Do you run at or above engine Redline? Why Not?

 OK As a tire design engineer and someone with RV tire experience let's see if I can clear up some of the partially correct and partially misunderstood information we see posted almost daily on one or more of the Internet RV forums.

If you have a Load / Inflation chart for "ST" type tires you will find that ALL ST type tires follow the same information. A chart published by company X may not have all the same sizes and Load Range tires as company Q but when both X and Q make the same size and the same Load Range the load capacity and the required inflation for the tires is identical.

 

The same applies to "P" and "LT" type. Now we need to pay attention to some other details on tire "type". Some tires made outside the US are not marked with a P, LT or ST before the size numbers. This means that they were made to some different standards. They may have been designed to comply with European or Asian standards to their numbers will be close but not identical to the US standards.  

 

The inflation number in the chart is the MINIMUM inflation required to support the stated load or to put it another way the LOAD in the chart for your tire is the MAXIMUM load the tire can support when inflated to the specific inflation found in the chart.

 

Some larger sizes with 19.5 or 22.5 size wheels make things a bit more difficult as some follow the US Standards while others follow European standards but there is no leading letter to help you know which so if you run tires of these sizes it is more important that you learn the Load capacity for the different inflation for these tires directly from the company that made those tires. The differences are not great but they are not identical so you can be off by 5 to 10 psi or maybe a couple hundred pounds load.

 

RV companies are required to select tires that can support the GAWR but trailers have different "assumptions" than do Motorhomes. For trailers the US DOT assumes a perfect 50/50 split of the load between two axles or 33% each when there are three axles on a trailer and also a perfect 50/50 side to side load split on any one axle. Neither of these load splits are realistic as those of you that have had each tire positions measured know that many axles have end to end load splits of 48/52 to 40/60%. Individual axles on multi-axle trailers can also have significant variation away from a uniform load split.

 

Motorhomes also have similar problems with the unbalance of each end of each individual axle. Engine placement and dual tire position on the rear axle require paying attention to other details when consulting Load &Inflation tables as "Single" load capacity is always different than when tires are is a "Dual" or side by side mounting as seen on most motorhomes.

Size and placement of water, fuel, propane, and holding tanks can result is 500# to 1,000# being in different corners of different model or different year motorhome so that is why you can't simply ask the RV owner parked next to you about proper tire inflation numbers.

The inflation number on most Certification labels is the MINIMUM inflation needed to meet the above requirement.

A complicating issue is that in 2017 RVIA (Gold oval sticker on the side of most RV trailers) added a requirement that the tire inflation be sufficient to support 110% of the GAWR. This partially addressed then known in-balance found on almost all RVs.

All the above ignores the advantage of having some "Reserve Load" capacity to allow for variation in loading of the RV that can occur if you carry more water than normal or load more "stuff" than normal etc. It also ignores normal engineering practice of not designing components to have some level of margin of load capacity. or for the occasions when trailers are towed at speeds above the tire design limit of 65 mph operating speed.

 

You can think of running at the max load capacity like running your TV right at the engine Redline. If you think that is a good practice to run with no reserve load then I guess you also must think it OK to run at engine redline for hours on end.

##RVT1058

Tire Recall in the news. Why aren't there more recalls?

 

I have seen this question on an RV forum. It was asked by someone complaining that there were no recalls of what he considered "crappy" RV tires. Other posts in the thread went on to say that complaints to the BBB or the tire importer won't accomplish much. I posted a reply pointing out that expecting the National Highway Traffic Safety Administration (NHTSA), that is part of DOT to recall tires when there had not been sufficient, usable complaints filed, was simply unrealistic.
NHTSA is the government agency charged with the responsibility of writing and enforcing the regulations necessary to achieve improved safety of vehicle systems and equipment. However they cannot order or even suggest that a vehicle or component be recalled without facts and data being collected and analyzed.

 A while ago I worked with a reader of my RV Tire blog, John B., who understood the necessity of providing the information NHTSA needs. He had suffered three tire failures. Luckily he discovered the failures before the tires had complete detachments. In his case there was no loss of air and no flailing of tire pieces. What he did have was a tire that was no longer round or having a uniform tread contour.

 
Now lets be sure we all have the same understanding of the terms. In this case a "Detachment" would be when a part of the tread or tread & belt package came apart from the rest of the tire. This type of failure can result in damage to the RV as pieces flail around hitting fenders and the side and undercarriage of the RV.

    John wanted to file a complaint with NHTSA and he wanted to be sure his complaints would be useful to the engineers. He understood that partial or incorrect information would result in no investigation and with no investigation there was no possibility of any action being taken to remove "crappy" tires from use. So John contact me and I walked him through the process of collecting all the details needed. He also wanted to  dissect his tire so he could ship the important parts to me for further examination.


When I received the sample I first cut the tread in the locations John had identified but found no serious issues.

I then called upon my 40 years of experience and took the time needed to closely examine and take measurements with special tools to identify a location that was more probably of interest. After cutting the section at the location of interest and found the separation between the belts that was almost all the way across. This separation allowed the tread area to bulge out to the shape seen in the picture of the tire at the top of this post.
For those interested these tires were not made in China as we decoded the serial and learned they were made in Mexico.

With the physical examination complete, John was able to file the three complaints with NHTSA. Now it is important to remember that NHTSA has budget constraints so investigations need to be prioritized. Obvious defects that result in physical injury would receive top priority. Also a single or small number of complaints will be of lower priority than a large number so if the only complaints NHTSA receives on these tires are the three from John there may not be any action. The same situation would apply to any complaint you might file BUT it is important to remember that if the majority of people with tire problems only post to RV forums or grouse to others around the campfire nothing will ever happen or result in the quality of tires improving.

Here are Links to John's information. Link 1     Link 2

A quick review of the complaint on file with NHTSA will show that the majority are of little or no value to NHTSA as the owner didn't provide the crucial information of a correct and complete DOT serial. Many complaints don't even provide the tire size or even the correct tire brand. I believe that if people spent half the time they do on RV forums but provided complete and accurate information to NHTSA we might all end up with better quality tires on our RVs.


BOTTOM LINE
If you have a tire problem you need to collect the facts - Size, Brand, DOT serial and collect some good sharp pictures in case NHTSA needs them. Then make the effort to file a complaint. Who knows, you might just be able to grab the interest of the engineers and have an investigation started.

