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Friday, November 17, 2017

When to replace tires? Can you drive on one "dual"?

Was reading a magazine aimed at motorhome owners and there was an item about a man that suffered a tire failure. It was one of his rear duals. The RV owner reported that he decided to drive to the nearest tire store where it was suggested that he replace all six tires. The tire dealer had to educate the RV owner about the life of tires in RV service being 10 years or less with many recommending that tires be replaced after six or seven years life.

The magazine did offer a brief explanation on how to "read" a tire DOT serial and learn it's age.

IMO the magazine missed an opportunity to further educate their readers with a warning of the damage that was probably being done to the mate of the tire that failed.

First off there is a good probability that the tire, being over 10 years old, failed from a belt/tread separation. We can't be sure, as the RV owner didn't have a TPMS, so we don't know if he could have avoided the problem of a "Blowout" or Run Low Flex Failure on the Interstate. We do know if there was a slow air loss, the tire that did not fail was being run with ever-increasing overload, for as the companion tire lost its air the load on that end of the axle was being transferred to the fully inflated tire.

In the tire industry, there are tables that provide information on how slow you need to drive as you increase the tire load above its normal load capacity.

Basically, you need to run no faster than 40 mph if you are running 107% of the rated load.
If you want to run 113% you can drive no faster than 30 and the max speed drops to 20 mph if the overload is +21%.

Since our RV owner was running at 200% load I would estimate that maximum speed he could travel without doing damage to the "good" tire to be no faster than 5 mph and even that is questionable as there are also distance limits for those conditions

If you have a tire failure, no matter the reason, you need to change out the failed tire and should not attempt to "limp" home on its companion. If you are concerned for your safety on the side of the road you need to be aware that driving over 5 mph means you need to have the companion tire also replaced. No matter its age. As always when changing tires in a dual position you must also match the pair as covered in THIS post.

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Friday, November 10, 2017

DId your RV company or dealer follow Federal law?

Been reading a number of posts on recalls, violations of federal safety standards in the RV industry and on tire and fire extinguisher recalls.
If you have been a reader of my blog you know that I am a strong proponent of filing complaints on failures of safety-related systems and of tires to NHTSA.

You can read a portion of the Federal Regulations on what RV companies are supposed to do HERE as far as the regulations relate to identifying who owns what tire in case there is a recall.
Obviously, if the S/N ( full DOT serial including the date numbers at the end) of a tire is not recorded when the tire is sold there is no reliable way for a tire company to contact owners of tires that are under a recall order.

Selling dealers have the following responsibility
"The National Highway Traffic Safety Administration (NHTSA) requires that tire dealers must provide every tire purchaser with a pre-addressed completed tire registration form OR complete the tire registration process electronically on behalf of the tire purchaser (49 CFR Part 574.8). This is not voluntary, tire dealers must do one or the other"

When a vehicle is sold (car, light truck, or RV) the tires are "sold" with the vehicle. The regulation says "tire dealer" but obviously just because a company sells vehicles and doesn't think of themselves as a "tire dealer" they are in fact selling tires.

However, I have found that few people have received the appropriate forms. Did you receive a form with the purchase of your last set of tires at the tire store, or with the tires that were shipped to you if you bought them online?  How about when you bought a new or used RV?

You say you bought your RV used so you don't think the tire ownership needs to be registered? Well, I know of nothing prohibiting a tire being covered under a recall just because ownership was transferred. The entire reason for this registration is simply to let either NHTSA or the tire company contact the owner of tires covered by a recall. The information is not used for any other purpose.

IMO the odds are that very few of you received the pre-addressed form or if you did you may not have bothered to complete the information.

Well HERE is a website that you can fill in the information and submit it online and don't even need a stamp.

I started this post and mentioned Fire Extinguishers. If you have following the blog you know about the recall and free replacement of potentially defective extinguishers. You may have discovered you have one or more of the covered units. Unlike tires, fire extinguisher ownership is not registered so there will be millions of extinguishers that are not replaced simply because people do not know about the recall and Kiddy company has no way of contacting the owners.

Do yourself a favor and register all your tires RV, Car or truck. Who knows you may just end up with a new set of tires if the ones you have are covered under a recall.

Friday, November 3, 2017

Summary for Newbies

This is a summary of a discussion I had on RV Forum on tire pressure that started with a question and some comments.

"I'll give my take on maybe why, we have ----- Travel Trailer owners ----- Motor Home owners that never had a TT ---- TT owners that moved up to a MH ----- MH owners that still act like they still own a TT -- maybe there is any other I can not think of right now.


I have friends that own MH's and suffer blow outs all the time and their tires look good at the pressure they are using and not a Run Low Flex Failure that will most likely cause a blowout.

