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.

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.

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

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

"65 psi is too much"

 "65 psi is too much" was the opinion posted by Tom when he said 

"Ok, I can’t find any info here so one last time. The tire pressure says 65psi cold on the side wall of the tires on my travel trailer. That sounds like a lot to me. What tire pressure do you run your camper at?" on an RVTips FB page. 

I'm not sure where he did his search. On the FB page, he did get some answers such as "60"  and "110" and "80".

 I responded:

 As an actual Tire Design Engineer, with 40 years of experience including decades of trailer ownership, I can explain why you need to follow the Science of proper tire loading and inflation. You have a Certification sticker or label on your TT that was applied by the RV company based on safety regulations. The sticker tells you the correct information for tire size, type, and Load Range. It also tells you the GAWR. You should not exceed the GAWR because you can break wheels, bolts, axles, hubs, springs, and related parts. Known strength limits of these parts were used to establish the maximum load you should put on your axles. The tire industry has tables that cover the dimensions and load limits for tires for different levels of inflation. These tables have been around since the 70's and all tires sold for highway use, in the US, are required to be capable of passing a number of different strength and durability tests. The test conditions specify both load and inflation levels for the different tests. The RV company has the responsibility to provide the information on the certification sticker such that the specified tires are rated to support at least 110% of GAWR when they are inflated to the stated inflation when the tires are at Ambient Temperature, i.e. "cold". You didn't offer what tire engineering experience you bring to this discussion so I don't know how you arrived at your conclusion that 65 psi cold (ambient) was "too much" but I sincerely doubt that the RV company would select LR-D tires if that level of load capacity was not needed. You can learn more about tires by reading some of the 500+ posts on my blog       www.RVTireSafety.Net

 

##RVT1029

Internet "Experts" have Dennis confused. Maybe I can help

 Dennis said:

Okay ... I've been RV'ng in Class-A's for 40 plus years and thought I had this figured out, but the "experts" have emerged to confuse me, once again. So, what is the definitive answer to the question of tire pressure ... should it be the coach manufacturer's recommendation on the placard in most every RV, or the tire manufacturer's inflation recommendation? I know for sure, the "cold pressure" stamped on the tire is NOT the recommended pressure. Please advise.

++++++++++++++++++++++++++++=

Dennis, I completely understand your frustration.

You need to remember that there are three entities that are trying to answer three different questions when it comes to tire inflation.

First the tire company.    They make tires that must meet various tire industry standards for tire dimensions and load capacity. The tire company must also certify to DOT that the tires they make are capable of passing a number of different strength and durability standards if they want to sell tires for use on US roads. These requirements are why you end up with the numbers molded on the tire sidewall. These include the Maximum load capacity for both Single and Dual-position. They must also identify the minimum inflation needed to support both the single and dual-position loads. We need to remember that the tire company does not know which RV or truck the tire will be mounted on. The tire company also doesn't know if the tire will be on the Front, in Dual on the drive, or even if it will be on a TAG axle so they can not give a single inflation number that meets all these requirements

Second, the RV Mfg has to meet some different DOT regulations. The primary one is that the Certification Label aka tire placard must specify inflation for the tire that would be sufficient to support the stated GAWR. Example: If the GAWR for a front axle was 6,000# then the tire they select must be capable of supporting 3,000# and the RV company must tell you, the owner, the MINIMUM inflation needed for the tire to be able to carry 3,000#. Since each axle probably has a different GAWR that is why you may have different inflations on the Certification label for each axle.

Third If the RV company wants to be able to meet RVIA standards the tire must be capable of supporting 110% of the axle load so in our example that means 3,300#. Here things can start to get messy. The inflation needed to support 3,300# is almost certainly different so the Tire placard would need to state the inflation for 3,300#. It is OK for the inflation on the placard to be higher than what DOT requires but it can not be any lower.

Finally, we get to you, the owner.

Option A. is to simply inflate to the number on the tire sidewall. This, in some cases, is significantly higher than the tire needs so you may get a hard ride and in extreme cases more rapid center wear.

Option B is to follow the placard inflation which on many Motorhomes is lower than the inflation number on the tire. So the owner gets confused.