 

##RVT1057

What margin should be used for Low Pressure warning on TPMS?

 In response to a reader from Australia with a question on TPMS Low Pressure Warning level, I responded...

We need to separate the settings specified by US DOT for regular passenger car applications and the more specialized world of Caravans (RVs for us Americans) (both motorized and pull behind).

DOT is trying to protect the worst case which is operating a tire in overload. Most cars have their tire inflation set based on the goals of the manufacturer to meet fuel economy goals while delivering acceptable ride & handling the expectations of the consumer. With our current Federal fuel economy standards, this results in fuel economy being #1 or very near the top of the priority list. This means that almost all tires have sufficient pressure to significantly exceed the load capacity requirements. I have seen some data that shows +30% is not an extreme level. So that means a loss of 25% of air pressure still keeps the tire out of the "overload" situation.

Now when we move to the Caravan market, there are no regulations for fuel economy. So marketing pressure of low cost takes priority. This means the RV manufacturer selects the lowest cost tires possible. While in many cases this means tires that just barely meet the DOT safety requirement tests, it also means the smallest tire size sufficient to support the load as this translated to smaller (lower cost) wheel well space and smaller (lower cost) wheels. This results in most tires needing to specify higher inflation (more load capacity) than if the tire were applied to a car. The bottom line is lower levels of actual reserve load capacity with data showing that the average user ends up overloading the tires or axle or both.

I am a strong supporter of tires & inflation that provides at least a 10% reserve load for motorhomes with towables goal of 15% reserve load being needed to address the Interply Shear forces seen in multi-axle towables. These are what I would consider MINIMUM reserves.

To achieve these smaller margins we need to recommend the Low Pressure setting of TPMS is closer to 0% air loss from the cold "set pressure". If we were to allow a 25% air loss before the TPMS Low Pressure sounded, the owner could have been driving hundreds of miles in an overloaded tire condition.

 

Here is a post on how I set the TPM system on my Class-C motorhome. 

 

##RVT1056

Why do ST type tires have more pressure gain than LT type tires?

 The main reason for ST type trailer tires to gain more pressure than the LT tires, is that they are forced to support more load relative to their size & inflation than LT tires are.

If you ever look at the Load & Inflation tables and find an LT and an ST type tire of identical physical dimensions you will see that the ST tire is “rated” to carry more load than the LT type tire is. The basic theory behind that increased capacity is that ST tires will be traveling slower because people should not be traveling as fast when towing as when just driving the car or truck. When ST tires were introduced they were limited to 65 MPH MAX in an effort to offset the damaging effects of higher loading than seen in LT tires.

Tire load capacity is basically a function of volume and pressure as seen here Load = K x (Air Volume x Air Pressure) with different type tires having different “K” factor. Tires in LT applications are required to support lower load as a percent of their volume and pressure so they do not have to “work”as hard so they do not generate as much heat.

More heat means a greater increase in pressure. (Approx 2% pressure increase for each increase in temperature of 10F)

Please note that the actual load calculation is much more complicated as the response to air pressure is not linear and different aspect tires i.e. 75 series vs 85 series etc have some different factors that are applied to the actual calculation.

 

 

Please note that the above is just one page of many used to calculate load capacity for tires not already published in the TRA Yearbook, so do not go and try to check every size. I show this to demonstrate that the calculations are complex.

##RVT1055

Interply Shear and am I spreading Fear?

On an RV Forum I saw some posts about tire failures:

There were some posts that mentioned the RV total weight capacity or Gross Vehicle Weight Rating (GVWR). Others were mentioning the total of the Tow vehicle plus the RV Trailer or Gross Combined Weight Rating (GCWR). While others were talking about tire load capacity.

One person responded with: "You need to pay more attention to just what acronyms are telling you. GCWR has nothing to do with tire inflation pressures. The tow vehicle and towed vehicle are individual vehicles and answer to their own standards and regulations.

Maybe you should ask Roger why, during his working years with tires, the interplay sheer problem was never corrected. (Interjecting a fear factor for attention purposes might be Roger's goal).

Tire tread separations are a fact that trailer haulers must respect. Fooling around with RV trailer tire inflation pressures by using less than what the vehicle manufacturer has recommended is, IMO, counter productive".

 

Since I was mentioned if the forum post along with the suggestion I might be spreading fear, I felt the need to reply.

Sorry, but Fear is not and never has been my intent. I covered interply shear and how I discovered that it was different for multi-axle trailers than it is in motor vehicles in my blog post of Nov 20, 2013 when I did the first of multiple posts in Interply Shear as it pertains to multi-axle trailers. The fact is that Interply Shear is well known in the tire industry but not the unique forces imparted on multi axle trailers. It was pure chance that I noticed the irregular path taken by a trailer doing a 180° U turn on freshly smoothed gravel (picture in the Feb 22 2018 post). That was my Ah-Ha moment. 

 

While I was retired by the time I first saw the gravel path, I still had friends that worked with the high power Finite Element and vehicle simulation software and called in a couple favors. I asked them to run a simulation of a truck pulling a tandem axle trailer through a series of "S" curves. All 8 tires were identical with the same load and inflation so we could end up with a comparison of trailer effects on tire belts vs the normal interply forces found in all radial tires in motor vehicles applications. What we discovered after the overnight computer run, was that in  simply S turns (similar to seen in the video) the belts on a multi axle trailer were developing 24% higher internal shear forces than the tires on the truck pulling the trailer.

 

This goes a long way in explaining why tires in multi-axle application have much shorter life and significantly higher failure rate than do tires in car or truck applications.

 

So you see, Science and facts can be used to point out why blindly following 50 year old design decisions and regulations that are the foundation for the current ST tire regulations can be significantly improved upon.

##RVT1054

I have too much tire pressure gain

Another post with a question on an RV Forum.

 I've often wondered why the tires on my travel trailer will gain about 10-12psi on a trip, and the tires on my tow truck only gain about 4-5 psi.  The tires on my trailer are Goodyear Endurance load range E, with a cold pressure rating of 80psi.  (The original Chinese tire bombs were load range D with a cold pressure rating of 65psi.  I had the same problem with them gaining pressure).  I usually start my trips with the pressure set at 75psi instead of 80psi because I'm nervous about the tires gaining so much pressure.  On a typical travel day in the summer (in Texas) the tires will usually go from 75 to about 87 or 88psi cruising down the highway at 65MPH.  I have a pressure monitoring system, and I have a lot of faith in it.  My trailer weighs 9950lbs according the the CAT scale.