Maybe Tireman9 will answer some of the differences between TP's of TT's and MH's to clear up some of the misconceptions people have."

Here is my answer to the broad question

Yes, the proper inflation for MH and "tow-able" RV are different. There is actually strong science behind why there is a difference. THIS blog post is a short explanation. You can read the technical info HERE.

The other issue is that some folks just do not want to hassle with vehicle maintenance required with RV ownership. They are used to cars that have thousands of man-hours of engineering design, testing and development to make those vehicles extremely reliable.

Some of us are old enough to remember that when we learned to drive we were taught how to change a tire, as 10,000 miles was good tire life. Many probably learned how to change oil or adjust a carb. Some even knew how to set the engine timing and change a set of spark plugs. Nowadays it's hard to even see the plugs but with their life now at 50,000+ and the ignition computer controlled and many tires good for 40 to 60k miles who needs to know how to maintain a car?

RVs have a number of systems that do need maintenance. The tires on RVs are an outlier. While top tier tire companies use essentially the same rubber compounds and tire building equipment. This blog post explains the concept of "Tiers" in the tire industry. If you have a MH you will probably recognize the brands and names of the tires on your RV. 

However if you have a "towable" and by this, I mean both 5th wheel and more basic "Travel Trailers" many of you probably have tire brands that don't even fall into the 5th tier level. Some of these off-brand tires may not have a lot of up to date engineering built into them but IMO the main problem is the selection of tire size and load/speed capacity made by the RV company.

Again IMO many RVs are sold based on low cost and the unit has lots of "bling". I don't think I have ever heard of a salesman touting the benefits of the tires that come on an RV. I doubt that many even know the size or maybe even the brand provided on the various units they sell.

So the bottom line is that tires on RVs need more maintenance than the tires that came on their cars. Partially because of their usage but also because there is such a small margin of extra capacity provided based on the tires being undersized to keep costs down. Maybe if RV companies spent more, or even some time evaluating better options, i.e. had better quality for the tire sizes and type they offered, the incidences of tire failure on RVs would be less frequent just as they are less frequent on today's cars. 


Friday, October 27, 2017

What effect does speed have on tire failure?

"What effect does speed have on tire failure?
I have an 8000 lb trailer with 4 Goodyear Marathons. The truck is a Dodge 2500 diesel so I can cruise at 75 mph subject to wind and hills.

How much if any does speed factor into tire failure? I replace tires every 4-6 years regardless of what the look like, and keep them inflated at the PSI marked on the tires."

Basically, increased speed means increased temperature in the critical areas of a tire. Here is a graphic showing the relative temperature in different areas of a tire.
with Red being the hottest and dark blue being the coolest.
When you increase the temperature of rubber you are effectively increasing the rate the rubber is aging. I believe we all understand that old rubber is not as flexible as new rubber. If it isn't as flexible you end up with cracks or cracks that grow every time the rubber is flexed. More cracks and larger cracks are what result in tire failure.

For the above reasons plus others, the major tire companies suggest that in RV operation tires never be driven any faster than 75 and you can see this max speed stated in their literature for many of their RV tires.

But wait, you say your tires have a "Speed Symbol" that relates to 87 or 99 or maybe even 130. HERE is some information on speed ratings. Just because you have a tire with a fast symbol doesn't mean it can be driven at those speeds for 20 or 40,000 miles. About the only thing you can count on is that tires with a "higher" symbol will provide better steering response than a tire with a lower rating.

Now back to the question of how fast can you drive or tow your RV. I previously mentioned 75 as the upper limit but for some tires, there are other things to consider. With ST type tires as found on many trailers, we need to remember that the load capacity is based on a formula that originally specified a max speed of 65 mph. In the past, there were Technical Bulletins that advised that inflation needed to be increased (but not above the sidewall max) by 10 psi to go 70 mph. In addition, load needed to be decreased by 10% if you want to go 75. Still, there was that 75 max.

Many trailer owners know that they need to replace their tires before they wear out. If they don't they are probably going to have some type of failure. But those same people will not replace the tires on their pick-up till they are significantly worn. Why?
The answer can be seen if you simply compare the load and speed ratings of an LT tire and an identical size ST type tire.

LT235/75R15 101Q LRC1985# 50 psi 99mph

ST235/75R15 110R LRC2340# 50 psi 106mph

So exactly what type of magical engineering has tire company X used to achieve both increased load capacity plus increased speed capacity when it is the air pressure that supports the load? If they have the ability to make that ST tire really perform at those speed and load conditions what is wrong with their LT type tires?

IMO what we are looking at is a marketing plan taking over the engineering reality.

