When is Minimum inflation the Maximum inflation?

Sometimes the information on inflating tires can lead to some confusion.  OK a lot of the time it can be confusing to many.

As many of you know I try and follow a number of RV forums and offer comments. I try and focus the comments to correcting serious or significant errors or misunderstandings, especially when Safety Related.

Recently there was some confusion about the Maximum allowable inflation for a tire. Some wanted to co-mingle some information from PSR or passenger car tires with information about large TBR, truck bus radials. The discussion then went way off track. Rather than limit my audience to those following that thread, I decided a blog post would be more appropriate. Also I wanted to be sure to have all the information up to date and accurate I contacted an "old friend" from the tire industry and he sent me this nice summary. With his permission I re-print it here.

First some definition of terms may be appropriate as "tire engineer speak" may confuse some.

"Seating pressure" this is the inflation needed for the tire beads to "pop" home against the wheel/

"FMVSS" these are various Federal Motor Vehicle Safety Standards. If you need some reading that will put you to sleep HERE is a link offered by NHTSA.

 "NHTSA" NATIONAL HIGHWAY TRAFFIC SAFETY ADMINISTRATION

"PSR"  Passenger steel belted radials or the normal tire you find on todays cars

"TBR" Truck Bus Radial  as found on most Class-A RVs

"LTR"  Light Truck Radials 

"CIP" Cold Inflation Pressure

OK here is the nitty-gritty

"First, the max pressure to seat beads as a matter regarding the technician’s safety is 40 psi, whether passenger, light truck, or truck-bus.  This bead seating pressure is totally independent of the tire maximum operating pressure.

It is important to clarify the differences in sidewall markings of the tires you bring up:

PSR & LTR Tires (load range E or less):  Since FMVSS 139, these tires have sidewall markings indicating maximum load AND maximum pressure.  Maximum load means max [static] load, and maximum pressure means max [operating] pressure (cold).  With respect to the minimum pressure that carries the maximum load there is a difference:

·         Passenger Tires:  These tires are usually marked with a maximum pressure that exceeds the pressure necessary to carry the maximum load marked on the sidewall.  For example, the tire may be marked with 44 psi max pressure, but only requires 35 psi to carry the max load.

·         Light Truck Tires (load range E or less):  These tires are usually marked with a maximum pressure that is also the pressure required to carry the maximum load.  For example, a load range E tire marked with max 80 psi would need that same pressure to carry the max load.

TBR Tires (and light truck load range F and higher):  On these tires the sidewall markings indicate maximum load AT a certain pressure (the word “maximum” is not used in regards to pressure).  Maximum load means max [static] load, but the pressure is not the maximum operating pressure (cold).  This marking just follows FMVSS 119.  Essentially, the pressure marking is informative, simply telling the reader the pressure that is required to carry the maximum rated load.

However, for most practical purposes, on TBR tires the pressure marking is typically considered the maximum pressure recommended in the tire while in ordinary service.  Certain situations may permit cold inflation pressure higher than the marking, usually in consultation with a tire manufacturer for a specific product, application, and service.

Regarding load-inflation tables:  As long as you are looking at the right table, this is where you find the pressures needed to carry certain loads for a given tire type, size, load rating, etc.  Note that for truck-bus, you might need to make sure the tire is a “T&RA tire” or an “ETRTO tire” since the tables can differ, even though the size codes are the same (such as 295/75R22.5).  Also, a tire manufacturer may have unique load-inflation table(s) associated with certain tire models, sizes, etc.

If an operator is running at max load, and the pressure to do that happens to be max pressure, then yes, they need to be diligent.  But it is manageable, and they owe it to themselves and to others on the road to do so.  Pressure loss through permeation requires minimal adjustment approximately once a month.  For predictable swings in temp, set the pressure when it is likely the coldest, and try to consistently check it during those times, such as early in the morning before setting out.  No one is saying that everyone in all circumstances needs to set pressure to +/- 0.1 psi every morning, noon, and night with three hours of ambient cold-soaking before taking measurements.  I agree with you about reducing load; not just due to the tire influence, but also drivetrain/axle and chassis wear and tear, braking performance, fuel economy, etc."

