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I promise this is my last response here--no matter what!! I've tried to stress that we're talking "theory" versus the real world. But to answer the question, "How do you create a negative pressure in a tire:"

Once the gut valve is removed (or the valve depressed) the air will leave the tire until the pressure inside the tire equals atmospheric pressure. [Put a pressure gauge on now and it will read "zero." In reality, though, the gauge is reading "zero differential between the pressure inside the tire versus the atmospheric pressure outside the tire. The TRUE pressure in the tire is (assuming you're at mean sea level and the temperature of the air is 59 F) 14.7 psi--the same as the atmosphere. You'll sometimes see "PSI" referred to "PSIG". This is a technical way for engineers to clarify whether a psi is referring to absolute pressure (PSI) or the pressure measured on a gauge (PSIG) which is, again, the difference between the pressure inside a closed system versus atmospheric. And, yes, this does mean that as the atmospheric pressure changes (high pressure weather systems/low pressure systems) the volume (size) of the tire does change e.g. when that rainy weather approaches and the atmospheric (barometric) pressure drops the tire expands (not enough, of course to measure) since the pressure inside the tire is "closed" and doesn't change--and thus the PSIG doesn't change.]
But I digress. Back to the tire: We've opened the gut valve. The liquid charge and some air remains inside. At that point removing one more molecule of fluid (a fluid can be either liquid or gas)reduces the pressure inside the vessel (tire.) Whenever the pressure of a fluid in a closed system is less than atmospheric it is expressed as a "negative" pressure, otherwise known as a partial vacuum. Since the atmospheric pressure is constant (for this theoretical discussion), as more fluid is removed from the tire the sidewalls begin to "collapse." This is literally the pressure of the atmosphere crushing the tire as the negative pressure differential inside increases. If the bead didn't break the tire would eventually completely come together inside. Remember the old saying "nature abhors a vacuum"? This is the colloquial way of saying all systems try to equalize--whether you're talking about temperature, pressure, electrical voltage, etc. This is why a pump is needed in the first place! Turning the tire upside down to drain the fluid would take forever, unless a way is provided for air to enter the tire to replace the draining fluid, thus continuously equalizing the pressure. Hence the "glug, glug, glug" of the inverted soda jug--the negative pressure created in the bottle as the liquid drains is relieved by the atmosphere forcing itself into the jug, pushing the the water out of its way momentarily in the process.
Pehaps some of the head-scratching is caused because we don't usually measure "negative pressures" in "psi," in the same way that we don't usually express atmospheric pressure (barometric pressure) in psi. Rather it is usually expressed in "inches of mercury" or "mm of mercury."
Enough already! :)

Liquid Tire Ballast
New Holland Tractors
Replies 65 Post by dan t