I have too much tire pressure gain

Another post with a question on an RV Forum.

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

 

 My response:

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

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

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

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

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

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

Some information that will help us.

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

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

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

 

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

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

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

##RVT1053
 

 

Excessive pressure increase?

 Another question from a reader with a "heavy 5th wheel trailer.

 

Hopefully, this isn’t a duplicate.  I tried to post a comment before I saw the email.  Thanks for all the excellent information on your site.  It is incredibly difficult to determine who is sharing good knowledge on other forums.  

I have read through many of your posts about hot tire pressures in particular, but I’m hoping to get your experienced input on a more specific example.  How much pressure increase is reasonable in load range H (4805 max load) tires inflated to 122-125 psi CIP?  I have tandem axles on a fairly heavy fifth wheel.  All weights are within limits (trailer, axle, tires, etc.), but with the wheel-position weights, one of the four tires is carrying c. 4,700 lbs, which is 53% of the total for that axle.  I know this violates your ‘85% of max load’ recommendation.  However, I’m seeing up to 32-35 psi increases (to about 157-160 psi) after 3-4 hours on the road when in the sun.  I have always pulled over for a break at that point, so I don’t know if it would have continued to climb.  According to the TPMS, the heavily loaded tire reaches these ranges first, but the other three aren’t far behind; accounting for shade, wind, etc. all four increase in psi and temps together fairly steadily throughout.  Generally, the temperatures aren’t much higher than 30 degrees above ambient, but I’m really wondering if there’s a point I should be concerned about the pressure increases.  I know it's been said the tires are engineered to handle pressures up to 100% over max CIP, but just trying to find the threshold of when one should begin to be concerned.  Once elevated to the point where I pull over to cool things off, I can only get about 30 minutes of drive time before they start touching that 157-160 psi level again. The tires are within specs, but close to the maximum.  I can add to the CIP as you recommend, but I’m limited to only a couple of PSI before I hit the max CIP of 125 psi.  My rims are rated to 130, but not sure I have the clearance for larger tires, and I’m not sure if I need to consider that.  The current tires are Goodyear G114 215/75R17.5.  Unfortunately, Goodyear documentation or customer support doesn’t provide much guidance on this.

Any thoughts would be greatly appreciated.

 

 

My reply:

Let's get a few things settled. The pressure on the tire sidewall is actually the MINIMUM required if you are loaded to the MAXIMUM load. Ya I know it's confusing but the reason is that increasing above that pressure will not give you any additional load capacity.

You gave ranges for your pressure. Why aren't you inflating all your tires to 125 psi? I am also wondering if you have had your pressure gauge checked at the 125 psi level where you should be running your tires.  Is the gauge a "stick" type or dial or digital? Digital is best as they are easier to read and I have found them to be more accurate too.

You didn't mention your travel speed but those tires are rated to a MAX of 75 and you should consider that max just as you do the "Red Line" for your TV engine speed. I would be limiting travel speed to 70 with 65 being better since you are loading your tires to their almost their max. Something to remember is that your tires are considered "regional" service not long haul so it would normally not driven at full highway speeds for many hours on end. Think of how a commercial trailer would be used. Most of the time I bet less than an hour loaded then an hour back to the warehouse empty. You are running at Max load for hours on end.

While I have seen reports from some people seeing a +25% inflation increase to me that suggests that the combination of load and speed is pushing your tires to their limit.

While a NEW tire might be capable of handling 100% over-inflation, static in a special test chamber, that would be way too high for a used tire running down the highway.

You didn't ask about tire life but with your conditions, I would suggest you figure on a 5-year life At that point you should be able to sell your tires for a good price. Maybe in the $100 range but the main consideration is to sell them before they fail under your operating conditions.

 

When you are operating at the limit, confirming all the details such as load, inflation and speed, need to be checked. 

##RVT989

 

Too much pressure increase

Reading an RV forum thread on TPMS usage. One comment jumped out at me.

"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."


I replied that inflation ratings for wheels are based on a "cold" pressure. Increase in pressure due to operation is considered by 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 but in reality the pressure stated on the tire sidewall is the cold pressure needed to support the stated load and 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 operation temperature to increase the pressure. "Cold" pressure is the only pressure you should be concerned with.

Tire pressure increases by about 2% for each increase in temperature of 10F. If you don't remember the Science from High School you can read THIS post.. If you are seeing a 21% increase in pressure (110 > 134) that means you are seeing about a 100F increase in internal tire temperature, which I would consider a bit excessive for normal tire operation.

If you are seeing a 25% increase in pressure (95 > 119) then you are getting a 125F increase in temperature which indicates you are working the tires even harder . 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 this "Key Point" of tire life as covered in THIS post.

