OK kids, today's science lesson is about the stuff we inflate our tires with. If you don't want to take the lesson simply skip to the "Bottom Line."
If you took engineering, chemistry or perhaps some other science class in school, you might remember something called the "Ideal Gas Law" PV=NRT. I covered the formula in THIS post, and the resultant "Rule of Thumb" is that for every 10°F increase or decrease in temperature the pressure in your tires will increase or decrease by 2% . Now we need to be careful and remember that some may be discussing passenger tires where 2% of 36 psi is rounded to mean 1 psi change for 10°F change but I am discussing tire pressures that may range from 45 psi to 130 psi so percentage is more accurate.
But that formula is based on basic scientific theory of a dry gas used in a non-expandable container.
The first "fly in the ointment" is that most people are inflating their tires with compressed air and air is a combination of different gases such as 78.084% Nitrogen, 20.946% Oxygen, 0.934% Argon 0.03769% other gases such as CO2 , Neon, Helium, Methane, Krypton, and Hydrogen. Note the concentrations of these trace gases along with various "pollution" changes over time so the specific percent varies slightly. Wikipedia has the details if you are interested.
The other major variable is the percent moisture. Water vapor varies between 1% and 5% depending on temperature and geographic location. It also varies based on the process of compressing air and the quality of maintenance the air compressor receives. In general the percent of water vapor will be raised in an air compressor, so unless special procedures are taken you are going to end up with a few percentage points of water vapor. Making this even more complex is that the effect of moisture on your tire pressure varies with temperature. This can range from less than 1 psi to almost 5 psi. I have a post on how you can make the air you use to inflate your tires "dryer" if you feel that is important.
In the equation the V stands for volume and since tires do expand a little bit as the pressure goes up and inflation gas isn't 100% dry or 100% of any one gas but a mixture of Oxygen, Nitrogen and other trace gases, these facts have an effect on how close to PV=NRT you will get with your tires.
Remember that even if you pay to "fill" your tires with Nitrogen you will never reach 100% nitrogen as there are always some other gases mixed in. In some experiments I have seen that the best you can reasonably expect to get would be about 95 to 98% N2.
Another problem is to know the actual temperature of the gas inside your tire. Most people have either an external TPM sensor or use an IR gun to measure the temperature of the tire and mistakenly assume that is the temperature of the gas inside the tire. About the only time you might obtain an accurate reading of tire inflation air temperature is many hours after last moving the tire and hours of storage in a constant temperature room so both in internal and external temperature is the same.
In an effort to get closer to the actual numbers I decided to try and do a quick experiment using an internal TPMS and an External TPMS on my Class-C motorhome. I ran into a number of problems.
Some of these problems include the fact that external sensors did not completely "wake up" in the first minute of operation so I was not able to easily get a zero point temperature reading.
Also we need to accept that despite claims to the contrary, TPM sensors are not calibrated laboratory grade instruments so they introduce some error into the experiment. Next we need to consider that external sensors are only providing the temperature of the metal in the sensor body which is affected by the outside air temperature. This will usually result in a cooler temperature reading than the actual temperature of the gas inside the tire.
I did collect some data but with all the variation I was able to identify I felt the experiment did not deliver any meaningful data. I did see variations in pressure readings between the internal and external sensors over time but the numbers were inconsistent.
What does all this mean?
BOTTOM LINE
1. Reality is much more complex than nice controlled laboratory experiments
2. Tire inflation pressure does vary with tire temperature and the pressure does increase as the temperature increases.
3. Dryer inflation gas will have less variation of pressure than "wet" inflation gas.
4. In my opinion none of the above observations is significant enough to change the "Rule of thumb" that we can expect our tire pressure to vary by about 2% for every change of 10°F.
5. We all need to remember that not every change or difference that is measurable is meaningful
6. Best advice I can give is not to get your shorts in a bunch about inflation.
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Showing posts with label Pressure. Show all posts
Showing posts with label Pressure. Show all posts
Sunday, January 24, 2016
Wednesday, October 7, 2015
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.
