Tuesday, November 17, 2009


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.


Robert said...

Have you done static torque calibration checks on both PTs?

And, what kinds of tires/tubes are you two using?

Robert Jordan said...

I have not done static testing, but plan to do that today on one of the units, at least.

I am running a 23c Maxxis Detonator on my PT wheel and a 23c Michelin Pro Race 2 on the front. Both wheels use butyl tubes - Kenda brand, I think.