More on TPMS Temperature reporting

Continuing my series on TPMS.  Last time I covered temperature reports when it was moderate to just cool outside. This report is different. On my way South to GA from OH it got downright cold.
 

I stopped for the night in KY and in the morning I obtained these readings.

Internal System.
RF 30   LF  32    RRO 32    RRI  32    LRI   30  LRO  32

External System
RF  32   LF  32   RRO  32   RRI  32   LRI  32   LRO   32

As expected the temperature numbers were essentially the same. To me this confirms that the sensors can report ambient temperature if given sufficient time to cool to ambient temp.

After about an hour running down I-75 I observed these temperatures when the outside temperature was in the 40's.

Internal System.
RF 86   LF  83    RRO  85    RRI  86    LRI   80   LRO  80

External System
RF  48   LF  48   RRO  46   RRI   44   LRI  57   LRO   55


These numbers clearly show that the external sensors are reporting significantly cooler temperatures than the internal sensors. With cooler ambient the difference internal to external is greater than when the temperatures were warmer. I previously suggested that people running external semsor TPMS might consider adjusting theit high temperature warning level down by 10°F to 15°F.  While the above might suggest lowering from 158°F to maybe 135°F I am not sure if this would be too cool for people that are traveling when the abmient is above 85°F.

For now you might just want to be aware of this difference. I am hoping that later this summer when I am traveling I will be able to get measurements when the ambient is above 85°F.

How accurate is your TPMS - Part two

Last week I posted on the question of "How Accurate was your TPMS".  This week I am continuing on my close examination of TPM systems.

We saw that the variation in pressure readings were Measurable but that IMO the differences were not meaningful.

I recently traveled from Akron, Ohio, to a large (2700+) RV Convention held by Family Motor Coach Association (FMCA) in Perry, GA. There was a large variation in ambient temperature during the trip with a low of 26°F to a midday high of 78°F observed. I felt that this trip was a good opportunity to take a first look at the temperature numbers reported by an internal TPMS vs. an external TPMS.

To start out let's look at the morning temperatures in GA before I started driving home.
 All temperatures are in °F
Internal System.
RF 61   LF  61    RRO  61    RRI  61    LRI   63   LRO  61

External System
RF  60   LF  62   RRO  60   RRI   60   LRI  62   LRO   62

I feel it is reasonable to say that for all intents and purposes the temperature readings are the same.

After heading out on my trip North  I stopped a couple times and recorded the readings.
The ambient temperature was 74.0°F
Internal System.
RF 108   LF  106    RRO  111    RRI  115    LRI   111   LRO  108

External System
RF  80   LF  84   RRO  84   RRI   84   LRI  88   LRO   85

I made a second stop and this time was able to learn the ambient temperature was 60.9°F
Internal System.
RF 95   LF  90    RRO  93    RRI  99    LRI   100   LRO  91

External System
RF  62   LF  68   RRO  66   RRI   66   LRI  73  LRO   71


I consider the above temperature differences to be both measurable and meaningful.

All the above data provides some interesting information.  All along I have thought and said that I felt that the temperature reading from external TPM would be affected and cooled by the outside temperature and rapid air movement around the sensor. Thinking about the heat flow the heat is generated in the outer edges of the belts under the tread in the radial tires. While some of this heat flows out through the tire sidewall the insulating properties slows this heat flow. Most of the heat energy flows into the air chamber where it is transferred to the metal wheel and then to outside air.

An internal sensor mounted on the wheel will be measuring the temperature of the air on the inside of the tire but since the sensor is in contact with the wheel it will be cooler than the hottest part of the tire.
An external sensor is depending on the heat to transfer from the wheel to the brass valve stem and then to the metal base of the sensor and finally to the thermocouple inside the sensor to report the temperature. But along the way heat is being lost to the outside air from the wheel outer surface and through the brass valve stem and from the brass sensor base to the cooler outside air.