Option C is what I and other tire engineers recommend. We have solid reasons for saying that the Rv owner needs to get on a truck scale, learn the actual load on the tires, and then learn the minimum inflation needed to support the actual load on the tires. We know this is the best for the tire because the data shows that a majority of RV have a tire or axle in overload and this is a major reason for tire problems in the RV world.

Option D This is a better version of Option C. This is even better but is not as easy as Option C. The data shows that almost no RV has the axle load split 50/50 side to side so simply taking the truck stop scale reading and dividing by two can be misleading. Some RVs have been found to be as much as 1,000# out of balance side to side. Option D means you need to learn the individual load on each tire position when the RV is loaded to its heaviest. This is almost impossible at regular truck stop scales. To learn the individual tire position load you need to get a company like RVSEF or Escapees to use their scales to get the individual weights. It is also possible, with a little work to find a platform scale at a gravel yard or some lumber or cement block companies have large scales. Here ere is a worksheet to help with the numbers. Once you learn the actual load on the heavy end of each axle you would use that weight to consult the tire Load and inflation tables to learn the MINIMUM inflation for all the tires on that axle. We also recommend that if possible you add 10% to the table inflation to give you a nice margin. Just do not exceed the max inflation rating for your wheels.

I hope this helps clear up your confusion.

 

##RVT997

How to adjust tire pressure when inflating in a heated garage

 I found a chart that covers how to adjust tire inflation when you are inflating tires inside a heated garage (heated to 65F) when the truck or RV will be moved to a very cold location. I am hoping that when you study the adjustment you will better understand what, if any adjustment is to be made with your vehicle.

• 

 

 

 ##RVT991

 

Setting Motorhome inflation for "Newbie"

Reading a forum post that said:

"We are moving across the United States for our FIRST trip in our 06 Mountain Aire. Don’t think that’s a great idea for a first trip, but that’s what’s gonna happen. LOL. I do not have access to a four corner scale. I do have access to a cat scale. I downloaded the Michelin tire pressure guide. From what I remember here, I take the axle weight and use the Michelin guide? I seem to remember someone saying to add 5 psi to compensate for the lack of a 4 corner scale?"

My reply:

You can read my RV Tire blog for more details for Motorhomes (but not towables)

1. If you have no scale reading follow the Vehicle Certification Label AKA (Tire Placard)

2. As soon as possible, get on a truck scale when you are at your heaviest (full of fuel, food, water, propane, clothes, people etc).

3. Using the axle load numbers: Confirm no axle exceeds GAWR (gross axle weight rating) (on placard) and the RV does not exceed GVWR (gross vehicle weight rating) (on placard). If you exceed either max weight rating you must move or eliminate some "stuff" in the RV.

4. If your weights are lower than GAWR then you MIGHT be able to lower your inflation a bit but there is some effort involved.

5. Since you will not have "4 corner weights" you will not know how much out of balance side to side you actually are. Some RVs have been found to be 1,000# out of balance on an axle.

6. I suggest at a minimum you assume one end of each axle is carrying 53% of the total axle load. You could use the 53% number to consult the Load & Inflation tables.  WARNING Michelin tables assume perfect 50/50 load balance which IMO is not realistic, so you need to calculate the individual axle end load ( divide Michelin number by 2)

7. Using the 53% load number and the calculated Michelin load number you now know the MINIMUM Cold Inflation aka MCI number. It is suggested you add 10% to that inflation number for inflation when setting tire inflation. This is your daily measured Cold Inflation Pressure aka CIP. This 10% covers you for day to day temperature and elevation changes.

8. If or when the measured cold inflation (or inflation reported by your TPMS) drops to your MCI number you need to add some air to get back to your 10% margin number.

9. I see no reason to ever bleed off pressure from your daily measured CIP unless your reading is higher than the inflation number molded on the tire sidewall. If you get that high you can bleed down to the tire sidewall pressure but never below the MCI.

Ya a bit of work but once you think about your goal - Stay above the MCI and below the inflation molded on the tire sidewall you are good to go. A TPMS makes the job of checking inflation on each day of travel much easier as you simply check each morning before you start out. TPMS will also give you a warning when or if you get a puncture od have a leaking valve and are losing air. A good TPMS will have some form of "early warning" when you start to lose a couple PSI while running down the road. This could prevent a serious tire failure.