Subscribe to the weekly RVtravel.com newsletter or one of our other newsletters about RVing. Great information and advice. Now in our 14th year. Learn more or subscribe.
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.
Subscribe to the weekly RVtravel.com newsletter or one of our other newsletters about RVing. Great information and advice. Now in our 14th year. Learn more or subscribe.
Monday, October 5, 2015
What is the optimum tire pressure?
Load inflation tables identify the minimum inflation for a number of specific load placed on the tire.
Tire sidewalls tell you the maximum load capacity for a given tire when inflated to the maximum pressure for that load range in that size tire.
Optimum implies we have common agreement on which performance requirements we feel are most important. Optimum also implies that there is agreement in the inherent trade-offs of the numerous performance characteristics.
If the only performance we are concerned about is load capacity and if we want the maximum capacity possible for the size and load range of a specific tire, then we have a situation where the optimum inflation is the maximum for the load range, which is also the minimum inflation specified in the tables.
If, however, we do not need to support a load that corresponds to the tire's maximum load, then it is possible that other performance criteria may be considered and as a result there will be trade-offs to arrive at a new optimum.
If we are talking about motorhomes, there are normally performance characteristics other than just load capacity. Some might be fuel economy, noise, ride comfort, tread wear, steering response, etc. Inflation pressure will affect each of these characteristics — some positively and some negatively. So clearly the "optimum" depends on the clearly understood and agreed on priority of these and other characteristics.
Bottom line
As a tire engineer I suggest that people select an inflation pressure that will provide at least 15 percent extra load capacity over the heaviest loaded tire on an axle. All tires on an axle should run the same cold inflation. The above will still provide acceptable ride and provide improved durability and fuel economy.
For multi-axle trailers I would consider improved durability, i.e., reduced chance of failure, to be of primary importance. So in this application, the "optimum" inflation pressure would be the pressure on the tire sidewall associated with the maximum load capacity. Even if you are not loaded to the max load you want to lower the "interply shear" forces as much as possible, as trailers induce much higher shear forces than seen in similarly loaded tires would if on a motorhome.
Subscribe to the weekly RVtravel.com newsletter or one of our other newsletters about RVing. Great information and advice. Now in our 14th year. Learn more or subscribe.
Tire sidewalls tell you the maximum load capacity for a given tire when inflated to the maximum pressure for that load range in that size tire.
Optimum implies we have common agreement on which performance requirements we feel are most important. Optimum also implies that there is agreement in the inherent trade-offs of the numerous performance characteristics.
If the only performance we are concerned about is load capacity and if we want the maximum capacity possible for the size and load range of a specific tire, then we have a situation where the optimum inflation is the maximum for the load range, which is also the minimum inflation specified in the tables.
If, however, we do not need to support a load that corresponds to the tire's maximum load, then it is possible that other performance criteria may be considered and as a result there will be trade-offs to arrive at a new optimum.
If we are talking about motorhomes, there are normally performance characteristics other than just load capacity. Some might be fuel economy, noise, ride comfort, tread wear, steering response, etc. Inflation pressure will affect each of these characteristics — some positively and some negatively. So clearly the "optimum" depends on the clearly understood and agreed on priority of these and other characteristics.
Bottom line
As a tire engineer I suggest that people select an inflation pressure that will provide at least 15 percent extra load capacity over the heaviest loaded tire on an axle. All tires on an axle should run the same cold inflation. The above will still provide acceptable ride and provide improved durability and fuel economy.
For multi-axle trailers I would consider improved durability, i.e., reduced chance of failure, to be of primary importance. So in this application, the "optimum" inflation pressure would be the pressure on the tire sidewall associated with the maximum load capacity. Even if you are not loaded to the max load you want to lower the "interply shear" forces as much as possible, as trailers induce much higher shear forces than seen in similarly loaded tires would if on a motorhome.
Subscribe to the weekly RVtravel.com newsletter or one of our other newsletters about RVing. Great information and advice. Now in our 14th year. Learn more or subscribe.
Wednesday, September 2, 2015
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.