Bottom line
You may be able to pull the trailer at speeds above 65 or drive the Class-A diesel pusher faster than 75 but you will be consuming the finite life of the tire and can expect a failure before you wear it out no matter how you maintain your tires.


Friday, October 20, 2017

Will confusion on how much inflation be resolved?

Hi Roger.

Do you think the tire pressure issue will ever be resolved to people accepting the values recommended by the tire manufacturers based on actual loading, or will some always be confused between the tables, the placard and quotes of Federal FMVSS Regulations from some on the forum? It seems so simple and logical, but some just don't get it. I still think a sticky written by you could settle the issue, but the moderators disagree. I suspect a concern for liability for the forum to advise something that contradicts the MH placard.

I have a question for you about my RV. I carry 5 psi above the minimum for the load on my steer tires on a 60* morning @ 1100' altitude. When traveling, I had a morning @ 5000' and temp in the upper 30's. This produced a pressure of 1-2 psi below the minimum. I was going to be traveling into a warmer climate (95*), so did not adjust pressures. This type of condition can happen when we are traveling. I don't want to chase the pressures, so if traveling into warmer weather, I just go and watch the TPMS. I don't feel this is a problem but wanted your thoughts.

Thanks for your thoughts and time.

Not sure if there ever will be a resolution to the Inflate to the placard vs Inflate to the actual tire load. I would liken this to the change oil every 3,000 miles vs change when the car's computer advises.

DOT has a goal of trying to make things simple and keep people safe. DOT also knows about the data that indicates that over half of the RVs on the road today have one or more tire overloaded (either too much load or too little air for the actual load) I really can't fault their approach as IMO many RV owners aren't willing to make the effort needed to learn the proper inflation and to then maintain it.

We have had tire inflation stickers in cars for many decades and there have been massive vehicle recalls because their tires were underinflated, I have read that many as much as 30% low. We don't hear anyone making a case that the accidents and fatalities were the driver's fault. That would be blaming the victim which isn't acceptable, even if true.

OK to your question. I am guessing your CIP is in the 90 - 110 psi range so IMO +5psi isn't enough to avoid the pressure fluctuation that results in your overload. This is why I suggest a +10% value for inflation margin over the table minimum number..
In my Class-C RV, with LT225/75R16 LR-E tires, I am lucky as my RV is rather light. I really only need 60 psi F & R based on my "4 corner scale readings. My certification sticker says 65 / 80 and my dealer delivered it at 64 psi all around. 

For my application +10% would indicate 66 psi and a  +15% margin > 69 psi. I run 70-75 
Even though I can be a bit "anal" about inflation I also wrote THIS post and I really do not mess around with my tire inflation.

My TPMS is set to warn before I would be in overload and I run across a scale once a year just to confirm I am not too far from my original "4 corner weights".

In 2014 I drove Ohio > OR > Calgary > Yellowstone >OH over a seven-week period with temperatures ranging from 90's to snow and elevation of 20' to 8,000' and never had to adjust inflation.

Thursday, October 12, 2017

Can Black tire "Covers" be used without causing damage?

Back in June 2011 I did a post that asked the question  "Tire Covers - Do they do any good?"  In that post I showed the numbers from a test I had run where I collected tire temperatures showing the effects of shielding my tires with white vinyl covers vs the tire temperature when exposed to direct sunlight for just a couple hours.

 I also covered the science of the damage that excess heat can do to our tires. The bottom line of that post was that white vinyl can help extend the life of tires by protecting them from both the affects of UV and the IMO more serious damage done to the internal structure of tires from excess heat.

I also did a rough check using dark trash bag covering a tire and in that case I found that the black cover actually resulted in a tire being hotter than when it was just in the Sun with no cover. Based on that limited data I have recommended against the use of Black or dark color vinyl tire covers.

On more than one occasion I have observed some Class-A RVs with what appears to be a mesh shield than hangs down off the side of the RV. This is different than the vinyl "bag" that hangs directly over the outside of my tires. I was able to collect a few data points while in Redmond , OR in 2014 at a large RV Convention, and that data suggested it might be possible to use this mesh material and not increase the temperature of the tires. Finally this Summer while at another RV Convention I struck up a conversation with a representative of ShadePro Inc who offered to send me a Tire Shade to test. In Aug & Sept i had some health issues and then I ran into difficulty with clouds here in NE Ohio but I was finally able to collect the data I felt comfortable with that would allow me to reach a conclusion of if this black mesh material could be used.
Here is a shot of my test set-up with a white vinyl on front, control sidewall in center and the black mesh shielding the rear. After 2 hours in the full sun 
In the shade a tire gave 92°F

In the sun the white cover was 126°F

The reference tire sidewall registered at 147°F

and the black mesh shade showed 136°F

Under the cover the front tire was at 114°

While behind the mesh shade the rear tire was only 101°F

The data shows that in this test the black mesh did a better job of keeping the tire cool than the white vinyl.

