Don Terrill’s Speed-Talk

So the mornings have been getting a lot cooler, 45f-50f and I set my cold tire pressure accordingly to 35psi. Well it still gets warm, upper 70f to lower 80s still. At that point they feel like hockey pucks. The pressure gain goes from 35psi cold to 44-45psi after a highway ride. Sometimes higher if it's over 80f.

So I air them done to 40psi. Much better. No additional pressure build up after doing that riding around. So it appears I'm underinflated cold and the heat build up is increasing psi along with the rising air temps during the day. Ive also noticed that my max tire loading is at 44psi. I assume if I'm not at max load I can air them down a bit and be ok?

That seems like a big rise in pressure with the small temp changes. Is the air very humid where you are at? Have you considered switching to nitrogen to fill the tires? The pressure increase with nitrogen is much less..

Just a thought.

bobalattie wrote: ↑Thu Oct 05, 2017 10:37 pm The pressure increase with nitrogen is much less..

No, sorry, all gases obey the same pressure vs temperature relationship.
https://en.wikipedia.org/wiki/Boyle%27s_law
The only thing useful about nitrogen in tyres is for aircraft so that it is impossible for inside of a tyre to catch fire when landing.

Circlotron wrote: ↑Sat Nov 18, 2017 5:36 am

bobalattie wrote: ↑Thu Oct 05, 2017 10:37 pm The pressure increase with nitrogen is much less..

No, sorry, all gases obey the same pressure vs temperature relationship.
https://en.wikipedia.org/wiki/Boyle%27s_law
The only thing useful about nitrogen in tyres is for aircraft so that it is impossible for inside of a tyre to catch fire when landing.

That's true, except for one thing, the nitrogen that goes into the tires is dry, but compressor air that goes into the tires is not. Moisture inside the tire (when using compressor air) causes a greater change in pressure in the tire with temperature change than nitrogen (dry) does. If dry air was added to the tire, instead of compressor air, the tire would have pressure fluctuation akin to nitrogen.

Guy at the tire store suggested the nitrogen too. He also mentioned most of the tire decals on the door are worst case scenario. The cold tire inflation takes into account being at max load for the tire so there is a safe cushion. If you're not at max load you can go down some on the cold pressure by a few pounds. My tires must have had a ton of moisture. They'd be 35psi cold in the am at 25F and by afternoon in the 70s after driving they'd be over 42psi. Rode like I had hockey pucks for tires.

30-32psi cold seems to be my sweet spot. That's with the nitrogen. After a highway drive the psi is in the mid to upper 30s. Much better.

The moisture in the air that was used to fill the tire is the likely culprit (as another poster stated) of the large swings in tire pressure vs. temperature.

The OEM that I worked at, used very dry air (less than 5% relative humidity as I recall) to fill the tires in order to avoid this problem.

Try deflating your tires and refill them with air that has gone through a good water trap after the compressor. This will usually result in air with a R.H. of 15% or less.

A normal pressure increase from the "cold" pressure to "hot" (after hours of highway driving on the same day) would be about 4 to 5 psi.

Deflate them hot and possibly more than once, as there will be liquid or solid water present when cold for there to be a pressure rise issue.

Boy this crap brings back memories of me sitting in both my Chemistry and Physics classes years ago.

I had to take 5 hours each of

Physics and Labs;
Calculus; and
Chemistry and Labs

for two f** semesters back in Jr. College! Talk about making ol' pdq67 a dull boy!! 30 hours of the stuff!!


The Ideal Gas Law
From Wikipedia, the free encyclopedia.

The ideal gas law, also called the general gas equation, is the equation of state of a hypothetical ideal gas. It is a good approximation of the behavior of many gases under many conditions, although it has several limitations. It was first stated by Émile Clapeyron in 1834 as a combination of the empirical Boyle's law, Charles's law and Avogadro's law.

The ideal gas law is often written as P V = n R T
where:
P is the pressure of the gas,
V is the volume of the gas,
n is the amount of substance of gas (in moles),
R is the ideal, or universal, gas constant, equal to the product of the Boltzmann constant and the Avogadro constant,
T is the absolute temperature of the gas."

pdq67