Subscribe to the weekly RVtravel.com newsletter or one of our other newsletters about RVing. Great information and advice. Now in our 14th year. Learn more or subscribe.
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.
Subscribe to the weekly RVtravel.com newsletter or one of our other newsletters about RVing. Great information and advice. Now in our 14th year. Learn more or subscribe.
Friday, January 16, 2015
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
Subscribe to the weekly RVtravel.com newsletter or one of our other newsletters about RVing. Great information and advice. Now in our 14th year. Learn more or subscribe.
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
Subscribe to the weekly RVtravel.com newsletter or one of our other newsletters about RVing. Great information and advice. Now in our 14th year. Learn more or subscribe.
Tuesday, July 8, 2014
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.
Subscribe to the weekly RVtravel.com newsletter or one of our other newsletters about RVing. Great information and advice. Now in our 14th year. Learn more or subscribe.
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.
Subscribe to the weekly RVtravel.com newsletter or one of our other newsletters about RVing. Great information and advice. Now in our 14th year. Learn more or subscribe.
Monday, March 3, 2014
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.
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.
Monday, February 24, 2014
Measurable vs Meaningful
It is important to understand the difference between these two terms. Sometimes people assume that if something can be measured it is important.Over the years as technology has advanced it becomes more and more easy to detect ever smaller quantities of material or forces. As an engineer I too often hear people expressing concern over some new discovery of x parts per Million or even x parts per Billion of some contaminant to air or water, or foreign material in some product. Some of these numbers are important and meaningful while others are not really significant so there should not be a cause for concern. The challenge is to know when some measurement is meaningful.
Some of you are asking yourself what this has to do with RV tires. Well if you have read many of my posts here in this blog or on various RV Forums you no doubt have seem me and others saying you need to know the actual load on each tire and not just guess. Also we tell you how important it is to have the proper inflation in your tires.
What made me think of a need for this topic is the occasional complaint I get from some that think we are suggesting a need to get their RV weighed every time they travel or add an item to their packing list.. They basically say they are not going to do that so ask "Why bother to weigh the RV at all?"
Lets step back and look at tire Load & Inflation tables. For highway use tires the tables have 5 psi increments and depending on the size tire that 5 psi increment can provide as little as 110# additional load carrying capacity for a small ST type tire to 320# or more additional capacity to a large Class-A tire. If we do some simple math that means on a single axle small Trailer you would need to adjust your inflation if you were to increase the load in the trailer by 220# but the Class-A RV with rear duals and a tag you might be able to tolerate (8x320#) or 2,560# additional load (assuming perfect load distribution).
If your hobby was going to yard sales and picking up cast iron lawn ornaments it might be possible for a couple good sized items at (115# each) to be meaningful so you would need to increase your tire inflation on your small teardrop trailer (assuming you were not already running the tire max). The owner of the 40' Class-A on the other hand could probably pick-up a couple dozen items that each weighed 105# and if the load was perfectly distributed still not be overloading the tires.
Now clearly we can count (measure) the difference in the additional load of the two 115# items and the load of twenty four 105# items. In both cases the additional load is easily measured but while 120# could be very meaningful to the small trailer owner an increase in load of 120# would certainly NOT be meaningful to the Class-A RV owner. Maybe you now have a better understanding of the difference between measurable and meaningful.
In a previous post, I provided information of the accuracy of pressure gauges, showing that I find that 10% to 15% of the gauges I have checked are off by 5 psi or more. While my personal gauges are certified accurate to +/- 0.5psi, that is way more accurate you you "need" if you are setting your cold inflation to give yourself a 10psi Safety Margin over what is needed for your normal full load. If however you are running loads that fall just below the load indicated for the cold inflation you are using, then BOTH your gauge needs to be more accurate than +/-5 psi and your scale reading needs to be accurate to less than half of the load increment in the table.
Example: Tables for a 255/80R22.5 show 4,805# @ 95 and 4,975@ 100 psi so your gauge needs to be at least +/- 2.5 psi from actual certified measurement and your load needs to be measured to an accuracy of at least +/- 85# if you are using 100 psi as your cold inflation.