I can think of a couple of reasons for this.
1. The vinyl cover was in direct contact with the front tire so heat was being directly transferred to the tire.
2. The mesh allowed better air circulation around the rear tire.
3. The fact that the black plastic was also in direct contact with the tire probably contributed to the poor results.

I was wrong to suggest that all black shields were worse than white covers, as this test shows that data is better than opinion when it comes to facts. This is one of the wonderful things about Science. 

Friday, October 6, 2017

"Safety Margin"

Some people ask "How much Safety Margin should I have with my tires?" While this concept is simple, the reality is quite complex.

If you want to skip over the "Why" safety is complex just jump to the "Bottom Line" below.

In engineering it is more proper to talk about "Safety Factor" and Wikipedia covers the topic quite well. " Essentially, the factor of safety is how much stronger the system is than it usually needs to be for an intended load". For tires this can become difficult to establish for unlike many materials such as steel or aluminum, tires being made of a number of complex organic compounds both natural and synthetic that have properties that can vary from batch to batch. Even how the raw materials are handled and stored can affect the end product. Also the "strength" of the tire rubber varies with both time and temperature history and as I have previously pointed out the temperature history is not established by just considering the ambient temperature as tire load, inflation and operating speed as well as even storage conditions play a part in establishing the temperature of the more critical components of a tire. Some of these factors can be controlled by the vehicle owner while others can not.

Another part of the calculation concerns the consequences of failure. With some products, the consequences are just an inconvenience say as when a pencil breaks or the ink in a pen stops flowing. With tires the failure can range from an inconvenience if the tire wears out faster than expected or property damage may occur or in extreme cases personal injury can result.

Over the past decades the tire industry has developed a series of guidelines as they try to anticipate the variation in service the vehicle operator will subject the tires too, but even here outside factors such as changes in speed limits or legal load limit changes can affect tires made years before these operating conditions were contemplated.

Top line tire companies have staff of engineers, chemists and statisticians who constantly monitor variations in raw materials and in the finished product. Different plants have different requirements as even something as mundane as the water source can have an affect on the end product. Test labs at each plant are constantly monitoring the quality and consistency of the products that plant makes. not every tire plant makes the same type of tires so along with sales volume requirements plant capabilities are taken into consideration.

Ya but you are thinking "So what? I just want to know the Safety factor of my tires."

Basically I and other tire engineers have tried to consider all of these factors and are constantly looking at tires that have been run on both test tracks and by end users such as yourself. We adjust our specifications to allow our tires to meet and exceed a list of special tests that over time have proven very reliable at predicting the potential for tire failure. While we shoot for zero failures we also know that due to factors out of our control that goal is never possible given the constraints of real life tire use.

I have seen some figures that show a failure rate in the range or 0.05% for many tires but I have also heard of some specific tires (brand, size, design) having a rate closer to 5% or even 10%.

The bottom line
The best I can do is to suggest that you obtain and read the product maintenance manuals for the brand tire you have or are considering of buying. You will probably find that the information across brands is pretty constant so I suggest you at least take a look at a couple different documents. Some of the top line tires have RV or truck application specific reference materials such as can be found from Michelin or Goodyear or Bridgestone or Maxxis or you can check some of the links on THIS post.
- If you have a Motorhome or pick-up slide-in camper you need to confirm the load on each tire position and using the highest loaded end of each axle and the Load Inflation tables from your tire company learn the MINIMUM cold inflation pressure.  I suggest you add 10% to the table number and use that for all tires on that axle for your minimum. 
- If you have a towable (trailer or 5th wheel) also confirm that no tire is loaded to more than 85% of the max load molded on the tire sidewall. AND inflate to the inflation molded on the tire sidewall associated with its maximum load capacity.
- Get and use a TPMS. I have written on how I would set the TPMS warning levels HERE.

- Inspect your tires. Motorhomes can have your tire dealer do the inspection. Trailer owners can follow THIS procedure at least once a year or every 5 to 7,000 miles if you travel that much.

- Never exceed 75 mph with any tire in RV application and if you have ST type tires with no speed symbol never exceed 65 mph.

In my opinion if you follow these guidelines I believe you will have a reasonable and realistic safety margin for your tires.

Friday, September 29, 2017

Who to believe on tire inflation?

I was reading an RV forum and found a number of folks offering their interpretation on what the proper inflation is for tires in RV application. Some insisted the only correct inflation pressure was what was shown on the "Tire Placard" or "Vehicle Certification label"
Here is my reply:

We have two separate issues to consider when selecting the minimum cold tire inflation pressure in RV application.

