Hot to cold inflation to pressure differentials

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I couldnt think of a proper topic for my concern. Say you have a racecar that you are diligent about checking the pressures on. Then, say you have a minivan that only gets inspected at rotations. How come the racecar will probably show low more often than the minivan?

They are both sealed tires on proper rims. So my attention goes to the sweep (or range) a tire/wheel sees when it is run on hot asphalt. Say the racecar gets really hot and goes from 32 psi doorjamb to max sidewall of 41 when driven aggressively. While the minivan might see 35 doorjamb and who know what when hot.


I am trying to get at, that the car driven aggressively or at higher speed will get hotter, psi will go higher, and it will thus cool more and somehow air will disappear. How is this?
 
Originally Posted By: mjoekingz28
I couldnt think of a proper topic for my concern. Say you have a racecar that you are diligent about checking the pressures on. Then, say you have a minivan that only gets inspected at rotations. How come the racecar will probably show low more often than the minivan?


OK, can you please provide the examples and measurements that you are referencing, and the time frames involved.

Or are you making up a scenario (again) and asking people to justify "something" ?

Are you stating a true fact, seeking a true fact, or fishing for something ?
 
Originally Posted By: mjoekingz28
I couldnt think of a proper topic for my concern. Say you have a racecar that you are diligent about checking the pressures on. Then, say you have a minivan that only gets inspected at rotations. How come the racecar will probably show low more often than the minivan?

They are both sealed tires on proper rims. So my attention goes to the sweep (or range) a tire/wheel sees when it is run on hot asphalt. Say the racecar gets really hot and goes from 32 psi doorjamb to max sidewall of 41 when driven aggressively. While the minivan might see 35 doorjamb and who know what when hot.


I am trying to get at, that the car driven aggressively or at higher speed will get hotter, psi will go higher, and it will thus cool more and somehow air will disappear. How is this?



My answer: what you have stated as factual doesn't happen. Therefore, no explanation needed.
 
OK, first, many race tires do NOT include an innerliner - the part of the tire that retains air.

Most rubber types will hold in air, but some rubber types do a better job than others. A family of rubber called halobutyls are the best (halo, from the halogen family of chemicals: Chlorine, bromine, iodine, etc.) Halobuytl rubber is formed into a sheet that is put on as the innermost layer - called the innerliner - where it finishes the seal with the rim.

Because race tires are only required to hold air for short periods of time and thinness is an advantage, most of them do not use an innerliner, so they leak much, much more rapidly.

When I was racing, one of the first chores every day was to air up the tires. I would bleed them down to where they needed to be about an hour before any on track activity - and if there was more than one activity in a day, I checked the pressures about an hour before and adjusted as needed.

Street tires ALWAYS have an innerliner - and there isn't universal usage of halobutyl as the innerliner material - sometimes it is a mixture. So some tires do a better job of holding air than others.

- BUT -

I saw some research that indicated that MOST air loss from tires is from the seal between the tire and the rim and around the valve hole - excluding punctures - and NOT from air leakage THROUGH the tire itself.

Pressure buildup due to operational heat?

There is a rule of thumb that says that normal should be no more than 10%. Anything more than 15% requires immediate attention - slowing down, more inflation pressure, larger tire, etc. - and between 10% and 15%, some action needs to take place but you don't have to do it within the next hour. A few days is OK.

And just so everyone understand: The 10% is not a goal. It is a maximum. The way to test this is to check during an extended highway run - an hour and a half. Check before you start, then an hour into the trip, then another a half hour later as confirmation.
 
Originally Posted By: CapriRacer
And just so everyone understand: The 10% is not a goal. It is a maximum. The way to test this is to check during an extended highway run - an hour and a half. Check before you start, then an hour into the trip, then another a half hour later as confirmation.


Somewhat related, would you expect that Sun exposure would affect your PSI readings to a noticeable degree? For example, you are driving south on a sunny day for an hour in the morning, so the tires on the driver side are in the Sun while the tires on passenger side are in the shade.
 
Originally Posted By: Quattro Pete
Originally Posted By: CapriRacer
And just so everyone understand: The 10% is not a goal. It is a maximum. The way to test this is to check during an extended highway run - an hour and a half. Check before you start, then an hour into the trip, then another a half hour later as confirmation.