BOTTOM LINE
I believe that the default high temperature warning for all or most TPMS is 158°F ( 70°C). With this initial data I plan on suggesting that the high temperature warning for external sensor TPMS be lowered by 10°F to 15°F.   I will be repeating this temperature comparison later this summer when I travel from Akron to Yellowstone. I will be looking for the numbers when ambient is in the 85°F to 95°F range to see if external sensors continue to be cooled as much at these higher ambient temperatures.

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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Tire Temperature always seems to be a hot topic

From a couple of RV forum threads.

Hot tire temperature is important but the problem is in learning what the actual temperature is.
The average temperature is not a good indicator and that is about all you can get by reading the Contained Air Temperature. While many seem to believe that their external TPM is giving them a reading of the CAT, I can tell you that the TPM will always read significantly cooler than the CAT and that the CAT is always cooler than the hottest and most critical location in the tire.
Rubber is a good insulator and does not transfer heat from the hot spots to the cooler locations very well. As a result you can have a tire failure from extreme heat in one location while another location only inches away can be perfectly OK.

I read that many TPMS have set the high temp warning point at about 158F. That is all well and good if you are measuring the actual CAT but I do not know how an external TPM will sense that temperature especially if it is out in the cooling air stream on a metal stem or at the end of an extension hose

I would always be more concerned about the pressure measurement as that is not going to be affected by cooling of the valve stem or extension hose. We know that for our purposes pressure does follow the "Ideal Gas Law" and it doesn't matter if you use air or Nitrogen the results are essentially the same unless you are measuring your pressure to the 0.1 psi.

So knowing that tire pressure will change by about 2% for each change of 10F in temperature of CAT we can get a reasonable estimate of the CAT and will note that out TPM is showing a lower temperature change than indicated by the pressure change.
I would not use the temperature reading from a TPM to set tire pressure. If I got a high temperature reading but the pressure reading was reasonable I would suspect a mechanical problem such as bearing or brake issue.

BOTTOM LINE
A motorhome properly loaded with the inflation set based on actual measured load on the tire and consulting the load Inflation tables will not result in an over-heated tire. NOTE Trailers should use different method to set pressure due to the Interply Shear issue of trailer axle alignment and should set the cold inflation to the tire sidewall pressure.

If the above isn't enough detail for you you can review the other posts that have Temperature as part of the topic.

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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.

+++++++++
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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Why does my tire lose pressure? Simple and Technical answers

 As people get ready to start traveling again, some are discovering that their RV tires have lost inflation pressure over the past few months while it was parked. There are a number of possible contributors to a loss of pressure.

Some of these have been covered in my posts on leaking valve cores or metal valve O-Rings or possibly punctures or even rim corrosion. This post will cover just two of the possible causes. permeation and Temperature change.

The simple answer is that tire pressure will change about 2% for every 10°F change in temperature.

The complex answer and mathematical proof follows:

1. Normal pressure loss due to gas permeation. This is basically what happens when the molecules of gas "leak" or travel through the rubber of a tire. New passenger and LT tires sold to "Detroit" have to pass tests and meet a specification of a loss rate of less than 1% or 2% per month when averaged over a number of weeks or months, with different companies having different specs depending on the application. TBR (Truck-Bus-Radial) tires as seen on Class-A RVs will have similar performance goals. Low Cost import tires sold at "Billy-Jo-Bob's Cheep Tire Emporium and Bait Shop" may not have any such spec so might lose pressure faster because of the use of lower cost inner-liner rubber. This topic could be an entire post of its own but is quite technical and is not based on just the molecule size of Oxygen.

2. Tire inflation pressure is directly proportional to tire temperature. The "Ideal Gas Law" is a good approximation of what really happens. The general terms of the Ideal Gas Law are PV=NRT
 P is the Absolute pressure, not gauge pressure, measured in pascals
V is the air Volume measured in cubic meters. While tires do change a little bit in volume the difference is not meaningful for the purposes of this discussion.
N is the amount of gas in the air chamber in moles. For the purposes of this discussion, we can ignore this number as we are not changing the amount of gas in the tire when we are comparing the pressure difference just due to temperature difference.
R is the ideal, or universal, gas constant. R for dry air is 287.1 for N2 296.8 and for O2 259.8 for water vapor it is 461.5 so you can see that moisture in your inflation gas can significantly affect the Pressure/Temperature ratio a change from air (79%N2) to 95% N2 is not significant for the purposes of this discussion. I did a post on how to make your own "air dryer" so you can approximate the properties of dry air or dry Nitrogen.
T is the absolute temperature in degrees kelvin.