I cover these points on Temperature, Inflation and Aging in various posts on my RV tire blog.

Your temperature increase indicates you are possibly overloading your tires and also possibly driving faster than desirable for your tire loading.

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

The poster then responded:

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

So I responded back:
 Few people realize that the pressure increase as a function of temperature is based on well established and confirmable Physics and that 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 very good insulator and 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 all the while the valve is moving rapidly around being cooled by the outside air.

I am aware of laboratory tests that goes 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 and it is the hot spots that can result in tire failing if hot enough for long enough.

I have no doubt that the TPMS was indicating only 20F above the cool 50F outside air temperature. If you are still concerned about the hot pressure of 134 on tires that have a cold pressure rating of 110 for its max load capacity rating.

As a tire design engineer with 40 years experience, I trust the science of the "Gas Law" and 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 and to always inflate the tire when cold to 110psi, as continued operation at current load and speeds will certainly result in pressure reading that are above the cold pressure of 110 psi.

Question on Maximum tire pressure

Got an email stating the writer was concerned and a bit confused on the Maximum inflation a tire can have. He had heard about people having "Blowouts" and after installing a new TPMS and seeing the pressure increase was concerned his tires might explode.

I can understand the confusion as people read the tire sidewall and in some cases see the words 'Max Inflation". This is NOT the max operating inflation but in reality is the inflation needed to support the max load the tire is rated for. It is also important to remember that unless a tire engineer is specifically talking about "hot inflation" we are talking about the "Cold" tire inflation when the tire has not been warmed by either running or being in direct sunlight over the previous couple of hours. Technically this means the tire is the same temperature as the surrounding air or "AMBIENT" temperature.

I previously covered the effect of temperature on tire pressure and the "Ideal Gas law" so we know that for each change in temperature of 10F the tire pressure will change by about 2%. This means that when the tire temperature increases by about 50F we can expect the pressure to also change by about 10%. We need to remember that the TPMS sensor is being cooled by outside moving air so the hot spot on the tire is actually much warmer than the indicated temperature. This is one reason why we many times see pressure increase by 15% or more.

True tire "Blowouts" are usually caused by too low a pressure which flexes the tire sidewall and results in fatigue failure of the steel body cord or melting of the Polyester cord in LT, P, or ST type tires. The separation of the tread and belts is sometimes mislabeled a "Blowout" which leads to confusion. Separations have a different cause than simple loss of air pressure.

Back to the original question. Tires are designed to handle significant increase in pressure, most in the range of 200% of the pressure marked on the tire sidewall.

BOTTOM LINE Always ensure your tires are properly inflated and the use of a TPMS is, in my opinion, the best way to  not be surprised by a puncture or air leak.


Send your questions to me at  Tireman9 (at) gmail.com


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##RVT785

Do you check your air every day? Why ???

An RV owner in Western NY said:
"OK ... I'm new at this, and I appreciate the importance of proper inflation. But short of damage to a tire I would think daily checking of PSI is more of a cause for air loss. Or is there something I'm missing? There seems to be a real fixation on this topic. I doubt school bus drivers place as much emphasis on this topic as RVers do."

You're correct. Too many fixate on daily air checks. In years gone by that was the best method we could use, but sticking valves can cause air loss and it is not unheard of for a tire to start leaking air right after an air check.
This post shows just how easy it is for a piece of grit to get into the actual valve core opening and could allow air to leak out slowly.

To me avoiding this potential issue is one of the major advantages to using TPMS, but for some reason I have never heard anyone mention this benefit. Since I run TireTraker TPMS I don't do a manual air check unless the readings go below the normal variation range of 3 to 5 psi around my "Set" pressure of 72 psi. Other than the start of my travel season, I may only do a manual check once a season.

Now if you don't want to run a warning system then a manual check each travel day is the only way you can know there is a slow leak. Now to me running without a good TPMS would be much like driving without any gauges or warning lights on your dash.  Would you feel comfortable if this

is what your dash looked like? Be sure to take a close look before you answer.


If you wait till your "thumper"
makes you suspect low pressure you are getting about as much information as checking engine oil by banging on the oil pan with a hammer.



If your IR gun

 makes you suspect high temperature you may already be too late and might have done permanent structural damage to the tire and shortened its life by many months or even years. Rubber is not a good conductor of heat so you will almost certainly not get the reading from the hottest location.



Just as there were advances in early cars when the temperature gauge was just a thermometer stuck in the top of the radiator,

today we have electronics capable of providing the current pressure in our tires so we can receive a warning as soon as it starts to leak air from the elevated hot pressure in our tires.



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Should you reduce pressure when driving on "Hot" Roads?

