Tire Pressure Leeway

[GTSeattle]

Running a Sprinter 3500 campervan with rear duallies. Since Mercedes doesn’t provide tire pressure management system, recently installed the TST valve end system on my six tires and it seems to be working great.

The only problem is now we are obsessed with our tire pressure fluctuations!

Question: What leeway on the low side and the high side of cold psi should be used to trigger alarms with the TPMS system? In other words, what kind of percentage of cold psi should cause alarm? I know there are a lot of calculations that go into properly determining your appropriate cold psi, but once you settle on that, are there any rules of thumb for when you should stop driving and let tires cool, etc?

Appreciate any insights or thoughts.

[mkguitar]

I live in AZ- it's hot.


TP is generally spec'd "cold".


If I drive to costco and my car says 36 PSI- I tell the tech that I drove 4 miles and they will generally set for 33~34 which when cool will be close to 36psi


on my tpms I set the alarm for 10 psi under the spec ( or my desired settings)


what I watch for is a gain in temps on 1 tire ( bad wheel bearing? hot brakes dragging?..why?)


or a SUDDEN loss of pressure like if I hit something which has holed the tire- that may give me a couple of minutes extra warning to get over.


try not to stare at it...


( this summer the tires on the sunny side were fully 40º hotter than the other side...)


mike

[markopolo]

Originally Posted by GTSeattle

Running a Sprinter 3500 campervan with rear duallies. Since Mercedes doesn’t provide tire pressure management system, recently installed the TST valve end system on my six tires and it seems to be working great.

The only problem is now we are obsessed with our tire pressure fluctuations!

Question: What leeway on the low side and the high side of cold psi should be used to trigger alarms with the TPMS system? In other words, what kind of percentage of cold psi should cause alarm? I know there are a lot of calculations that go into properly determining your appropriate cold psi, but once you settle on that, are there any rules of thumb for when you should stop driving and let tires cool, etc?

Appreciate any insights or thoughts.

I posted my thoughts on it here: https://www.classbforum.com/forums/f...tml#post111831

As for stopping driving to let tires cool - I haven't heard of that. My tires just seem to rise in temperature from cold and then plateau. They're internal sensors, not cap sensors though so might have less fluctuation than cap sensors.

[booster]

[QUOTE=mkguitar;118848]I live in AZ- it's hot.


TP is generally spec'd "cold".


If I drive to costco and my car says 36 PSI- I tell the tech that I drove 4 miles and they will generally set for 33~34 which when cool will be close to 36psi /QUOTE]


I must be misunderstanding this. If the vehicle had been inside Costco then I could see them setting the tire pressure lower because they would raise in pressure when they go out in the outside heat.


I don't see why they would set the pressure lower when the tires had been out in the heat and been driven on besides as that is when the tire pressure would be higher so that is what they would set.



Or are you talking about when you buy new tires? That would be the first example.


I prefer to set for the middle of the temp range for the current weather unless that would make the low end go lower than I like. I change 2psi per 10* change in the rear and 1.5psi per 10* in the front. So if it is going to be 50* outside I would increase pressure set at 70* by 4psi in the rear and 3psi in the front, so when they cool they will be at what I want.

[mkguitar]

I expressed that warm tire/psi relationship poorly and inverse of what is correct.

no coffee, up too late after a long day

I'd edit that and correct it if I could.


Mike

[booster]

Originally Posted by mkguitar

I expressed that warm tire/psi relationship poorly and inverse of what is correct.

no coffee, up too late after a long day

I'd edit that and correct it if I could.


Mike

We have all done that before, many of us quite often.

[Davydd]

My tire recommendation is 61 psi for all six tires on my ARV Sprinter. The tires are good for 80 psi so I haven't encountered a situation where they could get that high and don't worry about the upper psi. In the morning especially in these wild swinging temperature months in the fall my red warning light and beep will come on about 55 psi. I drive and it goes away in less than 5 miles and I don't worry about it. If it is persistently annoying I will adjust my tire pressure for the average weather of the time of the year. I keep my garage about 45 deg. in the winter so will adjust the tires to 61 deg. at that temperature before heading out in the Minnesota winter and they can still annoyingly drop in below freezing weather. I carry a 120v AC electric tire inflator since have AC power all the time. I can monitor the tire pressure of all the tires inside the cab. It would be a problem when driving along for awhile and the beeping and red light started up.

[wny-pat]

Tire Pressure – RV Weigh-Mobile Weigh Station

[GTSeattle]

Thanks all for your helpful posts and suggestions.