 

 My response:

Tire pressure will increase i.e. gain about 2% for each increase in temperature of 10F. I have covered this in detail in a few posts on my blog. Here is one post on that topic.

https://www.rvtiresafety.net/2016/01/does-tire-pressure-really-follow-gas-law.html

Since your TT tires carry a much higher load relative to the tire size they have to work harder than your truck tires. Inflation on trucks gives tires a 10% to 30% Reserve load while the inflation specified for TT tries may give 0% Reserve and the sad fact is that based on actual tire loading data a MAJORITY or RV trailers actually have a negative reserve (i.e. they are overloaded).

What can confuse the issue is when you change the Load Range you can run a different pressure  BUT a couple observations. When you increase the tire Load Range you only get an increase in load Capacity when you also run higher inflation.

Without knowing the scale reading for each axle and the actual tire sizes I can only give you generalizations. It is very unlikely that your 9950# is evenly split across all 4 tires and probably one or more is supporting more than 2,487#.

You should NOT get nervous about pressure gain as we tire engineers know the temperature will increase and also the pressure will increase.  You can email me directly at tireman9@gmail.com and I will be happy to work directly with you to resolve your questions and concerns.

Some information that will help us.

1. Complete tire size and Load Range info for both TT and TV

2. Scale readings for all 4 axles (TV & TT)

3. Cold inflation for the TV.  You said 75 for the 4 TT tires

 

==============================

Well I posted the above a couple weeks ago but have not heard back so either the OP has lost interest, or doesn't want to discuss the problem.

If you are reading this blog post then you probably care about your tire inflation and understand the normal pressure increase. If not my offer to help still stands, but please provide the requested information so we both don't spend time going back and forth.

##RVT1053
 

 

What is the Root Cause of a tire failure?

 "Root Cause" is the initiating condition or first cause. Someone having a temperature is not a Root Cause or Proof of an infection but is an indicator that there is probably an infection of some sort. A "Blowout" is not considered the Root Cause of a tire failure just the final condition.

In reality, tires fail for a relatively small number of Root Cause reasons that can be discovered, but it does require detailed and sometimes exhaustive investigation. By "Root Cause," I mean the initiating feature or condition that eventually led to the failure. Too often people confuse the tire's ending condition with the initiating or "Root Cause" reason for the tire to end up in the condition they are observing.

For the last few years of my 40-year career as a tire engineer, my primary job duty was to investigate failed tires that had conditions that were hard to understand or provided confusing and sometimes contradictory evidence as to the Root Cause for their condition. With that background, I have developed some guiding principles for the most likely reasons for tires to fail.

These are, in no special order:

1. External Damage. These include punctures, cuts, impacts, wheel and valve failures, and similar conditions.

2. Run Low Flex Failure. This is best described by the failure of the tire sidewall due to excessive flexing. The flexing is the result of operation when the tire is significantly under-inflated.

3. Belt & Tread Separations. This is when the belts of a radial tire detach from the body or from each other and/or when the tread rubber detaches from the top belt.

4. Manufacturing Defect. For the purpose of this discussion, this would be when components of a tire were not manufactured to the intended specification.

The "Why" for some of these are obvious, such as the External Damage category.

Others can be further traced to various contributory reasons.

Run Low Flex Failure This is a more accurate description of what some incorrectly call "Blowout". When a tire loses significant inflation air they flex much more than the body cord can tolerate. Excess heat is generated which can, in extreme cases, result in the Polyester actually melting. I showed examples of what this can look like link in THIS post. Polyester is used in most Passenger, ST type, and LT type tires. Steel body tires are used on most Class-A RVs, the steel can fatigue. I covered "Zipper" failures and steel fatigue in THIS post.

Belt & Tread Separations occur primarily because the rubber around the belt cords or between the belt rubber and the tread rubber isn't strong enough. Now it can be weak for a variety of reasons. Some might be design, some might be manufacturing and some weakening can be caused by cumulative improper service conditions.

Now let's talk about tires made by a reputable company, i.e., one that has tire stores and dealers with physical stores and sells tires that have a warranty of two or more years.

If this tire is subjected to thousands of miles of lower inflation or higher loading or operation at higher speeds or stored is full sunlight it is reasonable to expect the strength of the belt & tread area rubber to lose a good portion of its strength primarily due to the increased operating temperature of the tire. If this tire is in service on a tandem axle trailer then there is also increased Interply Shear which can overload the belt area rubber. The combination of the above may result in a belt or tread separation.

Another possibility is a Manufacturing Defect. These usually occur in small numbers as tires are built in batches so the substitution of the wrong type of rubber may cause tires to fail. It is important to understand that in almost all cases this type of "defect" usually shows up in the early life of the tire. It is also very important to understand that, unlike some lawyers, engineers deal in facts and logic. Simply having a tire fail is not in itself proof of a defect but all to often that is the position that those in the legal profession seem to jump to. That approach may result in a nice payout to the lawyer but will not result in product improvement as there has been no determination or identification of the actual initiating "defect".

Finally, there is the possibility of a design weakness. Now I do not believe that anyone is intentionally designing tires to fail but this sometimes may occur when the performance goals of the manufacturer are limited to meet the bare minimum for strength and durability and the focus is primarily on low cost. There is no absolute way to identify these tires but I do believe there are indicators when looking for tires that are made to a higher standard of quality and durability.

I would consider a tire company reputable if it had a chain of stores across the country. If the tires carry the name of the manufacturer and have a multi-year warranty the longer the better then they probably have higher durability requirements than those established by DOT.

For ST type tires I would look for tires with a Speed Symbol of "L" (75 mph) or faster. Now a side point about speed. I did a POST a little while ago on the topic and strongly recommend you read it. I do not think I would recommend any tire for general use that does not have a speed rating molded on the sidewall. Few realize that ST tire load formula is based on a 65 MPH max operation speed.

I do hope this post will help some to have a better understanding of what can contribute to tire failure and consider what you can do to lower the chances of having such a failure.

 

Over the next few weeks I will be posting pictures of tires with different conditions and I will identify what was discovered with a detailed inspection and identified as the most likely Root Cause.  Just to whet your appetite here is a video showing at time 1:12 a 

tire that was initially suspected of having some "Defect".  The physical evidence that proves what the Root Cause was can be seen in the video if you look closely and understand what you are seeing.  If you think you know the answer post it in the comments and the first 5 people to get the answer right with get their name published in my next post. Along with bragging rights.

