Topic: Why Air Down Your Tires Off Road

[H8PVMNT]

I will post another plus for airing down that hasn't been discussed yet and provide a real word example...  Yesterday my co-worker needed the work truck so I took the 4runner out on inspections.  I had to do a couple checks of ag land use and then there were these 5 grain bins that had been destroyed I had to go verify.  I was told access wasn't possible unless I went about 40 miles out of my way and came in through a few other ranches.  I was pretty close the way the crow flies so I though I would at least check it out, maybe hike if I had to.

It was all nice road until it climbed up the gumbo bluffs, where about a quarter mile of road had just schluffed off into the abyss.  It appeared safe, but it was all soft and full of cracks.  I knew it would be easy with the 4runner but I didn't have the option of tearing it up, so I went ahead and aired down to about 10 psi.

I barely left any tracks at all, much less tore anything up.

Funny thing is it rained that evening so there will be no trace, and the land owner will probably think I went there in a helicopter .

When you air down you are treading lightly, with less wheel spin and less impact on the terrain.

Now I understand this may not apply to rocks were you can't dig holes by spinning anyway, but I felt it was important to bring up.  It is a verifiable fact that a low pressure tire will do less damage than a street pressure tire.

[blackdiamond [OP]]

Yeah it was way old, like a cool late 60s mud terrain with some sweet weather checking.  A real museum piece for sure. To be fair he is a jeep guy.  He only did like 100 psi I think and the story was he heard a big bang in his garage, he went in to investigate and the tire had alligatored and peeled his rack and spare tire carrier right off the jeep.

Not really any different than using an old rust air tank other than you can’t use a rust air tank as a spare tire. Can you imagine the superior traction that could be had with a tire at 120 psi?  Someone should tell Interco to develop 120 psi boggers!

[Gnarly4X]

OMG... I started laughing so hard, I nearly peed my pants!!   

You guys crack me up!! 

Gnarls.

[Lewis Hein]

And just for clarification for the calculator police, I don't think it will top off 4 tires from low pressure to low-mid 20s psi, it DID top off 4 tires from trail pressure to low-mid 20s psi.  I even reseated a bead with it a time or two with the help of a ratchet strap.  Compressed air is compressed, so if you use a load range E for a spare maxed out you get a lot of air out of it.  I don't make a habit of posting speculation.

Calculator police here....

We start with the ideal gas law: PV=NRT. R is a constant always, and we will assume that V is the same for the spare as for all 4 tires. We will also assume that ambient temperature stays the same over the course of the inflation.

The 10-ply, load range E spare is pumped up to 120 PSI. That means it contains PV/RT or 120*V/RT moles of gas. How many tires can we inflate from 8 to 25 PSI with this setup?

For convenience, I'm going to say that V/RT = C, some constant. A tire of volume V at 8 PSI contains 8*C moles of air, and at 25 PSI contains 25*C moles of air. We thus need 17*C moles of air to air up one tire. How many tires can we air up to 25 PSI? We have 95*C moles of air available before the spare is down to 25 PSI. Therefore, we can write

95C = nTires*17C

nTires = 95C/17C = 95/17 =~ 4.

Note: this assumes that tires do not change volume as they inflate: This is obviously wrong! However, certain aspects of this error tend to cancel each other out. To know how many tires you can really inflate this way, try it or calculate more carefully. However, 4 is all we care about usually and H8PVMNT already showed that it worked.

H8PVMNT, the calculator police is on your side....

[blackdiamond [OP]]

Calculator police here....

We start with the ideal gas law: PV=NRT. R is a constant always, and we will assume that V is the same for the spare as for all 4 tires. We will also assume that ambient temperature stays the same over the course of the inflation.

The 10-ply, load range E spare is pumped up to 120 PSI. That means it contains PV/RT or 120*V/RT moles of gas. How many tires can we inflate from 8 to 25 PSI with this setup?

For convenience, I'm going to say that V/RT = C, some constant. A tire of volume V at 8 PSI contains 8*C moles of air, and at 25 PSI contains 25*C moles of air. We thus need 17*C moles of air to air up one tire. How many tires can we air up to 25 PSI? We have 95*C moles of air available before the spare is down to 25 PSI. Therefore, we can write

95C = nTires*17C

nTires = 95C/17C = 95/17 =~ 4.

Note: this assumes that tires do not change volume as they inflate: This is obviously wrong! However, certain aspects of this error tend to cancel each other out. To know how many tires you can really inflate this way, try it or calculate more carefully. However, 4 is all we care about usually and H8PVMNT already showed that it worked.

H8PVMNT, the calculator police is on your side....

Excellent work. Thoughts on deflate gate?  The NFL one, not the 300k one!

[Gnarly4X]

Calculator police here....

We start with the ideal gas law: PV=NRT. R is a constant always, and we will assume that V is the same for the spare as for all 4 tires. We will also assume that ambient temperature stays the same over the course of the inflation.

The 10-ply, load range E spare is pumped up to 120 PSI. That means it contains PV/RT or 120*V/RT moles of gas. How many tires can we inflate from 8 to 25 PSI with this setup?

For convenience, I'm going to say that V/RT = C, some constant. A tire of volume V at 8 PSI contains 8*C moles of air, and at 25 PSI contains 25*C moles of air. We thus need 17*C moles of air to air up one tire. How many tires can we air up to 25 PSI? We have 95*C moles of air available before the spare is down to 25 PSI. Therefore, we can write

95C = nTires*17C

nTires = 95C/17C = 95/17 =~ 4.

Note: this assumes that tires do not change volume as they inflate: This is obviously wrong! However, certain aspects of this error tend to cancel each other out. To know how many tires you can really inflate this way, try it or calculate more carefully. However, 4 is all we care about usually and H8PVMNT already showed that it worked.

H8PVMNT, the calculator police is on your side....

How did you determine the volume of the tire? 

Gnarls.

[Lewis Hein]

How did you determine the volume of the tire? 

Reread the post.... I am confident that a communications analyst can figure it out.

[H8PVMNT]

Those calculations reflect what I found in real time.  By the time you get done with the 4th tire breaking into the 20s you are all done.

[Gnarly4X]

...
The 10 ply 235/85 r16 load range E spare had a max inflation of 120 psi. 

What tire with a load range E has a maximum rated air pressure of 120 PSI?

Gnarls.

[H8PVMNT]

I don't remember it was some kind of generic commercial traction tire from the 1990s but it said 120 psi max right on the sidewall so we felt safe with that inflation.  Looking these days most of your load range E say 80 psi max so maybe we had something else there but 80 psi would be enough to get some drive home road pressure going.

I think the skinny Toyos on  our 1 ton said 120 psi max too.  They were those 7.50x16s. Wonder what load range those were?

[blackdiamond [OP]]

I aired my Sequoia up yesterday from the sub-30 psi that it was at for some unknown reason to 40 psi.  When I backed up the driveway and spun the rear tires I knew that the added contact pressure had resulted in superior traction (for leaving black marks at least) and it was clear that the tire was crowning as expected so I dropped them back to 35 psi.  The Smittybuilt compressor did an efficient job or airing up the 265/70/R17 tires to 40 psi.