How exactly do you calculate rolling resistance on the road? The truth is, without the help of one brilliant man, it would be a very difficult thing to do.
Nearly 10 years ago, Robert Chung developed a Vertical Elevation model for estimating a cyclist's CdA (drag coefficient). The vertical elevation model aka “The Chung Method” is widely used in a number of products and software today.
What’s interesting about the Chung Method is that there is a small section at the back of the document that discusses solving for both CdA and Crr (coefficient of rolling resistance) at the same time. In order to do this you ride up a hill of known elevation at a constant power, and then repeat the course at a different power output.
Once you process the data, you end up with two elevation profiles for each of the power outputs. Even though the course elevation is the same you will see different ending elevations when your CdA and Crr values are incorrect.
If you adjust the CdA and Crr values in the Chung Method equation, there is one combination where the two elevation profiles match identically at the final elevation of the course. When this happens, you’ve solved for both CdA and Crr simultaneously.
This was what we needed to start our on-road testing. Since there was little information on the specific protocol, I reached out to Robert Chung. We spent some time talking about the particulars of the protocol and how to complete a successful test.
Talking with Robert Chung
Robert Chung had a lot of great advice about the protocol. He suggested adding weights to the bike on different runs to give more data variation and suggested getting very accurate elevations.
This leads us to Part 7 of our Gravel Wheel Design Journey. Finding a course and figuring out how to get accurate elevation data.