After discarding the inconsequential terms we really end up with is a simple ratio P2/P1 = T2/T1 and we can even ignore the conversion of psi to pascals as the ratio of pressure is the ratio of temperature in degrees Kelvin. Sorry we do need to use Kelvin but you don't need to do a conversion as you will soon see.

Lets see how this woks out
If P1 is 100 psi and if  T1 is 70°F or 294.261°K and if the temperature drops to 60°F or 288.706 °K we can solve for P2/P1 =  288.706/294.261 which leaves us with the ratio ot the two temperatures as 0.9811 so we had a 2% drop in pressure for a 10°F drop in temperature

30°F would be 272.039/294.261 = 0.92448 or 8% drop in pressure for a 40°F drop in temperature from 70°F to 30°F.

You may see various web pages such as this one from TireRack or Wikipedia saying 1psi for each 10°F change but you need to remember that this information is based on the assumption that the "normal" inflation is about 35 psi in a passenger tire.

TPMS Accuracy & I had an air leak!

I sometimes see people worrying about temperature or pressure readings they see on their TPMS monitor. Based on the reported numbers, I have never been concerned with the minor variation but I felt I needed data to see if my opinion was justified.

As you can imagine I can get pretty focused on tire inflation pressure so it may not surprise you that in addition to two digital pressure gauges,
which have been checked and found accurate to +/-0.5 psi against ISO laboratory certified pressure gauges, I also have two TPMS on my RV. I am probably the only individual in the US and possibly the world to have an RV so equipped. The reason for having two systems is so I can give you this test report as I have relatively high confidence that with my frequent inflation checks with my digital gauges and adjustments to inflation I feel I would identify any problems with my TPMS. More on that later.

I installed my internal TPM about two weeks after buying my RV in 2008. At the time I was quite familiar with internal systems as I had been spending a significant portion of my work time involved with the OE versions of TPMS sensors. Some like this one had been damaged by improper mounting practice.


 My internal system is actually a car based system as it has an upper pressure range of 75psi but since I set my tires to 65 psi which is about 10psi above the minimum needed based on actual tire load, I felt I would not be seeing a big rise in inflation pressure. I was of course more concerned with the potential of a puncture and loss of air pressure. I subsequently learned that I would occasionally get a "high press" warning on one or two tires but have confirmed that my tires only get up to about 77 psi max.

I started this blog in 2011. After that I also increased my reading of various RV forums where I started to see more and more comments and questions on TPMS accuracy, I decided I needed to do some testing.

Here are the results from a number of tests.
 This data compares the two TPMS with my accurate hand gauge with the tires all set to 65.0 psi cold

 

 As you can see that while there is measurable difference in the readings I do not consider these meaningful and would consider the +/- 3 psi acceptible as I consider a TPM to really be a warning system based on a change in pressure and not a substitute for an accurate pressure gauge.

Running pressure shows a simmilar range of variation as seen above.

Running temperature is a slightly different story.Since each tire has a slightly different load and there is a significant difference in air-flow around the fronts vs the dual rears as well as a difference between the inner and outer dual, I would expect more variation and we see this in the test results shown here.

I note that the internal temperature is consistently higher than the external temperature reading which is to be expected with the external sensor both further from the heat source and being cooled by the external air flowing over both the sensor and in the rears over my extender hoses.

