speed vs fuel economy

 

 

 

 

What does "MAX PSI" on the sidewall mean?

 

I know I have answered this question in the past but maybe it was on an RV forum for a single-brand RV so not everyone has heard this. So here goes.

Many, but not all, tires have the words "Max PSI" followed by a number on the tire sidewall. I have no idea which lawyer wrote the requirement but he or she didn't understand how those words would be misinterpreted.

Too often I see people saying that the tire should never be run with a higher pressure or that this is the absolute only correct pressure for the tire, but these assumptions are incorrect.

I hope everyone reading this post understands that tire load capacity is related directly to tire inflation and if they want to increase the load capacity they will need to increase the inflation. While it is true that an increase in tire inflation is required if you want more load capacity, there is a limit. The limit is controlled by industry standards which are published and followed by all tire companies.

Each tire has a Maximum Load Capacity and to get to that capacity you need to increase the tire inflation BUT there is a limit as each tire also has a limit or maximum load capacity and increasing the inflation above the stated pressure WILL NOT increase the load capacity.

So you have a tire that says "Max Psi 65 psi" which means you will gain increased load capacity as you increase inflation from 35 to 45 to 60 and to 65 psi BUT any additional pressure above 65 psi WILL NOT RESULT in any additional increase in load capacity.

It is also important to know that tires are tested and can tolerate higher pressures due to being warmed up by running so the pressure on the tire sidewall only refers to the tire "cold" inflation so you should not bleed down the hot pressure.

 

Have a tire question? Ask Roger on his RV Tires Forum here. It's hosted by RVtravel.com and moderated by Roger. He'll be happy to help you.

Read more from Roger Marble on his blog at RVtiresafety.net or on RVtravel.com.

 ##RVT1098

Why is there so much confusion on tire inflation ?

 Why is there so much confusion on tire inflation versus max inflation versus my recommendation of +10%?

Here’s a question posted on an RV forum: Tom said, “So, I see some who are saying to set pressure to max cold pressure recommended, and others talk about ‘minimum +10%.’ …I’m confused.”

Starting at the end

Let’s assume you know the actual load on each tire from your measurement on a scale. (Yeah, I know about assuming. But every RV owner has been told at least once to learn their actual loading.) You take the load on the heavy end of an axle as there are almost zero percent RVs with the load exactly at 50/50% side-to-side.

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The load number is then found in the Load/Inflation charts for your size tire and you go up (to the right) until you find a block with at least, or more, load than what you measured on the scale. NEVER go lower than your scale reading. DO NOT average the reading from each end of the axle weight measurement. DO NOT try to calculate a pressure between the 5 psi increments. Then look up in the chart to find the PSI. That is the MINIMUM inflation you should ever run in the tires on that axle.

Add 10% to the tire inflation number

I suggest you add 10% to that inflation number to offer some “protection” in case the temperature drops. If you have added my recommended 10%, you will probably see that you do not have to add air every day the temperature drops 10 degrees.

RVs have certification labels aka tire placards that have tire size, type, load range and inflation numbers. They also have GAWR (Gross Axle Weight Rating), which is the MAXIMUM load you should ever have on that axle. The RV company is required by DOT to post on the sticker an inflation number that is sufficient to support 100% of the GAWR. The RVIA (RV Industry Association, a standards organization) sticker on the side of your RV now requires an inflation level good enough to support 110%, which is better than the DOT requirement.

Because of these load capabilities, most RV companies select the smallest (lowest cost for them) tire that can just barely meet these requirements. The result of this purchasing decision is that you will need to inflate your tires to the level needed to support the tire’s MAXIMUM load capacity – which is the number on the sidewall of the tire.

Side issue. The wording on the tire sidewall is confusing. The reality of what it means is that any given tire has a MAXIMUM load capacity and an inflation (minimum) required to support that load. What is not printed on the tire sidewall is the fact that there is no increase in inflation that will result in that tire ever being capable of supporting more load. Therefore, the “max inflation”  wording that was decided upon by some committee 50 years ago is inadequate.

Can you drive on a single tire if a dual fails?

