Why do radial tires come apart? “Interply shear” is the answer.
The entire reason we are doing this in-depth series is to better understand how to avoid tire failures. If you haven’t read the first two parts, you can read them here: Part 1: Is your RV tire temperature or pressure too high? Part 2: The air pressure supports the load.
The following is updated from my blog post November 20, 2023, and on RVtravel.com.
“Interply shear” and other Techno Babble
I put this topic off for quite some time as I was worried about overloading you with too much “techno babble.” However, I find myself having to constantly repeat this information in individual posts on the various RV forums that 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
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.
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. (Please ignore the YouTube advertisements.) Watch the section from time 0:46 to 1:07, which has the view important to this post on tire inflation. 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 molecular 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.
I would not consider this tire to have “failed,” as even with these small cracks the tires can deliver all the turning and ride forces the vehicle owner expects. However, with thousands of more miles, 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 take action.
Now you know this tire has failed and MUST be removed AT ONCE. Otherwise, the separation can grow and cause a belt to come off the body of a tire. This is sometimes referred to as a “blowout” by many folks.
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, as motor vehicles still have interply shear but at a much lower level.
The best we can do is to lower the interply shear. This force cannot be eliminated.
Roger Marble
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If you have tire questions, check out Roger’s Blog as well as his posts on RVtravel.com. There are hundreds of posts covering everything to do with tires.
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Thank you for the “details” of tire shear. I get it now and I know to do a better job on tire inflation.
Thank you, Roger! 🙂 Very helpful! Have a great week and safe travels! 🙂
So this kind of article just scares me. I was having fun with my RV until now. Thanks.
Steve, You don’t need to be afraid. I try and offer guidance to RV owners so they do not have to think or worry about the “Techno Babble”. Remember, I’m the professional so its my job to do the worrying for you.
Jim J. emailed Roger Marble and asked this question: “In regards to interply shear, which is the better tire, radial or bias ply? Most online comparisons talk about other differences between the two types, but not this one. While I suspect a radial tire would still be my best bet overall for towing, I’d like to know if there is any difference in how interply shear affects the two types.” Roger replied: “Jim, interply shear is a phenomenon unique to radial tires. Bias ply tires do not have this as they have no belts.” Thanks, Roger. 😀 –Diane at RVtravel.com