##RVT1052

 

Tire "Failed for no Apparent reason" I think not.

OK, as you know I follow a number of RV forums. Here is one that caught my eye. Maybe some can learn this lesson before they end up in big trouble.

 

The post started off with   "Well, I can assure that a tire can blow up for no apparent reason. LT275/70R18 Americus Commercial date code 3518. Truck was shaking a bit. Thought drive shaft. If any of you ever drove a bit heavy truck (I used to drive fire trucks) that sat for a long time and got a flat spot, well drive shaft or flat spot feeling.

Couldn't see anything wrong but decided to take my clunker over to (a local NAPA store) and let them have a look. Its about 15 miles. Missus following. Truck was shaking even worse. Missus said she saw the left front "flapping" (her words) and I queried why she didn't call me so I could pull over and check. Anyway I went to the tire and checked by feeling the inside and there was a definite bulge. So I decided to let that go and went in to the office and BANG! rubber off the inside of the driver side front blew off; about a foot long x 8 inch hunk. Tire still holds air. Always kept those tires aired properly.

Bought four Cooper Discoverer A/T All-Season LT275/70R18 125S Tires. The old Americus tires no longer had my confidence. I praised the Lord I was able to make it over to the NAPA without a blow out on the road.

Michelin's would have been my choice but they are outside what I wanted to spend. I checked Sams Club and the same tires were about $100 more a piece than the Coopers."

I wrote the following: I hope this truck owner and some that are reading this post can gain some knowledge.

Well there clearly was a reason for the failure as tires are not Magic. I do not understand what was meant by "blow up for no apparent reason"  If the owner is saying  that he did not understand the different technical reasons for a tire to fail, I can understand that, but as a minimum people should read and try and understand the information in THIS post as it covers the two main reasons for a tire to fail. I have posted this "Why tires Fail" on at least a dozen different RV forums, but as they say you can lead a horse to water but you can't make them drink.

 

When a tire fails, I strongly suggest that you should make at least a minimal effort to understand the "Why" otherwise the actions you take may not prevent another failure.

Think about getting in a car or truck and turning the key and the vehicle doesn't start. Would you simply replace the battery without confirming the reason it didn't start was because the battery didn't have enough charge? What if you were out of gas? Would replacing the starter motor "fix that problem? Of course not. You may not have thought about it but you already approach "Problem Solving" in a logical manner.

 

Tire failures should be approached in the same way if you want to avoid making the same mistake over and over

The vibration you felt was a significant hint that the tire was in the process of coming apart. Tires simply do not go from having a 100% solid construction to having the components such as the steel belts separate the next instant. 

Here is a more detailed post on why tires might fail.    

Here is a story of a tire inspection I did on a tire that was in the process of coming apart.

 As you can see there were a number of warning symptoms. Some were similar to the symptoms you experienced but failed to understand or act upon. 

 

Luckily you were not driving your truck with a significant load at speed when the front tire finally came apart. That could have been serious. I believe that a little self education can be a significant help in reducing potentially serious problems if you have a tire failure.

##RVT1051

Is the temperature reading of your TPMS correct? Probably Yes, BUT...

 IMO TPMS are of little value when it comes to temperature warning for tires. Same for IR guns.

I have written a number of times on TPMS Temperature reading really being a reading of the metal wheel and not the rubber tire as metal conducts heat while rubber is an excellent insulator. So your TPMS can be used to provide warning of potential problems with wheel bearings, breaks and other metal mechanical parts.

These posts use data from my ongoing, direct comparison of internal vs external sensor TPMS. My data suggests that there is no meaningful difference in pressure reading based on the test of 12 sensors.

When seeing people claim the TPMS temperature reading were useful for predicting impending tire failure, the engineering DNA in me kicked in and I devised a plan to test 12 sensors. These come from two different companies. One set of 6 external sensors is from TireTraker and one set of 6 internal sensors is from TechnoRV who provided the internal TST  system. My thanks to both companies for their support in my efforts to help educate the RV community about tires.

The question is: How do I make the test both fair and useful? For the pressure test, I decided to eliminate as many variables as possible and get all 12 readings from the same air chamber at the same time and compare them all against my personal digital hand gauge that I have checked against an ISO certified laboratory gauge.
Note: my hand gauge reads to 0.5 psi which is way more precise than anyone needs for checking tires in normal highway use.

Here is the test fixture I made.

6 Internal sensors are inside the tube and 6 external sensors are on the outside along with a pressure regulator, a safety pop-off, and a reference dial gauge that allowed a visual check plus the test port for my hand held digital gauge that I have confirmed accurate to +/- 0.5 psi against an ISO Certified master gauge.

Here are the results of my comparison test. The target pressure is 80.0 psi as reported by my handheld digital gauge.

Set A  1 reading of 78 psi,  5 readings of 79 psi
Set B  2 readings of 78, 2 readings of 79 and 2 readings of 80 psi

I also recorded the temperature.
Set A  4 readings of 66 F, one each of 64 and 68F
Set B  4 readings of 69 F  and 2 readings of 68

I have recorded the internal vs external pressure difference in reported running pressure with the following results.

I do not consider any of the differences in the readings of the internal vs external sensors pressure to be significant or meaningful for a TPMS, remembering that at Ambient I observed 3 to 5 psi differences when all 12 sensors were measuring an identical pressure, which is not too far off the claimed accuracy for TPMS.

 

BUT

The problem with TPMS temperature readings is that they are not able to read the hot spot of a tire. Here is a graphic representation to tire temperature differences.

Since heat can kill a tire, and tires simply do not fail based on their average temperature but they can fail if a single spot exceeds the ability of the rubber to maintain its strength. With this large temperature spread and the fact that the hottest spot in a radial tire is about 3/8" to 1/2" away from the air chamber and the internal TPM sensor is reading the average air temperature in the air chamber, I would ask why anyone would believe that a TPMS "High Temperature" reading is a reliable method of warning of an impending tire failure.

I started this post by saying I did not think that TPMS or IR guns were reliable tools for predicting an impending tire failure based on the reported temperature.  I will cover IR guns next week.

=================

##RVT1050

Simple question: Why do tires fail?