A vehicle MFG has DOT requirements they need to follow. They are to specify the minimum inflation needed in the tires they provide on the new RV. That inflation is required to be sufficient to support the stated GAWR divided by the number of tires on that axle. Basically this means the axle rating is used to establish tire inflation based on the published load & inflation tables from the tire company.

The owner of the RV needs to confirm the actual loading for their RV and should confirm they are not exceeding the GVWR or either GAWR or the individual tire load capability.

You will immediately note that the RV company is making an assumption that is just not a reflection of reality, that being that axle loads are always split exactly 50/50 side to side. While many RVs may have the load split only a couple percent off of 50/50 some have discovered one end of an axle may carry more than 1,000# more than the other. Others have learned their unbalance is closer to 40/60 and some have discovered that their tires are overloaded as soon as they fill their fuel tank before loading anything else into the RV. Obviously a responsible owner needs to do a better job than some RV companies are doing.

While some feel it is sufficient to just read the regulations about the vehicle certification label I and other tire engineers know that policy is not always the best way to get the best combination of ride, handling and tire durability.

Lets see what Goodyear says in their RV tire application web page.
"Correct tire inflation is a key component in tire care. The recommended maximum inflation pressures for your tires are indicated on the certification label or in your owner's manual. Since RVs can be loaded with many different configurations, the load on each tire will vary. For this reason, actual air pressure required should be determined based on the load on each individual tire.
Inflation pressure should be adjusted to handle the tire carrying the heaviest load, and all tires on the axle should be adjusted to this standard

Maybe you don't think Goodyear knows more than some RV owners. What does Michelin say in their RV documents Here on pg 4:
"The amount of inflation pressure required in each tire depends on the weight of the fully loaded vehicle, to include passengers, cargo load, fuel, and water."
" To determine proper inflation pressure, Michelin recommends weighing each wheel position of the recreational vehicle individually. Weighing each axle end individually will give a clear
indication of how the weight of the recreational vehicle is distributed across the axle.

Also in their tire maintenance document on tire inflation:
"Unlike commercial light truck and truck applications, we recommend weighing motorhomes at each wheel position of the vehicle (right front corner, left front corner, etc.). This is the only way to get a clear indication of exactly how the weight of the motorhome is distributed. Otherwise, one wheel position may be significantly overloaded even though the G.A.W.R. has not been exceeded. Tire inflation for a given axle should be set according to the pressure corresponding to the higher of the two end loads."

Others are allowed to have and express their opinions and their interpretation of how the regulations are to be applied, but it was my job as a tire engineer to understand and work within FMVSS regulations for some 40 years.

I will leave it up to you to decide if the only acceptable inflation is what is on the vehicle certification label or that inflation is learned by reviewing what major tire companies say in their  published guides.
Retired Desi

Friday, September 22, 2017

Wheel failures and "Silent Recall"

I have been seeing a number of posts on an RV Forum about cracked and leaking cast aluminum wheels on one brand of RV trailer. Some owners are reporting more than one failure.
There appears to have been a manufacturing defect in the casting process which results in stress/fatigue cracks that eventually may lead to air leaking from the tires.

Some folks are "lucky"(?) and are discovering the leak before there is a tire failure or the wheel fails and damages the RV. Others have reported damage.

It appears that the wheel provider is aware of the problem, as many are reporting that replacement wheels are being provided and the cost of dismount and mount of tires is being paid for with no argument from the supplier.

I have advised the readers of that forum of a few important things:

1. They should be filing complaints with NHTSA as IMO multiple failures of this type should be investigated with the potential for a federally mandated recall and free replacement.

2. If the wheel supplier is quickly replacing these failed wheels, it appears they are fully aware of the problem and should have reported it to NHTSA per federal regulation.

3. I am concerned that tires on these wheels may be suffering internal structural damage from being run under-inflated. This damage may result in tire failure at some later date. It is entirely possible that owners may not connect the loss of air due to the wheel problem with the tire failures.

4. If the supplier is aware of the problem but has not initiated a recall or made public the facts but are simply replacing the wheels if or when someone reports the failure, this is what is known as a "silent recall," which is operating against federal regulations.


Thursday, September 14, 2017

Can I change from 275 to 295 size tires?

Read this post on an RV forum:

"I need 6 new tires for '04 Dutch Star 38' on a Spartan chassis. Michelin tire models and sizing has me totally confused. Current Michelin size is 275/80R22.5. Dealers claim this Michelin size is interchangeable to 295/75R22.5 from brands that don't offer the original size. They tell me there will be no ill effects on the drive line or instrumentation."