[Calson]

I have learned to follow the recommended tire pressures from the manufacturer. They know what works best. I use a 12v air compressor that puts out 150 PSI to keep the tires at the recommended pressure. Dual rear wheel is also something where the PSI is usually lower than would be the case with SRW drive.

The "cold" pressure refers to not having the air inside the tires hotter than the ambient air temperature outside the tires. After driving if the vehicle sits for several hours the air inside the tire wil be the same as the outside air and that is when the pressure gauge provides an accurate indication of whether to add air or not.

With a vehicle parked outside overnight in the morning the tires that get the first rays of sunlight will be at a higher PSI than the ones in the shade. Best to check tire pressures before sunrise or very late in the day.

Underinflatted tires have more sidewall flexing and this makes them run hotter and can lead to tire failure.

[Telegrapher]

There's something called the "ideal gas law" that describes how pressure changes with changing temperature. It says that, all other things being held constant, when the absolute temperature (as measured in Kelvins, or degrees above absolute zero) changes a given percentage, the pressure will change by that same percentage.

Consider a somewhat realistic example: Suppose you start your day with the ambient temperature of 50 degrees Fahrenheit, and you've been parked all night, so the tires are at that cool 50 degree temperature. Then, later in the day, you're driving on warm pavement in the sun, and the ambient temperature has risen, so that your tires are now at a temperature of 100 degrees Fahrenheit. That 50 degree swing in temperature is approximately a 10% difference in the absolute temperature, so it will produce about a 10% swing in pressure. If you started at 50 psi in the cool morning, you'll be at about 55 psi in the warm afternoon. Or making a further approximation, each ten degrees of temperature change causes approximately a 1 psi pressure change, if you're starting at a baseline of 50 psi.

I can imagine days when the temperature swing will be much wider than this example.

The tire makers specify "cold inflation pressure", but they don't say exactly how cold it should be, and they don't expect you to adjust inflation pressure from one week to the next, as cold fronts and warm fronts pass.

My own policy is that I don't get excited over a pressure change that's less than 10% of the recommended inflation pressure. Yes, when I check and adjust inflation pressure, I get it as close to perfect as I can, but I realize it's going to change as temperature changes, so I don't stress about that, and I normally don't readjust it more than once every month or two, sometimes longer. On the average, I want the pressure to be nearly right, to keep the tire wear even. I especially want to avoid serious underinflation, because it causes excess sidewall flex, poor, handling, and excessive heat. But a few psi variation here and there is normal, and nothing to worry about.

BTW, there's almost a 5psi difference in ambient pressure between sea level and 10,000 feet of altitude. I've driven higher than that. I did not adjust my tire pressure. Theoretically, perhaps I should have, but it was close enough, and life's to short to obsess over tire pressure.

Measure it with a micrometer, mark it with chalk, cut it with an axe.

[Calson]

It is not cold tire pressure in the sense that the "tire" needs to be at any given temperature. The air inside the tire will get hotter as the tire rotates and the sidewall flexes along the road. The tires on my vehicles will be at least 5 PSI higher after 40 minutes of my driving down the freeway.

It does not matter if the air temp is 55 degrees or 80 degrees when I start out as the air pressure inside is relative to the air pressure outside (on the other side of the wall of the tire). So long as the air inside the tires is at the same temperature as the air outside the tire then any pressure reading will be accurate and can be used to indicate if the tire might need more or less air inside.

Tire manufacturers know all of this and they also understand that a tire supporting a given load and driven at highway speeds is going to generate a certain amount of heat and the design of the tire needs to allow for that without its failing.

The problem with the Bridgestone tires on the Ford Explorers was the result of the specification being changed to deal with suspension problems with the SUV and that resulted in lower recommended tire pressures and if the SUV was driven at speed with a full passenger load the tire was likely to fail completely.

Not all that complicated. If the tires on my SUV are supposed to be at 35 PSI per the vehicle manufacturer then I verify the tire pressures when the truck has been sitting in my driveway and not after driving it for an hour and then checking the pressures. When I put in air using a compressor (a hot fill) I will put in an extra 3-4 PSI and then wait and hour and check the tire pressure and if it is too high I will let our a little air. Usually when the hot air from the compressor that is in the tire has cooled to ambient temperatures the pressure is at the right level for the tire.

[booster]

Originally Posted by Telegrapher

There's something called the "ideal gas law" that describes how pressure changes with changing temperature. It says that, all other things being held constant, when the absolute temperature (as measured in Kelvins, or degrees above absolute zero) changes a given percentage, the pressure will change by that same percentage.

