Tire pressure - cold to warm climate

[Piker]

So every 10° of ambient temperature changes cold inflation pressure on our rig approximately 2 psi. (225-70R19.5 G load range rear - F load range front)



If I leave western PA on a January morning when it's 10° and end up in Savannah GA that night where it's 70°, the cold inflation pressure is going to be 12 psi over where I want it the next morning. Since the duals need to run around 105 psi cold because of the load on the rear axle, this actually would put me over the max cold inflation of 110 once I'm down south.



Maybe I'm splitting hairs... but what's the right thing to do here?


-cheers

[TonyMac]

I don't think you're splitting hairs, I'd start my day at the right pressure. We have some OTR guys here who could give you a professional opinion, but that's what I'd do. I'd never start the day over my max inflation pressure, that's simple. So why not take it down to the right pressure? OTR guys, what say you?

[vettenuts]

I have mine set right now assuming it will be 15 degrees when we leave New England (every 10 degrees results in approximately 2% change in pressure). I am figuring on the third morning of travel I will need to spend the time in the morning to adjust all my cold pressures.

[60sumtin]

iRV2 member Tireman9 is a retired tire engineer who has written an extensive ongoing blog on RV Tire Safety and within that blog 49 articles are related to inflation. The October 20, 2017 article may be of interest to you.

"In 2014 I drove Ohio > OR > Calgary > Yellowstone >OH over a seven-week period with temperatures ranging from 90's to snow and elevation of 20' to 8,000' and never had to adjust inflation."

[Gary RVRoamer]

this actually would put me over the max cold inflation of 110 once I'm down south.

You misconstrue the tire sidewall markings. The 110 psi is not the max allowed pressure - it is the minimum pressure needed to support the max load rating. Your tires are easily capable of handling 10% more than that, and probably 20%. Tire & Rubber Association standards pretty much assure that regardless of tire brand or model.


That said, it is appropriate to reduce the pressure somewhat if you know you will remain in the warmer climate for several days. If you check tire pressure every morning, you can always adjust if you feel you need to optimize, but it's not a necessity (see the Tireman9 article mentioned above). Do not adjust downward if you will be heading towards a colder climate. Wait till you get there.

[Piker]

Originally Posted by Gary RVRoamer

You misconstrue the tire sidewall markings. The 110 psi is not the max allowed pressure - it is the minimum pressure needed to support the max load rating. Your tires are easily capable of handling 10% more than that, and probably 20%.

Ohhhhhhhh!! That changes things doesn't it? Thanks for the info!



We run a TPMS, so tire pressure is something that I'm paying attention to all the time now. When we first got our rig, the first thing we did before we even got it home was get 6 brand new tires on it. The Goodyear shop went by the placard inside that gives tire size, load rating, and pressure and ended up putting the same load range F tires back on it... I found out later after doing some reading that we should probably weigh our rig... turns out our motorhome does not weigh anything near what the placard says. Unladen weight is said to be 16,000, but completely empty (except for me in it) it's right around 19,000 - 13k on the rear and 6k in the front. (There are no weight adding modifications to the RV - the placard is simply a lie) Gross weight is supposed to be limited to 19,800, but that simply doesn't work for the 4 of us if we want to take things like water and food and clothing with us when we travel for vacation. The actual axle ratings from Rockwell (Meritor) are 15k and 7K respectively.

Before our last trip I weighed the RV completely loaded and it was just under 21,500 with 6 people on board - the rear axle accounted for 14,300 of that - and this was BEFORE any water was transferred from the potable tank (half way between the front and rear axles) to the black and grey tanks which reside BEHIND the rear axle. F load range tires were simply not enough. I'm glad I checked...

We now have G load range on the back and feel better about the whole situation. I have been running 105 psi to cover 14,460 lbs over the rear axle per the tire mfg's load/inflation table. But I guess realistically it would be better to just run at 110 psi, which would be about 5% over and provide a little wiggle room?

The front should theoretically have G load range on it as well to cover some wiggle room, but I just had new F's put on the steer axle in 2017. When the RV is fully loaded, the tires are also fully loaded, so I keep them at 95 psi cold.

Now to my next question... while driving, tire pressure increases. Is there a percentage of increase that I should not allow the pressure to exceed while driving? The TPMS we use was originally set to alarm at 10% over the set point... the alarm was going off all the time when we were running across the arizona desert in July. I went to 20% over, and it was still pushing the limit in really high ambient temperature situations so I went to 25% on the alarm. This was with the F load range tires - they would easily get up around 118 psi in hot weather. I would slow down to keep the tires from getting any hotter and the pressure from going up any more.