So we see there are similar but different words on the sidewall of tires. Some have a stated Max cold inflation others do not. This is one reason why it best to have tire service done at a store that has the appropriate equipment and training to handle safe and proper mounting and inflation of the type tires you are working with. This does not mean you can't add 5 or even 10 psi to your tires but IMO if you need more than 20% of the CIP there is something wrong and you really need to consider having a professional inspect and re-inflate your tires. Inflating an improperly mounted, improperly repaired or damaged tire can injure or even result in death if not handled properly.
OK now back to our regular programming.
One other comment I have is that many times some think tire failure "Blowout" is caused by too high a pressure but this is essentially incorrect. Unless you have damaged the body ply cords, be they Nylon, Polyester, Steel or Rayon by over flexing and running significantly under-inflated, tires are designed to tolerate the normal pressure increase seen when running highway speeds at the approved load.

But if you have damaged or run the tires in overload or under-inflated for the actual load or perhaps at a speed higher than the tire rating you may have damaged the cord sufficiently that it has lost a portion of its strength so in that case even normal cold inflation may be too high. This is one reason any tire that has been damaged must be rendered un-usable or if it appears to be OK then inflated in a Safety Cage.


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Understanding Tire Failure

Yes, when a tire fails there is almost always evidence left behind that points to the Root Cause. I have shown this in tire "autopsy" posts on my blog.

Some "causes" are easy to see or understand:
Punctures, cuts and external damage from hitting potholes or other "road trash" can lead to leaking air (fast or slow) or in some cases broken body cord (see my blog post on sidewall bulges).

"Blowouts" with sidewall flex failures are most likely the result of air leak, not just low air pressure due to poor maintenance. The leak could be caused by puncture, cut, leaking valve, cracked wheel or the like.


Other failure modes are harder to detect, such as:

Belt separations come from long-term heating of the tire belt/body interface. This heating comes from a combination of insufficient air pressure for the load and excessive speed.

Now while separations may be harder to detect, many can be found if a little effort is made by the person doing the tire inspection.

I previously published a video showing one way to "measure" a defect.

Sometimes you don't need to "spin" or rotate the tire but just get a good look at it and roll it across the floor for a full revolution as one part may look OK while another portion clearly would only look straight to a drunk.

If any of your tires look like the above, I would bet $$ that they had a belt separation similar to what is seen here.

Driving on a tire in this condition is simply asking for trouble as it could come apart when you least expect it as you drive down the road.

As we move out of Winter and you "DE-Winterize" your RV, especially trailers, Please take the time to do a thorough tire inspection.  Spin or off-the-vehicle inspection is much better than a stationary on-the-vehicle "Look At".




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Is it "Safe" to add air to your tires?

Got a question I have never heard before but thought I would share it and the answer with you as there is some safety related information to be gained.
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Leih P.           asked
Just met an RV owner who was told by a Discount Tire salesperson that it was too dangerous for anyone to add air to a tire that was on the coach, as the risk was too great.  Apparently, from the rim or the tire exploding.  Even they weren't allowed to add air unless the tire was in a cage. 

Having to take the vehicle to a service center every time one needs air sure won't promote maintaining proper inflation.  Is it ok for the owner to check and maintain proper pressure, regardless of the tire size?

Thanks,  Leigh
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I replied,

Yes, Leigh, you can add air to your, passenger car, boat trailer, light truck, Class A, B or C Motorhome or RV trailer or dolly. It is even OK for you to add air to your wheelbarrow tire or lawnmower tires. :-)

Now having said that, there are a few times when the use of a cage is advised

1. I would consider the use of a safety cage as MANDATORY for initial inflation if covered by OSHA Regulations   i.e tires larger than "LT" or tires on multi piece rims 

2. You should also use a cage when re-inflating any tire that had been run flat or suffered a puncture and lost more than 20% of its air.

Basically running a tire when low by 20% or more, can damage the body cord. On tires with steel body cord such as most Class-A type this can result in broken body cord which can result in an explosion when the tire is inflated. Tires damaged due to puncture should also be re-inflated in a cage.

I think the salesperson at the tire store misunderstood some of his training. My suggestion is to ask to speak to a manager. If you get the same advice on always using a cage when adding a few psi to a tire then I suggest you find a different tire dealer.

Roger

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I do mention the use of a cage in this post.

I have contacted Discount tire but have not heard back yet. I believe the salesperson has some of their training information mixed up as I know of no regulation or even suggestion that simply adding a few psi to a tire that is already mounted and almost fully inflated needs to be done in a safety cage.