Consider a somewhat realistic example: Suppose you start your day with the ambient temperature of 50 degrees Fahrenheit, and you've been parked all night, so the tires are at that cool 50 degree temperature. Then, later in the day, you're driving on warm pavement in the sun, and the ambient temperature has risen, so that your tires are now at a temperature of 100 degrees Fahrenheit. That 50 degree swing in temperature is approximately a 10% difference in the absolute temperature, so it will produce about a 10% swing in pressure. If you started at 50 psi in the cool morning, you'll be at about 55 psi in the warm afternoon. Or making a further approximation, each ten degrees of temperature change causes approximately a 1 psi pressure change, if you're starting at a baseline of 50 psi.

I can imagine days when the temperature swing will be much wider than this example.

The tire makers specify "cold inflation pressure", but they don't say exactly how cold it should be, and they don't expect you to adjust inflation pressure from one week to the next, as cold fronts and warm fronts pass.

My own policy is that I don't get excited over a pressure change that's less than 10% of the recommended inflation pressure. Yes, when I check and adjust inflation pressure, I get it as close to perfect as I can, but I realize it's going to change as temperature changes, so I don't stress about that, and I normally don't readjust it more than once every month or two, sometimes longer. On the average, I want the pressure to be nearly right, to keep the tire wear even. I especially want to avoid serious underinflation, because it causes excess sidewall flex, poor, handling, and excessive heat. But a few psi variation here and there is normal, and nothing to worry about.

BTW, there's almost a 5psi difference in ambient pressure between sea level and 10,000 feet of altitude. I've driven higher than that. I did not adjust my tire pressure. Theoretically, perhaps I should have, but it was close enough, and life's to short to obsess over tire pressure.

Measure it with a micrometer, mark it with chalk, cut it with an axe.

I think you may want to read this discussion on tire pressures and how elevation/altitude affect the actual internal tire pressure as it is not what most of us have always been told.


Bottom line is that elevation does not change tire pressure, it just changes how the gauge measures it.


https://www.classbforum.com/forums/f...pms-10445.html

[parkgt]

After many years of traveling from the flat lands to 12,000 in Colorado I can attest to pressure changes no matter what the gauges say.

Low pressure ATV tires measuring 4.5 psi down low will be way to hard at 12,000 feet and have to be deflated back down to ride correctly. Those same tires set to 4.5 PSi at 12,000 feet will be near flat on return to low lands.

Bag of chips bought at 600 feet will be ready to burst open at 12,000 feet.

[booster]

Originally Posted by parkgt

After many years of traveling from the flat lands to 12,000 in Colorado I can attest to pressure changes no matter what the gauges say.

Low pressure ATV tires measuring 4.5 psi down low will be way to hard at 12,000 feet and have to be deflated back down to ride correctly. Those same tires set to 4.5 PSi at 12,000 feet will be near flat on return to low lands.

Bag of chips bought at 600 feet will be ready to burst open at 12,000 feet.

Those are kind of apples to oranges comparisons when next to vehicle tires.


ATV tires are more like a balloon than they are a vehicle tire that is belt reinforced.



Vehicle tires have a relatively constant volume in them and the perfect gas law mentioned in an earlier post by others says plainly that as long as the temp doesn't change, the pressure will not change at any fixed volume.


What you feel with low pressure tires as "hardness" is not much of an internal pressure change, if any, unless the tire can stretch like a balloon and I don't know if those tires are that flexible or not. If the volume did change, it would get bigger not smaller, so the internal pressure would be reduced, not increase.


When you push on the sidewall of the tire, that sidewall is seeing the internal pressure of the air in the tire pushing out and external atmosphere pressure pushing in plus your hand pushing. At see level you will about 14.7psi of atmospheric pressure and your tire gauge reads the difference between the air pressure in the tire and ambient atmosphere, so 14.7 plus 4.5 psi or 19.2psi. To move the sidewall in by pushing on it, you are only pushing against the 4.5 psi as the 14.7psi atmosphere is helping. Go to an elevation that is 5psi lower in atmospheric pressure and the 19.2psi in the tire is only being pushed back by 9.5psi of atmosphere so to push in the sidewall the same amount you would have to push with 9.5 pounds with your hand and it will feel twice a hard to push.