Friday, January 3, 2014
Tire Inflation accuracy
At a number of RV events I have tested different gauges against a digital 0.5 psi reading gauge that has been checked against QS9000 ISO/IEC 17025 approved master laboratory pressure gauge and found accurate to +/- 0.5psi. We have found that about 15% of the tested gauges were wrong by more than 5psi with a few off by more than 12psi. In general the most accurate were digital. The least accurate were the "stick" gauges with the sliding scale.
I have suggested that if you are accurate to within 5% of your goal cold tire inflation you are probably OK but obviously more accurate is better. The 5% tolerance is based on an assumption that you have added 10% to the minimum required per measurement of actual tire load and consultation with your tire companies Load/Inflation tables. It has also been suggested that using any gauge is better than not checking your tires at all.
Now how to know the accuracy of your gauge in day to day use. I suggest the following based on personal observation (no sponsorship or compensation received).
Get two digital gauges. One can be like Accutire MS-4021B as seen on the right
I have hose extenders as seen below
My master is packed away in a box so it does not get damaged or knocked around. Once a month I measure one front tire with both gauges and record the readings. The difference between the two readings should not change by more then 0.5psi from new. If I notice a change in tire inflation not explained by significant temperature change or puncture I will compare the daily gauge against the master gauge. I figure the chance of both gauges going bad the same amount and in the same direction at the same time is vanishingly small. The 4012 is available at $10 and occasionally for less than $7.
TPMS readings have not found to be as accurate as hand gauges. Their purpose is to be a warning of a loss in pressure. You should set tire pressure with the hand gauge. You should see a fairly constant difference between the TPMS and the hand gauge.
It is important to measure your tires when they are at ambient temperature. This means the air temperature. NOT exposed to the sun or even under a tire cover or having been driven on for at least two hours.
Tire pressure changes with temperature and you can use the rule of thumb of 2% for every 20F.
Thursday, September 12, 2013
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.
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.
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.
Tuesday, April 30, 2013
Gauge Accuracy - I was wrong.
Previously in my post on gauge accuracy I suggested that you could get your pressure gauge checked by visiting a tire store from a tire manufacturer or very large dealer. Well after doing more investigations it turns out I was expecting more from dealers than they actually provide.
Tire dealers are primarily in the business of selling tires and vehicle service. They use gauges every day to inflate hundreds of tires but they do not have ISO calibrated gauges as this would be very expensive and the difference between the gauges they use every day and a calibrated gauge might be measurable but is not really meaningful.
Now this doesn't mean you should not make an effort to confirm your gauge is reasonable accurate the question is "How accurate is accurate enough"? If your gauge is +/- five percent of the inflation pressure you're measuring then you are probably okay, especially if you're running at least five percent above the minimum inflation needed to carry your load as measured in individual tire load scales.
Still, I would suggest you get a digital gauge for your personal "master gauge." They cost as less than $15. Then use that master gauge to check your everyday gauge. Keep your master packed away and not rattling around the bottom of your tool box under the hammer and wrenches where it will get dirty and damaged.
If you check a front tire with the digital master and then your every day gauge and note the difference then when you compare them again maybe a month later you should see the same difference. It is very unlikely for both gauges to go bad the same amount in the same direction at the same time.
If both gauges are within two to three percent when you first get the new digital master you are probably okay. If there is more than a three percent difference then I would check both against some other gauge to learn which is wrong.
Hope this clears things up and sorry if I mislead anyone.
Tire dealers are primarily in the business of selling tires and vehicle service. They use gauges every day to inflate hundreds of tires but they do not have ISO calibrated gauges as this would be very expensive and the difference between the gauges they use every day and a calibrated gauge might be measurable but is not really meaningful.
Now this doesn't mean you should not make an effort to confirm your gauge is reasonable accurate the question is "How accurate is accurate enough"? If your gauge is +/- five percent of the inflation pressure you're measuring then you are probably okay, especially if you're running at least five percent above the minimum inflation needed to carry your load as measured in individual tire load scales.