In reality, tires fail for a relatively small number of Root Cause reasons that can be discovered, but it does require detailed and sometimes exhaustive investigation. By "Root Cause," I mean the initiating feature or condition that eventually led to the failure. Too often people confuse the tire's ending condition with the initiating or "Root Cause" reason for the tire to end up in the condition they are observing.

For the last few years of my 40-year career as a tire engineer, my primary job duty was to investigate failed tires that had conditions that were hard to understand or provided confusing and sometimes contradictory evidence as to the Root Cause for their condition. With that background, I have developed some guiding principles for the most likely reasons for tires to fail.

These are, in no special order:
1. External Damage. These include punctures, cuts, impacts, wheel and valve failures and similar conditions.
2. Run Low Flex Failure. This is best described by the failure of the tire sidewall due to excessive flexing. The flexing is the result of operation when the tire is significantly under-inflated.
3. Belt & Tread Separations. This is when the belts of a radial tire detach from the body or from each other and/or when the tread rubber detaches from the top belt.
4. Manufacturing Defect. For the purpose of this discussion this would be when components of a tire were not manufactured to intended specification.

The "Why" for some of these are obvious, such as the External Damage category.
Others can be further traced to various contributory reasons.

Run Low Flex Failure This is a more accurate description of what some incorrectly call "Blowout". When a tire looses significant inflation air they flex much more that the body cord can tolerate. Excess heat is generated which can, in extreme cases, result in the Polyester actually melting. I showed examples of what this can look link in THIS post. Polyester is used in most Passenger, ST type and LT type tires. Steel body tires are used on most Class-A RVs, the steel can fatigue. I covered "Zipper" failures and steel fatigue in THIS post.

Belt & Tread Separations occur primarily because the rubber around the belt cords or between the belt rubber and the tread rubber isn't strong enough. Now it can be weak for a variety of reasons. Some might be design, some might be manufacturing and some weakening can be caused by cumulative improper service conditions.

Now let's talk about tires made by a reputable company, i.e., one that has tire stores and dealers with physical stores and sells tires that have a warranty of two or more years.
If this tire is subjected to thousands of miles of lower inflation or higher loading or operation at higher speeds or stored is full sunlight it is reasonable to expect the strength of the belt & tread area rubber to loose a good portion of its strength primarily due to the increased operating temperature of the tire. If this tire is in service on a tandem axle trailer then there is also increased Interply Shear which can overload the belt area rubber. The combination of the above may result in a belt or tread separation.

Another possibility is a Manufacturing Defect. These usually occur in small numbers as tires are built in batches so the substitution of the wrong type of rubber may cause tires to fail. It is important to understand that in almost all cases this type of "defect" usually shows up at early life of the tire. It is also very important to understand that unlike some lawyers, engineers deal in facts and logic. Simply having a tire fail is not in it self proof of a defect but all to often that is the position that those in the legal profession seem to jump to. That approach may result in a nice payout to the lawyer but will not result in product improvement as there has been no determination or identification of the actual initiating "defect".

Finally there is the possibility of a design weakness. Now I do not believe that anyone is intentionally designing tires to fail but this sometimes may occur when the performance goals of the manufacturer are limited to meet the bare minimum for strength and durability and the focus is primarily on low cost. There is no absolute way to identify these tires but I do believe there are indicators when looking for tires that are made to a higher standard of quality and durability.
I would consider a tire company reputable if it had a chain of stores across the country. If the tires carry the name of the manufacturer and have a multi-year warranty the longer the better then they probably have higher durability requirements than those established by DOT.

 For ST type tires I would look for tires with a Speed Symbol of "L" (75 mph) or SLOWER. Now a side point about speed. I did a POST some time focusing on speed and strongly recommend you read it. I do not think I would recommend any tire for general use that does not have a speed rating molded on the sidewall. Few people realize that ST tire loading is based on the assumption of 65 mph Max operation speed

I do hope this post will help some to have a better understanding of what can contribute to tire failure and consider what you can do to lower the chances of having such a failure. 

 

##RVT1049

I just had a tire failure, for no reason

If you follow any RV forums you probably see a statement like this every few weeks. I can tell you that tires do not fail because of some RV-Magic. It comes down to actual Science.

Why do tires fail?

Simple question but as with almost every question about tires, the answer is not a one liner. This post. It's not short but it really isn't technical and I'm sure every reader will understand the concepts.

When looking at why tires fail we need to first exclude the obvious damage caused by external objects i.e. pot holes, curbs, chunks of metal, glass or rock or other trash and junk on the road. Second we'll exclude broken valve bodies, leaky valve cores, valve gaskets, extension hoses, and cracked rims.

What's left will be a structural failure which would almost 100 percent of the time will be either a detachment of different parts or components of a tire one from the other, OR a heat/fatigue failure in the mid sidewall due to driving with extreme low inflation.

Most tires are made up of 20 to 30 different components such as different steels (belts and bead are not the same steel) textiles such as Nylon, Polyester, Rayon, Aramid. Then there are the various different rubber components such as Tread, Sidewall, Innerliner, Steel skim, Wedge, Flippers, Chaffer, base tread, Inserts, and other bits and pieces.

Each component is selected for different reasons and contributes both advantages and disadvantages to the total. Each component must "stick" to its neighbors both in the un-cured and cured state. The interface between two different components is weaker than the individual components but it must hold together through millions of cycles over a 200F degree temperature range.

All would be just fine if a tire were made completely of inorganic materials such as metals, stone or even ceramics. There are numerous formulas for the strength and fatigue limits for these inorganic materials.

The issue with organics (wood, rubber or other materials made from oil such as plastics) is that their strength has a "T" or time function. If you build a bridge of stone, you can calculate the maximum load it can sustain and as long as the structure isn’t changed due to external damage its strength will be the same the day it’s built and 50 years later. If I build a pressure vessel of steel and put 150 psi in it and it doesn’t fail I would have every expectation for it to continue to hold that pressure for decades, again excluding external damage.

However, if I build a pressure vessel of organic materials (a tire) it might hold 150 psi the first day or maybe the first few days but at some point it can fail. This principle is not something many engineers think of today because they don’t design bridges of wood but 100 or 150 years ago when wood was common they learned that a bridge that was strong enough to hold a railroad train to drive across, it could fail if the train parked on the bridge.