I got this question and thought that others might be wondering about the same topic.

Jim K asked

 Message: I will be traveling in the desert for the first time and I am
wondering if I should reduce the tire pressure before I go. The hot road will
increase the pressure and I am afraid of damaging my tires.
++++++++++++++++++++++


Hi Jim,

No you don't have to worry about hot roads.
IF you run the correct cold pressure


Now you didn't say if you have a standard RV trailer or a Motorhome so I will give you a summary for each application.


Trailers: You should set the Cold Inflation to the pressure on the tire sidewall. If you look at the sticker on the side of your trailer you should find the tire size, type, Load Range and pressure recommendation from the manufacturer. In almost all cases the recommended inflation is the inflation on the sidewall of the tires.
Have you confirmed you are not overloading any of your tires? Simply guessing or looking at the tires is not good enough you need to get the trailer on a scale and at a minimum get the total load on the tires. Now you can't assume the load is equally distributed side to side or axle to axle Measurements of thousands of trailers suggests you need to assume at least 53/47 to 55/45 split axle to axle and split side to side so you need to calculate the heaviest load based on an estimate of 27% to 30% of the total being on one of the 4 tires. A better method is to get individual tire loading. You can learn more HERE.


Motorhomes are a bit different than towables. Here you need to get the "corner" loading as the side to side difference is affected by the placement of things like generator, water tanks, refrigerators etc. The Front Rear loading is obviously different and for most motorhomes the number of tires on each axle is also different. You can use the information on your placard but a better method is to get the actual tire loading and then using Load/Inflation charts establish the MINIMUM cold inflation then add 10% to get your Cold Set inflation. THIS post has some info and a link in it.


Bottom Line

When tires are designed, we know that some vehicles will be driven on hot roads. Tires will normally run +20°F to +50° above ambient. You should run a TPMS to get warning of air leak due to puncture. If you are driving in the USA you should have no problems.
Most TPMS also have a high temperature warning that is set for 155°F to 160°F. If you get a warning at this temp but the pressure is above your set pressure by about 10%, simply slowing down should lower the temperature. If that doesn't work you can still stop for 10 minutes while you do a walk around to be sure nothing unusuall is going on.
If you are traveling to Saudi Arabia, the Sahara or Australian outback then we need to take some additional steps and precautions.

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Tires and Burst Pressure, What causes Blowouts?

Recently saw a post on another blog Tires and Burst Pressure, What causes a "blowout". 

Now there was nothing really wrong with what was posted, it was just not as accurate as it could have been.

I have many posts on tire blowouts and their most likely cause, but as long as I continue to hear people claim or read blog or forum posts on blowouts followed by partially correct information I will continue to try and provide the facts based on the thousands of tire autopsies, I have done in my career.

Tire Maximum Pressure

Many times people incorrectly think the pressure molded on the tire sidewall, similar to what we see in this picture,

to be the maximum tire pressure a tire can tolerate when in reality you should consider it the minimum pressure needed to carry the load indicated 

Tire companies design and test their tires to tolerate pressures much higher than the number molded on the sidewall. For regular passenger tires, it is probably close to or above 150psi. Light truck tires will probably be a bit higher and truck tires higher yet. In almost all cases I am aware of, there is a good possibility that the wheel might fail from high pressure before the tire simply explodes from high pressure.

SAFETY WARNING  DO NOT TEST THIS YOURSELF. The explosive force can take out concrete block walls and could kill you.

The correct terminology for the pressure on the tire sidewall is: The tire pressure molded on the tire sidewall associated with the maximum load capability for the tire. Now this is admittedly a real mouthful so I can understand it being shortened to "Tire pressure associated with max load" or even Tire pressure molded on the tire sidewall. but it is incorrect to say or think that the pressure molded on the tire is the maximum the tire can stand.

Now it is important that you not confuse "The tire pressure molded on the tire sidewall associated with the maximum load capability for the tire." with the number that might be part of the tire "Safety Warning" 

many passenger and light truck tires have a separate warning about tire pressure not associated with the tire load capability. This has to do with the pressure used to "seat" the beads. This is the "pop" you hear in the tire store when the tech is inflating your new passenger tires.

Important Tire Safety information. If you do not understand the safety warning you should never be inflating tires from flat. Leave that job to the professionals.

Bottom Line

Tires can handle pressures higher than the number molded on the tire sidewall that is associated with the maximum tire load, so do not bleed down your tires when the hot tire pressure increases above the number molded on the sidewall. Pressure increases of 10 to 20% are not uncommon. Always inflate your tires to specified pressure when they are "cold". That means at air temperature, not having been driven on or in sunshine or even partial sun for at least two to three hours.