CdA also coming out on high side from what I expected ( seen everything from 0.24-0.31)

I have more done out and back routes as dont have a good circular route and am fnding the CdA generally low on downhill and higher on uphills. I understand that it should not theoretically change. This could be wind effects but seems to be a pattern over a number of weeks testing on different routes.

Other considerations.

  1. Are you sure bike is running smooth
  2. Tyres, Crr etc
  3. What’s the surface like? Very rough?
  4. Are you using gps or actual speed sensor
  5. Are you sure elevation data is correct, e.g.

I think it’s actually hard to test precisely with this method and in the podcast when they discussed aero testing the trainerroad team actually mentioned they were not able to get accurate results.

I also suggest stepping back, go to the website and play with their power sim calculator for ranges of cda that would be possible to produce over a given route (relatively constant gradient) for given power and other variables. They actually also have a cda calculator on there but have had mixed results with that too.

Maybe you could try attaching some object to your bike, to see if you can detect the increase in drag?

It can be v accurate if done correctly under right (no wind and no traffic) conditions…however from what I understand not that difficult to get ball park figure fairly easily…and I think I’m well off.
Been over the bike and no issues, CdA been checked by others and on best bike split and all near enough same. Road surface not too bad.

I just keep.coming back to my PM is either over reading OR at least I’m not consistent with my pedalling which has big impact with single sided PM.

Tomorrow I’m borrowing a mates Kickr for a half hour test whilst also running my PM in parallel. See what that shows.

Hi all,
Just a follow up from my posts above and thank you all to those who have responded.

I popped round to a mate’s house yesterday and did a custom made workout on his Kickr whilst also recording my 4iiii.

Interested in people’s thoughts and analysis…I know my opinion but don’t want to influence the opinion of anyone else!

See below - the first photo is 10 mins warming up before I calibrated the Kickr. The Kickr was in resistance (not ERG mode) for this.

The next is workout post calibration. Kickr in ERG mode

The final one is another ramp type test. Kickr in ERG mode again.

The line marked ‘Power meter test’ is the Kickr.

The line marked with date and time is my 4iiii.

Pre calibration Test 1

Post calibration Test 1

Test 2:

For test 1 and test 2, looks like kickr is reporting target power, not actual. I don’t have a kickr but if you search the forum it’s a pretty common topic.

That said… looks safe to say the pm is reading high compared to the kickr. Those are pretty big diffs in avg power. That makes sense given your aero test results, since a higher power given observed test speed means less aero = higher CdA.

I’m not totally familiar with Kickr as power looks…to steady…however it does fluctuate slightly and I did add 3sec smoothing to the graphs.

Turn off the ERG Mode Power Smoothing. It’s a terrible lie from Wahoo, and they set it as the default, which is a huge mistake, IMO.