I pre-rode the course today on my cyclocross bike. It's bone dry and very rough, and my feeling is that it would be quicker on my hardtail mountain bike, with its higher volume tyres.
I wanted to properly check which bike was faster though, and I decided to check this by performing a quick Chung method virtual elevation test on the CX bike, with it's new tyres, and compare the results against the MTB, which I had tested previously. The results, plotted above, show that the MTB is indeed fastest on the grass field that I did the testing on.
Why Chung testing?
Chung testing, also called virtual elevation (VE) testing, is a method for determining the performance of bicycle using its power meter. It's often used by time trialists and triathletes to determine improvements to their aerodynamic efficiency, which is characterised by the CdA metric. The method can, however, also be used to determine rolling resistance changes though, and I have performed off-road rolling resistance tests in the past using this techniques, which are documented here.
The use of the Chung method for this type of testing is that is allows one bike to be compared to another, to quickly determine the relative efficiency, regardless of whether that efficiency improvement is coming from rolling resistance, aerodynamics or weight.
The traditional way of doing an VE analysis is to iteratively adjust the parameters, either the CdA or the Crr (rolling resistance coefficient), usually using the Golden Cheetah software, until the VE profile becomes flat. When the VE profile is flat, you know that you have a combination of CdA and Crr that is representing the performance of the bike correctly.
The alternative method, which I have used here, is to keep those parameters (CdA, Crr and weight) fixed for the analyses for both bikes, then look at the relative flatness of the two VE profiles.
In the plot above, it's clear that Bike B, the cyclocross bike, has a rising VE profile relative to Bike A, the MTB, when using the same values for CdA, Crr and weight as used for the MTB. This rising VE profile means that the analysis 'thinks' Bike B should be climbing, because more power is needed to propel the bike than would be needed for a flat profile. This shows that Bike B, the CX bike, is slower on the grass field I tested them on.
The beauty of this method is that it doesn't care whether the benefit is coming from rolling resistance, aerodynamics or weight. Instead, it only shows the net results of changes to those three. Also, as with all Chung testing, there is no need to hold a fixed power, which is a method I often see athletes and journalists trying to perform a comparative test. The Chung method allows you to ride at whatever power you want, as long as it's reasonably similar for one test and another.
A few caveats
I should add a few caveats, because it wasn't an ideal test, performed in perfect back-to-back conditions. Firstly the two bikes were tested on different days, so potentially the ground conditions and wind conditions were different. Qualitatively though, the ground conditions were similar on both days and although I felt there was slightly less wind for Bike B, this should favour the Bike B apparent performance. Therefore, as Bike B is showing worse performance, it won't change the conclusion that Bike A is the best one to use.
Secondly, the two bikes had different power meters. This not ideal, and in a perfect world I would test the same PM on both bikes. However, I mitigated this potential bias by applying a 10W correction (10W reduction in power) to the powers from Bike B, based on comparative testing that I did previously (here and here). All results are shown with this correction applied. hence it would require a 16W correction for Bike B to be as fast as Bike A, and I don't think they are that far apart.
Finally, in a perfect world I would have done some repeats, like an A-B-A-B type test protocol. However, I didn't have time.
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