Still, I would suggest you get a digital gauge for your personal "master gauge." They cost as less than $15. Then use that master gauge to check your everyday gauge. Keep your master packed away and not rattling around the bottom of your tool box under the hammer and wrenches where it will get dirty and damaged.
If you check a front tire with the digital master and then your every day gauge and note the difference then when you compare them again maybe a month later you should see the same difference. It is very unlikely for both gauges to go bad the same amount in the same direction at the same time.
If both gauges are within two to three percent when you first get the new digital master you are probably okay. If there is more than a three percent difference then I would check both against some other gauge to learn which is wrong.
Hope this clears things up and sorry if I mislead anyone.
Tuesday, April 23, 2013
Have you read a tire lately? Tire Marking
Have you read the information provided to you by the tire manufacturer? If you have, did you bother to record the important numbers in your log book so you only need to do this job once in the life of a tire?
There is a lot of information molded into the sidewall of your tires. Most of this is required by law. Most of this is important for you to know so you can look up the correct inflation for your tires or if buying new tires be sure to get replacements that can carry at least the same as the original tires.
Here is some information from one tire.
First the SIZE
This tire is a "Passenger" type as it starts with the letter "P" other types might be "LT" for Light Truck, or "ST" for Special Trailer. Small tires intended for passenger cars migth not have the "P" if made to European specs. Large tires like 19.5 or 22.5 rim dia have no letter and are "TBR" of Truck Bus radials and are found on Class-A RV. The "114S" is the Service Description which is a Load Index (from a published table but using the actual Max Load is better). The "S" is the Speed Rating or max operating speed. Like the Red Line on your engine. Not all tires have this description.
The DOT Serial
This has important information used in determining the tire age. Other information such as the location of the tire plant that made the tire is part of this code ( first two characters 8X in this example). If there is a recall, this code is used to identify which tires are covered by the recall. NOTE that the last portion, the 4 digit date code, 3908 in this example is only molded on one side of most tires. Every tire sold for use on the highway in the USA must have a full DOT serial including the date code molded on at least one side. This tire was made the 39th week of 2008.
Tire Materials
This is really just FYI and is more like truth in advertising to let you know the materials used in the sidewall and center of the tread of your tires. In this case there are two ply of Polyester in the sidewall and in the tread there are two ply of polyester + 2 ply of Steel + 1 ply of Nylon. Most TBR tires will only have 1 ply of steel in the sidewall.
Safety Warning
This is for the person mounting and inflating the tires for the first time. Do not confuse the inflation number here ( 40 psi) with the inflation number associated with the load (44 psi). This is the max inflation to seat the bead. If you have ever watched a tire being inflated it is the "Pop" or "Bang" first heard. If the tire doesn't seat by this inflation then it should be deflated, re-lubed, re-centered and re-inflated. People can die if they ignore this warning.
Bottom line
If you get new tires and they match the Size and Load & Inflation information you are good to go. If ANY of these numbers are different you need to be sure you completely understand why and that you are not getting a tire with lower load capacity.
There is a lot of information molded into the sidewall of your tires. Most of this is required by law. Most of this is important for you to know so you can look up the correct inflation for your tires or if buying new tires be sure to get replacements that can carry at least the same as the original tires.
Here is some information from one tire.
First the SIZE
The DOT Serial
The Load & Inflation information
This is the maximum load capacity for the tire when the cold inflation is set to this pressure. In this example (2601 lbs) when the inflation is set to( 44 psi max press) when the tire is at ambient temperature. NOTE for LT and TBR type tires there is a second lower load limit for dual application (tires side by side on the same axle as on the rears). If you have two axles and ST type tires I personally suggest you not exceed the "dual" tire load as you need a safety factor in your tire loading due to extreme side loading unique to tandem axle trailers.
Tire Materials
Safety Warning
Bottom line
If you get new tires and they match the Size and Load & Inflation information you are good to go. If ANY of these numbers are different you need to be sure you completely understand why and that you are not getting a tire with lower load capacity.
