with RV tire expert Roger Marble
I had a question about what inflation to run when parking your RV or other vehicle 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 RV?”
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 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.
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 rearrange.
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 decrease these shear 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.
Read more from Roger Marble on his blog at RVtiresafety.net or on RVtravel.com.
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Having recently replaced 6 Michelin XRV tires that were 17 years old with 31,000 miles on them I take most of this advice with a grain of salt. The most useful piece of equipment you can have when it comes to your tires is a tire pressure monitor system. I don’t leave on any trip before I check the pressure and temperature of every tire. Then periodically while traveling I check them again to see how they are doing.
Hey Roger,
If I can add my two cents, to avoid the Interply Shear. I take it a step further, at the end of the RV season after I winterize the interior of the coach. I break out the hydraulic jack and raise the coach off the ground and place it on automotive jack-stands (6 tons rated each) under each axial . I have not experienced the dreaded flat spot shake.
Another advantage of having the coach off the ground, come RV season lubing the wheel bearing is a breeze with the tires of the ground. Our coach axials are equipped with grease fittings. I just attach the grease gun, spin the wheel and fill those babies up.
Thanks,
Agustin “Naka” Nakamoto
Las Cruces, NM
I always store at rated pressure. In addition, once I’m unhooked I lower the landing gear until the hitch is 2 inches above level and then lower the rear Jack’s until the entire rig is level. This leaves the tires in contact with the boards (I won’t leave it in the grass for an extended period) but with most of the load being carried by the hydraulic leveling Jack’s.