Tire pressures...again!

[mike5511]

I got my rig weighed by Trey Selman, RV Safety, at the HDT Rally in Hutch, KS. Weighed each wheel position and had a good discussion while looking at the air pressure/weight tables. Very educational and well worth it! My only advice here is, never, ever under inflate! If you don't know your weights, just air them up cold to the max listed on the sidewall........but it is worth your while to learn what your actual weights are and adjust the pressure accordingly.

[Msmoto]

Truck/RV tires drop about 2 psi per 10 degrees Fahrenheit in ambient temperature. However, once you begin driving when it is 20 degrees they will warm up quickly to normal operating temperature and be inflated properly.

https://www.fleetequipmentmag.com/un...-air-pressure/

[Tireman9]

Originally Posted by Msmoto

Truck/RV tires drop about 2 psi per 10 degrees Fahrenheit in ambient temperature. However, once you begin driving when it is 20 degrees they will warm up quickly to normal operating temperature and be inflated properly.

https://www.fleetequipmentmag.com/un...-air-pressure/

Yes tire pressure changes by about 2% for each change in temperature of 10F.


Please tell me you are not depending on tires to heat up to get to your goal tire inflation pressure. Tire pressure in the charts are never the hot running temperature but the cold (at ambient) temperature.

[wolfe10]

Originally Posted by Tireman9

Yes tire pressure changes by about 2% for each change in temperature of 10F.


Tire pressure in the charts are never the hot running temperature but the cold (at ambient) temperature.

YUP, PV=nRT (the Ideal Gas Law) is REAL and was "discovered" more than a century and a half ago. Said another way, physics is physics.



Change temperature with other factors remaining the same and you WILL change pressure. Tire engineers work with this on a daily basis.


The only exception I know of is tire SALESMEN who want to sell you Nitrogen. They are unique in the universe in that they "feel" they can defy physics (ya, stepping off soap box now).




And absolutely-- cold when discussing tire PSI is AT AMBIENT TEMPERATURE, before driving and sun not shining on it.

[RVPioneer]

I’ll never understand why tire pressure is so hard to understand.

[Msmoto]

Originally Posted by Tireman9

Yes tire pressure changes by about 2% for each change in temperature of 10F.


Please tell me you are not depending on tires to heat up to get to your goal tire inflation pressure. Tire pressure in the charts are never the hot running temperature but the cold (at ambient) temperature.

I check my pressures at about 60 degrees in a heated garage during winter travel, but run slightly higher pressures than given by RV data. In my LTV, cold, ambient pressures are 63/65 f/r. In summer these are checked with ambient temp of from 70 - 80 degrees as garage is not air conditioned but I will go 64/66 if at 80 degrees. I like to look at tire manufacturers data based on load, then ad about 10 % for additional safety factor but never exceeding or actually coming close to maximum pressures on sidewall.

Hope this relieves any anxiety.... LOL

Oh, BTW, I have checked tire temps with IR thermometer, especially from side to side to detect other issues. When I used to run tire pressure monitors on my Class A Moho, I think I saw temps of 120 as cold pressures were I believe as high as 110 psi.

[KanzKran]

Originally Posted by Msmoto

Truck/RV tires drop about 2 psi per 10 degrees Fahrenheit in ambient temperature. However, once you begin driving when it is 20 degrees they will warm up quickly to normal operating temperature and be inflated properly.

https://www.fleetequipmentmag.com/un...-air-pressure/

From the article: "Those load/inflation tables are all based on an ambient temperature of 70 degrees F."

I've seen mention of tables being based on 70F ambient, but for the life of me, I don't see how that's in any way relevant. If it takes 100 psi to support the load with the tire shape and contact patch the tire manufacturer designed for at a particular load, why would it make any difference what the temperature is?

If it's 0F ambient when setting out, it takes 100 psi (a made up number). If it's 90F first thing in the morning, it still takes 100 psi. You may have to adjust the pressure before departing after a big temperature swing. That's common for me as I leave snow country one day and am in south Florida or Texas two days later, and especially vice-versa where I have to air up in the morning.

But why are inflation tables 'based on 70F ambient', and what difference does that make? Or is that just something the author misunderstood?