I'm thinking about our situation heading south to florida in January... if I start at 110 cold, I could have 122 cold by the time I get into 70 degree weather... add normal pressure increase from driving and I could easily go over the 132 psi (20%) mark. Maybe I'm not thinking about this correctly either?

I have not put any miles on the new G load range tires yet to see how they react regarding pressure. I presume that the F load range tires were overloaded and probably ran hotter... and thus pressure went up more?

cheers

[MN_Traveler]

I might be the odd person out here - but I will say the following. The point of the tire pressure vs load charts is to define the minimum pressure needed to support that load. does not matter how you get to that setting, or at what temperature (the charts say nothing about temperature). The point of the morning "cold tire inflation" recommendation is an assumption that from that point on the tire pressure will always rise (temps go up during the day, and the tires get warm due to road friction). thus it is "safe" to check the inflation during the morning and set it to the minimum. this common recommendation, however, completely misses the potential use of TPMS.

I have left MN when it was something like ten below, and by the time I hit southern IL, the temperatures were substantially warmer ... enough that my TPMS was alarming for an over-pressure. Granted, the tires could have taken the pressure, but the ride was getting noticeably harder and handling degrading ... so I bled the tires to adjust the pressures in all tires downward to where I know they typically ride when warmed up (cold pressure is usually 85-90 psi, running temp is usually something like 100-105psi). Heading south, temps continued to rise, and pressures in the tires were fine.

Coming back was exactly the opposite, southern Missouri was around 45 degrees, and my tire pressures started the day at 90 psi. By the time I hit Minnesota, the temps were pushing -10 again ... and I watched as my running pressures continued to drop ... pushing the 85 psi min (while running!). I was sweating bullets: did not want to stop running for long enough for the pressures to drop. I did NOT want to get out in the -10 weather to air up the tires. The last example is given to drive home that IF I had started that day with the tire pressures at their recommended 85 min psi, the "start the day with the cold tire inflation pressure" would have failed, and I would have been running on tires that were under pressure when I hit the colder weather back in MN.

Bottom line: if you have a TPMS, you can be aware of what the tire pressures are, and easily adjust the pressures in the tires .. upwards or downwards, during the day, as long as you do not fall below that minimum inflation vs load limit from the inflation charts.

I will likely get flamed for this because it is not common practice ... but I think this is just the physics of the situation.

[Mr_D]

Originally Posted by Gary RVRoamer

You misconstrue the tire sidewall markings. The 110 psi is not the max allowed pressure - it is the minimum pressure needed to support the max load rating. Your tires are easily capable of handling 10% more than that, and probably 20%. Tire & Rubber Association standards pretty much assure that regardless of tire brand or model.


That said, it is appropriate to reduce the pressure somewhat if you know you will remain in the warmer climate for several days. If you check tire pressure every morning, you can always adjust if you feel you need to optimize, but it's not a necessity (see the Tireman9 article mentioned above). Do not adjust downward if you will be heading towards a colder climate. Wait till you get there.

Been posting that info for years!

[Piker]

Wait a minute... let me play devil's advocate here...

I always knew that the 110 psi number on the sidewall wasn't the maximum pressure pressure the tire could handle... I've always known that pressure goes up from friction/heat when you drive. I just figured it was the maximum COLD pressure you'd inflate the tire to. In essence, since the tire's max load capacity is at the cold inflation number on the sidewall, then why would there ever be a need to put more air pressure inside a cold tire than that figure? If that's true, then the number does kind of represent a "max" cold inflation value? Again, maybe I'm overthinking this.

If you don't go any further than saying the number on the sidewall is the minimum pressure needed to support the maximum weight, then don't you open up the door to put ANY pressure into the tire that you desire as long as it's greater than the cold inflation number on the sidewall?

Are the tire pressure/loading charts the same? They recommend a minimum pressure to support a certain weight? Ok, then I'd like 200psi in my tires cold? Obviously that doesn't make sense...

The trouble here seems to be that there is not really a max psi rating when the tire is in use and up to temp. I guess I had always assumed that the cold inflation numbers vs load capacity were established as maximums so that you'd never see pressure climb above a dangerous level under even extreme conditions. I mean really, before TPMS, who ever worried about how much pressure was in their tires after they got hot?

Again, maybe I'm overthinking this, but it seems like there's a lot of grey area here. If I knew what to set the alarm on my TPMS for high pressure I'd feel alot better.

Note: not at all trying to be a troublemaker here... just trying to understand.