 This question comes up every few months and the basic answer is NO it is not safe to travel more than a few hundred feet when one tire in a pair of duals fails.

However, I also know that some locations on the side of the road are not safe and if you decide that moving is the best option what can you do?

 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 if you want to try and prevent a second tire failure.

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 +18%.
The above also limit the "max travel time at those speeds to 2 hours with a minimum 1/2 hour "cool Down" time each 2 hours.


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.

BOTTOM LINE
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 MUST 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 for OC.


Many in a thread said "slow down" but I doubt that any were thinking of less than 20 mph.  Some may have driven considerable distance at some "reduced speed" but is each case the companion tire should have also been replaced.

How are tire load ratings calculated?

 I had a question asked recently on an RV Forum about tire load ratings.

Mike posted:

Tireman9 — I’ve seen your posts with this “minimum inflation” statement in other threads in addition to this one. I never was able to find documentation in the Michelin files that states that, UNTIL I finally realized that the “Maximum load & pressure on sidewall” statement in their Michelin Truck Tire Data Book and Inflation Charts establishes the relationship between weight and psi for each 5 psi increment in the chart.

I like to weigh our MH at least once a year because we’ve been making some changes to it, and our trips vary from short to fairly long with different loading as a result. Long story short, I use a simple Excel sheet to determine psi requirements based on scale weights (it’s just easier for me and I like using Excel). Well, this simple little Excel lookup routine turned into a bit of a pain until I realized that the weight increments for each 5 psi increase are not consistent. In fact, they are all over the place.

The way I found this out is I was calculating psi # for our MH’s weights in two ways. One was to find the corresponding psi for my actual scale weight, and then add 10% to the psi. The second way was to increase my scale weights by 10% and then find the corresponding psi for that weight. When I compared the resulting psi’s between the two methods, they were different in many instances, but not consistently so.

So, the question is why do the single tire increments vary from 140 lbs. to 230 lbs., and the duals vary from 230 lbs. to 410 lbs. Can you explain? Does setting tire pressures really have to be this exact? And which way is correct? You would think that both would have the same results, but nah-Baby-nah.

My response:

The simple answer is. The load formula is not linear, as you can see here.

As you can see, there are some values that are exponential.

Adjusting the load for dual position

On the question of adjusting the load for dual position, there are more instructions we tire engineers must follow:

I suggest:

1. Learn actual tire loading on a truck scale when the RV is loaded to your expected heaviest.

2. Assume one end of an axle is supporting 51% to 52% of the total axle load (this estimate is not exact). This is why “4 corner weights” are preferred if you are near the load limit.

3. Consult load and inflation tables to learn the MINIMUM cold inflation.

4. Add at least 10% to the inflation in #3 and use this new number for your “cold inflation goal.”

5. Set your TPMS Low-Pressure Warning level to the inflation in #3 above.

I think that if you follow these instructions you can stop using your Excel calculations, which can be misleading … and Go camping.

'Defective’ tires: Just what is the ‘defect’?

 I have heard people claim they had a “defective” tire when they post about a tire failure. I have even addressed claims from lawyers and supposed “experts” that a group or even thousands of tires had “a defect” that caused tires to fail. However, for some reason, they can almost never point to or identify the specific component or material that contained said “defect”.

I spent the majority of my 40+ years as a tire design engineer looking at and investigating tire failures. I can say that I have identified a few tires that contained a “defect.” But before I continue we need to be sure we have a common understanding of the word “defect” and what we mean when we use it.

Definition of “defect”

Merriam-Webster says the meaning of “defect” is “an imperfection or abnormality that impairs quality, function, or utility: shortcoming, flaw.” I note that they are not saying that an item has a defect if it has failed to perform. They are talking about a specific “imperfection or abnormality” in an item.

I sometimes offer the following in an effort to clarify the concept. If we discover a dead body, should we always assume that the person was murdered? Of course not. But jumping to a conclusion about the most extreme cause seems to be what some want to do while ignoring the need for a thoughtful and reasoned collection of facts and evidence.

