Time to Share Some "Gravel Fun"

*edit 18Feb2020: There's a late-breaking addition to the list after I recently completed a test on the new Continental Terra Speed. See the list below and in the spreadsheet

OK...yeah, I know...it's been awhile since I wrote something here :-)

But, it's a new year (relatively speaking) and I've got some stuff I'd like to finally share. So, below is my first go at presenting hard surface rolling resistance data on tires intended for mixed-surface riding, A.K.A "gravel riding". I'm sure I'll be opening myself up to criticism from certain corners of the interwebs for looking at this, but I'll discuss below some of my reasoning on the subject and try to put the information into the proper (usable) context.

So, without further ado, here's a quick list of what I've tested so far, in descending order of Crr (full spreadsheet is located at the link to the right, or here):

                          TIRE                                          CRR           POWER (pair @30kph)

• Continental GP5000S 700x23c                  .0029                 20W
• Specialized Turbo Cotton 700x28c             .0031                 21W
• Continental GP4000S 700x23c (control)    .0035                 25W
• Challenge Strada Bianca Pro 700x30c       .0036                 25W
• Compass Snoqualmie Pass EL 700x44c    .0036                 25W
• Challenge Strada Bianca Pro 700x36c       .0038                 27W
• Challenge Gravel Grinder Pro 700x36c      .0041                 29W
• Continental Terra Speed 700x40C              .0043                 30W*
• Compass Snoqualmie Pass 700x44c         .0043                 30W
• Panaracer Pari Moto 650Bx48c                  .0047                 33W
• Challenge Gravel Grinder Race 700x42c   .0047                 33W
• Compass Bon Jon Pass 700x35c               .0048                 33W 
• Challenge Gravel Grinder TLR 700x42c     .0050                 34W
• Panaracer Gravel King SK 700x32c            .0051                 35W
• Challenge Gravel Grinder TLR 700x38c     .0051                 35W
• Compass Steilacoom EL 700x38c               .0056                39W 
• WTB Byway 650Bx47                                  .0056                39W
• Challenge Gravel Grinder Race 700x38c    .0057                40W
• Vittoria Terreno Dry 700x40c                       .0057                40W

Explanation:

Before diving into the actual results, it would be good to review a few notes about some of the test conditions and how the results are reported:

• The tires listed above (unless otherwise noted) have been tested at a pressure predicted to correspond with a tire "drop" (i.e. deflection under load) of 15% of the inflated casing height. There will be more on how that pressure is calculated below. The reason for doing so is that the tires in this category can vary in size by quite a bit, and it makes sense to compare their performance in a more "apples to apples" condition than with a fixed pressure (as I have done previously with road tires of similar size to each other).
• The power for a pair of tires is shown compared at 30kph, unlike  the previous reporting for road tires at 40kph. This is to account for the generally lower average speeds encountered in mixed-surface riding. The spreadsheet reports values for 20, 30, and 40kph instead of the road spreadsheet reporting of 30, 40, and 50kph
• The top 3 tires listed are basically road tires. The Continental GP5000, although a 23C tire is listed mostly because I haven't shown a result for that yet (and some information linked to below indicates that the performance of the larger sizes is basically identical when run at Berto pressure). The GP4000S is just shown as a "control" and comparison to my previous road only results (still linked at the right). Lastly, the 28C Specialized Turbo Cotton is also another road tire I haven't shown results for in the past...but, in this case, I consider it to be the first of tires I would consider for "light gravel" use (and have used it as such). On rims of 20-21mm internal width, those tires measure nearly 29mm wide.

Discussion of Berto Pressure calculations:

Quite a long time ago, after discussing the subject with tire engineers, Frank Berto took on the task of measuring a range of tire sizes to determine the pressure required to result in a 15% deflection of the tire casing for a given load. The assumption was that this deflection point resulted in a consistent performance for a given tire size and load...and, if anything, was at least a good "starting point" for determining a preferred pressure. The results of those tests are shown in the chart by Berto above.

Because I wanted to use the charts for a wider range of tires and for sizes in between the shown lines for tire sizes, I decided to see if I could come up with a "universal" Berto pressure equation.  To do so, I calculated the slope and intercept dependencies on tire size and wheel load. This resulted in a "pressure intercept" and "pressure slope" for each tire size curve. I then plotted these intercepts and slopes versus tire size in order to come up with a curve fit for each (and they were surprisingly linear). This exercise resulted in a  "universal equation" to solve for pressure for any size and load. Now it's not necessarily predictive of actual pressures one would run (since that can be highly surface dependent) but it's a way to "normalize" for comparison purposes. That equation is embedded in the spreadsheet.

As an example, BicycleRollingResistance.com did a comparison of 4 different sizes of the Contintental GP5000 tires: https://www.bicyclerollingresistance.com/specials/grand-prix-5000-comparison , and in an interesting comparison there, the rolling resistance measured for all 4 sizes was within 1W when "normalized" to a measured 15% tire deflection. Perhaps ol' Frank was on to something ;-)

Anyway...I think I'll just throw this info out there for now to hopefully stimulate some discussion, and will probably go into further depth on the subject in future blog posts (I promise!)