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#1
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Trailer Tires Overheating.
Mark Browne wrote:
Yes, but now the others on this group have a better understanding of the factors involved. I am not going to bother to run the numbers but the partial pressure of any "normal" quantity of water vapor in a tire is not going to change the tire pressure by an amount easily measured by anyone outside a laboratory. It will have no significant influence. Moisture will however contribute to corrosion and oxidation at elevated temperatures in an oxygen bearing atmosphere. How much of a factor this is in an application where tires are changed every few minutes anyway is debatable. I think your racer/writer/engineer friend might be disappointed when he fails to see much, if any, measurable difference in his tire pressure. Rick |
#2
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Trailer Tires Overheating.
"Rick" wrote in message ink.net... Mark Browne wrote: Yes, but now the others on this group have a better understanding of the factors involved. I am not going to bother to run the numbers but the partial pressure of any "normal" quantity of water vapor in a tire is not going to change the tire pressure by an amount easily measured by anyone outside a laboratory. It will have no significant influence. Moisture will however contribute to corrosion and oxidation at elevated temperatures in an oxygen bearing atmosphere. How much of a factor this is in an application where tires are changed every few minutes anyway is debatable. I think your racer/writer/engineer friend might be disappointed when he fails to see much, if any, measurable difference in his tire pressure. Rick Rick, I believe that what you are neatly trying to side-step in your consideration is the presence of liquid water. If all we were talking about is water vapor, even at 100% humidity, then I would completely agree that you are right. Unfortunately there *can* be liquid water trapped inside the tire. Some of this comes from tire mounting compound, some from air compressors without suitable dryers, some from water inside the tire. This trapped water inside the tire can be standing on the surface, or inside the rubber. It is rather difficult to make a blanket statement about how much effect each source can contribute. This makes an unassailable mathematical analysis equally difficult. Not to worry - others have done it and I have read the reports. In a Formula or NASCAR setting moisture can raise tire pressure about 4 PSI in the corners. This is enough to mess up a finely tuned race car chassis. Whatever *it* is, either it works, or it does not. If *it* does not make cars go faster or safer, most people don't put a lot of time and money into it. Real race teams that have real physicists and engineers on their staff go to considerable effort to control the presence of water inside the tire. Tire moisture *is* a significant problem in racing; people worry about going into a corner at 200 miles an hour and having their car go squirrelly in the middle of the turn. I spend a fair amount of time working around race tracks and see a lot of people shoot their mouths off about how things *should* work. The nice thing about racing is that most of this stuff gets sorted out on the track. If you would like to field a car and fill the tires with normal air to prove that there is no difference, by all means go ahead. The nice thing about racing is that people that know what they are talking about go fast, and clueless people watch 'em go by. It all gets sorted out when the rubber hits the road! Mark Browne |
#3
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Trailer Tires Overheating.
Mark Browne wrote:
I believe that what you are neatly trying to side-step in your consideration is the presence of liquid water. If all we were talking about is water vapor, even at 100% humidity, then I would completely agree that you are right. Unfortunately there *can* be liquid water trapped inside the tire. Some of this comes from tire mounting compound, some from air compressors without suitable dryers, some from water inside the tire. This trapped water inside the tire can be standing on the surface, or inside the rubber. Wasn't this discussion about using nitrogen in the tires? If the racers are so particular why are they using wet air from a cheap air compressor and still worry about the effects of moisture? Why are they using practices and processes that are known to add worrisome quantities of liquid water to a component that is so sensitive to moisture? Which way do you guys want this? Perfectly predictable tire pressures will be impossible to obtain when the mounting is done with shade tree techniques as you describe. If there is so much water in a tire despite the best efforts of "real physicists and engineers on their staff (who)go to considerable effort to control the presence of water inside the tire" then something is missing in this equation. In a Formula or NASCAR setting moisture can raise tire pressure about 4 PSI in the corners. I find this a bit hard to believe. If it is water vapor it will respond exactly like the nitrogen or air and the pressure rise is due to the temperature increase of the tire. That is one of the gas laws that no one seems to want to follow. If there is liquid water in the tire a whole new set of conditions exist that are still unlikely to produce the effect you describe. Look at it another way. If you can attribute that 4 psi increase to moisture then why not control the amount of moisture in the tire ... dry the tire and add a measured amount of water so that you can predict the pressure increase and regulate accordingly? But if you can dry the tire then why worry about moisture? If you cannot dry the tire then you can measure the dew point of the filling gas and calculate the weight of the water in the tire and predict accordingly. It sounds to me like you guys are just using "rules of thumb" and are surprised when you get surprised. What is the normal tire pressure on one of those cars? Thirty - forty psig? Let's say the tire started out at 35 psi, for the water to boil away and increase the pressure of the tire to 39 psi its temperature would have to increase to somewhere around 285 degrees F. Do you run your tires at those temperatures? For the tires to increase 4 psi in the second or two of cornering due to boiling water the carcass temperature of the tire would have to be damn near glowing since heat does not transfer instantaneously to the water, nor does it cool instantaneously as the car leaves the corner. If the starting temperature and pressure of the tire in the pits was 35 psig at 80 degrees F and it heated up to 200 degrees in the corner its pressure would raise to around 46 psig ... at that pressure any water would still be water until it reached over 290 degrees ... I have not even mentioned the fact that the heat to vaporize the water comes from the tire and the other gases filling the tire. The transfer of that heat actually cools the tire and the gas ... a minute amount, yes, but the effect of water has to be an equally minute amount and I have yet to be shown the mechanism whereby "normal" amounts of water in a tire will produce the effect you describe in the conditions in which tires operate. If you can explain how race car tires somehow work differently than other machines I would love to hear it. I am very open minded but this smacks of voodoo engineering to me. Rick |
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