Tires can fail for dozens, if not hundreds, of different contributing factors, but most do not involve design or manufacturing mistakes. When I read a post on an RV forum or receive an email about a “failed” tire, I do not start with the assumption that there was a manufacturing or design mistake. I do start with a close examination of the tire, wheel, and valve, as many times the evidence is in front of our faces. The question is, are we willing to take the time and make the effort to look for the evidence that is almost always there?

Why people might use the word “defect”

I believe that many times some people use the word “defect” as they do not have the knowledge of how to do even a basic inspection. That is understandable. But if we want to avoid a repeat, wouldn’t it be best to learn the real reason for the failure and take steps to try and prevent a re-occurrence? What bothers me, though, is when I hear lawyers or reporters use the word possibly to grab the attention of those reading an article. I believe that a headline saying “Another RV tire failed, facts to follow” would not gain much readership.

In my posts on RVTravel.com and in my RV Tire Safety blog,  I have provided numerous examples of failed tires that were initially claimed to be “defective” such as the following examples.

A tire that was run with low inflation.

One that has an external cut.

Or this tire with a plug in the sidewall.

Or this tire failure below that was due to leaking air. This failure even ended up on YouTube, but the video did not mention the evidence of the tire having been run with very low air pressure.

Currently, there are news articles mentioning failures that occurred many years ago on some RV tires that might have been improperly installed on some heavy RVs. While I have no details, I do recall reading the word “defective” in the article, but no mention of what the “defect” might have been.

Did a “defect” really cause the tire failure?

I would suggest that when you read a “news” article about tire failures and find the word “defective,” you stop and see if the author mentions what the imperfection or abnormality was in the tire that caused the failure.

Why a safety margin on tire inflation is important

 If you have read any of my posts here at RVtravel.com or on my blog RVTireSafety.net or on any individual post on the various RV forums I follow, you have probably seen me suggest you add a “margin” (safety factor aka reserve load to some folks) to either the load capacity or the minimum inflation recommendation, or even to both. You may wonder what are my reasons. I have seen some people try to chip away at my load margin or the minimum inflation I suggest.

Reason for adding extra margins

I do have a reason for adding these extra margins. It has to do with the basic nature of the rubber and other materials used in tire manufacturing. Adding margins to other items such as bridges, buildings, or even the cars and trucks used to haul a trailer or load in the bed, is also fundamental to the nature of Engineering Design.

Engineers and others in scientific fields such as chemistry, medicine or physics all know that the properties of the materials we specify have some level of variation in their properties. For tires, I know and have seen firsthand that you cannot have an absolute number for the strength of a material. Each component of a tire, as seen above, be it rubber, steel, polyester or others, has a set of properties depending on which specific material I select. If we take a sample of material, be it steel or natural rubber (NR) and ethylene–propylene diene monomer (EPDM), we are presented with stated strengths. Even what we call “steel” is available in more than 3,500 different “grades”, each with different properties and levels of strength ranging from 30,000 psi to 72,000 psi.

When designing a tire we can basically choose from one type for the wire used to make the “bead” (the part that keeps the tire on the wheel when inflated—see above) and a different type of steel used in the belts. But then we can also select from a variety of wire cables which is a configuration of individual steel strands. Each cable has its own set of properties of strength or flexibility or even the ability of rubber to adhere to the cable.

As you can see, there is an almost infinite variety of combinations of materials a tire design engineer can select from. There are other things that can also be selected such as the temperature and time to “cure” the rubber, which can also affect the strength and durability of a tire.

All tires for U.S. highway use must pass DOT tests

Now all tires sold for highway use in the U.S. must be certified by the tire manufacturer to be able to pass specific DOT tests. So passing those tests sort of established a minimum “strength.” Tire companies can select to exceed those minimums but generally to exceed the minimums we would need to use more expensive materials or constructions. Since the tire companies want to stay in business, they also need to keep the costs of making a tire in mind. It is also well-documented that if you take a sample of steel cord and pull on it till it breaks, you never get a single result but will get a range of numbers. The same variation is observed in the strength of rubber and polyester, as used in tires. Now to ensure we produce tires that are acceptable to the public and pass the DOT tests, tire companies also have their own internal minimum performance standards. BUT I do not think that all companies design or make tires that perform identically for every type of test possible.

