Airing down

glwright

Member
I will agree with the others that for an empty truck, you can go about half the highway pressure for general offroading. Note that "highway pressure" is NOT what is posted on the door sticker, it's what's reasonable for highway travel for the load condition of the truck. I commonly run 55psi in the front and 45psi in the back of my truck when empty, and with the truck camper on the back (and usually at or slightly over GVWR) more like 60/70psi. When empty, I will have no problem dropping to ~30psi front and ~25psi in the rear for long distances on rocky or washboard roads. I've been down to ~15psi in the sand empty with no issues. With the camper, I drop the front to 30-35, and the rear to 35-40. Lower than that with the camper is for "emergency" situations.

FWIW, using ANY 12v comperssor to air up to 80psi is going to take a long time!! Fortunately, you almost never need 80psi in your tires, regardless of what the idiot light says... Even with my truck camper on the back, I run about 70psi max in my 255/80R17's, but if I air down, I only bother to air back up to about 50psi on electric. That gets me enough pressure to get up the road and use a better compressor to top off the tires. 10 or even 20 miles with 50psi in the tires even when you'd rather have 60 or 70 isn't going to kill them. Just keep it a little slower if it's really hot out.

Finally, your fears about pulling a tire off the bead are probably unfounded. When you air down, you should always drive a bit more carefully, but modern tires and wheels have pretty good resistance to debeading. I've been airing down for a LONG time, and I have NEVER pushed a tire off the rim. I've had my Jeep down a 4.5psi for a day at the dunes and had no issues, even with some more spirited than usual driving. I had a previous 3/4 ton truck down to ~10psi at the dunes to see how it did and was a little worried, but for nothing. In general, the lower you go, the slower you go and you'll be fine.
I keep seeing the 55 front, 40 rear set up here and other forums, mostly for towing/empty there. I'm working on getting Forscan to adjust where the idiot light starts to scream at me. I'll probably bump back up to 60 for towing the stock trailer or the skid steer. Never had the thought of breaking a bead cross my mind until I saw your post. I don't plan on going down to single digits any time soon though! When I run the compressor I'll probably try to get it back up to about 50 ish before hitting pavement. Thanks for the info.

My method of airing down to a safe starting point.
•Place a 1" square piece of steel under the center of your tire.
•Air down until both sides of the tire touch the ground.

This system provides a great safe starting point for airing down. I have used this system on vehicles from 4000 - 12000 lbs with great results.

As for compressor
If it plugs in its a POS
Avoid duty cycles
Amp draw is a direct relationship with performance. In most cases higher amperage higher performance.

Pictured below the rear of a 10700lbs truck

View attachment 568116
I like your test of the square stock under the tire. Might find some and play with that a little this weekend. The plug in compressor is a decent option for now. As I make adjustments and tweaks to my truck I'd like to find a truck mounted solution that I can link to one of my up fitter switches.
 

luthj

Engineer In Residence
Airing down is going to depend on loaded weight and tire size.

First: Air the tire up to road pressure. This is determined by weighing each axle, and consulting the tire MFGs load tables.

Second: Measure the sidewall height on each axle (ground to wheel edge).

Third: Reduce tire pressure until sidewall is 25% shorter. This is the ideal pressure for most off road usage. Keep your speed to about 30% of the tires normal speed limit (its the letter on the sidewall next to the weight class).

For deep and soft sand, you can go lower, but mind unseating a bead. Also note that at very low pressure you need to drive slow, or the tire can overheat.
 
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LandCruiserPhil

Expedition Leader
Airing down is going to depend on loaded weight and tire size.

First: Air the tire up to road pressure. This is determined by weighing each axle, and consulting the tire MFGs load tables.

Second: Measure the sidewall height on each axle (ground to wheel edge).

Third: Reduce tire pressure until sidewall is 25% shorter. This is the ideal pressure for most off road usage. Keep your speed to about 30% of the tires normal speed limit (its the letter on the sidewall next to the weight class).

For deep and soft sand, you can go lower, but mind unseating a bead. Also not that at very low pressure you need to drive slow, or the tire can overheat.
Im open to all methods and admitted nerdy when it comes to airing down.
Here are my results and honestly Im not comfortable.
I do like the 30% of rated tire speed, where did you get that recommendation from?

315/75-16
Start 7.25"
End 5.5"
Final air 7.5psi

275/70-18
Start 5.625"
End 4.0"
Final air 6.6

How much more down are you comfortable with in the sand?
What do you consider hot or overheated for a tire?
 

