I bleed off the high air pressure so I don't exceed the "Max Pressure"

The above statement or a variation of it seems to be made about once a week on one or another of the various RV forums I frequently review. It appears that many people incorrectly believe that exceeding the 'Max Pressure" number molded on the tire sidewall is going to result in tire failure and explosion.
It may help to let people know about some of the testings that tires undergo during the development phase.

First some background. The "Max Inflation" statement on your tire sidewall really is the inflation associated with the Maximum Load capacity. If you actually read your tire sidewall you will see that is what it says. So the number of PSI is really the minimum needed to support the stated load. It is important to understand that we are always talking about the "cold" inflation and not the inflation of a tire that is running down the highway or has been in direct sunlight or driven on in the prior couple of hours. "Cold" inflation does not mean the tire needs to be refrigerated but it means the tire is at the prevailing Ambient temperature.

With the introduction of aftermarket TPMS that report both tire pressure and the temperature of the sensor, people are now being exposed to numbers, they have no experience with. They see tire pressure rising as they drive down the highway. Some may see a 10% rise in pressure from the pressure they set their tires to just a half-hour earlier. Others may see a 20% or even a 25% increase in pressure and for some, this increase was a cause for concern. Not because they had any working knowledge of tire operating temperature or pressure but simply because the pressure was higher than they expected. A few people have decided that they need to bleed down the high pressure because they thought the pressure number of the tire sidewall was the max it should ever see.  Of course, the action of bleeding down the hot pressure was exactly the wrong thing to do as that meant that the tire would no longer be operating at the pressures expected by the tire engineer when they originally designed and tested the tire specification.

Obviously, this raises the question of how much pressure a tire can tolerate before the owner should be concerned. Well, I am going to give you some numbers but these are only examples. I cannot speak for every tire companies process of specifications but if we start with a few guidelines I think you can get comfortable and hopefully, you will believe that tire design engineers do have a reasonable idea of what they are doing.

First off we are only going to discuss regular production street tires that have passed DOT testing. The numbers I will use would basically be seen on new tires. I cannot advise on a tire's capability after it has been damaged or run for many thousands of miles. Damaged tires can fail when re-inflated as seen in THIS video. Heavy truck multi-piece rims MUST be inflated in a safety cage. If you suffered a puncture and drove any distance with the tire underinflated you may have permanently damaged the body cords which is why you should always tell the tire service person you drove on the under-inflated tire so they know to use a cage or other restraining safety equipment. Age and miles can reduce the strength of any tire but we do consider this degradation when approving a design. Another consideration is the availability of high-pressure air. Most home compressors or those that supply air to hoses made available to consumers have an upper limit of about 150 psi so we don't expect any consumer to have access to or to use industrial air pressure which can be 400 psi or even special inflation units as used on aircraft tires.

Basically many tires are tested to 300 to 400% of their rated inflation or above 200 psi whichever is lower. I had a number of tires that could contain over 300 psi when new but if they had been damaged the same tire might fail at less than 200psi but I also never, in my 40 years saw a tire fail from normal pressure increase and I was involved in the testing and evaluation of thousands of tires.

The BOTTOM LINE is Please do not bleed down your hot inflation pressure. You should ONLY set and adjust inflation when the tire is cool and at Ambient temperature. Doing otherwise MAY result in you having a failure days or weeks later because you were running lower pressure than what was needed for the tire load you were applying.

Quick post on Belt Separation not a "Zipper" failure

Starting to catch up on posts on various RV forums after 10 days on a beach in Jamaica not thinking about tires BUT here we go.

Post about this tire that lost it's tread.
 

There was a reply about "zippering"

---

"Your tires look like they threw the whole tread off. I always assumed that the treads I see on the road are bad retreads from truck trailers, but it appears that this is not the only source of thrown treads.

Note that Michelin has documented a "zipper" condition on their motorhome tires. It consists of a circumferential crack on the sidewall just where it meets the tread. It could be the start of a delamination of the whole tread, which is what you seem to have.
You might look this up on the Michelin website. It might result in a warranty claim.
The only good way to protect against such a problem is by inspecting the tire regularly, which is not easy to do with an inside dual, especially since the zipper starts on part of the tire only and doesn't start on the full circumference. I wouldn't have any confidence at all that a TPMS would catch such a problem."


While the reply was well intended it didn't in my opinion offer the correct  answer. A closer examination finds areas that were rubbed smooth.