THE REST OF THE STORY

I previously mentioned that I felt I would be able to see a failing TPM sensor and I can report that I was able to discover a leak of  about 8 psi a month because I pay attention to the range of readings of my TPMS. I do not write down the numbers every time but I do pay attention to the range of numbers observed. I can also confirm that my external system gave me a warning as it is set tighter than the internal system. The leak was at the TPM sensor itself but since my system has a three year warranty I have been able to get a free replacement sensor.

BOTTOM LINE:
You can expect some variation in both pressure and temperature between different TPM systems. If you use a good hand gauge to set your pressures and just make a mental note of the range of pressure readings right after setting the tire pressures and on your first couple of trips, I think you will be able to recognize when there is a leak or an unusual reading that warrants additional investigation.

Why do tires fail?

There are basically two reasons that explain the vast majority of tire failures.

1. Over-load / under-inflation which are almost the same thing since it is the inflation air that carries the load not the tire.
Under-inflation: A tire operating at less than 80% of the inflation needed to carry the load, is considered to have been run flat and there is a good chance that there has been permanent internal structural done to the tire if driven on in that condition. Related to this is having the tire inflated to a level that just barely is rated for the actual load on the tire. Obviously you can be under-inflated because of cut or puncture or a valve leak or if you use an inaccurate gauge. If you run sufficiently low in pressure at highway speeds for a just a few miles you can have a Run Low Flex Failure or more commonly a "Blowout". Steel body tires can have "Zipper" failures due to fatigue of the steel body ply.

Over-loaded: Few people realize that by design, most passenger vehicles have 13% to 20% or more "Reserve Load". That means that they are actually under-loaded by that much for a vast majority of the time. Most RV trailers on the other hand have tires selected that are at the tire max load and can just barely carry the actual load while motorhomes seem to have some reserve load but not as much as passenger cars.
Data on actual loads measured shows that over half of RVs measured (out of many thousands) have one or more tires overloaded based on actual inspection.
While at a campground last week I got into a discussion with the guy parked next to me with a 5 week old RV.  I bet him a beer that at least one of his tires was at least 10% low. We found all his tires were between 18% and 21% below spec. I did enjoy the beer. :-)

2. Heat. Heat damage occurs at the molecular level and degrade the ability of rubber to flex and stretch and not break the chemical bonds. Once cracks form the rubber does not repair itself, the cracks just continue to grow. If they grow enough eventually you may have components come apart. Heat comes from a few different sources. This heat is generated by the flexing of the tire with the hottest region being at the belt edges (edge of the tread) in radials, not the sidewall if the tire is properly loaded and inflated. Increased speed generates more heat. Sometimes the heat is generated faster than it can be transferred to the surrounding air. Over-loading generates more heat. Under-inflation generates more heat. Having 0% reserve load generates more heat than having 10% reserve load which generates more heat than being 15%  under-loaded etc.  This heat can soak into the structure of the tire and actually accelerate the aging of the rubber in the tire. As rubber ages, it looses it's flexibility so this contributes to the breakdown of the rubber at the molecular level mentioned above.



Part of Organic Chemistry is chemical reaction rate.

For every 18F increase in temperature the rate of aging doubles. Heat also comes from being in the sun when parked. So if the RV is parked with tires in direct sunlight you can see the tire achieve 36F increase or more which means it is aging at more than four times the rate it would have if in full shade.
If you want to understand the technology behind this accelerated aging due to heat I suggest you can read some of these sources if you have a few hours.
http://en.wikipedia.org/wiki/Reaction_rate

http://www.chm.davidson.edu/vce/kine...tionRates.html

http://www.chm.davidson.edu/vce/kine...fReaction.html

http://chemistry.about.com/od/stoich...actionrate.htm

Here are some specific references on tires
http://www.rubberchemtechnol.org/doi...5254/1.3547913

I provided temperature data in my post on  white tire covers which protect tires almost completely from this heat damage. Many times the cumulative damage from excess heat can result in a separation of the belts and tread from the rest of the tire. Remember aging of rubber makes it less able to tolerate flexing.

Hope this helps others understand the causes of the vast majority of tire failures.