Somewhat related, would you expect that Sun exposure would affect your PSI readings to a noticeable degree? For example, you are driving south on a sunny day for an hour in the morning, so the tires on the driver side are in the Sun while the tires on passenger side are in the shade.



Yes, I've heard reports that a psi or 2 is common, but I don't have any personal experience on the subject.
 
Originally Posted By: Quattro Pete
Somewhat related, would you expect that Sun exposure would affect your PSI readings to a noticeable degree? For example, you are driving south on a sunny day for an hour in the morning, so the tires on the driver side are in the Sun while the tires on passenger side are in the shade.


My new car is the first one to have dashboard readouts of tire pressures. It has been interesting to watch the pressure changes related to temperature, speed and indeed, sun.

If one side of my car is facing the sun, the pressures on that side will run about 1 psi more than the shade side.
 
Originally Posted By: Quattro Pete
Originally Posted By: CapriRacer
And just so everyone understand: The 10% is not a goal. It is a maximum. The way to test this is to check during an extended highway run - an hour and a half. Check before you start, then an hour into the trip, then another a half hour later as confirmation.


Somewhat related, would you expect that Sun exposure would affect your PSI readings to a noticeable degree? For example, you are driving south on a sunny day for an hour in the morning, so the tires on the driver side are in the Sun while the tires on passenger side are in the shade.



I can tell you with certainty that this effect does happen on my PARKED car. When I walk out at the end of the day and the car has been parked north-south, the tires on the west side (in full sun for a couple of hours previously) show a pound or two higher on the dash readout than the east-facing tires.

Within 15 minutes of driving, the pressures even out, though, so I doubt you'd ever see that effect WHILE driving. The delta-T from the solar heat is quite a lot less than the delta-T between parked and driving 70 mph.
 
I will "guess" leakage at the bead to wheel interface as the bead area gets MUCH more loading and distortion on track than it would on a DD.
 
Tires have improved immensely in their ability to hold pressure, over my lifetime. 30 years ago, a weekly check of pressure would always require that all the tires be topped up.

Now, I only deal with pressure when the tires get rotated.....And then, more because the front and rear pressures are different. Sometimes, a mid winter top up, just because of the very low temperatures.

But they just do not lose air like they used to. Just sayin.
 
Originally Posted By: CapriRacer
There is a rule of thumb that says that normal should be no more than 10%. Anything more than 15% requires immediate attention - slowing down, more inflation pressure, larger tire, etc. - and between 10% and 15%, some action needs to take place but you don't have to do it within the next hour. A few days is OK.

And just so everyone understand: The 10% is not a goal. It is a maximum. The way to test this is to check during an extended highway run - an hour and a half. Check before you start, then an hour into the trip, then another a half hour later as confirmation.



Nice, thankyou.
 
Originally Posted By: Quattro Pete
Originally Posted By: CapriRacer
And just so everyone understand: The 10% is not a goal. It is a maximum. The way to test this is to check during an extended highway run - an hour and a half. Check before you start, then an hour into the trip, then another a half hour later as confirmation.

Somewhat related, would you expect that Sun exposure would affect your PSI readings to a noticeable degree? For example, you are driving south on a sunny day for an hour in the morning, so the tires on the driver side are in the Sun while the tires on passenger side are in the shade.

Originally Posted By: 440Magnum
I can tell you with certainty that this effect does happen on my PARKED car. When I walk out at the end of the day and the car has been parked north-south, the tires on the west side (in full sun for a couple of hours previously) show a pound or two higher on the dash readout than the east-facing tires.

Within 15 minutes of driving, the pressures even out, though, so I doubt you'd ever see that effect WHILE driving. The delta-T from the solar heat is quite a lot less than the delta-T between parked and driving 70 mph.

I confirm this phenomenal. I checked tires pressure of my S2000 one night, the next morning around 10-11 AM the rear tire facing the sun was 1 PSI higher. Next morning without garage opened, both rear tires had the same pressure.
 
My Sierra 1500 goes up about 4 psi on average, cold to after driving an hour in warmer temps. This is starting off with pressure at about 39 psi.

I can depend on the various factors, but 4-5 psi is what I see.
 
Originally Posted By: CapriRacer

Pressure buildup due to operational heat?

