Airing down/up hot vs cold temps.

[Desmolicious]

First off, my search-fu must be off because I tried to look for the answer to this question and just got bogged down...

I've always been told to use cold tire temps as the baseline - the 'accurate' temp if you will.
My Bronco HE was delivered with 43psi all round... I lowered that to 39 cold initially after reading the owner' manual.
After it still riding like it has no suspension - and seeing that Goodyear recommends 36 psi as max - I lowered it to 36 psi cold.

After I drive for a little while, the tire temps obviously increase, and as such so do the pressures. My 36 cold becomes 39 hot.

So, the point of this - airing down when off roading. My tires are now hot and I want to air down. A few psi difference does not really matter off roading at the slow speeds I will be doing, so an indicated 20 or 22 psi would be fine. But when I air up to get back onto paved roads, do I air up to an indicated 36 or higher to match the hot tire temps?

Shouid I just try to maintain the same psi no matter the temperature?

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[Techun]

If everything is hot and I'm inflating I would inflate to 39, in hopes that would result in a cold 36 like you said.

It may or may not...there are some variables and you may have to adjust later if you care.

 

[DALOLA]

The "hot & cold" is referring to the tire, not the ambient temps. A "cold" tire is a tire that has not been driven in a time frame to have sufficiently heated the air inside. A "hot" tire is one that has recently been driven enough to raise the temperature of the air inside, vs ambient. A cold tire is typically whatever the ambient temp is, and a hot tire is 10-40 degrees higher, as the flexing of the tire during operation heats the air inside. So, pending your activity, you can compensate for the pressure accordingly.

 

[Desmolicious]

The "hot & cold" is referring to the tire, not the ambient temps. A "cold" tire is a tire that has not been driven in a time frame to have sufficiently heated the air inside. A "hot" tire is one that has recently been driven enough to raise the temperature of the air inside, vs ambient. A cold tire is typically whatever the ambient temp is, and a hot tire is 10-40 degrees higher, as the flexing of the tire during operation heats the air inside. So, pending your activity, you can compensate for the pressure accordingly.

Thanks, but I guess I did not explain myself properly. Cold tires to me - and you! - is at the beginning of the day when the truck has not been used for hours. Hot is when they have been driven on for at least 30 minutes - unless you do burn outs then it will be a lot shorter!

But I understand what you are saying - if my 'hot' psi was 39 (started cold at 36), then when I air up/reinflate before hitting paved roads I should air up to 39 to be at the same 'hot' psi as the tires have heat in them.

 

[DALOLA]

Thanks, but I guess I did not explain myself properly. Cold tires to me - and you! - is at the beginning of the day when the truck has not been used for hours. Hot is when they have been driven on for at least 30 minutes - unless you do burn outs then it will be a lot shorter!

But I understand what you are saying - if my 'hot' psi was 39 (started cold at 36), then when I air up/reinflate before hitting paved roads I should air up to 39 to be at the same 'hot' psi as the tires have heat in them.

Yeah, we're both saying the same thing. The exception might be your trail use, if it's a slow speed trail, it's likely your tires are not actually "hot", as speed is the biggest factor to generating heat in a tire. After a slow trail, I would reset the pressure as "cold".

 

[Desmolicious]

Yeah, we're both saying the same thing. The exception might be your trail use, if it's a slow speed trail, it's likely your tires are not actually "hot", as speed is the biggest factor to generating heat in a tire. After a slow trail, I would reset the pressure as "cold".

Good point.

 

[Scoop]

This is classic "overthinking"!

 

[Brian_B]

After a couple of air-ups and downs its not too hard to see what the delta is between cold pressure and hot. And be able to adjust accordingly.

 

[Q1svt]

…. Isn’t the air compressor putting out ‘HOT’ air , some would say very hot air.

Guess if you aired up w/ comp on trail, you’ll be adjusting after a night home to get back to daily driving air pressure.

 

[Desmolicious]

This is classic "overthinking"!

Let me think about that before I craft a reply.

 

[dgorsett]

Let me think about that before I craft a reply.

While you're noodling that out add this variable: I check pressures on my Motorhome judiciously before each trip. I notice up to a 4 psi difference side to side sunny versus shady side, at ambient temperature before driving. And back in my school days we referred to STP, standard temperature and pressure which I think was 70 or 72 degrees at sea level. I see you're in SoCal, so maybe you can get that easily.

 

[Brian_B]

…. Isn’t the air compressor putting out ‘HOT’ air , some would say very hot air.

Guess if you aired up w/ comp on trail, you’ll be adjusting after a night home to get back to daily driving air pressure.

Not everyone uses an onboard compressor though. If you were using a CO2 tank or air from a big compressor tank it would be extremely cold.

 

[Q1svt]

Not everyone uses an onboard compressor though. If you were using a CO2 tank or air from a big compressor tank it would be extremely cold.

… so you are telling the OP to fill to cold 36# ?

After I drive for a little while, the tire temps obviously increase, and as such so do the pressures. My 36 cold becomes 39 hot.

So, the point of this - airing down when off roading. My tires are now hot and I want to air down. A few psi difference does not really matter off roading at the slow speeds I will be doing, so an indicated 20 or 22 psi would be fine. But when I air up to get back onto paved roads, do I air up to an indicated 36 or higher to match the hot tire temps?

 

[Scott R Nelson]

I drop to 19 or 20 before heading into the dirt and aim for 38-40 when I air back up. The next morning they'll show 36.

I can't really tell the difference of 1-2 psi on either end. I just want to be in the ballpark and that works for me.

 

[Brian_B]

… so you are telling the OP to fill to cold 36# ?

Some 2Dr's run around with that pressure cold, or higher. Seems high to me but not my ride.

I'd say "chalk test" if someone asked what pressure they should run.

Just responding to the statement that implies that refilling is always with hot air - it may be often the case, but not universally.

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