I was hoping one of the aero experts here could help me figure out how to use a recent Powertap race data file to calculate/estimate my Cda.
The file is from a recent flat out and back 40k TT where I was under threshold power for a variety of reasons but nevertheless ended up with a 57:44 on just 235 watts. Wind was ~5-7 mph. That got me to wondering where I sit aerodynamically. I an 5’9" and ~140 lbs. Is there a way to use the program to find Cda and perhaps thereafter predict times based on additional power?
Thanks in advance for all of your help and expertise.
Curious as to what setup you used. Is it the bike in your profile pic? I’m the same height/weight as you and looking to get my w/kg up this winter, but your performance is giving me some hope.
Do you have Golden Cheetah installed somewhere? If so it is a pretty
simple matter to load your file and play around with Aerolab. Should
be enough for you to ballpark your CdA. To really nail it down will
require a lot more care in setting up your data. I don’t think there is
anything you can do in post processing to get more accuracy given
those inputs, but perhaps those more expert than I will correct me.
I was hoping one of the aero experts here could help me figure out how to use a recent Powertap race data file to calculate/estimate my Cda.
My name is Andy Froncioni and I’m the Aerolab developer. I used some previous work by Robert Chung, Alex Simmons, Andy Coggan, Jim Martin… oh, heck… by lots of people to code it up. I’d be happy to help.
The file is from a recent flat out and back 40k TT where I was under threshold power for a variety of reasons but nevertheless ended up with a 57:44 on just 235 watts. Wind was ~5-7 mph. That got me to wondering where I sit aerodynamically. I an 5’9" and ~140 lbs. Is there a way to use the program to find Cda and perhaps thereafter predict times based on additional power?
Thanks in advance for all of your help and expertise.
Usually, race data doesn’t work that well with Aerolab. Wind is not very compatible with virtual elevation unless you have a way to measure airpspeed and yaw. Even if it’s a windless day, you need to use some careful loop, out-and-back, or half-pipe (both loop and out-and-back) protocols to determine your Crr and CdA. You need to carefully select your bike course and follow some simple rules.
That said, many people are already using Aerolab and other spreadsheets to help improve their position. It just requires a little focus and a bit of patience. Otherwise, you have just about all the tools you need to do it.
The file is from a recent flat out and back 40k TT where I was under threshold power for a variety of reasons but nevertheless ended up with a 57:44 on just 235 watts. Wind was ~5-7 mph.
Nice result. Congratulations.
You can get a ballpark estimate but 5-7 mph wind is a lot of wind so if you want to do better than ballpark you’ll want to do special dedicated test runs.
That said, if your route was straight out and straight back and the wind was constant and at a constant angle in both directions you might be able to narrow the ballpark a bit, though it’s a bit of work. Basically, under those (pretty restrictive and usually unrealistic) conditions you’d be adding and subtracting the same small wind component to the out and back legs so that the VE would match. That’s possible but not a part of Golden Cheetah’s Aerolab so you’d either have to be desperate or have a really high boredom threshold to try this. Or maybe both.
Thanks very much Andy. I guess I’m a bit dense at this but once I load the data file into the program and go into Aerolab, what do I do next?
Aerolab tries to reconstruct your ride elevation profile by using guesses of your CdA and Crr through the use of Newton’s Second Law.
Just quickly:
Enter the mass of rider + bike.
Enter rho, the air density. Use a wizard like this one, but remember to convert to kg/m^3. The number should be close to about 1.2.
Did you use a crank- or hub-based power meter? Crank → use 0.98 for eta. Hub → use 1.0 for eta.
Estimate your coefficient of rolling resistance. I guess for a ballpark value, start with about 0.006 .
Slide the CdA slider back and forth until the elevation profile looks like the one you rode. When it does, you’ve found your CdA.
Obviously, protocol is everything. Get familiar with the interface with your dataset, and then when you’re ready for a half-pipe protocol, you’ll know what to do.
