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Sunday, January 24, 2016

Does tire pressure follow the "Gas Law" ?

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?

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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Tuesday, January 12, 2016

Quick posts: Do you need an Air Compressor? Inflation vs Ride comfort?

I recently read a question from a Class-A motorhome owner about the need for carrying your own air compressor. My thoughts and reply to the question:

 If you run the 10% extra pressure (above the minimum required to support the actual tire load) I and others suggest, and if you keep an eye on your pressure every travel day with your TPMS, you should get plenty of warning on when you will need to add air. While +/- a couple psi is normal for temperature variation, if you see a 5% loss one day and another couple the next, etc., then that should be a warning that you have a leak, which indicates some problem that needs to be addressed sooner rather than later. Once you see -5%, simply stop at the next fuel stop and use the high pressure that is available at truck stops.

People need to remember that it is normal for tires to lose between 1% and 2% of their inflation pressure each month (adjusting for temperature and barometric pressure variation). This is the result of the molecules of air moving out between the spaces between rubber molecules. I also note the concern about most small compressors not being able to inflate a large tire that has lost a lot of air.

If you have lost sufficient air to be more than 10% below what is needed to support the load, you really do have a problem that more than likely needs professional service, not simply a "top off" of air. If you have lost 20%  or more, you should not be driving on that tire and also should not be inflating your tire outside a service center tire safety cage, as running a tire very low may have damaged the steel body cords to the point that the tire might explode when being inflated and result in injury.

I also read a question about running the exact air pressure needed to support the tire load. Following this practice creates a couple problems:

1. Every time the temperature changes 10°F, your pressure will also change about 2%, which could mean you would be running your tires under-inflated. Not a real big deal, but every mile of operation in overload may consume part of the tire life with the potential of having an early failure.

2. I believe that you would soon tire of constantly adjusting your tire pressure up or down one or two psi every day. When you get tired of this constant effort, I am afraid there may be a tendency to forgo checking and adjusting the air. This could lead to a failure when you go an extended time without checking your pressure. This is why I suggest the 10% margin.

Of course this raises the question from some about increased inflation causing hard ride. Now, running higher pressure could lead to a harder ride, but while I believe that running more air than the minimum needed to support the load may theoretically contribute to a harsher ride, I doubt that there are many riding in Class-A RVs who can feel a 5 psi or maybe even a 10 psi difference in a controlled blind test. This 5% or 10% translates to a 1 to 3 psi difference in a car, and I have seen many people unable to notice changes of 5 to 15 psi (20% to 50%) in passenger car operation.

See my post about not getting your "shorts in a bunch" about air pressure.

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Sunday, January 3, 2016

Summer tires, Snow tires, All-Season tires getting you confused?

Lets talk about passenger car tires for a moment. Yes this is an RV tire blog but a vast majority of people even including RV owners probably have a vehicle that can use passenger type tires. Occasionally I also run across trailer owners that want to tow in the snow so want a "traction" tread design"

I saw a post where someone asked about "All Weather" tires. This is really a marketing term probably offered in an effort to create the impression of somehow being different than the competition.

In reality there are three types of tire when it comes to weather. Snow, Summer and All Season.

"Snow" does have a specific meaning and tires must pass certain criteria to be called "Snow" and to use the snowflake symbol. I like this definition "Tires designed for use in severe winter conditions can handle both snowy, slippery roads and low temperatures. The mountain snowflake symbol 
 indicates a winter tire meets the minimum requirements for providing traction in harsh conditions, though some tires exceed the symbol's requirements." In some heavy snow areas and mountain roads there are requirements for vehicles to be equipped with chains or for the tires to have the snowflake symbol (check with the Highway Patrol if in question).

All-Season tires are the normal tires on most new cars sold today. These tires also known as M&S tires or M+S tires. The M and S stand for Mud and Snow. They do not perform as well as real snow tires in the snow but can handle many situations where snow is present. Here is a web page that does a nice job of covering this type performance.

Finally we have "Summer" tires. These are usually  ultra high performance tires ie tires with a "V: speed or higher. I have seen some summer tires have their tread rubber shatter due to their use in extreme cold weather.

Personally I would suggest these guidelines:

1. Snow (with the symbol)  Whenever snow, ice or freezing temperatures are expected and you must travel on roads that may not be salted or plowed. These will have shorter tread wear when used in the dry
2. Ultra-high performance or Summer tires (no siping in tread blocks) - should never driven when temperatures are below about 20°F and preferably not below 35°F  These tires will have shorter tread wear in the dry
3. "All-Seasons"  Not as good snow performance as real snow tires and not as good dry performance as "Summer tires.  These are a balance and trade off and will deliver better wear mileage than either Snow or Summer tires

I found test results for the three types of tires. The tires were all from the same tire company and of the same size and used on the same vehicle. I have adjusted the results where 100 is the best. a lower rating is not as good.

Summer tire             M&S tire           Snow tire
In snow  0 to 40 mph
     28                         81                           100
In Snow  40 to 0 mph
     44                         84                            100

In Wet   0 to 60 mph
      100                     77                             94
In Wet   60 to 0 mph
      100                     74                             87

In Dry   0 to 60
       100                    100                          97
In Dry  60 to 0
       100                     92                           78
In Dry  Cornering
       100                      97                          94

 The above would seem to indicate that Summer tires are a good choice as long as you never travel when temperature is below 40°F but there are some downsides to the summer type tire. Cost and tread wear.

Cost can range by over 30% between same size tires with the summer tire usually being the most expensive. Your costs would vary depending on what size you are selecting. In some cases you might even need to change wheels to get a summer tire that was appropriate for your vehicle.

Tread wear All Season  would be best rated 100 with the summer tire  rated 80 the snow tire as low as 50 so this difference compounds the cost difference.

I hope this has been of some value to readers that need tires for their passenger car, SUV or small pickup.

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