For those not sure what an Inflation Cage looks like or what it does here is a short video  from Ken-Tool doing an OSHA test.

One final comment. I did once receive a passenger tire for inspection that had exploded after being repaired. In this case the tech had damaged the tire when dismounting it. This is why I suggest that a safety cage or at least some type of restraint be used when re-inflating from 0 psi after a repair.


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What is the Best Trailer Tire?

I frequently get asked "What is the Best tire for my trailer?

I don't want another blowout"

My answer:

 I previously did a post on "Best Tire".


Sorry there is not enough data to provide a more focused answer.  I am sure some are hoping I would provide the one size and one brand that will solve all their tire problems.
I would point everyone back to this link from an RV forum with data from NHTSA. As you can see the numbers of RV owners that make the minimal effort to file a complaint sends the message to NHTSA that there are no significant problems with RV tires. I also know from looking at some of the complaints the quality of the information is spotty even if a complaint was filed.

Examples include wrong size, wrong tire brand, missing or incomplete DOT serial. Expecting information on actual load, speed etc is unrealistic and expecting an accurate description of the real failure is just a dream.

Engineers react to and work with facts. I have been trying to educate a few folks on another forum that melted body ply cord, as seen in tires with a Run Low Flex Failure


is physical evidence of a tire having been run under-inflated at highway speed. The discussion ended with one poster accusing me of hiding the facts and lying to keep my job. Of course he had no idea why the body cord melted but was certain I didn't know either. That ended my participation on that thread.

I know of no data to indicate that anyone is intentionally making "bad" tires. I do know of data that confirms the majority of RVs have tires in overload and that a significant percentage of tires on the highway are under-inflated. I share with others the  personal observations that a good number of trailers are exceeding the speed limit of their tires.

Multi-axle trailers place unique and high side loading on tires. This cause internal forces that contribute to the short life for tires in this application. This is a fact based on engineering and all tires are subjected to this type of structural load. It is just a fact that multi axle trailers are worse than single axle RVs.

Bottom Line
If you want to minimize the probability of having a tire failure on your multi-axle trailer, all I can suggest is you read my post on "Best tire", inflate your tires to the sidewall inflation 100% of the time, at least once confirm your side to side and axle to axle unbalance is not giving you an overloaded tire, use TPMS w/metal valve stems and do a free spin inspection at least once a year.

 and cover your tires with WHITE cover whenever parked in the sun for more than a day. You will probably still only get 4-5 years out of your tires because of the internal stress load but hopefully they will be 5 years with no problems.


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What is the "Best" TPMS

Yes along with   What is the "best" brand tire or Best RV or maybe even the Best pizza, people seem to think that there is a single answer to almost every brand selection that everyone will agree on.

 As an engineer I feel that just as Henry Ford was wrong when he decided everyone would want a black car, there is seldom a "Best" anything that is the best for everyone. However, having said that, you probably still would like some guidance in selecting a TPMS.

NOTE: I am certain that ANY brand TPMS is better than not having a system.

So here is what I suggest you use for a selection system. I think this will be a good system to use even as the technology moves forward.

I think there are four major categories to use when shopping for a new or replacement system.

1. Performance:  What does the system do and how well does it do it?

2. Cost:  This includes more than just the initial purchase price as there may be the cost of installation and what about the batteries?  When looking at cost I suggest you do a comparison based on the expected replacement cost of replacement sensors if you can't install batteries yourself.
Example - non-replaceable batteries last 5 to 6 years so include the cost of a set on new sensors vs the cost of replacing batteries every two or three years. (three sets at $1.50 per battery for example). Selection of an internal mount system will include the cost of the initial dismount and mount and balance of tires and a second dismount, mount and balance when the sensors need replacement a few years later. This cost could exceed the cost of a TPM system so is an important consideration

3. Warranty:  This may be the easiest as I prefer a longer warranty to a shorter one. I believe that car manufacturers do a much better job of designing and building their product than does the RV industry. Given the prevalence of 3, 5 and even 10 year warranties for cars vs the normal  few months offered by those selling RVs products with longer warranties are more likely to be better than those with short warranties.

4. Support & Service:  How easy is it to contact the seller? Are there videos on YouTube showing how to install and program your TPMS?