Friday, March 29, 2013
Dual Tire Hose Extenders? Tips to avoid problems.
Some general comment on extensions for owners of Motorhomes and dually pick-ups.
When I was racing, I towed a 26' enclosed trailer with a 1-Ton dually and had steel braided hose valve extensions. We ran about 78,000 miles with no problems. When I got my Class-C my first add-on was TPMS followed by steel braided extensions. With only 16,000 miles there have been no leaks or other problems.
Now there are TWO main things that I would consider a MUST.
First you need bolt in metal valves.
Second you need a hard attaching point on the outer end of the hose to prevent movement of the hoses. The hoses come in different lengths so you should run the shortest hoses that allow you to easily bolt the outer end down and you need to support the end of the hose when pushing on it with your tire gauge or air chuck.
I have seen a claim that a metal valve failed because of fatigue from the hose but without significant movement of the hose there should not be enough to stress the valve stem.
Metal valve stems should be replaced or at least have all the rubber O-rings and gaskets replaced whenever you change a tire. The rubber parts age just as the rubber on your tire ages. Replacing when you replace tires eliminates the need of keeping track of the age of the O-rings and seals. There is a rubber interior to the hose and this will also age, so when I replace my tires I will get new hose extenders along with new valve rubber parts.
I have seen spring clips that are supposed to retain the steel braided hose but they don't look too solid to me. I have not looked at the rubber "grommet" that fits the hand holes but have to wonder if they hold the braided steel line solidly. Clearly they do not support the hose end so there will definitely be some movement.
You can see in my set-up that I have the Wheel Master 8001 hoses firmly attached to my hub cap. I do hold the extender hose when ever using a gauge or air chuck.
Valve extender hoses can work and make adding and checking the air on dual applications much easier. All you need to do is to pay attention to a proper installation that minimizes any movement or rotation of the hose extender.
When I was racing, I towed a 26' enclosed trailer with a 1-Ton dually and had steel braided hose valve extensions. We ran about 78,000 miles with no problems. When I got my Class-C my first add-on was TPMS followed by steel braided extensions. With only 16,000 miles there have been no leaks or other problems.
Now there are TWO main things that I would consider a MUST.
First you need bolt in metal valves.
Second you need a hard attaching point on the outer end of the hose to prevent movement of the hoses. The hoses come in different lengths so you should run the shortest hoses that allow you to easily bolt the outer end down and you need to support the end of the hose when pushing on it with your tire gauge or air chuck.
I have seen a claim that a metal valve failed because of fatigue from the hose but without significant movement of the hose there should not be enough to stress the valve stem.
Metal valve stems should be replaced or at least have all the rubber O-rings and gaskets replaced whenever you change a tire. The rubber parts age just as the rubber on your tire ages. Replacing when you replace tires eliminates the need of keeping track of the age of the O-rings and seals. There is a rubber interior to the hose and this will also age, so when I replace my tires I will get new hose extenders along with new valve rubber parts.
I have seen spring clips that are supposed to retain the steel braided hose but they don't look too solid to me. I have not looked at the rubber "grommet" that fits the hand holes but have to wonder if they hold the braided steel line solidly. Clearly they do not support the hose end so there will definitely be some movement.
You can see in my set-up that I have the Wheel Master 8001 hoses firmly attached to my hub cap. I do hold the extender hose when ever using a gauge or air chuck.
Monday, March 4, 2013
How to "Top Off" your inflation pressure
Hopefully all the readers of this blog understand the importance of having the correct inflation pressure in ALL their tires. Be they on a 45' Class-A, your toad, a tow-dolly, regular passenger car or anything in between. The question is how do you get a few more psi of air for your tires when you check in the morning.
There are a number of different situations and I can't possibly address them all here, but I think you can review these suggestions and find a plan that will work for your situation.