-cheers

[Crasher]

Originally Posted by Gary RVRoamer

You misconstrue the tire sidewall markings. The 110 psi is not the max allowed pressure - it is the minimum pressure needed to support the max load rating. Your tires are easily capable of handling 10% more than that, and probably 20%. Tire & Rubber Association standards pretty much assure that regardless of tire brand or model.

You are probably correct just like a 20,000# axle can occasionally carry 22,000# without a problem. When I contacted Michelin a couple years ago, the tech told me that the sidewall psi was in fact the minimum pressure needed to carry the maximum load, however, he added that it was also the maximum cold pressure the tire should have. I'm not sure why it is ok to see 18% higher than that when the tire is at heated running temp and not ok to start out above the minimum cold psi.

[jackreno]

Originally Posted by Piker

(225-70R19.5 G load range rear - F load range front)



If I leave western PA on a January morning when it's 10° and end up in Savannah GA that night where it's 70°, the cold inflation pressure is going to be 12 psi over where I want it the next morning. Since the duals need to run around 105 psi cold because of the load on the rear axle, this actually would put me over the max cold inflation of 110 once I'm down south.


-cheers

You are running to much pressure on your tires, 80 to 85 lbs in front and 90 to 95 lbs in back, you will still have enough carrying weight for your motorhome unless you have it packed with rocks..


Your ride will be softer as well..

[Piker]

Originally Posted by jackreno

You are running to much pressure on your tires, 80 to 85 lbs in front and 90 to 95 lbs in back, you will still have enough carrying weight for your motorhome unless you have it packed with rocks..

Your ride will be softer as well..

Ok... now I'm lost. If my front axle has 7000 lbs on it when we're loaded up, and my tire chart says single tire at 90 psi can carry 3450 (6900 total) then am I not in the ball park?

Same with the rear axle... tire chart says in a dual setup at 105 psi each can carry 3615. That's 14,400 total... and loaded up our rear axle is 14,300.

What am I missing?

Empty... yes that would be more pressure than necessary.

-cheers

[MN_Traveler]

Originally Posted by Piker

The trouble here seems to be that there is not really a max psi rating when the tire is in use and up to temp. I guess I had always assumed that the cold inflation numbers vs load capacity were established as maximums so that you'd never see pressure climb above a dangerous level under even extreme conditions. I mean really, before TPMS, who ever worried about how much pressure was in their tires after they got hot?

I think I dont understand what you are trying to say. If you are saying the load chart indicates maximum inflation, then if they say, say 85 psi, that would mean 65 is ok (which does not make sense, because then you could to 45, 35,etc ... which would be BAD).

Agreed ... the grey thing here is that there really is no where (at least that I have seen) that an absolute maximum pressure a tire can take (cold or hot) does not appear to be published anywhere. the best I think we have is something like the sidewall max pressure ... then adjust up 10-20 psi and assume that is the max for the tire?? Agreed - it would be really nice to be able to set the TPMs to that pressure as a max.

On the other hand (and I dont know if others have the same experience) my tire load pressure is 85 psi. When I had my rig new I ran the max sidewall pressure of 120 psi. The thing was extremely squirrely (sp?). reducing the pressure make the thing a dream to drive. So ... maybe the issue with high pressure is more on the handling front?

[Piker]

Originally Posted by MN_Traveler

I think I dont understand what you are trying to say. If you are saying the load chart indicates maximum inflation, then if they say, say 85 psi, that would mean 65 is ok (which does not make sense, because then you could to 45, 35,etc ... which would be BAD).

LOL... yes that would be BAD. What we have here, is a "failyah to communicate..." I did not mean to imply that the number on the sidewall is the maximum pressure... but rather its both the minimum pressure to carry the maximum load, and also the maximum COLD inflation pressure that you would ever set your tires at to carry the maximum load because additional pressure will not yield additional load capacity beyond that point. Does that make sense? Kind of like the maximum of the minimum. Wow, we're really blurring lines here now...


If I'm maxing out a tire on it's load capacity, and it says 110 on the sidewall cold inflation to carry that load... do I put 150 psi in the tire? No, I put 110 in. If I put more than 110 in, can I carry additional load? No, so why would i put additional pressure in.



Here's what I want from the tire manufacturer... I want a calculator online where I can input my tire's loaded weight, it's configuration (dual or single), and that spits out three pieces of information:
1) recommended cold inflation pressure for the specified load.

2) tolerance on that recommended cold inflation pressure (ie ±X psi)
3) the tire's maximum allowable pressure under a working load.


Simple.


Right now, I'm told what minimum cold pressure i need to carry a certain weight, but not specifically how much over I can or should go... there should be a tolerance? Maybe there is and I have missed it somewhere?



-cheers