This is usually an indication of long term separation probably after operating for hundreds or possibly thousands of miles with a small separation at the edge of the two steel belts.

 I commented that "IMO the picture indicates not just a "tread separation" but the top belt separated from the bottom belt. This is a long (thousands of miles) term progressive failure. Many times a close thorough inspection can discover the signs beforthe tire comes apart."
I also advised "RE "Zipper" comment.  A classic "Zipper" failure occurs not at the tread sidewall jusnction and certainly not in a textile body tire as the one seen in the picture but is a steel body tire that has been run severly underinflated. This results in steel fatigue in mid sidewall as seen in the picture in THIS post.

##RVT832

"Only" drove 750 miles with companion dual tire low on air

Quote:

Originally Posted by Charlie What should I do? I had checked tire pressure the day before and only drove about 750 miles when the right rear inside duel blew resulting in damage to the MH and the tow. Only the side wall blew and the tire stayed on the rim. This is the first time in over 25 years of RVing that I have ever had tire trouble. I only had about 14,000 miles on these tires that are less than 2 years old. I bought the best tires they recommended. I will not say the brand but it begins with M. I had pressure set at 120 psi and they were filled when the tires were cold and had not been driven for over a month, Should I be concerned about the other tires?

Should you be concerned? It depends. Why did the tire fail? If the failure was a sidewall flex failure (sometimes called a "zipper") from running low then there was most likely an active leak, i.e. puncture or leaking valve core would be on the top of my list. In this case the other tires are no more likely to fail next week than they were last week with one exception to be covered in a moment.

If you take the failed tire to a heavy truck tire dealer for the brand involved they should be able to confirm the mode of failure.

Since you were not running a TPMS (tire pressure monitoring system), all we know is that probably sometime between checking the tire pressure and 750 miles later (2 days?) one tire had lost most/all of its air. We also know that the outer dual was run with possibly 100% overload so it probably sustained internal structural damage that shortened its life (months or years). Since we do not know how many miles at what % overload, it is impossible to predict -- so the suggested best practice is to replace the companion dual.

One thing we can do is to use this as a learning experience. With a TPMS a driver would get early warning and many times can stop before a tire is damaged and, by extension, avoid damage to the RV and even not damage the mate in a set of duals. A single warning in the life of a TPMS (years) could save much more than the cost of the system.

Now as to the other possible reason for a tire to lose air. It is possible for a piece of grit to get into the valve core and allow air to leak. I suggest you review the posts on "Valves" (list to the left) and you can see an actual example of a leaking tire and the grit that caused it. So, as I have pointed out in the past, the very act of checking air pressure can sometimes result in a leak and potentially a failed tire.

##RVT797

Zipper "Blowouts" on Class-A RV

I noted in an RV forum someone was changing from one brand tire to another brand. One reason given for the brand change was because they had suffered two "zipper" failures with their 22.5" steel body radials. As I do many times when reading RV forums, I posted a link to an earlier post in this blog where I had suggested that a "zipper" failure can occur with any brand steel body radial.

As I try to support my statements with more than just my opinion I dug out an article from Tire Business magazine. In the article it was pointed out that "circumferential fatigue ruptures - known in the trade as 'zippers' in steel radial truck tires" are a failure that can have serious safety consequences and are not just an inconvenience to the RV traveler.

The article goes on to point out that "a zipper rupture is a tire failure in which the tire blows out in the upper sidewall, leaving an even line of broken steel cords exposed, which gives it the appearance of an open zipper. They are typically 10 to 12 inches long but can extend up to 36 inches in length."

The large amount of flexing can occur when a tire is significantly under-inflated. When one tire in a pair of duals looses air it can mean the failure takes longer to become obvious and in the meantime structural damage is being done to the tire that had not lost air but is now being asked to carry up to 100% more load than normal.

What makes this type of failure dangerous is that it is entirely possible for a number of steel body cords to have failed but the rubber in the sidewall has not ruptured at the now low inflation pressure. If that has occurred and the RV owner happens to discover a tire with low pressure then decides to "fix" things by re-inflating the low tire, the damaged tire may now rupture with explosive force when higher pressure air is introduced to the tire.

If you wonder what can happen when a tire explodes a simple Google search of "tire explosion" will return just too many results. Or you can watch this compilation video and ask yourself if you would want to be adding air to any of these tires.


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