Tire pressure Increase

Russell G. said

"Hi This has been a question that I have ask and got a different reply as many time as I have ask the question. If a tire is rated for 80 psi as shown on the side wall is aired up to 80 psi. Then driven on a hot day the pressure goes up to 120 psi is that tire over pressure? If not then why? If so then what are we to do? Right now on my truck my tires are at 65 psi the tire shows 85 psi but when loaded pulling the camper it goes to 80 to 85 psi I asked the guys at the tire store and got no where with them. I like your blogs on tires and have got a lot of good info. off of them keep up the good work."

Thanks for reading Russ. This is a good question. I and others many times warn about not exceeding the max pressure of a tire or wheel but we are always talking about the pressure when the tire is "cold" and by that we don't mean you can cool your beer by setting it on the tire. What is meant by "cold pressure" is when a tire is at the same temperature as the surrounding air and not in direct sunlight. You know, the temperature in the shade.

Without going into the details of the "Gas Law" and looking at what is meaningful not just measurable, we find that  using 2% pressure increase for each 10°F temperature increase works out pretty well for real life. This brings us to the obvious question of what is a "normal" temperature increase. Tire temperature will increase as you drive faster, or as you increase the sidewall flex by lowering the pressure or increasing the load. Being in the sun or near the exhaust pipe can also increase the temperature.

The normal temperature increase over ambient is in the 30°F to 60°F range so I would expect to see a 6psi to 12psi increase in a tire starting at 65 to 70 psi. Now if you checked in the early morning while the day was still cool and then were driving in the heat of the day when it was 20°F hotter then we might need to add another 4% so we might see a 16% increase.

If your normal cold inflation is 65 psi you could see a 10 to 12 psi increase on a hot day. In your question you were asking about a 40psi increase which is 150% of the cold inflation. If you saw that level of increase I would suspect there is a problem and that you have a bad gauge or the tire is very overloaded or you have excess moisture inside your tire because the shop did not drain and filter the air from their compressor.

Your increase from 65 to 85 sounds high. I only see about half that in my Class-C. I run about 10 psi over the inflation required for the actual load and I drive at 60 to 63 mph and I see about 8 psi increase.
Have you confirmed your gauge and TPMS readings are accurate?
Are you sure 65 is not leaving you overloaded?  Remember you need to inflate to at least the minimum inflation shown for the heaviest tire on an axle and then I suggest at least a 10% increase for a reserve inflation factor.

BOTTOM LINE
Tire & wheel manufacturers account for a normal pressure increase as outlined above when they design tires & wheels, so you should not be concerned unless you are seeing a large increase in pressure. If you do see a pressure increase like 25% you need to find out why. Remember you should not be bleeding down your tires when they are hot.

Adjust tire pressure for better ride?

Got a question from Bob G. from somewhere in TX 

"Roger,
Let me start off by saying I have really enjoyed your informative articles on the Internet.  Your writing style is one that even the inexperienced can understand and learn from.  Keep up the good work, we need the help!

My question deals with inflation pressures.  I know in the past we have been instructed to inflate according to the manufactures weight limit charts.  With all the tire monitors giving us "instant" pressure readings could the cold inflation numbers be modified slightly where the manufacturer
Inflation Number is achieved after the tires warm up?  I know the NASCAR guys do this.

The real reason I am asking about this is a problem I currently have with "expansion joints" on the highway.  We are currently in the middle of a TX-AK-TX trip and my fillings are starting to fall out!  If my front pressure is set at a cold pressure of 75, which is chart recommended for my axle loading, the bump from expansion joints can be tolerated.  As the tires heat up and pressure goes to 85 and above the bump becomes more of a BANG which hits the front end so hard my dash has broken mounting screws in the past.  Now that I have secured it to the chassis the dash is developing a horizontal crack.  With the aid of my TPMS  I could adjust my cold pressure to reach 75 or 80 hot pressure after a few warmup miles on the tires.  I am currently seeing a temperature rise of 60 to 70 degrees above ambient between cold and hot tires.  This reading was taken with a laser temp gauge measuring in a valley between the tread.  I don't rely on myTPMS for accurate temp as it is a TireMinder with the screw on sensors.