I educated some fellow tire engineers about this “T” function when I proved, through lab experiments that it was possible to fail two tire with high pressure above its stated max 18 days after it was initially inflated.

The other condition that affects and changes the “T” in the equation is temperature. Organics experience constant change (loss) in properties as the temperature increases. The rate of change (aging) doubles about every 18 degrees F of increase in temperature, so it's not easy to calculate or predict the time it will take for an organic structure to fail unless you can control the temperature over time.

Bottom line. It’s a combination of temperature (heat) and time that causes tires to fail. A tire that spends its life in Flagstaff, Ariz., could probably last twice as long as a tire that spent its life in Phoenix if all other operating conditions were identical so even knowing the state where the RV was used isn't sufficient.

As the owner of an RV you can significantly affect one factor and that is the heat generated internal to the tire. When you run fast or overloaded or under-inflated you are running hotter. This means you are speeding up the rate of "aging" of your tires and can expect to see a tire fail before it wears out. You can also protect and shield your tires from the heat generated when in direct sunlight while your RV is parked. HERE you can read about an experiment I ran on the benefits of shielding your tires from Sun exposure.

##RVT1048

Should I move from Load Range D to LR-E ?

 Following an RV Forum on trailers.

Originally Posted by NavyL

If the trailer came with Load Range D tires from the factory, and you replace them with Load Range E tires - I just don't see the need to run the Load Range E tires at the full 80 psi. Tire manufactures have inflation pressure v. weight charts for a reason.

My reply:

If you change from LR-D to LR-E (or from LR-C to LR-D) you will only get an increase in load capacity, or an increase in the Reserve Load, if you run higher than the Certification Sticker inflation psi. If you look at the Load & Inflation tables (they are almost all identical for ST and LT type tires) and look at your actual scale weights you can calculate your current Reserve Load and see what inflation you would need to get to a Reserve closer to 20% or 25%. Due to the unique forces ("Interply Shear") tires must tolerate on multi-axle trailers, I and other tire engineers suggest at least a 15% Reserve with 20% being better. On my personal RV I run 20 to 25% Reserve Load.

While we are talking about the certification label, I advise people to snap a picture of the label, or labels if more than one, along with a shot of the complete tire size and Load Range info on your tires PLUS the full DOT serial including the data code portion of the DOT serial. That way if the printing on the sticker fades or your tire gets cut and destroys itself you have a record so you can file a claim if you purchased "Road Hazard" warranty on your tires as I have, or need to check against a tire recall notice, you can answer the question of what size and Load Range you are currently running. Keep a picture of the Weight Slip too. Its a lot easier to have all this tire related info in one place on your phone.

##RVT1047

Videos from Goodyear worth watching

 I would strongly recommend every RV owner watch the four videos from Goodyear. 

https://www.goodyearrvtires.com/helpful-videos.aspx

Note: If you have problems running the videos you can try using VLC Video Player. Download for free HERE

 While they focus on RV applications, the information can still be informative and educational for anyone that owns a vehicle that has tires. Watching these will also give you a break from my run-on sentences in my sometimes over-detailed posts.

PS I know Tim Miller, the Goodyear Engineer in the video. While I never worked directly with Tim we did share some knowledge about tires in RV applications. 

 

##RVT1046

Is a 10 ply tire better than a Load Range E tire?

 Had a question:

I've read many comments on various RV forums about truck tires vs. "motorhome" tires as it pertains to cost.  Some also mention the firmness of the ride as one of the comparison differences.  Upon reading several tires' specifications I've seen that some have 16-plies ( for example: Michelin xza2 energy 295/80r22.5) whereas others have 18.  This leaves me wondering if the number of plies can help predict ride firmness?  

Does the advertised number of plies refer to the sidewall or to the portion of the tire that touches the road surface? (I'm guessing these can be different.)

Also, what other factors and/or specifications contribute to a tire's ride firmness that may help us compare them?

 

 My reply:

When people talk about the "ply" they are usually referring to the sidewall as the number of layers under the tread is always more as there are usually two to four or more additional layers under the tread of radial tires.

I suggest you read the tire sidewall of your motorhome tires. I believe you will see that those tires say something like "Sidewall 1 Ply (or layer) of steel".  The use of "ply rating" in advertising is just perpetuating the confusion that started in the 70's with "Ply rating" such as "6 for 8"  or even "8 for 12" when better and stronger cords were introduced in Bias truck tires. 

With the switch to radial construction, you will find that most radials from car to light truck and even heavy truck have just one "ply" or "Layer" in the sidewall. The letters for "Load Range" replaced the "ply rating" advertising because some people don't understand the concept of "rating".  When you are looking at large radials with "Load Range" of F and higher all that those letters are telling you are the inflation level the tires can tolerate. 

If you look at the published Load & Inflation tables you will see that some sizes are available in a number of different Load Ranges.

While these are LT tires they show both Inflation, Load Range and Load rating.

 

 

Here you can see that some sizes only come in one Load Range while others come in many. You will also note that a Load Range C, D, or E are only rated for the same number of pounds if the Inflation is the same.

One tire I designed was a Load Range E version of a Load Range D tire and after running all the required tests it was discovered that for this specific tire I only needed to change the number of strands of wire in the bead area (where the tire attaches to the wheel) to meet all the strength requirements. So you can see that it is impossible to make a broad statement on the tire construction to provide any useful information on the possible ride qualities of two different Load Range tires.

If you were to conduct a controlled ride test of 295/80r22.5 size tires with different Load Range but ran the same inflation level you would not be able to feel a difference in ride. However if you were to change the inflation level to achieve greater load capacity you might feel a difference but in that case wouldn't you expect a heavier loaded vehicle to have different "ride"? 

My original question in the title of this post is intenionally misleading as you will find that the load capacity of a "10 ply rated" tire and a Load Range E tire of the same size from the same drsign are really identical.

##RVT1045

Another post on Cold inflation. Is there just TMI?

A few times each week there are posts on various RV forums asking about tire inflation. There continues to be many  readers confused by the words "Max Cold Inflation" on the sidewall of many tires. This wording, while confusing, is mandated by regulation from DOT so don't blame the tire companies. You are more than welcome to write to      National Highway Traffic Safety Administration   at 1200 New Jersey Avenue, SE ,    Washington, D.C. 20590          and ask them why they require the marking say "Max xx psi" when they know that all tires warm up when in use and that many will have inflation above the stated "Max" soon after we start driving and that many people then bleed the tire pressure down to get below the stated "Max". This bleeding of hot air pressure has resulted in numerous tire failures.