Tire inflation not the same for all trailers

I have written a number of times on the topic of Interply Shear. This is the force that is tring to tear the belts of radial tires apart.
There are some highly technical papers on the topic and you can review tham after a simple Google search on the term. You can also look here on my blog for the posts where the term is tied to a post by simply checking the list of topics on the left side of my blog. Basically this force comes about when the belts in a radial tire are forced to change shape. This means either when a tire rolls and the footprint changes from curved to flat as the footprint rolls into contact with the ground. This force increases when external forces from cornering are also applied to tires.
So why do trailers seem to have such high Interply Shear forces? Well it's not all trailers as the cornering forces of single axle trailers are much lower than the forces of tandem or triple axle trailers.

What tipped me off to this was an observation at a campground that happened to have a freshly smother gravel driveway and a multi-axle trailer happened to make a 180° turn as I was walking by. I noticed that the gravel marks were not a smoth curve but there was a series of turns interupted by discontinuities.

 

A short time later I saw a video from Keystone RV Company of wheel lug nut torque (you should watch the full video some time)
 While watching their video I recall some special high side load tests we ran in our tire test lab.

Here is a short out-take from the longer Keystone video that shows what happens to multi-axle trailer tires.

In the video you can see how the two tires are fighting each other with one bending in as the other bends out. You can imagine that if the turn is made on gravel at some point the high sideload would result is a sudden breakaway or slip. That's what I observed on the gravel turn.

Back at work I had some high powered Finite Element computer programs run to simulate the side load of a multi axle trailer and the results showed that the side loads on a trailer could be 24% higher than on a standard vehicle even with identical radius and loads on the tires.

This post shows why tires get different side loading.

Further analysis showed that increasing the tire inflation could lower the extra shear, but sadly not eliminate it.

Bottom line
In a number of posts I have recommended that Motorhomes set their inflation based on measured static load plus a margin of at least 10% additional PSI. This would also apply to single axle trailers. BUT for Tandem and triple axle trailers I strongly recommend that the tire cold inflation be set to the inflation molded on the tire sidewall associated with the tire maximum load capacity. I also recommend that the measured static tire loads on these trailers be no greater than 85% of the tire maximum with a 20% margin being better.

You can learn some facts about Max tire pressure by reading your tire

I found significant discussion on tire 'maximum pressure" on a forum thread on a new Michelin tire. One problem is that the new tire is not yet available in the market so getting confirmation on what the tire has for markings on the sidewall was based on second hand information from various Michelin sources. Despite these problems I felt it might be instructive to review the relationship between load capacity and inflation pressure.

The following was posted:

The Michelin LTX M/S2 P235/75R15XL 108T carries its rated load at 41 PSI. Yet the maximum pressure stated on the tire is 50 PSI. There is no load capacity increase above 41 PSI.
Tire Rack has a good explanation of the maximum load for a tire
Tire Tech Information - Tire Specs Explained: Maximum Load

As I have previously reported the numbers associated with the "complete" size nomenclature are very important. In the above thread there were a number of readers confusing the inflation associated with the max load capacity with a statement on the tire about safety and tire inflation. When discussing Load and inflation it should be easy to read the tire as the max load and the inflation needed to support that load are clearly stated together.

There is also a "Safety Warning" on many passenger and similar info on LT tires that includes a statement about "max inflation" but that number is really the max pressure that should ever be used when inflating a tire to ensure proper seating of the tire bead against the rim. This number is not associated with the load carrying capacity of the tire.

Here is a sample Safety Warning from a Performance 16" passenger tire



Here is the Load and inflation information, again for a passenger tire



Now while the above are from a passenger tire that was convenient for me to get pictures of, I know that you should see similar on your ST or LT type tires.

I think you may learn a bit more about your own tires if you take a moment to read the information provided. Knowing what tires say on the sidewall can help avoid making mistakes when assumptions are made.

 It might be a good idea to take a few minutes to go and read your tire sidewall. You might be surprised what you can learn.


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Quick reminder - Don't get your shorts in a bunch about tire inflation.

