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Friday, February 21, 2020

Why inflate your tires to their max, when parking for long time?

I had a question about what inflation to run when parking your RV or vehicles for a long time. I initially said that would lower the Interply Shear.

Then, I was asked
"Could you explain interply shear on a parked R/V?"

Any time a tire is deformed (loaded) the cured rubber in the belt area moves away from the "as cured" shape. Even if not rolling, the area that is now flat on the road has been "bent" from the as-cured original curved state.
This bending causes shear (tearing) forces between the belts, which, at the molecular level, can result in bonds breaking between the carbon, hydrogen and sulfur atoms. If the bending is sufficiently large, the tear gets larger.
If you have higher inflation in the tire, the bending is less than when you have lower inflation which takes us back to decreasing the molecular tearing.
Other things happen too. "Cold Flat spotting" where a portion of the tread ends up flatter than the portion not loaded. This difference can result in vibration and shaking once you start driving. The "flat spot" also has to "work itself out" when you start driving. Again the change in shape when driving is rapid which can result in those broken molecular bonds. Slower changes in shape as when you inflate a tire allow the rubber to move or even "flow" a bit so the atoms have time to re-arrange.

If you ever played with "Silly Putty" as a kid you have experienced this effect. When you slowly pulled the putty it would stretch but if you yanked it quickly it would break. Silly Putty is a form of synthetic butadiene rubber which is very similar to the synthetic rubber used in tires

The whole concept of getting a longer tire life and better belt durability is to decreases these shear (tearing) forces that come about because of the changes in the shape of the belt package.
Now I don't expect people to inflate your tires every night or even every time you go camping for a few days. But if you are parking for a month or more over the Winter, it might be worth the effort. Only you know how much effort it would take to adjust your tire pressure and if you think a few extra weeks or months of tire life is worth the effort.


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