As readers of my blog you know you should NEVER bleed down the pressure in hot or warm tires and the only time you might lower tire pressure is after you have been parked away from direct Sunlight and for at least two hours, and have moved to a location that is much warmer than where you were parked the day before your travels.

 Despite the information in my posts, some folks want to play the game of trying to adjust the tire pressure for the location they are traveling to. This is not the proper method of establishing your goal "set pressure" even if the expected "future" ambient is significantly different than the ambient where you are starting your travels.

 I'm not sure why this topic seems difficult to understand but maybe I'm just too close to the topic to see the confusion. I have over 25 blog posts that mention "Cold Inflation". THIS link will display a dozen of those posts if you need a review.

 I suppose one problem may be that with the introduction of TPMS many people are now getting a bit of "Information overload"  as they watch the pressure and temperature readings from each tire go up and down as they drive. This may be the TMI I spoke of in the title of this post.

I suggest you just stick to setting the pressure on the morning (before driving and generally before sunlight has hit the tires) of your travel day. Don't try and second-guess what the ambient temperature will be tomorrow and hundreds of miles away.

I have posts on how I suggest you learn what your "Set Pressure" should be. The procedure is a little different for motor vehicles (cars, trucks and motorhomes) vs trailers.

Generally trailers need to be running higher inflation and/or lower load levels than motor vehicles because of the higher level of Interply Shear that is inherent in multi-axle trailers.

For all users, I advise that the "set" pressure be at least the minimum in the load inflation tables for the measured load +10%.

For multi-axle trailers, I suggest you follow the inflation shown on the Certification sticker but you should confirm your actual axle load is no more than 85% of GAWR if you want a chance of getting better tire life.

Motorhomes should follow the Certification Sticker inflation until they have confirmed the actual loads on their tires, then consulting the load Inflation tables identify the MINIMUM tire pressure  they should ever run. I recommend you then add at least 10% to that inflation with a +15% Reserve load capacity being better.

Remember we are trying to always protect & prevent the tires from ever being in an overload or low inflation pressure condition when we are setting the temperature.

A tire with inflation higher than x psi will generate less heat than the same tire with the inflation lower than x. As you drive and your tire heats up, the pressure will rise but with the increase in pressure the amount of heat generated will decrease so the pressure will stabilize.

 ##RVT1044

Can you change a flat tire?

 

Have you ever given this question much thought? Your answer will depend on your answer to a number of very important questions that need to be considered first.

1. Do you have a spare? A lot of RVs don’t have a one. Their only option is to call a service and hope the service company has the correct size and Load Range (D, E, G etc) tire

2. If you have a spare, is it inflated? Given the number of folk who seldom check the tires already on the ground a majority simply forget to check the spare or don’t check because it isn’t easy to do.

3. If it’s inflated, do you have enough pressure to carry the load for the position where you are going to mount it? Your car or Toad probably has the same pressure in all 4 tires but your RV may have different inflation Front vs Rear. You probably need to be sure you have the spare inflated to the max on the tire sidewall so you can bleed it down to the correct amount for the position.

4. Do you have the necessary tools? Wrench, sockets, long breaker bar, torque wrench, jack, jack stand, steel plate to support the jack, Safety warning triangles, flares, safety vest, and lighting to see what you are doing in the dark? How about waterproof tarp to sit on while doing the job? The steel plate needs to be big enough to support the jack if you didn’t park on a hard road surface.

5. If you think you have all the correct tools, have you made sure by actually unbolting a wheel?

6. Do you have the strength to loosen and retighten the nuts? Have you ever actually tried to loosen all the lug nuts? Do you know the torque specs? Do you have a torque wrench that is big enough for your RV? I have a full toolbox and air impact wrenches in my shop but I doubt I could loosen the nuts on a Class-A. Just watch the first 45 seconds of this sales video and ask yourself if this would be you? Note I am not endorsing that product. I just liked watching the guy jump on his wrench


One other thing to consider. If the nuts have been on for a few years there is a good possibility it will take much more than the OE specs to loosen. I have broken Craftsman and SK sockets on passenger lug nuts because they were put on too tight.

7. Finally do you have the strength to lift the tire & wheel to get it on the wheel studs? 22.5 tire and wheel is over 100#.

I suggest that if you think you are going to change your own tire you need to do a few things.

READ YOUR OWNERS MANUAL and be sure you understand what you are about to do. This job is definitely NOT for everyone.

1. Pick a nice day and with the RV level, wheel chocks in place and the jack stand on a hard surface, first just see if you can loosen all the lug nuts and then re-tighten to the factory specs. Don’t do just one nut or one wheel but do them all. I also suggest you just loosen and tighten one nut at a time for safety sake as we don't need to have the wheel pop off the RV when loaded. Be sure to have someone around watching just in case.

2. See if you can move the spare out of storage and to get it back into storage again.

3. Remove an outer tire and the inner dual and put it back on again as this isn't the same as doing a front single.

4. Most important be sure you clean the threads and torque the nuts to proper specs. I find that WD-40 is good on the threads and does not mess up the torque spec.  You do know the spec for the torque of the lug nuts. It may be as low as 75 Ft Lbs or over 150 Ft-Lbs depending on the vehicle.

5. Ask yourself if this is something you want to do while at the side of an Interstate In the rain, at night?

If you don’t feel up to the job you will need to plan on having a service do the job.

If you don’t have a lot of space for a spare tire mounted on a wheel you might consider having a used tire of the correct size just in case the service company doesn’t have your size. If informed most can do a tire change for you and you will save some big bucks too. You can always pack stuff inside the tire if there is no wheel.

Finally be sure to check the air on the spare every month, even on your toad. Please be safe if you decide to do this job. If you haven't changed a tire for a few years have an experienced person with you

##RVT1043

Your tire says "Max xx psi at Max Load of yyy Pounds" Will your tire blow up if you go above xx psi?

 The simple answer is No. Your undamaged tires are not going to Blowout or Explode or Blowup if you see a pressure greater than xx psi on your TPMS or on your hand gauge.

There is a lot of confusion out there because people do not understand the reason for the confusing wording that is mandated by DOT.

There are Federal regulations on the words and information that must be molded on the tire sidewall. This wording has been around for years with some unchanged since the 1960's.