I know I could change my shocks to something softer on the compression stroke to take care of the "bang" but my ride and handling characteristics are really good everywhere else.

What say Ye Great Tire Zen Master?
Bob G"

 

Thanks for the compliment Bob, and for reading the blog.

You pose a reasonable question Bob, but you fail to understand that the load/inflation tables are based on the fact that the pressure does increase as a tire is run. 

If you were to lower your “hot pressure” to match the inflation in the table you would probably soon experience tire durability problems and certainly see a hit in fuel costs and your rolling resistance, which is the force it takes to roll a loaded tire down the road, would immediately increase.

If you are seeing 60 to 70 degree increase over ambient you may be running right at the low limit for tire inflation. You didn’t say how old or what size your RV was or the number of miles on it but shock absorbers do wear out and many times even when the RV is new there are better riding shocks available than the low cost ones that might have been selected by the RV manufacturer.

You are correct about the temperature information from your external TPMS. The temperature you are seeing will most certainly not reflect the tire internal temperature. Watch my blog as I am conducting a test right now to collect the facts. I hope to post the report soon.

Bottom Line

Never bleed down hot pressure. You should consider the inflation on the load tables to be your minimum cold inflation. I usually recommend people run plus 10 % over the table long as they are not exceeding the inflation molded on the tire or the max inflation rating for the wheel. If you can’t run the +10% then you probably have tires and wheels that are lower capacity than what you need based on your actual loading.

What do you think about temperature guns? Part 1

I get asked that question.

This can get technical so if you don't want to get bogged down, the answer is - IR guns are OK for metal objects or for surfaces with uniform temperatures but not for getting accurate tire tread area temperatures.

Now for those that want the details.
I see some who claim on RV forums that the guns are a good tool to help determine the proper tire inflation for their RV. I own an infrared (IR) gun I got from our favorite cheap tool store, Harbor Freight. So you might expect I use it all the time. While I did use the gun for my Tire Cover Test, I have only used it once on my tires and that was during my study to collect data so I could offer this observation on using IR guns for monitoring tire temperatures.

During my career as tire engineer I have used needle probes on Indy racing tires for drivers like Mario Andretti and Al Unser. These probes were designed to penetrate into the tire to the top of the body cord which is the hottest location. If I did not probe deep enough I would get a misleading low temperature. An error of just a few degrees could be critical.

I have also been involved with the use of thermal imaging systems costing tens of thousands of dollars, to conduct tire design tests, so I know from personal experience that they can provide information on the operating temperature of a tire under load and at speed. What I also know is that different parts of a tire can have a very large temperature variation. If a tire is going to fail or be damaged due to excess heat, it is not the average temperature of a tire but the temperature of the hottest spot that may cause the damage.

To help you understand what I am talking about let’s look at some thermal images captured by a laboratory quality IR system. These images are from a book “The Pneumatic Tire” by Gent & Walters, distributed by NHTSA in 2006. The 707 page book is aimed at tire engineers and chemists, so it is definitely not a quick read while at your campsite. While I did consult a very little bit with a couple of the engineers and scientists that contributed to its writing, the majority of this information came from very experienced members of the tire industry community, some of whom worked for different major tire companies and others who have PhD’s in their field and others who teach college and graduate level chemistry or tire mechanics courses. These guys know their stuff.

IN THIS SERIES OF IMAGES, you can see the variation in temperature both with inflation level and across various portions of the tire being tested. I have added yellow circles approximately ½” diameter. I think you can see that it is essentially impossible to find a uniform temperature over such an area especially when increased temperature differential becomes more important.

The lab IR equipment has a very high resolution which means it can differentiate temperatures across very small areas, 0.10” or smaller. To help, green is 90°F and white, or hottest, is 140°F. It is important to also remember that rubber is a good insulator and does not transfer heat rapidly, so it takes minutes at a constant load and speed for the higher temperatures generated deep within the tire structure to migrate and stabilize at its surface which is constantly being cooled by the surface air.

