Optimizing Bicycle Tire Pressure on Gravel - Part III (The Part where freshly-waxed chain break-in time is revealed.)

This is Part III describing studies to determine which tire pressure gives the least rolling resistance on gravel.  You should read Parts I & II 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).

In Part II, I made some improvements in the testing protocol, but I began to question the reproducibility of the data.

This study, Part III focused on reproducibilty of the protocol. The changes from Part II are:

1) Silca Pump used with advertised accuracy of 1%.
2) Wheel circumference was remeasured using new pump and a 10 revolution loaded-roll-out on smooth pavement. (Average of 5 roll-outs.  All pressures tested had measured circumferences with std dev of less than 0.03%.)
3) New test loop was in a traffic-free, wind-protected dip, with no paved sections.
4) 8 laps per test (vs 4 in Part II). Total distance per test was 2.21 miles vs. 2.09.
5) Freshly-waxed chain

The Test Variables

Only the tire pressure was varied.  All other variables were held constant.  Three pressures were tested: 20, 30, and 40.  The 30 psi test was repeated 4 times to test for reproducibility.

The results are shown in terms of Virtual Elevation (VE). (See Part I for short explanation of VE). The steeper the slope of a Test, the more power is being lost versus other Tests. [Note how much cleaner these data are than in Part I and II.  All of the laps are clearly visible. The VE of the dip is more consistent than Part II (due to lack of paved section?)]

Analysis of the data revealed that the break-in of the freshly-waxed chain overshadowed all of the other variables! The negative impact of using a freshly-waxed chain was about 14 watts during Test 1 versus Test 6. Even after 9 miles of riding there appears to be break-in losses. The abrupt change in slope in Test 3, when the pressure was lowered to 20 psi, suggests there could be some reductions in rolling resistance, making the break-in losses appear less severe.  When the pressure was increased to 40 psi in Test 4, the abrupt change in slope in the opposite direction indicates more loses versus the 20 psi Test.  The flattening of the slope at the end of Test 4 is a bit puzzling. The slope at the start of Test 5 could be viewed as a continuation of the curve begun back in Tests 1 and 2. The last 3.5 miles of riding appear consistent, indicating the break-in is complete.

Conclusion

The objective of showing reproducibility was not achieved in Part III. (Sorry...Part IV is coming.)

The freshly-waxed chain (SRAM 12 sp with SILCA Super Secret Hot Wax) was shown to take at least 10 miles or about 45 minutes of riding to reach a steady state. 

Also see 7:26 minutes into the Silca video: Chain Waxing? Avoid These 7 Common Failures! (youtube.com)

GCN beat me to it.  This Chain Waxing Mistake Makes You SLOW (youtube.com)