I do need to separate out the few folks that discover they have a flat tire or one that has lost more than 20% of the minimum required pressure. NOTE: Tires that have lost 20% of their air are considered "flat" by the tire industry. You have a problem. Maybe a puncture or a failed valve or a tire that has been damaged. You should not drive on your flat tire. You need to change it if you have a spare and the proper tools and experience to do the job safely, or have it changed by a tire service truck and technician.
Now the rest of you who just need a few psi to get back to your goal inflation that provides the inflation needed to carry the load plus a few psi "cushion", there is a way to handle your situation. The options depend on how much air pressure you need.
If you need 80 psi or less one option is to carry a small "tankless" or "pancake" compressor rated at 100 psi and 1 or 2 Cubic Feet per minute (CFM). These are available for $20 to $60 at discount tool supply companies or auto parts stores. Some are 120V and some are 12v and can run off your battery or on-board generator. Just be sure you have enough extension cord or air host to get to all your tires.
NOTE You will have a tough time ever inflating a tire to the pressure the compressor is "rated" for so don't buy a 100 psi compressor is you need 100 psi.
If you have a larger rig like a Class-A, you probably need 90 to 120 psi and a compressor rated at 125 to 150 psi and 2 CFM or higher. If you have air brakes you may have enough on-board capacity and just need the appropriate fittings and host. There are small compressors on sale at less than $100 that claim to be capable of 125 psi.
One other option for those only a few psi low. Drive to the nearest service station at slightly reduced speed (10 mph under the speed limit would be max) and follow these instructions on how to inflate a hot tire.
1. Record your cold inflation.
2. Calculate how many psi each tire would need to reach your goal cold inflation.
3. Drive at reduced speed, hopefully no more than 10 miles, to the service station with air available. You might want to call ahead to be sure they have enough space or long enough hose to reach your rig. Not all service stations can accommodate a Class-A with a toad.
4. Measure your now warm inflation pressure
5. Add the psi needed from step #2 above plus 3psi to learn your temporary "warm" tire inflation
6. Inflate your warm tires to the temporary goal inflation calculated in step 5.
7. Confirm you have the needed inflation the next morning after the tires are at ambient temperature and adjust accordingly.
If you follow these steps I think you will find that your tires have the proper inflation or 1 or 2 psi more so you can set the inflation at your exact goal cold inflation using your digital gauge.
If you have any concerns then have a service truck come out to top off your tires.
Remember DO NOT DRIVE on any tire that has lost 20% or more of its air.
There are a number of different situations and I can't possibly address them all here, but I think you can review these suggestions and find a plan that will work for your situation.
I do need to separate out the few folks that discover they have a flat tire or one that has lost more than 20% of the minimum required pressure. NOTE: Tires that have lost 20% of their air are considered "flat" by the tire industry. You have a problem. Maybe a puncture or a failed valve or a tire that has been damaged. You should not drive on your flat tire. You need to change it if you have a spare and the proper tools and experience to do the job safely, or have it changed by a tire service truck and technician.
Now the rest of you who just need a few psi to get back to your goal inflation that provides the inflation needed to carry the load plus a few psi "cushion", there is a way to handle your situation. The options depend on how much air pressure you need.
If you need 80 psi or less one option is to carry a small "tankless" or "pancake" compressor rated at 100 psi and 1 or 2 Cubic Feet per minute (CFM). These are available for $20 to $60 at discount tool supply companies or auto parts stores. Some are 120V and some are 12v and can run off your battery or on-board generator. Just be sure you have enough extension cord or air host to get to all your tires.
NOTE You will have a tough time ever inflating a tire to the pressure the compressor is "rated" for so don't buy a 100 psi compressor is you need 100 psi.
If you have a larger rig like a Class-A, you probably need 90 to 120 psi and a compressor rated at 125 to 150 psi and 2 CFM or higher. If you have air brakes you may have enough on-board capacity and just need the appropriate fittings and host. There are small compressors on sale at less than $100 that claim to be capable of 125 psi.
One other option for those only a few psi low. Drive to the nearest service station at slightly reduced speed (10 mph under the speed limit would be max) and follow these instructions on how to inflate a hot tire.
