Chung Method Aerodynamic Testing

I've always wanted my own wind tunnel. Now I sort of have one.

A couple of years ago I read Robert Chung's presentation, "Estimating CdA with a Power Meter," on aerodynamic testing using nothing more than a known road profile, internet weather info, a bathroom scale, and a power meter.

I remember finding it interesting at the time; but I didn't follow up because I didn't have a power meter on my TT bike. And I didn't really care what my CdA was on my road bike.

A few weeks ago, I read about Colin Griffiths's recent Chung testing in the UK. I now have a Power Tap wheel that I can easily move to my Cervelo TT bike, so re-enter Chung testing.

Basically, Robert Chung took the equation for all the forces acting on a rider: power, air drag (CdA), rolling resistance (Crr), and gravity:

w = wrr + wPE + wKE + waero
w = Crr m v g + s m v g + a m v + CdA ρ vair 2v / 2

and he solved the equation for slope:

s = w/(m g v) – Crr – a/g – (ρ CdA v2)/(2 m g)

Then he used slope and known horizontal position at each time interval to build a virtual profile (using a spreadsheet). By adjusting values of CdA and Crr until the virtual profile matches the real world profile, you are able to solve for both.

Lucky for me, I'm an engineer and a surveyor (lucky in this example, anyway - most of the time I'd rather be a rocket scientist or the base player for the Stones). And lucky for me, a few years ago I designed and staked out a new subdivision street about a mile from my house, so I know the EXACT profile of the road. And lucky for me, the developer has barely sold a single lot in the subdivision, so there is ZERO traffic. And lucky for me, the profile is a perfect U shape with cul-de-sacs at each end so that I can turn around at both ends without touching the brakes. And lastly, the road is very well protected by tall pines, reducing any minor breezes that might interfere with my results.

So as a test run, today I Chung-tested my Tarmac, and I got perfect results. Here's the procedure:

Get on the web and get the temperature, pressure, and humidity for the test location.

Dress for riding and weigh yourself with your bike, bottles, everything.

Ride a known profile (really all you need to know is the elevations of the high points and low points).

Keep EXACTLY the same position on the bike for the duration of the test.

Do not ever touch the brakes - the formula can't account for deceleration due to braking).

Record several runs over/through the known profile.

Return home and record weight and weather data again and average start/finish numbers.

Set up a spreadsheet to plot a virtual profile of your course using the Chung Method.

Adjust the CdA and Crr until you get a constant amplitude and crest height.

Here's a screen shot of the spreadsheet I created to do all of this. Download it from my eSnips account if you want a copy.

It worked like a charm. I did three runs of the course in each of three different positions: 1 - on the hoods; 2 - on the "horns" (hands wrapped around the tops of my Shimano shifters and elbows sort of low; and 3 - in the drops.

I did a trial-and-error adjustment the CdAs for all three positions to get an almost perfect and consistent profile. What little profile variation I saw was likely due to a very light breeze or hitting a rock in the road. A little tweaking of the Crr (rolling resistance coefficient) got the amplitudes right.

Here's what it looked like when I was done. Remember, this is a VIRTUAL profile. It's not measured elevations, it's calculated elevations assuming all the different forces on the rider. It looks to be so accurate and precise that I could literally use it to perform asbuilt surveys on finished roadways (Causey will find that idea intriguing, I think).

The fact that it worked is cool enough. But now comes the fun part: using the new technique to play around with different positions and equipment on my TT bike.

I also learned something very interesting and useful that I will use while training and racing on the road bike. I've always wondered how much more aerodynamic it was to ride in the drops as compared to on the horns. On the horns is so much more comfortable and seems more powerful, too. I turns out that my CdA on the horns is a LOT lower than on the hoods (somewhat expected), and only very slightly less aero than riding in the drops (I was surprised the difference was so little).

So there will be no more training or riding in the drops for me. The almost immeasurable benefit isn't worth the more aggressive, less comfortable position. I'll just ride on the horns. I guess I'll only use the drops for standing and sprinting.