"Interply Shear" and other Techno Babble

I have been putting this topic off for quite some time as I was worried about overloading you with too much "Techno-babble" but I find myself having to constantly repeat this information in individual posts on various RV forums I monitor so figured it would be easier to do a complete post that I could direct people that want to understand why the loading of some trailer tires is much more complex than the average person thinks.

To make this less painful I will give the Bottom Line info first, so those not interested, can stop reading before I put them to sleep.

BOTTOM LINE
When a radial tire is loaded, the belts and body have to bend from a round shape to a flat shape in the area that contacts the road. In addition when you turn a corner the forces generated to move the RV sideways have to be transferred through the tire structure.

This causes additional bending of the belt and body structure. The more the bending the higher the stretching of the rubber. With enough stretch, microscopic cracks form and existing cracks get bigger. Eventually with enough cycles and enough force the cracks may grow and join up with the possibility of tire components separating which could lead to a tire failure. You can lower the stretching if you lower the bending and you can lower the bending if you increase the inflation.


So now on to the Engineer Speak and Techno Babble

If you own a multi-axle trailer these forces can be much higher than those seen on a tow vehicle, motorhome or car, where the tires are not close together but at the corners of the vehicle.
I found an excellent video that shows the results of these forces at Keystone RV. Watch the section from time 0:46 to 1:07 and note that the tires on one axle bend inboard while the others are forced outward.

Special consideration for multi-axle trailers. Warning, this gets technical.
When not driving in a straight line there are special side loads on multi-axle trailers because the tires are fighting each other because they are not "pointed" to the center of the radius of the turn. These loads cause interior structural tearing. Sometimes 24% higher loads than those seen in tires on non-trailer application. Initially tearing is at the microscopic level but with time and repeated cycles these forces grow which can lead to small cracks at the belt edges as seen here at the arrows.

 If not spotted these cracks continue to grow to almost the full width of the tread as seen below.

 If you are lucky you will see the bulge in the tread as seen here and now you know this tire has failed and MUST be removed AT ONCE as the separation can grow and  can cause a belt to come off the body of a tire.
You can lower these forces by either decreasing the load 24% on the tire (probably not something you want to do or may not be able to do) or you can increase the inflation to stiffen the structure and decrease the slip-angle. In this case you could increase the tire inflation from the minimum inflation needed for the static load to the inflation associated with the max tire load as molded on the tire sidewall. BUT you need to be sure you are not exceeding the max rating of the wheel.

So the best recommendation I can give to trailer owners is to run the inflation molded on the tire sidewall. For owners of a TV or motorhomes, I recommend you run the inflation needed to carry the actual measured tire load plus at least a 10% margin.