It is also important to remember that the DOT requires that ALL tires be capable of passing the tests—not some or an average or even most. But 100% of the tires made must be capable of passing each and every DOT test. To ensure that all new tires are capable of passing the tests, tire companies use statistical analysis of test variation in an effort to be confident that production tires will pass the DOT testing.

Finally, we come to you, the user. We know that a significant percentage of RV users do not set or even maintain tire inflation necessary to meet published minimums. We also know that many have no idea of the actual load they have on their tires. While some may keep driving speed below 65 mph, some will actually boast of towing at well over 75 or even 80 mph.

A tire’s strength decreases with use and time

In previous posts here and on my blog I have covered the fact that after a tire is used and as time moves on, its ultimate strength decreases. In an effort to decrease this variation, I have been advising that people not run at the lowest possible inflation for the load on their tires. The lower the inflation you run, the more heat is generated, which in turn lowers the ultimate strength of the rubber.

Degradation of rubber strength is not an on-off switch but a continuous process. The more you drive at higher heat, the more strength is “consumed.” The more pot-holes you hit, the more damage you do to the tire structure. This, in turn, can result in a decrease in the maximum strength of your tires.

Load & Inflation tables give you a guide for the MINIMUM inflation for a tire loaded to the stated number and operated at a given speed.

More speed means more heat.

Lower inflation means more heat.

More load means more heat.

And it is HEAT that ultimately can result in a tire coming apart.

However, if you run more inflation than what the tables show, that will decrease the heat.

 

 

In a future post, I will cover what I feel is the improper use of the word “defect.”

How much air pressure is too much in RV tires?

 With the introduction of Tire Pressure Monitoring Systems (TPMS), many RV owners are presented with what to them is new information on the status of their tire pressure and temperature.

We can start off with a question:

“I have a serious concern with the G-rated tires on my 5th wheel trailer. When I run the cold max pressure at 110 PSI, I get TPMS readings up to 134 PSI rolling down the road. Lately, I have been running 95 PSI cold and am getting 115-119 PSI rolling. The tires and rims are rated for 110 PSI. I can live with 120 PSI, but the 134 PSI concerns me.”

The inflation ratings for wheels are based on a “cold” pressure. An increase in pressure due to operation is considered by wheel manufacturers and I would not be concerned as long as the proper cold inflation is used along with appropriate limits on load and speed.

Tire wording “Max Pressure” can be confusing. In reality, the pressure stated on the tire sidewall is the cold pressure needed to support the stated load. That load is the maximum load the tire should be subjected to. I advised the owner that he should NOT underinflate his tires and plan on seeing the operation temperature to result in an increase in the pressure sufficient to support the load. The “cold” pressure is the only pressure you should be concerned with.

Tire pressure increases with increase in temperature

Tire pressure increases by about 2% for each increase in temperature of 10° F. If you don’t remember the science from high school, you can read THIS post from my blog on Tire Safety. If you are seeing a 21% increase in pressure (110 > 134 psi), that means you are seeing about a 100° F increase in internal tire temperature. I would consider that increase a bit much for normal tire operation. In my 50 years of designing, testing, and working with tires, I only remember measuring a temperature rise of 100° F in Indy race car applications.

If you are seeing a 25% increase in pressure (95 > 119), then you are getting a 125° F increase in temperature. This indicates you are working the tires very hard. This extra “work” that is generating a greater temperature increase is not good for long-term tire life. You are “aging” the tire rubber faster. Some might want to review the “Key Point” of tire life and how increased temperature can shorten life as covered in THIS post.

Possibly overloading tires and/or driving too fast

Your temperature increase indicates you are possibly overloading your tires and also possibly driving faster than desirable for your tire loading. It is recommended in tire company data books that your operating speed for any tire in RV application be no greater than 75 mph. The “Speed Rating” is just a short-term rating and should never be considered acceptable in day-to-day RV applications.