IdaSHO

IDACAMPER
There is really only one hard fast rule.... If it comes off the bead, underinflated 😁
Beyond that, trying different pressures and monitoring the results is the best way to go about it.

If you really want to know how your airing down a tire under a heavy truck, you really need to monitor tire temps.
Load ratings are based upon temperature afterall. :sneaky:

We run cheap external (valve stem) TPMS monitors on our truck.
Provides real time PSI & temperature, and is good peace of mind.

With our own setup, full pressure and rolling down the road wet and loaded we run 55PSI front 65PSI rear.
Same setup off the blacktop (if we decided to air down) is typically 35PSI front 40PSI rear.

It makes a big difference in ride and flotation.

Could we go more(lower psi)? If we were to get stuck or need additional flotation, absolutely.
But the pressures I run aired down I feel are a great compromise for our setup.
 
The formula for the temperature of the air inside the tire is below. Pressure is in psi, temperature is ABSOLUTE scale: i.e 273 + temp C or 460 + temp F.
You measure T and P cold in am.
T(hot) = T(cold) x (P(hot) +14.5)/(P(cold) +14.5)
It is a simple derivation from the universal gas law.
A Michelin tire engineer at Eurosatory 2010 told me the maximum recommended temperature for at least their military tires is 75C = 167F.
 

LandCruiserPhil

Expedition Leader
There is really only one hard fast rule.... If it comes off the bead, underinflated 😁
Beyond that, trying different pressures and monitoring the results is the best way to go about it.

If you really want to know how your airing down a tire under a heavy truck, you really need to monitor tire temps.
Load ratings are based upon temperature afterall. :sneaky:

We run cheap external (valve stem) TPMS monitors on our truck.
Provides real time PSI & temperature, and is good peace of mind.

With our own setup, full pressure and rolling down the road wet and loaded we run 55PSI front 65PSI rear.
Same setup off the blacktop (if we decided to air down) is typically 35PSI front 40PSI rear.

It makes a big difference in ride and flotation.

Could we go more(lower psi)? If we were to get stuck or need additional flotation, absolutely.
But the pressures I run aired down I feel are a great compromise for our setup.
Tires in the SW run easily 145° at highway speeds in summer months. Our testing has seen the same temps aired down at 45mph and off road wheeling. What do you consider a safe tire temp?
 

LandCruiserPhil

Expedition Leader
Here is what Michelin said years back for street pressure-

I worked for Michelin Tire Corporation for 7 years and Yokohama Tire Corporation for 11 years. I have given numerous tire seminars on tire maintenance and especially how to determine the correct tire pressures. So here goes.
The pressure on the sidewall of the tire is the maximum pressure at the published load at approximately 55 mph. (The speed can vary somewhat but it is not important for our discussion).
The air pressure is required to support the load that the tire must carry in such a manner that the tire flexes at the designed place on the sidewall of the tire.
If the load on the tire changes then the air pressure should change accordingly to keep the tire flexing at the proper place.
The reason for correct air pressure is to prevent the tire from overheating. It was put together with heat and it will come apart the same way. An under inflated tire will eventually self destruct due to excessive heat build up. An over inflated tire will ride harshly and be more likely to burst upon impact. Sorry for the long explanation but here is the bottom line.
To determine the correct air pressure, check the pressure when the tire is cold. Run the tire for several miles at highway speed. Stop and immediately check the air pressure in the tire. It should be higher than we cold but no more than 10% higher.
Now here is the hard to believe part. If the pressure is more than 10% higher you must ADD AIR and test again. For example if you start with 50 psi cold. If the pressure is 60 when hot, you have exceeded the (10%) in this case, 55 psi maximum safe heat build up pressure. You must ADD AIR. In this case I would add 5 psi which would take the tire to 65 psi when hot. After you run the tire again you will find the pressure to actually drop because the tire will run cooler. The heat build up causes the tire pressure to increase when under inflated.
On the other hand, if the 50 psi cold pressure does not change when hot. You have more air than needed. You can remove 5 psi or so and test again when they return to cold. Like the next trip you take.
So a fully loaded rig will require more air in the tires than one with empty tanks and a light load on board. Always error on the side of over inflation. Thus the maximum sidewall pressure indicated on the tire is usually used. It usually is more than needed. Each axle has its own requirement based upon the load on that axle.