There is a rule of thumb that says that normal should be no more than 10%. Anything more than 15% requires immediate attention - slowing down, more inflation pressure, larger tire, etc. - and between 10% and 15%, some action needs to take place but you don't have to do it within the next hour. A few days is OK.


Thanks for this informative post, but I'm not quite understanding one part. I presume you mean taking immediate action if the pressure has increased by more than 15% ... so the slowing down part makes sense instantly (I assume get off the highway altogether not just slow down by 8-10mph/?)

But what did you mean by "more inflation pressure"? Are you saying that if the pressure has increased 15% adding more air will somehow help (by cooling it down??) or did you mean to say that the 15% increase might have been the result of the tire being underinflated in the first place and heating up because of that??
 
Originally Posted By: Darris
Originally Posted By: CapriRacer

Pressure buildup due to operational heat?

There is a rule of thumb that says that normal should be no more than 10%. Anything more than 15% requires immediate attention - slowing down, more inflation pressure, larger tire, etc. - and between 10% and 15%, some action needs to take place but you don't have to do it within the next hour. A few days is OK.


Thanks for this informative post, but I'm not quite understanding one part. I presume you mean taking immediate action if the pressure has increased by more than 15% ... so the slowing down part makes sense instantly (I assume get off the highway altogether not just slow down by 8-10mph/?)

But what did you mean by "more inflation pressure"? Are you saying that if the pressure has increased 15% adding more air will somehow help (by cooling it down??) or did you mean to say that the 15% increase might have been the result of the tire being underinflated in the first place and heating up because of that??


What I am saying is that the buildup in inflation pressure is an indication that excessive heat is being generated - and that's bad for a tire. By adding air you are reducing the amount of heat being generated, even though the operating pressure will have gone up. If that exceeds the maximum pressure of the tire, something else must be done - like a higher load range or a large size (that is, one with a higher load carrying capacity.)
 
Originally Posted By: CapriRacer

What I am saying is that the buildup in inflation pressure is an indication that excessive heat is being generated - and that's bad for a tire. By adding air you are reducing the amount of heat being generated, even though the operating pressure will have gone up. If that exceeds the maximum pressure of the tire, something else must be done - like a higher load range or a large size (that is, one with a higher load carrying capacity.)


Interesting, so the remedy for rising tire pressure is more pressure? It makes sense from the cooling point of view. Should one do this on the spot or stop driving amd wait for the tire to cool somewhat? Or add the cold air imediately, then wait a while, bleed it back down to what you started with than ad a few PSI and see if that makes it more stable?

With regard to changing to a higher Load Rating - if one is already running a tire that meets or exceeds the factory spec, does that mean the tire in question is no longer fit to perform up to it;s designation or that in fact the car maker spec'd the wrong tire and never bothered to disclose it?
 
Originally Posted By: Darris
Interesting, so the remedy for rising tire pressure is more pressure? It makes sense from the cooling point of view. Should one do this on the spot or stop driving amd wait for the tire to cool somewhat? Or add the cold air imediately, then wait a while, bleed it back down to what you started with than ad a few PSI and see if that makes it more stable?

With regard to changing to a higher Load Rating - if one is already running a tire that meets or exceeds the factory spec, does that mean the tire in question is no longer fit to perform up to it;s designation or that in fact the car maker spec'd the wrong tire and never bothered to disclose it?


You can add air to any tire any time. The only caveat is when the tire is really hot and there might be some issues with the tire. Safety first.

What I would suggest is adding 5 psi, then see where it ends up. What you are looking for is a buildup less than 10%.

Also, if the tire on the vehicle is per the specs, but is running too high, then there is a problem that needs to be addressed. This is very unlikely in a car or a truck, but could be a problem for trailers as it is known that many trailers weren't spec'd with large enough tires.
 
My race car's tires don't lose air at any kind of alarming rate. The difference is that I set the race car's tire pressures when it is hot, while my street cars' pressures are set cold. Typically, the weather gets hotter over the day at the track. So the race car tire might start at 28psi cold, get to 35psi hot in the morning. But as the day warms up, I bleed air out of the tires to keep them at 35psi hot. Then it goes home and sits. Next time we go to the track, the tires will need some air in them so that we start out where we started last time.

robert
 
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