Recently was asked why some dealers want to go with the minimum inflation needed to carry the load.

I think the most likely reason for specing the minimum inflation is the belief that customers are more likely to complain about harsh ride even when they are getting better tire life and better MPG. They can feel ride immediately but not see the benefits of inflation to minimum +10% psi at once.

I am also in favor of +10% so you don't find yourself chasing your tail every day by adding 1 or 2 psi when it gets cooler when you find yourself 1 or 2 psi low, or bleeding off 1 or 2 psi when the weather turns warmer.

With +10% it would be easier to discover you are low a couple psi and simply wait till the next fuel stop, where there should be high pressure air available.

For those that don't know how to inflate a warm tire here are the steps:

1. Measure the pressure when the tire is at ambient temperature (not warm from driving or being in sunlight).
2. Note the number of psi you want to add to each tire to get to your goal inflation.
3. When you get to a fuel stop measure the warm pressure.
4. Add the number of psi from #2 to the warm pressure in #3 and add air till you get to at least this new warm pressure goal.

This "rule of thumb" will work for pressure changes of 5 psi or less. If you find you need to add 5 psi there may be something wrong,  i.e., a leak -- and this needs to be investigated ASAP.

Don't get hyper about being 1 or 2 psi off. Remember, if you have a 10% cushion, you are good to go as long as you are within a few psi of your goal.


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What temperature for CIP "cold inflation pressure - Part two

The following is based on a couple posts I made on an RV forum but wanted to share with the rest of my readers

I had posted my comments on the thread from my Aug 25 post on CIP in the hopes of clarifying questions about adjusting inflation for various changes in ambient temperature, but there have been comments that indicate the need for more details when talking about tire temperature so here is a bit more info on the topic.

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We really have two different but related temperatures to consider.

One set of temperatures is the external set. This would be the "Ambient" temperature of the air around us and the temperature of the road surface.
 The other and much more important temperature is the temperature of the tire structure.

Now with the correct instruments, it is possible to measure the temperature of the tire structure. This is done at test tracks and at race tracks using a small needle probe that sticks into the tire rubber 1/8" to 1/2" deep depending on the tire type & size and gives a reading the engineers use to make adjustments to race car suspension or to tire pressures. Since good & accurate tire "Needle Pyrometers" cost a couple hundred bucks, few individuals have them. Some think a simple IR gun from Harbor Freight is OK but all that does is give the approximate surface temperature of a tire which is always cooler than the important internal structural temperature. Rubber is an insulator so heat does not move through a tire structure from hot to cool very well so you can mislead yourself if you only use surface temperature. The IR gun is fine for conductive materials such as metal in a hub or brake drum.

Heat is generated at the molecular level not from belts rubbing against each other. This heat moves slowly from the hottest location (belt edges) to cooler surfaces. There is the surface of the air chamber and the surface on the outside of the tire.

The air on the inside of the tire gets warmed from the heat transfer from the inside surface of the tire. This air is also cooled by transfer to the metal wheel and the turbulence inside the tire means the temperature of the inflation gas (air) is fairly uniform.

The inside temperature is related to the outside temperature in that cooler outside temperature allows faster heat transfer from inside to outside so the inside temperature will be a little lower when the outside is lower but not in a 1:1 relationship.

Tire pressure will increase as described by Boyle's Law PV=NRT. I did the math proof in my blog post of March 13 2014 .

So after all that we are left with the facts that:

1. A tire generates heat based on deflection (tire inflation & load) and rate of deflection (speed)
2. More deflection due to more load or less inflation or both, results in more heat in the tire structure.
3. More speed means greater generation of heat. This is one reason tires have speed ratings.
4. If you lower the CIP you will generate more heat and this will hurt the rubber and eventually weaken it.
5. With sufficiently low inflation or sufficiently weakened rubber the heat will increase at a faster rate than it can flow out of the tire structure and can go into "run-away" increase which can result in a sidewall flex failure or "Blowout" due to shorter term low inflation or possibly a belt separation due to long term heating and weakening of the belt rubber.

I hope this clarifies and answers any additional questions.

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What temperature for CIP "cold inflation pressure?