Items 2 through 4 are best left to you to learn and investigate. Some people don't care about YouTube videos and others don't worry about warranties, so lets focus on "Performance"

Here are some questions to consider:
a. Does the system "talk" to your on-board video display? This may lock you into one brand to the exclusion of all others.

b. Accuracy:  Some people are overly concerned with the accuracy of the TPMS. Personally I have observed that after setting my tire cold pressure using by calibrated digital hand gauge I see that the TPMS gives slightly different numbers for almost every tire BUT the difference is usually only a couple psi. While this may be measurable I don't consider it meaningful. You might want to review the concept of Measurable vs Meaningful. You could also review my comparison of two systems HERE.

c Temperature vs pressure checking. Most aftermarket systems will signal both pressure and temperature. Some people think temperature is very important but as a tire engineer I believe that if you have the proper load and inflation you do not need to worry about temperature variation and can expect to see operating temperatures range from +15°F to +50°F above ambient. Many TPMS have a warning temperature of between 150 and 160°F which is OK in my opinion for a warning level. It is very unlikely for you to have a tire get hot without having a loss of air precede the increase in temperature.

d. Early warning:  I consider this an IMPORTANT and desirable feature
Imagine your cold pressure is 100 psi. Your low pressure warning is probably -15% or 85psi. When driving you might have a hot pressure of 120 psi. Now suppose you get a leak from a puncture or possibly a valve stem gasket leak. Would you rather get a warning when you loose 3 or 6 psi over a few minutes down from the 120 psi HOT pressure, or would you think it OK to only be warned after you loose 35 psi down from the hot pressure to the "Low Pressure Warning" level.?

In my opinion getting that "Early Warning" allows you to slow down and start looking for an exit or safe place to pull over as you monitor the air loss over then next 5 to 20 minutes you might have before you get down the the minimum pressure needed to carry the load.

Sometimes the rate of air loss can start small but increase over a few minutes as the hole in the tire gets bigger. If you only get the single warning it may already be too late to save and repair the tire.

I hope I have given you some things to consider when selecting the "Best for you" TPMS.



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Autopsy of a failed tire

I continue to see posts on various RV forums saying "I had a blowout on my tire. I checked the inflation every day and I didn't hit anything. It must be because it was a (fill in the brand)  or  because it was made in (fill in the location). I'll never buy that brand or tires made in that location again cuz they are junk."

If you are a follower of this blog you have seen this topic discussed previously but clearly there are folks out there who just do not want to be confused with evidence that doesn't support their pre-concieved notion of why tires fail.

Lets follow an "autopsy" of a failed tire and see if we can improve people's understanding of how to "read" a  failed tire so we have a better idea of the real reason it failed. If we don't properly understand the "WHY" then we probably will not take the appropriate action to prevent a re-occurrence.

 Here we note the size as 8-14.5. This is a low platform trailer tire. Many times used on Mobile Homes or construction equipment trailers. It is a bias construction and we can see the body ply at an angle.







We see this tire is a Load Range F (12 Ply rating) tire.
 


rated to carry  2835# at 100psi cold.

Looking at the tread we see the tire "looks" great.
 






As we continue to read the tire sidewall we see it was made in Thailand..
 




Now for some, that is sufficient. They say it wasn't made in the USA so it is a piece of junk. Lets continue and see if they are correct in their analysis.


 
The DOT date is 2812 so since this tire failed mid June 2013 or about 2613 the tire is only 1 year old and clearly since it came from Thailand there were probably 3 or 4 months for shipping and handling in a warehouse. Clearly the tire age is not the problem.

The first two characters of the DOT"2B" can be translated using out decoder web site  and we learn the tire was made by Deestone Ltd in Samutsakom, Thailand.

Continuing on we see the tire is Nylon body which would be consistent with a lower cost, bias, heavy duty construction.

Now we get to the interesting part. Looking at the actual area of the failure.
 In the picture at the top of this post you can see that both sidewalls have significant damage.

 As we look at the area of the tire that actually failed we see frayed cord.  This is what Nylon and Polyester look like when they fail in tension. Looking closer we see that not all cords are frayed.


These three cords have the look we see when we cut Nylon rope and fuse the ends with a lighter.


Warning Technical Information to follow:  Nylon and Polyester both melt in the range of 395°F to 495°F depending on the treatment and exact material involved. These materials also lose about half their strength when heated to over 300°F.