You should note that different portions of the tread pattern have very significantly differences in temperature with the highest temperatures deep down in the bottom of the grooves, slots and sipes of the pattern. To obtain these higher temperature readings it is necessary for the IR sensor to “see” down into these tread pattern features while also not including the much cooler temperatures only 0.10” to 0.25” away and to be able to record the temperature while the tire is running. It is also important to notice that as you move away from the tread pattern and down to the smoother sidewall, the temperature gets much cooler. So if you try and use your hand held gun on a more temperature uniform portion of the tire you will no longer be reading the hot locations.

IF YOU HAVE USED YOUR HAND HELD IR GUN to learn the temperature of your tires, I think you can see that you have been mislead, as the area used to sense the surface temperature was too large to capture just the hot area in the tread area, or you targeted the more uniform but much cooler sidewall of the tire. In either case your readings do not provide a reliable or sufficiently accurate reading to be used for tire inflation setting measurement.

I am sure that some still think that they have made the effort to be very precise when taking temperature readings. They always shot the exact same location (i.e. the same tire lug for they vary in size and therefore vary in temperature) on each tire, were only a ½” or less away from the tire surface to get the hot location in the bottom of a slot, and that the time to measure all their tires was always less than 30 seconds after stopping and always in the same sequence to address the surface cooling that all tires go through, or maybe not.

So, in my next post, I will show the results and analysis of the test of pressure variation effect on temperature I ran with the manager of race tire development at a major tire company, who was gracious enough to assist me in doing a comparison of my TPMS that reads the contained air temperature (CAT) using a probe mounted inside the tire vs. a hand gun vs. his professional 0.1° F reading probe which gets into the rubber and provides a reading closer to the source of the temperature.

Tire Covers - Do they do any good?

I have seen some posts and comments about tire covers. One post caught my eye.

The owner was complaining about the cover degrading in the sun so he was of the opinion the cover wasn't worth the cost. I have to wonder how he failed to properly interpret the proof that the cover was protecting his tires. How would he answer the question: Which would you rather have degrade due to being in the sun? Your tires or the low cost cover?

As an Engineer I always prefer to have data rather than just someone's opinion. So I set up a quick test on my own RV.

As you can see I have covers for my tires. They were the second purchase after buying my rig. The first was a Tire Pressure Monitor System.





The Test
With one side of the RV in full shade I checked the temperature of the side of the unit.
You can see I recorded 85.1°F.




Then I took the temperature of the side of the RV in the full sun.

Here we see 107.9°F








Next the temperature of the white tire cover in full sun.

We get 98.6°F
I believe the cover is cooler than the side of the RV because air can circulate behind the cover.


I then removed the cover to see what the tire temperature was under it.


We see 99.5°F
Only 1 degree hotter than the cover.







I then waited 30 minutes to see how hot the black tire got while in full sun.

We see the black tire was at 136.1°F.








KEY POINT
There are three things that can "kill" your tires. Ozone, UV and High Temperature. The Ozone & UV directly attack the surface of the tire making it crack when flexed. Temperature works no only on the surface but deep down inside the tire structure.
Increased temperature causes continued and accelerated chemical reaction which "ages" a tire faster than when the tire is cool. A rule of thumb would be that the rate a tire ages doubles with every 18°F increase in temperature. We can see the result of old rubber on the surface. What we don't see is the more brittle rubber of the internal tire structure. As rubber gets more brittle with age it also looses strength.

NOTE Overload and Underinflation can overheat a portion of a tire to the point the rubber and reinforcement materials loose all their strength.

Based on my simple test it would appear that by covering my tires I am significantly reducing the artificial aging for all the daylight hours my RV is parked and the sun is out. If I didn't have covers my tires would be "aging" FOUR times faster than with the covers in place.

I don't have black tire covers so can only guess at their temperature. While they may provide protection from UV I would be surprised if they can offer much temperature protection.

So I will let you answer the question of Tire Covers being a good investment or not.