I occasionally see posts on the need to do an adjustment of your Cold Inflation Pressure ( CIP )when the temperature is not at some "standard".

Many times the "standard" is stated to be 70F or 68F. Neither of these are correct.

From a tire design standpoint CIP means when a tire has not been warmed up either by being in direct Sunlight or from having been run.
We are not talking about some chemistry lab experiment but real life. This is defined by the Tire & Rim Association, the organization that published the standards book for tire dimensions and recommended Load & Inflation for all kinds of tires, wheels and valves.
 These standards are primarily intended to provide "interchangeability" as we want to be sure that every 15" tire fits properly on a 15" wheel of the appropriate type. Or that the valve will properly seal against air leaks by having the hole in the wheel of the correct diameter.

Now you don't have to get all wound up with temperature probes or IR guns to confirm a tire has not been warmed up. Just follow the guideline that when checking or adjusting tire pressure to your CIP, the tire should not have been driven more than 2 miles in the previous 2 hours, AND that the tire has not been in direct sunlight or otherwise artificially warmed up in the previous 2 hours.
"Cold" really means when the tire is at the temperature of the surrounding air or what is called "Ambient" temperature.

So unless you are taking your RV to the Antarctic or to the Sahara desert you can use the above as a simple guideline. Even if I were planning a trip from the top of Pikes Peak to Phoenix in a single one day drive I would not worry about making adjustments based on expected temperature. Tires are designed to have a large tolerance for pressure increase due to variations in the surrounding temperature.

Remember the rule of thumb is that tire pressure will only change about 2% for every change of 10F so even going from 30F to 100F might only result in a pressure change of 14%. Now you would probably need to adjust the CIP the next morning before setting out but again you would set the CIP when the tire is in the shade and not driven on for a couple of hours.



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When is Minimum inflation the Maximum inflation?

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

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

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

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

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

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

 "NHTSA" NATIONAL HIGHWAY TRAFFIC SAFETY ADMINISTRATION

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

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

"LTR"  Light Truck Radials 

"CIP" Cold Inflation Pressure

OK here is the nitty-gritty

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

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

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

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

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

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

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

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

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

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

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


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Basic tire inflation procedure - Motorhomes

Thought a short post covering the important points of setting and ensuring you have the necessary inflation on your motorhome tires was in order.

 NOTE Trailers have slightly different needs for establishing the "set" pressure than motorhomes but the rest of this post would apply to towables too.

1. Ensure your gauge is sufficiently accurate. To me this means +/- 3 psi of the real pressure. This level of accuracy is easy to confirm yourself if you follow these steps:
      A. Get and keep a "Master" gauge. This is not used for checking the air in your tires but is used to confirm that your daily use gauge is sufficiently accurate and to allow you to adjust the reading of your daily gauge accordingly. I have provided an example of what a low cost "Master" gauge could be in THIS post. The master gauge should NOT be kept in your tool box but in a box or case inside the RV where it is protected from damage.
     B. At least once a month, measure the air in one of your tire with the Master gauge and immediately get a reading with your daily gauge. You now know how much to adjust your reading when setting your tire pressures. If your daily gauge reads 3 psi lower than the Master then you need to set your tires 3 psi high when using your daily gauge.

2. Learn the actual load on each position of your RV. This has been covered in a number of earlier posts that have the label "Load". Please review those posts if you do not know how to learn your "corner" loading. The minimum inflation for all tires on any one axle is based on the inflation needed for the heaviest loaded end of the axle. ALL tires on an axle should have the same cold inflation.

3. Establish your "Set" or "Goal" pressure pressure that you should have each morning of a travel day. I suggest this be 10% above the minimum needed to carry the load on your tires you learned in step 2. above

4. Set your TPMS to provide warning if you have lost air to the point of being 10% below the Set Pressure. This should provide adequate warning to allow you to safely get off the road or at least out of traffic. If you don't already have a TPMS then get one. If you don't think you need a TPMS then I would challenge you to try this experiment. Get some black tape and cover all your dash gauges and see how comfortable you are traveling down the highway without having any of the factory provided warning instruments you have become accustomed to.

