Joel Campbell (1735-1828): His Life and Times: Optimizing Gravel Tire Pressure using Chung Method

I am preparing for an upcoming gravel triathlon. What is the optimal tire pressure for me and my set-up on gravel? What if I find myself alone on the bike course with no one to draft behind? ...What is my optimal body position?

Generic "answers" to these questions can be found on the internet, but can I find the optimal conditions for me and my set-up?

I performed the seven tests below in under an hour. Three of the tests were duplicates to verify that the tests were reproducible (1, 4, and 7). The test conditions and results are shown below. They are shown in terms of a "Virtual Elevation" (VE). If the slope is upwards for a test, it means that extra power is needed for that test (like going uphill). Conversely, if the slope is downwards for a test, it means less power is needed for those conditions.

In summary, it appears that a tire pressure as low as 30 psi is more efficient for me than 40 or 50 psi. As one would expect, getting into the drops is more aero (negative slope in VE) than being on the hoods. Even greater savings is seen when I bring my hands in close on the bars (Test #6).

What amazed me was how quickly I was able to do these tests (under an hour) AND despite less than ideal conditions (it was windy, course was flat, and I did not vary alter speed very much) I had results that yielded valuable information.

Test Protocol

Each test was 6 laps on a quarter mile gravel loop. The loop was flat. (Better results are obtained on loops with a change in elevation.) I averaged about 14 mph for all laps. (Better results are obtained if the speeds are varied.) I used a calibrated Garmin Speed Sensor (more accurate than GPS). I used a single-sided Quarq spindle power meter. (Better if two-sided.) Tire pressure was measured using a Specialized floor pump with gradations every 5 psi....not that precise and probably not that accurate either. I captured T, RH, and P at the start and end of the test protocol using www.localconditions.com. I weighed myself and loaded bike using a Tanita floor scale. Each test was saved as a Garmin workout. All seven workouts were exported from Garmin Connect as TCX files and imported into Golden Cheetah. The seven workouts were combined and analyzed in the AeroLab Chart as shown above.

The Chung Method

Virtual Elevation is the output of the Chung Method. The method solves the Power Balance equation for slope, using guesses for Crr and CdA. Slope multiplied by velocity gives the change in elevation which are strung together and plotted. This elevation is referred to as "Virtual Elevation" as all of the unaccounted for "power" in the Power Balance, whether it is related to elevation gain or not, is converted to elevation. Chung has shown that this method is good at exposing even minor changes in Crr or CdA, even when the data is crappy...as mine are. You can read his paper here. [Crr is the Coefficient of rolling resistance and CdA is the Coefficient of drag area.]

Data Analysis

Stringing all of the test data together allows a visual analysis. I guessed a Crr and CdA which made the duplicate runs (1,4,7) as flat as possible. If the test is reproducible, 1,4, and 7 should all be the same. In fact, each of the 6 laps within a test should be the same. If you look closely, you can count the 6 laps in each test. Test 1 had some anomalies on the first two laps. I did not do any practice laps. This was the absolute first time I had ever biked this loop. There was a conduit over the road which I rode over on the first 2 laps. All subsequent laps, I biked around it. It is interesting that this method is sensitive enough to show that. Test 4 had relatively good data. Test 7 appears OK for the first 3 laps, then trends up. The wind was gusting more, so that could be part of the cause. Considering how windy it was, I am surprised I got any meaningful data at all from these tests. With these differences in 1,4,7, I would want to do this test again on a calm day to confirm the results.

A note on Test 3: On the fifth lap, a camper pulled out and I had to go onto the shoulder to go around him. On the sixth lap, he was still there. All of that is visible in the virtual elevation plot. Now I know to redo the test if that happens again.

Crr on Gravel

My testing qualitatively showed that my set-up at 30 psi had less rolling resistance than 40 psi or 50 psi. My tires are tubeless Panaracer GravelKing SS TLC 40 (40-622) (700x38c) tires. I use Silca Sealant. My total weight (biker + bike) is 71.7 kg.

BicycleRollingResistance.com has tested these tires. They show that the rolling resistance decreases as psi increases....the opposite of what I found in real life!! Of course, their testing is done on a drum that mimics a paved road at 18 mph and 94 lbs load and 70-73 °F. Their Crr are 0.00498 at 54 psi, 0.00528 @ 45 psi, 0.00576 @ 36 psi, and 0.00674 @ 27 psi. You would expect the Crrs to be much higher on a rough surface like gravel.

The Silca Tire Pressure Calculator suggests (for my weight and tire) about 43 psi for Category 1 gravel (well-packed), 40 psi for Category 2 Gravel (not packed), 35 psi for Category 3 gravel (very rough) and about 31 psi for Category 4 gravel (off-road). This is getting closer to the pressures suggested by my testing on what I would call Category 2 Gravel.

My data are not really good enough to quantify a value for Crr for my conditions, but we know it is greater than 0.005.

CdA on a Gravel Bike

The range of CdAs for cyclists vary from 0.2 m² (time-trialers) to over 0.7 m² (riding upright).

The Chung Method may not always be able to quantify the value of CdA, but one can visually see a change in CdA from one set-up to another. In Tests 4,5,6, all conditions remained the same except for body position as seen below.

I have also used AeroTune to measure CdA on a timetrial bike and find the Chung method not only easier to perform, but much easier to get a visual feel for the accuracy of the results.