Optimizing Bicycle Tire Pressure on Gravel - Part II

This is Part II describing studies to determine which tire pressure gives the least rolling resistance on gravel.  You should read Part I first. Be aware that the results are unique to my weight, tire, and road surface conditions.

[For my genealogy research readers:   You are at the correct location.  As you know I get distracted at times.  More genealogy research results will come soon.  My most recent post on Y-DNA is here.]

In Part I, I found the surprising result (to me) that my drag at a tire pressure of 30 psi was less than my drag at 40 or 50 psi. I calculated the power savings to be about 10 watts. That is similar to the savings I obtained by getting into an aero position (all at about 15 mph).

There were some flaws with that study, which I tried to correct in Part II. The following changes were made to the study for Part II:

1) I used a two-sided power meter instead of one-sided.
2) I measured wheel circumferences at all tested pressures. (Lower pressures have smaller circumferences.  The circumference is a speed sensor setting.)
3) The new loop is at a dip in the road.  This allows for speed variations and easier identification of laps.
4) I started and ended tests at V=0 vs. in-motion starts and stops in Part I.
5) Test on a less windy day.

This study still had some flaws which I think I can address in Part III or IV. They are:

1) My pressure gauge is neither accurate nor precise.  (A new one is on order.)
2) There is a section of paved road at the bottom of the dip. This results in some noise in the data seen at the bottom of each data curve.
3) I only made 4 laps per test (vs. 6 in Part I), but the laps were twice as long as in Part I.

The Test Variables

Only the tire pressure was varied.  All other variables were held constant.  Three pressures were tested: 27, 32, and 37.  The 32 psi test was repeated at the beginning and end to test for reproducibility.

The data are reported as 'Virtual Elevation' (VE) versus distance. An increase in slope indicates more power was needed for that test condition versus the base case. Test 4 is the base case.

The data are a bit cleaner than the data from Part I, however the test is not reproducible from Test 1 to Test 4.  In my next study, I will do a duplicate at the beginning of the study (Test 1 and Test 2) and again at the end, to nail down the variability of this test.

If I use Test 4 as my baseline and choose CdA and Crr that will make Test 4 level, it can be compared to Test 2 and 3. In this case 37 psi is a 3 watt penalty and 27 psi is a 3 watt savings versus 32 psi.

I lack 100% confidence in these results.  However, I have changed my default riding pressure to 28 psi in front and 30 psi in rear, so I do have some confidence that what I am seeing is real.

Part III will be coming soon!