A recent poll of RV owners responding to a question on tire inflation number on the tire sidewall indicates that 18% think the inflation number molded on a tire sidewall number is the absolute highest a tire should ever have in it. Another 18% think that inflation is "the best" inflation for the tire 2% think it's the lowest pressure the tire should ever have. I am very disappointed with this level of confusion.

Here is the reality:

Each type and size tire and Load Range has a stated Maximum load it should ever be subjected to. The number is molded on the tire sidewall in both pounds and Kg. The tire industry has published tables that provide the MINIMUM inflation a given tire needs to support a stated load. The tables clearly state that the inflation number is the inflation measured before the tire is driven or warmed by direct sunlight. This is called "Cold Inflation". Not "Refrigerated" inflation and not some laboratory 68F or 70F "standard, but the inflation that would be the same as the surrounding ambient air. Some people know this as the "Temperature in the shade".

The confusion comes about because until recently vehicle owners never knew the operating temperature and pressure of their tires. However with the introduction of aftermarket TPMS as used by many RV owners, they now have those numbers presented to them.

What is missing are two things. One being training by the selling dealer as to what inflation is needed to support the stated load and second an explanation of what the words on the tire actually mean.

I am not sure if the RV salesman has ever received the training other than to tell the customer the information is in the Owner's Manual.

Hopefully when an RV owner reads "Max Load"  they understand that they should never load the tire more than that.

The confusion comes with the inclusion of the word Max as it relates to tire pressure.

IMO the wording would be much better and more logical if the tire said "Max Load yyyy pounds at xx Psi cold".

I would leave it up to the people at DOT to try and explain why they were not consistent across all types of tires with the wording on load and inflation limits, but I have no idea who to ask. as I expect them to pass the question off and say "Ask the tire manufacturer" but the manufacturer is only following the regulations established by DOT.

I have a large number of posts in my blog that mention inflation. If you have questions I can suggest you review my posts as the questions are asked a few different ways and I provide the answer with what I believe is a consistent interpretation of the intent of the requirements.

 

##RVT1042

Why did I have three Blowouts? A question on an RV forum

 So I came across another post on an RV forum. I am sorry to report that many of the posters did not understand the concept of Cold Inflation.  The OP asked:

Why did I have three Blowouts?

I’m pulling my hair out. I’ve had three blowouts in 18 months of ownership of our 2020 Keystone Outback. We have ST225 75 R 15 Load E.

The blowouts have been on 3 of 4 points on our dual axle TT.

First blowout was road hazard / excessive speed (I was routinely doing a few mph over on the interstate), and second and third appeared to be heat related in that both tires were running hotter than others and PSI had climbed to 95 on our max 80 PSI.

I feel like I’m inflating my tires every few days when leaving a camp site. This can’t be normal. When starting this trip from SC to FL, I found them all down 3-6 PSI in GA after I had inflated all to 80 (cold) two nights before in the SC storage yard.

Before this last blowout they were all sitting around 74/75 PSI in the morning and I topped them up to 80 PSI according to TPMS. I had barely left the campground when each had climbed to 82. One went to 95 within an hour and temp was running mid to high 70s and the other three were in mid 60s. I pulled over and let air out of them all. But the eventual blown tire just kept running hotter and then popped within 2-3 hours later. Center tread separated completely from tires.

No I haven’t weighed my trailer but I’m just a guy with a wife and three kids 5 and under, no modifications to my trailer, etc, and I don’t pack every inch of floor space with gear that then spills out at the campsite upon arrival, so I can’t imagine I’m overloading.

I don’t understand why I’m having to top up my tires all the time, and it’s not unclear why the last two blowouts have occurred when I’ve been trying to stay on top of the cold PSI and staying under the speed limits.

We are “on the road” so I’m contemplating buying whatever replacement tire I can get, and then asking my local RV service center to check bearings and alignment upon our return. Could either of these be the underlying issue?

Then if that isn’t the case, I’ll consider investing in a complete set of tires with a better reputation, in case the current tires are truly bad.

I will welcome any thoughts or comments!

My reply:

• There were 3 tire failures. A puncture is not the fault of the tire as any tire can be punctured or cut. An increase of 20 to 25% in pressure is an indication of a combination of excess load and excessive speed. A dragging brake or wheel bearing problem can also generate excess heat which can affect both TPM temperature reading as well as increase tire pressure. We do not know what the Ambient was or the TPM pressure readings were which might provide additional clues as to the reasons for their failures. Others have posted that bleeding air out of a hot tire is definitely the wrong thing to do. There is the potential that this action resulted in two tire failures depending on the actual loads and how much air was actually let out. We do not know the actual truck scale load for each axle so this is important information that is missing. 80 psi on the tire is the minimum inflation needed to support the load number on the tire sidewall. It is NOT the max operating pressure. The ONLY pressure we need to be concerned with is the "cold" inflation which is the inflation measured before the tires are driven on or exposed to direct sunlight for the previous 2 hours. 

   

  Inflation pressure when the tires are stationary and out of direct sunlight will change about 2% for each change in the ambient temperature of 10 F. A 6 psi drop with no other cause would indicate a drop in Ambient of about 37°F for a tire inflated to 80 psi. We do not know the Ambient at the time of tire measurements but that is a considerable drop in temperature so I think there is probably some other reason for the reported pressure loss. We do not have to fill every space with "stuff" to end up overweight. The RV Certification sticker indicates the maximum load for each axle when the RV is fully loaded. This load GAWR should not be exceeded. If a scale indicates the RV has an axle at GAWR then the tires MUST be inflated to the pressure stated on the sticker BUT we can still have one tire overloaded as most RV have a side to side imbalance of their axles. They also have an imbalance between the two axles so that is why we need to confirm the actual load on each axle. This can be learned on truck scales as long as we get readings for each axle which requires careful parking on the platform scales. I have seen air loss due to small tread punctures, leaks around rubber valve stem, Leaks between TPMS and the valve stem. leaks through the aluminum due to casting errors. and leaks between the tire and the wheel due to improper mounting. I have posted example of leak through the valve core. See the link below. I and a couple other actual tire engineers follow some of the RV posts. Plus there are many self-appointed "experts" so you do need to always consider the source of the information you find on the Internet as not everything you read here is the truth believe it or not. https://www.rvtiresafety.net/2012/11/why-do-valves-leak.html

  

##RVT1041