Saturday, December 10, 2022


Radial tire belt “interply shear” and tire failure


RV Tire Safety
with RV tire expert Roger Marble

Originally posted on Airstream forum, but the answer applies to anyone running radial tires.

“From my reading, if you see a post where the tire failed with the tread coming off AND the tire carcass still held air pressure, that is likely to be an interply shear failure. It is my understanding of the issue, that interply shear (twisting of the tire) breaks the bond of the tread from the tire carcass. This may start as a small section that can be seen as a bubble under the tread. Then rolling the tire expands the failed area until it all comes apart.

“For what it’s worth, I’ve seen plenty of these in various posts. I suspect it is also possible for the tread to separate from the carcass and in the process of self-destructing, punctures the tire carcass resulting in tire deflation. Do I understand the consequences of a high level of IPS?”

My reply:

The short answer is YES.

OK, now to the questions of interply shear (IPS) (nice acronym BTW).

If you have reviewed my numerous posts that mention IPS you can learn the background and the steps suggested to lower this destructive force.

All radial tires exhibit this force. It is a function of having belts under the tread that are at a high angle relative to the low angle body ply. Here low angle is about zero with the body ply running radially from bead to bead. Belts are generally in the range of 60 to 70 degrees relative to the body ply. The two belts ply or layers run in opposite directions and for the width of the tread. NOTE:  Different tire companies use a different reference for the “radial.” Some call that 90 degrees and they say their belts run in the 20- to 30-degree range, but the result is the same as only your reference changes.

Do tires ever fail due to IPS? Yes, it is these forces that initiate microscopic cracks which grow over time and use. Air loss or not is not a controlling factor as air loss can occur because the belts have separated from the body, which allows tearing of the rubber between the body ply cords, which then leads to air loss. This can occur in fractions of a second so the air loss is indistinguishable from the belts and tread detaching from the body. The rapid loss of air can sound “explosive,” which leads many to use the catch-all term “blowout”.

In THIS post the two PRIMARY reasons for tires to fail are covered. We are not talking about air leak here.

It is the air pressure that supports the load, not the tire construction. (Yeah, the tire does support some of the load but maybe only 5% at best, so we are discounting that.)

In general, a stiffer tire can generate higher cornering force than a tire with low inflation. Cornering force is not just from the contact area. This is well known in the racing community as our tires generally run higher pressure than we would run on the street. I know this from first-hand experience running and winning numerous road course events in my Camaro. (Six-time winner of the 24-hour race at Nelson Ledges, Lap records at 6 different tracks including Lime Rock, Mid-Ohio, Watkins Glen and others.)

Click to enlarge.

I ran real “DOT street tires” as required in my class, not special-purpose-built racing tires that wouldn’t last 15,000 miles of street use. Those other tires were only available from race tire dealers. Most of the time I ran 34 to 36 psi cold vs. an estimated 20 to 22 psi, which is what I would have needed to simply support the actual load – so clearly more contact area from lower inflation did not provide race winning results.

We don’t need to get into the sales (price) and marketing decisions of RV companies on what size, type or brand tires they supply. We as RV owners are trying to get the best durability and overall performance from the tires we run on our RVs.

Tire durability (not coming apart) is our number one goal. You can choose to follow our recommendations or not. All I ask is that you not complain if or when you have a “blowout” that has the root cause of the failure traced to a failure to follow my recommendations. Lowering the IPS force can be accomplished by increasing the margin between the tire load capacity at a given inflation and the actual load on your tires.

You can accomplish this with larger tires or by unloading your RV, but not everyone can do those things. This leaves increasing the tire pressure. Especially on multi-axle trailers, you need to do all you can to increase the margin, and running the inflation molded on the tire sidewall can be done by, and is recommended for, trailer owners.

Read more from Roger Marble on his blog at




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4 years ago

This year I’ve had two TOWMAX ST tires fail on my 5th wheel. Both had a bubble, the size of a silver dollar, under the rubber sidewall that popped open. Below that you could see the belts and could hear air escaping. Both held up long enough to find a safe place to put on the spare. What I suspect happened is that the belts started to separate, then air began to leak through the belts, caused the bubble which eventually popped. When it popped I was travelling down the highway just under 60 mph and it sounded like a piece of wood had been dropped into the truck box. The tire load I had on them was 500 lbs under the limit per tire at maximum recommended PSI. I now run Goodyear Endurance tires so we’ll see what happens.