I was concerned with this possibly incorrect information. In this case the two sizes have the same load capacities so that was not my concern.

Some folks replied to the original question by suggesting he use THIS or similar web sites to learn about tire sizes. I did have a little problem of using such a "tire size" site as the information is aimed at car application and does not include special information regarding dual tire application. Also the resulting numbers may or may not match published industry standards.

I pointed out my concern about the Minimum Dual Spacing which is a published dimension in industry standards and has been covered previously in this blog.

Some folks reported that they have changes tire sizes and had no problems and implying that any change might be OK.

I provided this graphic to help people understand what this dimension is.

As you can see the wheel offset and even wheel thickness can have an affect on this spacing. I also pointed out this example.  275's have a MDS of 12.24 and 295's have a MDS of 13.19.
Let's assume that with your wheels and with the 275's you have a physical 1.00" clearance down at the bulge in the sidewall near the road. So if you don't change wheels and put wider 295's on the new clearance will be   1.00 - ( 13.19 - 12.24) or  1.00-0.95  or a final clearance of 0.05"which is clearly too small.

I hope that those considering changing tire size consider not only the load capacity, which is very important but also the dimensions.

Tuesday, September 5, 2017

Tire Terminology

Sometimes people use their own terms when asking a question. Some RV forums have people answering questions using incorrect terms which can mislead others who may be reading  the question or answer being offered. This is just a "Failure to communicate" which doesn't help anyone.

Here are some common terms with their correct definition or example of proper use.

Load Range - A letter code  E, F, G, H etc that established the inflation and max load capacity for a given tire size. (this replaces the old, out of date "ply rating")

GAWR  Gross Axle rating is the MAX design load capacity on an axle. You should never exceed that number

GVWR Gross Vehicle Weight rating is the MAX design weight of a vehicle. You should never exceed that number. Note that the GVWR may be lower than the mathematical sum of the Front GAWR plus the Rear GAWR

Tire Load capacity This is the number of pounds a tire can support at a specified inflation. The Load/Inflation tables establish the various steps in load capacity associated with a level of inflation. Sometimes people incorrectly use the term "Tire Weight" which is incorrect. A tire itself may weigh 45# but it may be able to support 2,800# when properly inflated according to the Load & Infl tables.

CIP  or Cold Inflation Pressure is the measured Psi in a tire when the tire has not been warmed from running or from being in direct sunlight for the previous 2 hours. Technically we are talking about the Cold Inflation Pressure whenever we are discussing tire "inflation pressure".

Hot Inflation Pressure. Sometimes we may want to discuss the pressure in a tire when it has been warmed from running or from being in Sunlight. If this is the case, the term "Hot Inflation Pressure" should be used to be sure all understand the condition of the tire being discussed. You should not be bleeding down the inflation of a hot tire except under special, specific conditions

TRA or Tire & Rim Association is the US group that published the Load /Infl tables for tires made for use in the USA. There are similar organizations with similar tables in Europe (ETRTO) and Asia (JATMA). The numbers of pounds (Kg) and the associated PSI(Kpa) are many times the same or slightly different. The difference is due to using SI or Metric dimensions and rounding and conversions between units. It is legal to sell tires in the US that were designed to ETRTO or JATMA standards but the tires must still have the symbol "DOT" and the individual tire serial code molded into the tire sidewall and must still be certified by the tire manufacturer to be capable of meeting all the appropriate DOT regulations.

DOT  U.S. Dept of Transportation. The regulatory agency that specified the performance requirements for tires intended for use on public roads.

"DOT Certified". This incorrect phrase is many times used by those that do not understand the regulatory process. The DOT does not certify any tire. It is the tire manufacturer that is responsible for "Certifying" that all tires they sell are capable of meeting the DOT Federal Regulations.

FMVSS Federal Motor Vehicle Safety Standards You can read Sub part B of these standards HERE.

Tandem axle. This is found on many trailers where each axle has one tire on each end of each axle. Here is a write-up on Tandem vs single axle trailers if you are not sure of the description.

Tire loads are ALWAYS based on the load on tires on a specific position ie RF, LR, RR or LR etc. Front tires are "Single". Rear tires on most Motorhomes mounted side by side are "Duals". The tables show load capacity for individual tires in "single" position or in "Dual" position. You need to pay attention as some companies publish their own tables and show Axle loads or axle end loads which can lead to confusion. Tires on one end of an axle do not "know" what the load is on the other end of the same axle is. RV companies however assume axle loads are perfectly balanced end to end at 50%/50%. This allows the RV company to select the smallest (lowest cost?) tires capable of supporting 50% of the published GAWR but if one end of an axle loads that tire to 55% or 60% of the GAWR we would be looking at a tire overload situation.

