In 2015, when we redesigned our road and tri-wheels, we studied the relationship between tire pressure and aerodynamics. Our thought was that a change in tire pressure would affect the tire size and shape, and therefore affect the aerodynamic performance of the wheel. To test this, we needed an accurate pump.
Here is a video about this article if you prefer that format.
We reached out to a good friend, Josh Poertner who owns Silca, and chatted with him on tire pressure and pumps. We found out that a standard floor pump was accurate to +/- 8 psi. This means that if we set two wheels at a theoretical 90 psi, we could have one at 82 psi and the other at 98 psi. A standard pump would not be accurate enough to test the small differences in tire pressure that we were looking for.
The most accurate pump in the industry is the Silca Superpista Ultima which has an impressive +/- 1psi accuracy. Still, this wasn’t accurate enough for our specific need. Josh offered a test rig nicknamed, “The Truth” that would get us to +/- 0.1% accuracy. That worked for us.
What We Found Out. The Good And The Bad
The Good- after testing, we found our prediction correct. A change in even 5 psi could result in a difference of 30 seconds over an Ironman. This was a significant finding that we knew needed further investigating.
The Bad- We found once tire pressure was set with 0.1% accuracy, removing the pump head from the presta valve released a puff of air. That puff of air could result in a change of up to 5 psi. To investigate further, we needed even more accuracy for setting tire pressure.
How We Fixed Tire Pressure Loss When Removing a Pump
When we started designing the gravel wheels, we started by solving the problem of air loss when removing the pump. We designed our own custom version of “The Truth.” We built a custom fitting, composed of a small ball valve that allows us to stop air flow. This fitting attaches to a presta tube so we can close the air flow before removing the pump head, therefore, no “puff of air” and we have accurate tire pressure readings. Problem Solved.
Why Did We Need Tire Pressure To Be So Accurate?
To accurately study rolling resistance and aerodynamics, we had to have a way to accurately set the psi of the wheel. The first test conducted was how tire pressure affects the height and width of tires on rims with different internal rim widths. This sounds like a good topic for Part 3 of our FLO Gravel Wheel Design Journey, doesn’t it? Stay tuned for next week.
The puff can come from the hose or the tube. It has a lot to do with removal technique. So depending on pump, style, removal, etc, you can lose pressure in the tube.
I thought the “puff of air” comes from the pump? How does this affect tire pressure readings? Even if the the hose was a mile long, all pressure loss would come from initially inserting the gauge then the pressure between the tire and gauge would be equalized, once the gauge is removed the tire stays at the same pressure and the gauge goes to 0 (that’s 0 gauge pressure).