# Tire inflation not the same for all trailers – more on Interply Shear

RV Tire Safety
with RV tire expert Roger Marble

I have written a number of times on the topic of Interply Shear. This is the force that is trying to tear the belts of radial tires apart.

There are some highly technical papers on the topic and you can review them after a simple Google search on the term. You can also look on my blog for the posts where the term is tied to a post by simply checking the list of topics on the left side of the blog.

Basically this force comes about when the belts in a radial tire are forced to change shape, such as when a tire rolls and the footprint changes from curved to flat as the footprint rolls into contact with the ground. This force increases when external forces from cornering are also applied to tires.

So why do trailer tires seem to have such high Interply Shear forces? Well, it’s not the case in all trailers, as the cornering forces of single-axle trailers are much lower than the forces of tandem- or triple-axle trailers.

What tipped me off to this was an observation at a campground that happened to have a freshly smoothed gravel driveway and a multi-axle trailer happened to make a 180° turn as I was walking by. I noticed that the gravel marks were not a smooth curve but there was a series of turns interrupted by discontinuities.

A short time later I saw a video from Keystone RV Company of wheel lug nut torque (you should watch the full video some time).

While watching their video I recalled some special high side load tests we ran in our tire test lab. [Editor: Wow! Check it out!]

Here is a short out-take from the longer Keystone video that shows what happens to multi-axle trailer tires.

In the video you can see how the two tires are fighting each other with one bending in as the other bends out. You can imagine that if the turn is made on gravel at some point the high side load would result in a sudden breakaway or slip. That’s what I observed on the gravel turn.

Back at work, I had some high-powered Finite Element computer programs run to simulate the side load of a multi-axle trailer and the results showed that the side loads on a trailer could be 24% higher than on a standard vehicle even with identical radius and loads on the tires.

Further analysis showed that increasing the tire inflation could lower the extra shear, but unfortunately not eliminate it.

Bottom line
In a number of posts I have recommended that motorhomes set their inflation based on measured static load plus a margin of at least 10% additional PSI. This would also apply to single-axle trailers. BUT for tandem- and triple-axle trailers I strongly recommend that the tire cold inflation be set to the inflation molded on the tire sidewall associated with the tire maximum load capacity. I also recommend that the measured static tire loads on these trailers be no greater than 85% of the tire maximum, with a 20% margin being better.

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

##RVT834

## 4 thoughts on “Tire inflation not the same for all trailers – more on Interply Shear”

1. William Fouste

My understanding of Roger’s advice from reading many of his posts, is to cold-inflate to the maximum pressure molded on the tire. I myself, inflate to 5 psi less, to soften the ride somewhat as my tandem axle trailer tires (load D) are also rated over the actual weight on my tires.

2. Irv Kanode

Aargh!! just when I think I have tire inflation figured out based on your articles, you throw a curve ball.

I have a dual axle trailer with a CAT scale loaded weight of 4160 lb or less than 1300 pounds per tire.

The tires are: GOODYEAR® ENDURANCE SPECIAL TRAILER (ST) TIRES ST205/75-14. The Goodyear Load/Inflation chart lists 30lbs of air for 1300 pounds. I cold-inflate to 50lbs because that’s what the trailer sticker said for the original lower quality tires.

The max pressure is 65lbs which is listed as 2040lbs which is roughly twice the load I’m putting on them. I’ve been watching for uneven wear on the center of the tire due to my “over inflation”.

So should I be at 30lbs, 50lbs or 65 lbs?

Thanks,
Irv

1. IckyPic7

I would love to see Mr. Marble’s answer to this question.

2. Roger Marble

You are really lucky that your RV company put 4 tires on it and that it is so lightweight.
If you read my blog you know I am a strong proponent of inflating tires on multi-axle trailers well above what the minimum needed to support the measured load. While you could go with the 35 psi and meet the load requirement I would suggest you go with at least 50 psi. This should be a good compromise between lowest Interply Shear (65) and softest ride (30)