I feel these steps are the minimum every motorhome owner should take



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Is monitoring tires "Rocket Science?"

I get asked this question quite a bit. I think that after reading various posts on tire inflation and load, some people want to complain a bit and are hoping to use the excuse that it is too much work to monitor tire pressure every travel day.

Here is an example from a trailer owner:

"I've noticed that 50 psi cold at sea level at ambient 65F is 54-55F at 6,000 to 7,000 feet.
  While touring should I deflate to 50 psi cold at 7,000 feet if I'm hanging around there for a day or two?"


I am hoping that if I share my personal experience, it will provide a down to earth view on the topic that some want to turn into "Rocket Science"

I just finished a two-month cross-country trip.  Ohio > Oregon > Seattle  > Calgary, Canada > Yellowstone > Ohio in my Class-C MH with LT type tires that are 7 years old but have always been covered whenever parked for more than a couple of days, Always running inflation about 15 psi above what is needed for actual max load based on 4 position scale weights that are confirmed to have not changed significantly each year with a trip across CAT scales.
 Fastest I ever run is 70 mph but most of the 7,400 mile trip was with cruise set at 62mph. In other words, the tires have had a good life with good care.

Elevation ranged from about 25' at Olympic Nat Park to 8,000'+ in Rockies. Morning temperature ranged from 33 F to 94 F. I have TPMS (both internal and external) so I am able to constantly monitor both pressure and temperature.

I adjusted the tires once during the trip. I think I needed to add from 1 to 4 psi in the 6 tires. Since I run a nice inflation cushion of +25% I don't get all bent out of shape when the cold inflation is off by 3 to 5 psi from my goal.

You don't have to make the task of monitoring inflation a big deal. While I may set my pressure to +/- 0.5 psi, you certainly don't need to be that fanatical, and I would consider it completely acceptable to be +/- 2% for  the average user as long as the "goal" cold inflation has an appropriate safety margin.

With TPMS I simply hit the button a couple times a day and get a real time reading of tire temperature and pressure. Yes, the temperatures vary and so do the pressures, but unless I see a sudden drop in pressure and there have been no external changes such as a sudden rainfall or one side constantly hotter than the other after spending hours with one side in full sun, I just do not worry about it.

Relax.  Let your TPMS monitor your tires and enjoy the scenery.

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What is "Cold" Inflation? Quick Post

The definition from US Tire & Rim Association is

Cold Inflation Pressure is the inflation pressure taken with tires at the prevailing atmospheric temperatures and do not include any inflation build-up due to vehicle operation

 In real life this means before the tire has been driven one mile. It also implies that the tire has not been exposed to external warming from being in Sunlight.

We have shown that when a tire is in sunlight for as little as 30 minutes the temperature can rise by almost 40°F which will give you a higher than Ambient pressure reading.

You should wait at least 2 hours with 3 preferred after end of the day driving with the tires in full shade, to give them a chance to equalize temperature with ambient air temperature.

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Quick Summary here

I have been following a number of RV forums and want to do a bit of a summary for those that don't have the time or interest to read a number of posts on this Blog: Remember more than half of the thousands of RVs that have been checked have been found to have one or more tire and/or axle in overload. So nect time you camp you can be pretty sure that one or both of the RVs parked next to you are overloading a component.

- You should have a Digital tire gauge. I have posted the results of my tests of dozens of gauges that RVrs were using and the bottom line is that about 15% were off by more than 5 psi which  IMO makes that tire gauge a good door stop or tent peg. I have provided information on how to do your own gauge "Calibration Check" to confirm the accuracy of the gauge you use daily and how to maintain your "Master Gauge"

- If you follow the advice from the two Tire Engineers on this thread you will be setting the pressure based on the appropriate method here:
     - On your TV inflate to pressure based on the Door sticker and Owner's manual for "Fully Loaded" operation. BUT get on a scale at least once to confirm you are not overloaded.
     - If you have a multi-Axle trailer you should be running the pressure on the tire sidewall. I have written in my Blog and provided the technical justification and need to follow this advice. You should still go across a scale and calculate the actual tire loads to confirm you are not overloading one or more tires
      - If you have Motorhome you need to confirm your unit's "corner weights". Use one of the worksheets I have provided links to. Use Load/Inflation chart from your tire manufacturer, to confirm you are not overloaded and identify the heavier loaded tire on each axle. Using the pressure for that load add 10% (without exceeding the wheel max) and that is your Cold set pressure for all tires on that axle.
      - If you follow the above there should be no reason to be chasing your tire inflation around unless you have a major change in Morning temperature ( More than say 30°F ) as you would only need to "Top Off" the air once a month at most.