1. Record your cold inflation.
2. Calculate how many psi each tire would need to reach your goal cold inflation.
3. Drive at reduced speed, hopefully no more than 10 miles, to the service station with air available. You might want to call ahead to be sure they have enough space or long enough hose to reach your rig. Not all service stations can accommodate a Class-A with a toad.
4. Measure your now warm inflation pressure
5. Add the psi needed from step #2 above plus 3psi to learn your temporary "warm" tire inflation
6. Inflate your warm tires to the temporary goal inflation calculated in step 5.
7. Confirm you have the needed inflation the next morning after the tires are at ambient temperature and adjust accordingly.
If you follow these steps I think you will find that your tires have the proper inflation or 1 or 2 psi more so you can set the inflation at your exact goal cold inflation using your digital gauge.
If you have any concerns then have a service truck come out to top off your tires.
Remember DO NOT DRIVE on any tire that has lost 20% or more of its air.
Friday, October 26, 2012
Gauge accuracy
One part of the process is using a sufficiently accurate gauge. I consider +/- 2 to 3 percent or +/- 1psi when measuring passenger tires to be sufficiently accurate.
Over the past few years I have checked more than 40 different gauges and have observed a 14 to 18 percent failure rate in two groups of 20 or so gauges.
I have two digital "Master " gauges as seen to the right, which read to 0.5psi increment. Both have been checked against ISO certified laboratory gauges and one reads 0.5 psi high and the other 0.5 low. I compare these gauges to each other to confirm they have not gone out of calibration before I conduct my gauge test. I figure the chance of both going out of calibration the same amount and same direction at the same time is very unlikely.This year while at the Gypsy Journal Rally in Celina, OH, at my Tire Basics seminar, I offered to check gauges. Here are the results.
Actual Indicated Type Gauge Error
82.0 81 Dial -1%
70.0 69 Dial -1%
77.0 75 Stick -2.6%
76.0 81 Dial +6.6% Fail
77.0 92 Short Stick +19.5% Fail
74.5 81 Long Stick +8.2% Fail
79.5 78 Long Stick -1.8%
77.5 78 Long Stick +0.6%
68.5 72 Long Stick +5.1% Fail
68.0 68 Dial =
68.5 74 Long Stick +8% Fail
As you can see we observed a 45 percent failure rate with all the "fail" gauges giving a high reading which could result in the operator unknowingly operating their tires in an under-inflated state yet thinking the tires were properly inflated. Given the general tendency for RVs to be overloaded in the first place or to be operated right at the limit of the load capability for the specified inflation this is definitely not the better way for a gauge to be wrong.
Having your own accurate "master gauge" is not unreasonable given the low cost of good quality digital gauges. You don't even need a special dual foot master gauge.
Do your own "calibration check" by using the "master single head gauge to check a front tire. Then use your "truck" dual foot gauge (as seen on the left in the top picture above ) to get a reading on the same tire to confirm the reading. If the readings match you can be confident all your readings will be sufficiently accurate when you use your dual foot gauge on the rear tires.
I would suggest you get a digital gauge and then visit the tire store from one of the major tire companies and ask if they would check your digital against their master certified gauge. You could also ask when their master was last calibrated. A competent certification includes affixing a dated calibration sticker to a master gauge. If they cant tell you when it was calibrated just say "thank you" and go to a more competent tire dealer.
Finally don't just throw your master gauge in the bottom of your tool box. I suggest you keep it in a box of some type. Even a Tupperwear type container can protect your master gauge.
Also don't forget to always use a cap or screw on TPM sensor to keep dirt out of the valve core area of your valves. If you don't understand why you need a cap on the valve I suggest you subscribe to this blog as I will be posting a story showing exactly why you should always protect the valve core.
Edit: I have conducted similar evaluations at FMCA Conventions in Perry, GA in 2016 and Madison WI in 2015. At those events I tested about 50 gauges and found between 10% and 16% were off by more than 5 psi.
Monday, September 24, 2012
Tire pressure Increase
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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.
Wednesday, September 19, 2012
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"
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.
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