Why inflate motorhome tires differently than trailer tires?

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RV Tire Safety

with RV tire expert Roger Marble

I found the following comment in a thread on an RV owners’ forum after there were comments about the advantages of inflating trailer tires to the tire sidewall inflation number but inflating motorhome tires based on the measured load on the tires: “Such a hard concept for most to understand.”

As an actual tire design engineer – not just someone that has used a lot of tires, or bought or sold a lot of tires – I feel I might have a slightly better understanding of the science behind why tires fail.

I try to make the information easy to understand but I find that many simply refuse to accept the fact that my 40 years’ experience that includes thousands of failed tire “autopsies” might qualify me to give sound advice.

If you simply look at the experience of three groups of tire users, excluding punctures or pothole impact breaks:

1. Regular motor vehicles: People get about 40,000 to 50,000 miles before the tires “wear out.” Less than 1% experience tire failures.

2. Class A and Class C motorhome users: Many only drive 5,000 to 8,000 miles a year. It is recommended that starting at 5 years of age, tires be professionally inspected. This does not mean a simple walk around to look at the tread depth, but close inspection with good lighting – maybe even using a pit to allow the inner sidewalls to be inspected. Annual inspections thereafter are recommended and replacement at 10-year tire age “no matter how good a tire looks.” This group also has a low structural failure rate not traceable to air leak or impact.

3. RV trailer users’ “towables”: Based on numerous reports of higher structural failures, i.e., belt/tread separations, and some strange patterns left in loose gravel where a trailer was turned 180 degrees, I had some computer simulations run and the numbers provided an explanation for “why” towables have a much worse structural failure rate.

The forces inside the tire structure are significantly higher (+24%) in trailer application (i.e., towables) than in motor vehicle applications. This force is identified as Interply Shear and it shows up as trying to separate the top steel belt from the bottom steel belt in radial tires.

While it would be possible for RV trailer companies to make design changes to trailer suspension to allow for “passive steering,” as seen on large cement trucks with a tag axle, I doubt they would go to the expense simply to extend tire life.

While lowering the actual load on a tire in trailer service can lower the Interply Shear force, I doubt that it is possible to lower the load by 40% to 50%. One thing trailer owners can do to lower this force is to increase the cold inflation to the tire sidewall “max.” Sorry to say you cannot reduce the Interply Shear to zero, as this is the nature of radial tires.

 

Read more from Roger Marble on his blog at RVtiresafety.net.

 ##RVT905

 

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Tom
1 year ago

Appreciate the insight provided in this article by author and commentors.

Drew
1 year ago

Maybe the best cure for tire failures of any kind is to simply be a better and more cautious driver- giving due consideration to the forces acting on the tires and using driving patterns to help mitigate some of those.

Jeff
1 year ago

Right On, as usual Roger!

I was recently chastised for running my 5th Wheel tires at their Cold Pressure rating. (Sailun 17.5 H Tires) at the Cold Tire Pressure of 125 psi. Of course I have a TPMS on the tires and monitor them closely when traveling. Have not had any problems to date and have a little more than 8000 miles on them so far.

RVers need to do their research and listen to the experts!