In trailer application with two or more axles they may be some load transfer between the axles such that the ends on the right side may share some of the side to side unbalance but I know of no case where all the side to side unbalance is shared equally between all the tires on one side of a trailer.


Wednesday, August 30, 2017

Why does a sidewall "blow out" when you run on low inflation?

The title for this post is essentially the exact question I was asked.
"Tireman9, can you explain why, when the tire pressure drops below a rating for a tire, that the sidewall of the tire blows out with great force? What pressure drop will cause this type of damage? When the tire blew it had enough force to damage the motorhome with the rapid escape of air that made a large bang when it blew."

My answer:
OK, let's see if i can cover the details of why and how a tire sidewall fails due to being run without proper inflation.

The mechanics are essentially the same, be it a textile (usually Polyester) tire as are most P, LT, and ST type tires, or for tires with steel body cord as most commercial grade LT tires and "TBR" Truck-Bus Radial" tires. These cords are referred to as the "body ply".

I think we all realize that tire sidewalls bend when loaded. This can be observed by simply looking at the bottom (near the road) vs. the rest of the tire sidewall. The amount of bending is essentially just a function of tire size, load and inflation. This bending includes some stretching of the outer surface of the tire and of the rubber surrounding the body ply. This stretching results in some heat being generated. You can test/experience this heat generation yourself with a simple test of holding a rubber band against your lip and stretching and releasing the rubber band rapidly. Your lip is sensitive enough to feel the temperature rise of the rubber band.

Now the rubber used in tire construction can tolerate some temperature rise. The heat generated can transfer to outside air at about the same rate it is being generated. This is what happens for hopefully tens of thousands of miles and hundreds of thousands of revolutions, i.e., sidewall flexes.

So what happens if there is a leak of inflation air? Or if the tire was not properly inflated in the first place? With lower air pressure the amount of bending increases and with an increase in bending we see more heat being generated. Increased heat generation means increased temperature of the rubber internal to the tire structure. Since rubber is a good insulator, heat transfer can be slower than heat dissipation to the outside air so the temperature can continue to rise ever faster.

The strength of the rubber decreases with an increase in temperature which allows more bending. With slower heat transfer from the internal structure to the outer surface and increased heat generation as more air leaks out, I think you can see how it is possible to get to a point where there is something like a chain reaction or "runaway" temperature increase.

The above heat generation can also result in the polyester experiencing a rise in temperature with the associated loss of strength. You have seen the effect of high heat by holding a match near the end of a piece of Nylon or Polyester rope and see the textile melting. In the steel body ply tire the increased bending can result in a fatigue failure of a steel cord. You can test the fatigue with a steel paperclip. Simply bend the paper-clip a few times and it will break. In the case of a tire the number of bends to failure can easily be in the thousands.

So what you are seeing when the sidewall fails is the result of dozens, hundreds or thousands of miles of excess rapid flexing of sidewall rubber and sidewall cords. If the bending is great enough the materials that are being bent simply fail. An explosive loss of air can occur if/when the force of the remaining inflation exceeds the strength of the body cord. Since there is an almost infinite combination of load, inflation, bending and speed involved, it is impossible to give an exact answer as to when the sidewall will finally fail.

If we have a large tire we can have significant force involved even at relatively low pressure. 20 or 30 psi can easily generate over 30,000 pounds of force.


Tuesday, August 22, 2017

New GY Endurance only available in higher Load rating. What to do?

Hello Roger,
At the end of next season I plan to replace our GY Marathon tires after 5 seasons and about 20K miles and have been thinking of the new Endurance tire. However, the specs. are somewhat different between them and am not sure what to do.
Our Marathons are LR-D ST225/75R15.  I always run them at sidewall max. pressure 65 psi. The max. load rating is 2540 lbs and gives us about 30% reserve load capacity. The required rim width is 6-7”. The standard tires for our make and model of travel trailer are an unknown brand LR-C and the Marathons were an available option so we upgraded to them. No issues whatsoever with them. I do not know what the max. psi rating for our rims is without taking one off. They are 6-lug.

I like the idea of having a 225 wide tire vs 205 because I *think* it may provide better handling from lateral forces in curves. I have our TV & TT set up for better handling including shocks on the TT. It performs very well in twisty mountain roads at speeds up to 65 mph.

If we were to get the new Endurance tires in an ST225/75R15 width, it means going to LR-E tire which have a max. 80 psi rating. The load rating increases to 2830 lbs. Required rim width is still 6-7”. The increased reserve load capacity would be fine. I have no idea if our rims are rated for 80 psi and would have to remove a wheel. I believe in the practice of running the ST tires at the sidewall max. pressure to get max. reserve load capacity.