- Don't be too anal about inflation. Variations of 1 to 3 psi day to day are normal. You should not need to make adjustments unless the Cold pressure is 4 or 5 psi away from your goal. (Multi-axle trailers might try and get no lower than 3 psi below tire sidewall pressure)

- TPMS are a WARNING device and like Insurance we should all be using them. Also just like Insurance they cannot "Prevent" bad stuff from happening but with luck they may give sufficient advance warning to lessen the impact of the bad stuff that does happen.

- If you are parked for more than a day you need to cover your tires that will be in direct sunlight with WHITE tire covers. This protects against both UV and higher temperature damage which can shorten tire life.

Can I change from ST to LT tires on my Trailer or 5ver?

This question is a real "Hot Button" on a number of RV forums and blogs. People ask this question because they want a better or alternate selection of brands or they want to improve the durability of their tires.

The answers given seem to range from "Sure, Why not" to "Absolutely Not, Never do it" and some will even offer that they think you are breaking some law if you make any change from the type, size, Load Range or cold inflation from the OE tire information shown on the tire Placard.

As we all should know by now answers about tires are never simple and straight forward and changing tires is definitely one of the more involved answers.

First off, I am not a lawyer but an Engineer and as such I form opinions based on data and facts. So here is the answer based on my 40 years experience as a Tire Design Engineer.

Yes you may be able to change tires  

    

 there are some things you MUST do to ensure that any change you make will actually improve your probability of having better tire durability.

My plan is to provide an outline of the steps you need to take before you make any change. I will try and include each step and each of the points of data you need to collect and evaluate. If you skip a step you may end up with a less durable tire selection which could lead to tire failure, RV damage and even an accident.

Before we start you need to consider that the most conservative approach is to make no change and to simply use the tire Construction (Bias or Radial), Type (ST or LT) and size and Load Range and inflation as specified on the placard and specification documents. This represents the RV manufacturers recommendation based on a number of assumptions as well as some legal regulations the RV MFG must follow plus in many or some cases a desire on the Mfg part to keep their costs as low as possible.

So if you still want to move forward here are the steps you need to take:
1. You need to know the actual load on each tire. This is important because A. we will be basing some decisions on the tire loading    and     B. It is possible that there is a significant unbalance in the tire loading which may be the cause of poor tire durability. With sufficient unbalance it may be impossible to provide a tire selection that would lower the probability of having problems.
To learn the actual individual tire loads you need to either find a company such as RVSEF or agency that has individual scales or to follow the steps outlined on worksheets such as This one  or This one. I have hears some people say that they have been able to get the individual tire loads from their state police or state DOT.

2. Knowing the ACTUAL LOAD on each tire you need to confirm that no single tire is loaded more than the max load molded on the tire sidewall. This is an absolute rule. If any tire is overloaded you should not move the trailer until you either change the load or change the tire.

3. Assuming no tire is overloaded we want to make sure that all tires on en axle are inflated to the same inflation. For multi-axle trailers you can lower the internal tire structural shear forces (the forces trying to tear the tire apart) by running the inflation molded on the tire sidewall. Sometimes this is stated as the Max PSI and other times it is stated as the PSI for the max load. For our purposes we will consider this the proper cold inflation you should always run. 

4. We should have "Headroom" or "Reserve Load" or "Safety Margin" on the tire loading. I suggest 15%. However I know that for many trailers 15% extra capacity above the actual load is very hard to do. Especially since some RV MFG manage to so under-size the tires that even when the trailer is empty they may not have 15% margin.

5. If you don't have at least a 10% margin I would strongly suggest you need to consider changing tires to something with more load capacity when inflated tot he sidewall PSI.

OK so you have some homework to do. We will continue this in the next post.


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