Yeah, I’m gonna get technical. Some people do not understand why RV trailer tires have such a high failure rate and assume it’s because the tires are “made in China.” Sorry, but there is a technical reason and it is called “Interply Shear.” This is the name for forces inside the tire structure that are trying to tear the tire apart. This force is there in tires whether they’re made in China, Germany, or even Akron, Ohio. Every time you turn a corner, the trailer tires are dragged around the turn, and it is this “dragging” that results in the internal stress.
I do not want to overload 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 I figured it would be easier to do a complete post that I could direct people to 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 (first)
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. Then there is 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.
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. That is due to the fact that they are not “pointed” to the center of the radius of the turn. These loads cause interior structural tearing. These are sometimes 24% higher loads than those seen in tires on non-trailer applications. Initially, tearing is at the microscopic level. However, 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. 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.
How to lower these forces
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 tow vehicle or motorhome, I recommend you run the inflation needed to carry the actual measured tire load plus at least a 10% margin.
Roger Marble
Check out my Blog www.RVTireSafety.Net
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Roger, you describe the “Interply Shear” forces on radial tires (which I suspect is much of the trailer tire market). How does this apply to bias ply trailer tires? Do bias ply tires handle the stress better? Would this be a reason to use bias ply in spite of shorter tread life and increased rolling resistance?
Interply Shear is a function of Radial Tires so Bias tires do not have the same shear forces. Also you can not have a ‘Belt Separation” when there is no “belt” in the tire.
Since I have not run the computer simulation on Bias tires I do not know how they react to turning forces. It is true that tread life with bias will be much reduced and fuel economy will suffer significantly with bias vs radial construction. With the larger, heavier RVs we have in the market today we might end up with 1 or 2 year life with bias tires on a new RV.
While not exceeding the maximum inflation pressure of the wheel, a load range tire, or tire having a higher inflation pressure than the wheel’s maximum rated pressure should never be installed on that metal wheel.
Thank you, Roger, for your unending patience throughout your tireless explanation of the physics of tires. As I told my uncle who was a physicist, “I am uncontaminated by any knowledge of physics.” However, regularly following your column is beginning to reduce the accuracy of my claim. Thank you for expanding both my understanding of tires and the safety of me and my family while driving our RV. 🙂