Cervelo: P3 vs P2K

I moved all my components over from the P2K frame to the P3 frame and did another round of Chung testing.  I'm not surprised at the results - the frames are identical except for the seat tube and seat stay shapes.

My average of three Chung test days on the P2K was CdA=0.231.  My CdA on the P3 was measured today at 0.232 in perfect, calm conditions - very clean data.  At least I'm finding the method consistent; and apparently I did a good job of getting my setup the same on the P3 as on the P2K (most drag is the result of body position).

Any changes from here on out will be with body position.

More on Chung Testing

It's been impossible to do much testing in the past two months due to weather.  I can't get out there in a skinsuit if the temp is below 55 degrees - it's just too uncomfortable.  And any breeze at all invalidates the data; so that eliminates lots of days.

But yesterday I was able to get another set of data collected on my P2K.  So now I have three data sets with the same setup.  My CdA results from three different days on the same test course are 0.228, 0.230, and 0.235.  Based on n=3, my average CdA is 0.231 and the standard deviation is 0.0036.  These numbers reflect skinsuit, TT helmet, shoe covers, no gloves, Zipp 303 front wheel, and power tap with Kinlin XR-300 (30mm) rear wheel.

My Cervelo P2K frame was slightly damaged in a crash last year, so I'm about to move my P2K components over to a Cervelo P3 frame a teammate recently gave me.  I'll set it up exactly the same as my P2K, except I'll run a Zipp 404 on the front instead of a Zipp 303.  The only difference between the new test setup and race setup will be the absence of my disc wheel during testing.

These aren't my bikes, but depict my old P2K frame and new P3 frame:

I'll be interested to see how much difference the frame change and 303 vs 404 change make to my CdA.

Stumped

My last post presented my nice neat Chung results. Everything turned out exactly as expected once I measured my test hill and input the real numbers.

But now I have a problem. A teammate tested with me on the same hill at the same time. He used a power tap just like me, he weighed in and weighed out after each test segment just like me. The weather conditions were the same. The course was the same.

But when I processed his numbers, the only way I could get the virtual hill to be the right height was to use an unrealistically large Crr, 0.007. That yeilded an unrealistically low CdA for him - 0.190 m^2 (he's 6'3" and about 180 lbs - I don't think he has a CdA under 0.2).

So I looked at all the inputs. Most were identical to my tests as mentioned above. Same spreadsheet, too.

I looked at weight: he tested using two different bikes, so his weight was actually about 2 pounds different between runs, but the two runs still yeilded exactly the same Crr. That tells me the weights were right (and we used the same scale).

What about distance/speed? We both calibrated our Power Taps using a three-revolution weighted rollout, averaging three rollouts, before the test. And I checked the distance measurement of the hill versus both of our data sets and everything matches there. So I don't think the problem is distance/velocity.

That leaves the Power Tap power measurement as the only other source of error I can think of. I thought I had a culprit. To obtain a data set that would give me a Crr for him on my hill near the expected 0.005, I was required to lower all of his power numbers to 95% of their measured values. That got the hill height right, but his CdA would still be around 0.210 on his TT bike and around 0.260 on his road bike. I don't believe his Power Tap is wrong AND he has an unrealistically low CdAs. I'm stumped - but only temporarily - I'll figure it out.

My next steps - let him do runs using my Power Tap and put him on my calibrated Computrainer to see if the Power Taps read the same. Then we'll go from there.

Chung TT Bike Results

This plot shows the results of my recent Chung Method aerodynamics testing with my Cervelo P2K. I wore full aero gear (skinsuit, TT helmet, shoe covers, aero front wheel), but used my standard Powertap wheel on the rear.

Because there is very little air resistance on an 8% climb at 150 watts, I was able to accurately discern my Crr using the climb side of the virtual profile. Then with a little iteration, the descent side of the profile, where CdA greatly overwhelms Crr as a force against the bike at 43 mph, allowed me to determine my CdA. I found it to be very close to the CdA result I got about a year ago when I did extensive coast-down testing on a 1/2-mile 3% descent. I found it to be 0.232 then, and this week's testing shows0.230. That's a pretty good number, but I'm not a big guy (5'9" and 146 lbs) and I ride in a very aggressive position, so it's not completely unexpected.

The first set of runs is with the setup I used for most of this season. For the second set of runs, I moved my aerobars from 4" apart to 3-1/4" apart (moved each of them inward 3/8", or 9.5mm). It was definately enough of a change to feel on the bike. I felt narrower. I didn't expect to see a difference in CdA from the Chung testing, but I did. My next task is to do repeat runs on different days to check precision/repeatability.

I trimmed all my data away except for the climb and the descent for each run, and I reset each run's virtual elevation at the known elevation.

The Chung Method is No Joke

I did TT tests yesterday at my Brookhollow course. When messing with the data last night, I came to the conclusion that my test hill must be four feet higher than the 83.1 vertical feet it was designed and staked out to be. Increasing the vertical drop by 4 feet was the only way I could get everything to work out with reasonable Crr and CdA numbers.

I couldn't stand the suspense, so I took a survey instrument (my trusty Topcon GPT-3000 W, shown at left) out there this morning to check it out. Guess what? The contractor built the road with 3.51' more vertical drop than it was supposed to have according to my design (I guess he wanted to save some cash on fill dirt).

The Chung Method is definitely no joke if you use it right. I don't know what Garmin Slipstream pays for tunnel time, but I think I have a comparable tool now for free.

I moved each of my aerobars inward by 3/8" yesterday, and apparently I was able to discern the difference in CdAs from that tiny change (which, by the way, would mean 11 seconds in a 40k TT). If I can see that type of change, this process is going to be fun (and fruitful).

More details to come.

Chung Spreadsheet - Better Acceleration Calculation

The CdAs I calculated for my road bike seemed a little lower than what I would have expected. After reading 106 miles on my Power Tap odometer yesterday for a Claxton Century course that was advertised as 104 miles, I figured the problem might be my PT odometer. I did a very accurate 3-revolution, weighted calibration of my PT wheel this morning and found the circumference to be dead on 2100 mm. I had had the PT programmed to 2098 mm. That means I'm going 2100/2098=1.00095 times as fast/far as I'd thought. Not only is that probably insignificant, it would LOWER my CdA calculations, not raise them. (A quick check revealed that my fastest CdA - when I was in the drops - was lowered by 0.001 m^2 by accounting for the PT circumference change). So I can scratch that as a significant source of error, although the fact that I picked up any change in CdA at all from a 0.095% change in speed is surprising - this method really does provide incredible data resolution).

Then I read Dr. Chung's comment on my last post: He noticed that I had used a simple approximation of acceleration in my Chung Method spreadsheet. To calculate acceleration, I used: change in velocity from T1 to T2 divided by the time interval a = (t2-t1)/dt.

That seemed correct to me. But he suggested I use a more robust approximation suggested by Adam Haile: a = (t2^2-t1^2)/(2*t2*(t2-t1))

As you can see in the highlighted columns in this screen shot of my spreadsheet, the two methods result in very similar, but differing, values of a.















It turns out those differences are enough to change my CdA calculations significantly. My road bike CdAs for hoods, horns, and drops changed from 0.310, 0.245, and 0.240 to 0.338, 0.258, and 0.255, respectively. Those numbers seem more realistic, although they are still lower than I would have guessed.

My Crr changed from 0.0054 to 0.0055 (a difference that is probably not within the resolution of the method).

I think I have everything ready now to do some baseline TT bike tests. Then I'll begin to tinker with my position and see what happens.

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.