Also from the article: "Having said that, you need to be aware that cold weather can create a different challenge. As an example, a tire that has cooled down after running all day and measures 100 PSI at 70 degrees F will lose pressure if it sits out overnight and the temperature drops to 20 degrees F. When the tire is checked, you will find only 90 PSI in the tire because when the temperature drops, so does the tire pressure. Every loss of 10 degrees F equals a loss of two PSI. You may think that the tire is underinflated but is not. As soon as it warms back up to 70 degrees F, the tire pressure will be back to the specified 100 PSI."

That author seems to be suggesting running at inadequate pressure while it warms up. Or maybe that if you delay departure until it warms up, it'll be where it needs to be. I can't quite figure out what Mr. Cohn is saying here. Regardless, if my pressure get too close to the 100% table values for my axle loads, they get air before departing, but it takes a pretty good ambient swing, as I air them to 110% normally.

[RVPioneer]

^^^The author of that article is a buffoon.

[wolfe10]

Originally Posted by RVPioneer

^^^The author of that article is a buffoon.

He is dangerously INCORRECT.


This is no place for "alternate facts"!

[brobox]

Unbelievable how much incorrect information has been given in this post. If MH owners are airing their tires from some of the advise given in this post, I am surprised there are not more blowouts than what we hear about. Read Tireman's posts and forget about the rest. He is a certified expert, the others are "I heard it on the internet."

[garysfb]

When I first posted several weeks ago I had no idea how much ego, emotion and testosterone would come into play. And way too much time spent on a fairly simple concept. Tireman you rock, common sense mixed with facts and experience.

So following the KISS method here is my takeaway. Weight the coach fully loaded for a trip including fuel, DEF and water, self and DW. Determine pressures based on tire manufacturers chart. Add 10 percent, but not to exceed tire manufacturers max pressure's based on max loads. Check pressures prior to departure and don't worry about a few pounds low because of the 10 percent cushion. Maybe in extremely hot ambiant temp bleed off a couple pounds, but that in my experience would be rare. And you're done.

Once pressures have been set, all you do is check the TPMS and go. Simple!

[drwaynesmith]

Simple, yes, correct, no. "Over-inflate your tires by 10 percent at whatever temperature you happen to call home, and then forget about it wherever you travel" is not correct. If the tires should be overinflated by 10%, the manufacturer would say so. Their tables already take into account typical daily temperature fluctuation and tire heating.

Inflate to the correct pressure and keep it that way. If you travel from 90 degree southern summer weather to 30 degree northern winter weather, your tire pressures will drop by over 10 psi. Add air to bring them up to the correct pressure when you are north in the winter. When you return south, remove the extra 10 psi. Don't over-inflate by 10 psi the entire time you are south just because you might go north some year.

Many people don't see major temperature changes in their travels, they tend to follow the temperature. So they don't need to adjust pressure much, if at all. But if you go from southern summer to northern winter, you do need to add/remove air when temperatures change that much. Don't just over-inflate all the time.

Originally Posted by garysfb

When I first posted several weeks ago I had no idea how much ego, emotion and testosterone would come into play. And way too much time spent on a fairly simple concept. Tireman you rock, common sense mixed with facts and experience.

So following the KISS method here is my takeaway. Weight the coach fully loaded for a trip including fuel, DEF and water, self and DW. Determine pressures based on tire manufacturers chart. Add 10 percent, but not to exceed tire manufacturers max pressure's based on max loads. Check pressures prior to departure and don't worry about a few pounds low because of the 10 percent cushion. Maybe in extremely hot ambiant temp bleed off a couple pounds, but that in my experience would be rare. And you're done.

Once pressures have been set, all you do is check the TPMS and go. Simple!

[RVPioneer]

Here we go again.

[drwaynesmith]

Originally Posted by RVPioneer

Here we go again.

Somebody had to say it. Truth dies in silence. Not sure why "inflate your tires to the correct pressure for the weight they are carrying" is so controversial. This is about temperature fluctuation though, I think people who haven't lived in the north, like you in Oregon, haven't experienced what happens to tire pressure when it gets cold in the winter. Up north, most everyone knows you have to add air in the winter, let it out in the spring. Actually though, with car tires, some people prefer to let it run flat in the winter, for better snow traction. That might be OK for a squishy car tire, but I wouldn't recommend it for a 110 psi steel sidewall truck tire. Unless you like the adventure of a sidewall blowout when things warm up again.