Attached is by Tire and Rim Association for street tire pressure
 

Attachments

luthj

Engineer In Residence
Im open to all methods and admitted nerdy when it comes to airing down.
Here are my results and honestly Im not comfortable.
I do like the 30% of rated tire speed, where did you get that recommendation from?

315/75-16
Start 7.25"
End 5.5"
Final air 7.5psi

275/70-18
Start 5.625"
End 4.0"
Final air 6.6

How much more down are you comfortable with in the sand?
What do you consider hot or overheated for a tire?
It sounds like you have very large tires for the load you are carrying. In which case your lower tire pressure is determined by bead retention. I would not go lower than ~15psi except for deep sand, where 10 psi (with care not to unseat the bead is needed). LT tires are much stiffer at low loads, so they need very low pressures (at low loads) to get lots of flex.

The 30% speed number is a rough estimation based on the tires heat generation. At correct highway pressure a tire will flex between 5-15% of its sidewall height. Going to 30-40% total flex roughly triples the heat generation. Thus the speed needs to be reduced by 3x.
 

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IdaSHO

IDACAMPER
Tires in the SW run easily 145° at highway speeds in summer months. Our testing has seen the same temps aired down at 45mph and off road wheeling. What do you consider a safe tire temp?
Like most gauges, the actual temp is (nearly) irrelevant.

What you are looking for is dramatic changes in temp, and tire temps relative to the other tires.
 

LandCruiserPhil

Expedition Leader
It sounds like you have very large tires for the load you are carrying. In which case your lower tire pressure is determined by bead retention. I would not go lower than ~15psi except for deep sand, where 10 psi (with care not to unseat the bead is needed). LT tires are much stiffer at low loads, so they need very low pressures (at low loads) to get lots of flex.

The 30% speed number is a rough estimation based on the tires heat generation. At correct highway pressure a tire will flex between 5-15% of its sidewall height. Going to 30-40% total flex roughly triples the heat generation. Thus the speed needs to be reduced by 3x.
What tires and weight does your system work for then?
 

luthj

Engineer In Residence
What tires and weight does your system work for then?
The only caveat is bead retention at low pressures. If you have 1,000lb on a tire rated for 3,000lb, there is going to be a significant contribution of sidewall to load carrying (as opposed to just internal pressure). That also means that significant flex is going to put the tire pressure too low for good bead retention.

This is part of the reason that going with very wide tires doesn't improve ride as much when aired down (though it does improve flotation). The same applies to using tires with a much higher load rating than the vehicle requires.
 

IdaSHO

IDACAMPER
OK How much change are you talking? Our testing has not seen much temp range at all.
I have found that direct sunlight alone will change both PSI and TEMP (as you noted above, they are VERY much related) by as much as 20 degrees & 10PSI when stationary.

When rolling direct sunlight doesnt seem to have much effect at all. Air temp seems to normalize things considerably.
The TPMS units I use are also valve stem units, so that very well could be the entire difference.


But once stabilized and rolling I would consider a quick temp swing of more than 10+ degrees (F) to be of concern.
Especially if the other tires do not show the same change.
 

Happy Joe

Apprentice Geezer
... just thought that I would chime in with a little empirical experience;
Over many tires, vehicle and loads; airing down doesn't seem to do do all that much good until I reach 12-15 psi. (my typical street/highway pressure, unloaded) is in the mid 20s psi range)... when airing down I normally go for 10-12 psi.
Tire retention is largely related to the size of the ridges (Safety Beads) on the inside of the tire rim (varies by brand: bigger/taller is better).
Driving technique can push the tire off the rim relatively easily ; Don't try to drive by pushing the tire sidewall into the obstacles unless you are using bead lock rims.... (one of my friends is really bad about this).
Some tires (SuperSwampers) have really stiff sidewall and will show little sign of flex/change in the rim to ground measurement, even with out air (valve completely removed/ don't try to drive it this way) on very light vehicles.
For snow wheeling I typically start at 10 psi, for powder sand I might also go this low; for just rocks I rarely bother to air down until the going gets close to extreme.
The lowest pressure that I have run with was around 4 psi (Very deep powder snow).

Besides a fair compressor; carry a ratchet strap to wrap around the circumference of the tire to hold it to the rim (to minimize leakage) while you are trying to re-inflate it after you push if off the rim.

My recommendation; get high traction tires and install selectable lockers... so that for most moderate trails/terrain you don't really have to mess with airing down and back up.

Enjoy!
 
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