If we were to go with an Endurance tire in a 65 max. rating, it would mean going to a LR-D 205 wide tire and the max. load rating drops to 2150 lbs. I do not want to reduce the reserve load capacity that much and would rather not have to go to a skinnier tire.

What to do if going to Endurance tires? If our rims are only rated to 65 psi, could I simply reduce the LR-E pressure to 65 psi? Have not looked at load tables to see what this does. Or stay with LR-D and have skinnier tires and substantially less reserve load capacity? Are tires rated for and inflated to 80 psi going to result in better handling or perhaps an undesirable harsher ride for the trailer? Not sure why GY did not make the Endurance a direct one-to-one swap for Marathons. It looks like GY no longer offers the Marathon?

Comments would be greatly appreciated.
My reply:
Your 30% reserve load capacity is good. I wish other trailer owners had as much. You may not see any benefit in increasing the reserve load above the 30%. The reserve is based on actual scale readings, yes?
I would not go down to the ST205 @ 65 psi.
I see no problem with running the Endurance ST225 LR-E @ 65 until or unless you confirm higher rim psi rating. It may not be marked on the wheel and I doubt the wheels are "brand name". When you change tires you can look for anything that might ID the wheel such as part number. If the wheels are only 65, then 65 is OK.
Yes, in my posts on Interply Shear I have suggested increasing inflation, but your CIP should not exceed the rim rating (all ratings are for cold inflation pressure).

LR-E ST225's have same load capacity when inflated to 65 psi as do ST225 LR-D at 65 psi.
Hope this info helps. 

Tuesday, August 15, 2017

"Run Flat" vs "Blowout" protection

Saw a thread on run flat fixtures to be used on the front of an RV to provide some protection against losing control if a front tire were to fail. A lot of discussion and conjecture but not many facts. Before I start it's important to be sure we all are on the same page with these terms.

"Run Flat" capability in today's tire market is a reality for passenger car tires. Tires designed and so labeled can provide mobility at reduced speed for various number of miles even after a complete loss of air. Some advertise 50 miles at 50 mph. I know of no true run flat tires for LT, Truck or RV application.

"Blowout" is a generic term used by many to describe a tire failure. This term does not provide information to tire engineers on the condition or possible cause of the failure.

"Rapid Air Loss" or "RAL" is any sudden loss of all the air in a tire.

"Belt or Tread Separation" -- This is when the belt and or tread portion of a tire separates from the rest of the tire. The carcass or body of the tire many times will still hold air.

"Run Flat Device" -- This is some equipment that fits inside a tire. It may prevent the beads from De-seating from the rim or may limit the collapse of the tire after the loss of air.

"Steering Stabilizer" -- This is vehicle equipment that may lessen the "jerk' of the steering in the event of an RAL.

I do understand the concern of some about the loss of control of a large RV in the event of a front tire failure. Yes, I have seen some of the videos and they are spectacular, but a front tire failure does not have to mean you will have a crash.

Back in the early '70s I worked extensively on a new type of tire from Firestone, the LXX, which was intended to replace 10.00-20 and 11-22.5 size tires but used a 26.5 rim. I even spent a week in Texas at the test track intentionally puncturing front tires with a 2" diameter hole to test vehicle control. While the tire worked as designed, the cost of new wheels made the tire design not viable in the truck tire market.

Here is a picture in the Akron newspaper at the time showing the tire with my then boss Ed Henry.

The main challenge for vehicle control is to either limit the "drop" when a tire suffers an RAL or to limit the steering response which can send the vehicle into another traffic lane.

There are a number of systems on the market:

Tyron is designed to keep the tire on the wheel. It does not appear to lessen the collapse of the tire.

Rodguard inserts for 14" to 17" tires appears to partially support a flat tire.

Safe-T-Plus steering stabilizers take a different approach to vehicle control.

Hutchenson seems to focus on extreme situations, such as military applications.

I recall hearing about multi-piece inserts like this one from TAC but was not able to find current info for 22.5 RV size tires.

IMO the best approach to preventing a loss of vehicle control is three steps:

1. Use a TPMS and have it set to tightly monitor your inflation. Test your sensor to confirm it reports pressure loss at the level you expect. It may not be set correctly and a test can confirm.

2. Closely inspect your tires at least annually and if possible even have your tire dealer check for out-of-round or lateral wobble of the tread and bulges in the tire sidewall. These can be early warning of separations or other structural damage to a tire.

3. Study and review the safety videos on how to maintain vehicle control in the event of an RAL.
   Here is one from Michelin, and another, and yet another.