Now move to how tires work to support the load of the vehicle. Tires sit on the road surface, not atmosphere, so the full internal tire pressure is pushing down on the road surface through the tire patch on the road. The weight carried would be the pressure times the patch area, allowing for clear area where there is no contact because of tread. The weight of the vehicle doesn't change appreciably with elevation, so the contact patch will also not change size with elevation either unless the tire has a very amount of open area on the tire patch area like V lug tires might have as they allow atmosphere to be under the tire helping support the load and that atmosphere pressure will change with elevation. On a very low pressure, heavily lugged, tire you might feel more harshness in the ride, but on a slick nothing in the contact area would change so ride would stay the same. On a high pressure load range E tire at 70-80psi, and minimal area with no contact, you would have very little ride change.


It would be interesting to measure the tire contact patch for the 4.5 psi tires at sea level and 12K feet to see what it looks like at each and what it looks like if you let the extra 5psi an atmospheric referenced gauge will tell you to get to a 4.5psi reading again.


As a point of reference, and from the link I posted earlier, factory TPMS systems almost all use absolute pressure reference, which means they measure they are reading the tire pressure you would see on your gauge plus atmosphere. That is why they will not give you an alarm when you go up to high elevations as that internal pressure doesn't change as you go up. I don't ever recall an owners manual that said to adjust tire pressure for elevation, just temperature if anything said at all.


Bottom line, I think, on the 4.5psi tires is that you might feel a bit less hit if you let out the 5 psi difference in pressure on sharp bumps that are using the tire for a spring, but then you will underinflated if you go down a smooth road and might be squirelly or hot tires get hotter than they should. On van or car tires, it is best to just leave them as is up or down in elevation.

[JonK]

The idea that psia determines the contact patch with the road due to the road surface having no air pressure strikes me as perhaps a bit off. The example of the ATV tire being harder at high altitude to me makes the case that psi is what matters, the trail will experience just as much resistance as your thumb to the "harder" tire at altitude since presumably when your thumb is against the tire there is supposedly no air pressure there, it's just a different kind of contact patch. But, either a contact is a contact patch or something else is going on other than psia.

The major cause of confusion here is the starting pressure, about 5psig for the ATV versus around 60psig for our B. If we assume 5psi atmospheric pressure decrease that means the pressure differential between the inside and outside of the ATV tire has doubled, as we can feel by pressing it with our thumb. Conversely for our B, the change in the inside versus outside pressures would be less than 10%, and it too would change the contact patch though I doubt most of us would notice.

Ultimately, the tire companies give us pressures in psig for a given tire load on a given tire, our tire gauges are psig, and it all works as proven by tire longevity. If I go up in elevation and back down, I see no need to adjust pressure because it doesn't change enough for long enough to constitute a hazard.

[booster]

Originally Posted by JonK

The idea that psia determines the contact patch with the road due to the road surface having no air pressure strikes me as perhaps a bit off. The example of the ATV tire being harder at high altitude to me makes the case that psi is what matters, the trail will experience just as much resistance as your thumb to the "harder" tire at altitude since presumably when your thumb is against the tire there is supposedly no air pressure there, it's just a different kind of contact patch. But, either a contact is a contact patch or something else is going on other than psia.

The major cause of confusion here is the starting pressure, about 5psig for the ATV versus around 60psig for our B. If we assume 5psi atmospheric pressure decrease that means the pressure differential between the inside and outside of the ATV tire has doubled, as we can feel by pressing it with our thumb. Conversely for our B, the change in the inside versus outside pressures would be less than 10%, and it too would change the contact patch though I doubt most of us would notice.

Ultimately, the tire companies give us pressures in psig for a given tire load on a given tire, our tire gauges are psig, and it all works as proven by tire longevity. If I go up in elevation and back down, I see no need to adjust pressure because it doesn't change enough for long enough to constitute a hazard.

I don't think there is conflict in what you say compared to what I wrote. Yes the the differential between the pressure inside and out side the ATV tire is double, as I also said and that is why the tire feels a lot harder when you push on it.


On the thumb vs the contact patch, yes you are correct there would be no pressure on the tire under you under your thumb, but the air pressure will be pushing on the backside of your thumb so that force is still there. But even if the pressure wasn't still there you would still have a big force difference to push on the tire and feel it is harder because the tire defects over a much larger area than you thumb when you push on it and all that area would see the atmosphere in both sea level and altitude conditions.


I agree on a 60psi tire, 5 psi would not be all that bad, but if it also got cold after you let the 5 psi out, you could easily be 10 psi or more off of what you want, if it stayed cold when you went down to low elevations.



For most of us, the biggest issue probably would be the psig reading TPMS systems going off all the time at high elevations if the pressure limits were set pretty tightly. As mentioned it is not an issue for the factory TPMS systems because they read absolute pressure, and that won't change.