You need to confirm your pressure is 110 psi AND that your gauge is giving an accurate reading at that level.

This was the reply:

“Today drove 250 miles and my 95 PSI tires were running 115-119 PSI and the tire temps were at 20F above outside ambient temp of 50F and 71F tire readings. I still contend that 134 PSI is way too dangerous for tires to run on 110-rated tires.”

So I responded:

Few people realize that the pressure increase as a function of temperature is based on well-established and confirm able physics. A TPMS is not reading the actual tire temperature but is actually reading the temperature of the brass valve stem and the metal base of the TPMS itself, which is being cooled by outside air.

Air is a good insulator

Air is a very good insulator. If you think about it, you have a small column of air running up the inside of the valve stem which makes it difficult for the heat to travel up the center of the stem and past the valve core itself. This is all while the valve is moving rapidly around being cooled by the outside air.

I am aware of laboratory tests that go against what “common sense” might indicate. That being that the air inside a tire is not uniform in temperature but it is always cooler than the hot spots of a tire. It is the hot spots that can result in a tire failing if hot enough for long enough.

I have no doubt that the TPMS was indicating only 20° F above the cool 50° F outside air temperature. You should not be concerned about the hot pressure of 134 psi on tires that have a cold pressure rating of 110 psi for its max load capacity rating, that is, as long as you have confirmed the actual load you are placing on your tires is no greater than 90% of the load capacity shown in the Load & Inflation tables for your tire. By “confirmed” I mean with actual scale readings for each axle when the RV is fully loaded

As a tire design engineer with 50 years’ experience, I trust the science of the “Gas Law” and the knowledge that air is an insulator and metal conducts heat from a hot source to a cooler one.

I don’t know what to advise other than to decrease the operating load and speed and to confirm your hand gauge is accurate. Also, always inflate the tire when cold to 110 psi, as continued operation at current load and speeds will certainly result in pressure readings that are well above the cold pressure of 110 psi.

How are loads calculated

 Had a question asked on an RV Forum

Mike Posted

 

Tireman9 - I've seen your posts with this "minimum inflation" statement in other threads in addition to this one, and I never was able to find documentation in the Michelin files that states that UNTIL I finally realized that the "Maximum load & pressure on sidewall" statement in their Michelin Truck Tire DataBook and Inflation Charts establishes the relationship between weight and PSI for each 5psi increment in the chart. I like to weigh our MH at least once a year because we've been making some changes to it, and our trips vary from short to fairly long with different loading as a result. Long story short, I use a simple Excel sheet to determine PSI requirements based on scale weights (it's just easier for me and I like using Excel). Well, this simple little Excel lookup routine turned into a bit of a pain until I realized that the weight increments for each 5psi increase are not consistent. In fact, they are all over the place.

The way I found this out is I was calculating PSI # for our MH's weights in two ways. One was to find the corresponding PSI for my actual scale weight, and then add 10% to the PSI, or the second way was to increase my scale weights by 10% and then find the corresponding PSI for that weight. When I compared the resulting PSI's between the two methods, they were different in many instances, but not consistently so.

So the question is why do the single tire increments vary from 140lbs to 230lbs, and the duals vary from 230 lbs. to 410 lbs. Can you explain? Does setting tire pressures really have to be this exact? And which way is correct? You would think that both would have the same results, but nah-Baby-nah.

Simple answer is, The Load formula is not linear as you can see here.

 

 

 

 

  there are values that are exponents

 

 

 

 

 

As to adjusting for Dual position there are more instructions we tire design engineers have to follow.

 

 

 

 

 

I suggest:
1. Learn actual tire loading on a truck scale when the RV is loaded to your expected heaviest.

2. Assume one end of an axle is supporting 51% to 52% of the total axle load (this estimate is not exact. This is why "4 corner weights" are preferred if you are near the load limit

3. Consult Load & inflation tables to learn the MINIMUM cold inflation

4. Add at least 10% to the inflation in #3 and use this new number for your "Cold Inflation goal"

5. Set your TPMS Low Pressure Warning level to the inflation in #3 above.


Go camping.