Looking even closer at those cords it is clear the ends are not frayed but fused. So we now have the physical evidence that parts of this tire saw temperatures approaching or exceeding 400°F. The only way to generate that kind of temperature is with rapid, extreme flexing.
 We see rapid flexing at highway speeds. We see extreme flexing when a tire looses say 50% or more of its proper inflation.

The heat in a tire comes from the bending of the cord material and radiates outward to the cooler surface.  It is illogical to think that only a few cords somehow flex enough to see high temperature and if we look closely all around the area of the most damage there are a number of cords showing similar heat damage.

So we now have physical evidence that proves operation at highway speed for a number of miles while the tire is significantly under-inflated.

Knowing this, I think you can understand that there was probably some other cause such as a leaking valve or puncture or cut of the tire that allowed the air to leak out over time. If the tire had in fact been properly inflated we would not see the 360° circumferential flexing and signs of high heat.

How do we avoid this type of failure?  Will changing the brand prevent air loss?  Will buying tires made in a different country prevent them from getting too hot when over flexed? Clearly neither of these "fixes" will prevent a reoccurance of this type of failure.

I would suggest a TPMS could provide warning if there is an air leak. That warning might be early enough to allow the driver to stop before the tire fails and maybe even before the tire is significantly damaged.

BOTTOM LINE
I would say that 95+% of tires I have seen with this type of "blowout" failure were in fact Run Low Flex failures. Having inspected many thousands of tires in my 40 years as a tire engineer, I have accumulated enough experience to be able to observe and find the physical proof of high heat damage similar to what we see here. Many times I can find and identify this proof in less than a 30 seconds. It's easy once you know what to look for.

Hopefully the next time you meet someone that says "I just had a Blowout" you will be able to identify the evidence that points to a run low flex so the person learns they need to know why the tire lost air. If it were say a leaking valve it certainly will do them no good to buy a new tire and mount it only to have it suffer the same fate.

What is involved in doing a tire inspection?

 Some of you may wonder how I can provide an opinion on what caused a tire to come apart when all I see is a picture or two. 

For many decades my primary job was to perform tire inspections and "autopsies." I estimate I did more than 20,000 inspections, so I was able to build a good experience base for my observations.

My findings were many times presented to a customer that did not want to accept responsibility for the failure, so I needed to be very thorough and to provide an easy to follow explanation.

 Here is an example of just one such inspection.

The tire was reported to have "blown out on a local road” The lack of facts and data did not help in the analysis of the tire.

However I was able to establish that the tire suffered a sidewall puncture, lost air and after running significantly under inflated at highway speeds for a number of miles the cumulative damage to the tire resulted in a flex fatigue failure.

Photo -1 above

                Shows the 2 piece condition, which is indicative of, flex fatigue failure.

Photo -2

Shows the result of melted body cord – Body Cord melts at 350°F to 400°F

Photo Tire-3

Shows the abrasion on the interior from running a few miles at essentially zero inflation

 Photo 4 shows a larger portion of the interior. Here I noticed the location of a possible small sidewall puncture.

 

Picture 5 is a close-up of the area of interest

  

 Picture 6 is what the outside looks like.

The question is ... How do I prove there was a puncture?  If I used a sharp probe like an awl some would think I might fave made the "puncture" myself. Over the years I learned a trick. If you take a paper clip and straighten it out, you can use it like a probe. The un-bent clip isn't strong enough to hold and doesn't allow you to force the end of the clip through the sidewall of a tire but if there already is a hole, the paper clip can be guided through the already made puncture.

Pictures 7 shows the clip on the inside

Picture 8 shows the clip on the outside at the suspect location.

               

Findings

                The tire was not defective but failed due to being operated significantly under inflated. The under inflation was caused by the small sidewall puncture.

Corrective Action:

                Use a TPMS. Instruct driver to be more attuned to ride & handling degradation, as this tire should be operated with 80 psi under normal conditions. A slow leak would have taken some time and resulted in significant changes in handling and vehicle response.

BOTTOM LINE

Some punctures are hard to find. Sometimes the puncture occurred in the heavily damaged portion of the tire so it is impossible to demonstrate the puncture. Melted polyester body cord is physical proof that a tire was run with only 10% to 20% of the proper inflation.