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The science of Airing Down?

P

Plays For Jeeps

NAXJA Forum User
Location
Allentown PA
I know the purpose of airing down. I just don't know what to air down to.

I have heard between 12-15psi on street tires. I also heard that you tires should be aired down to 1/3rd of the factory rec.

What do you guys air down to at the trails head?
 
what tires do you have?

I run about 10 lbs depending on what im running with 33x12.5x15

with my brothers old xj with 35x12.5x15 we would run 3-5 lbs
 
I have 31 Muds and run about 12psi.
 
I aired my 32x11.5r15 bfg MT's to 15 pounds.

I air my 315/75/16 (35") MT/R's down to 12 pounds.

I sorta go by look. If I'm starting to fold the wheel over onto the sidewall a lot, I add a couple psi of air. If my tires arent forming around the rocks I put them on, I remove a few psi.
 
I think last time i was at JV i was at 4? and i needed less....

however the tires im running have a 3/4" thick sidewall...
 
I usually go to 12psi with my 33's. I have yet to have any trouble.


I do really like my Oasis bleeders.


Hale
 
I run my 12.5 x 33 TSL bias on 8" rims at 10# with no trouble.
 
How about for smaller tires? I run LT235/75/15 (8 ply) all terrains. I run 32psi on the street. Ive never aired them down since I only wheel logging roads to get to camping spots and mountain bike trails. Now that Ive just lifted my jeep a few inches though, I'd like to try airing down and getting into a bit more "serious" wheeling.
 
larger tires have more volume, and can run less air pressure.
smaller tires, the opposite applies.
in my 235/75-15 bfg at's I ran 22psi...
I ran 20psi in my 31x9.5-15's last weekend and they worked awesome.
I don't like the idea of pintching the tire between a rock and the rim, so I run on the higher side of what you 'might' get away with when not using beadlocks.
 
I've heard you should run half of the psi on the trails that you normally run on the highway.

So if you run 30 on the highway, 15 on the trails.
Click to expand...

You can't use this for every situation! I run my Q's at 6-7 PSI...but they would overheat BADLY as well as handle like sh!t at 12-14 PSI on the street. Every situation is different and unique.

A good rule of thumb for getting your tires aired down to a good and low pressure is to lower the pressure until you have a sidewall height that's 75% of what safe street pressure height is. This should happen right when small psi changes make big sidewall height changes...it's the sweet spot in your specific rig and tire/wheel combo.

As always, your tire size vs. rim width and ability to air back up will be a factor also.
 
clunk said:
How about for smaller tires? I run LT235/75/15 (8 ply) all terrains. I run 32psi on the street. Ive never aired them down since I only wheel logging roads to get to camping spots and mountain bike trails. Now that Ive just lifted my jeep a few inches though, I'd like to try airing down and getting into a bit more "serious" wheeling.
Click to expand...
8-ply rated,but NOT 8-ply!
 
any pressure works it all depends if can air back up before you hit the highway on my 33's i run 7 psi on trails if i have air if I have to do some driving on pavement to get to some air then i run like 15. on stock size tires you could go pretty low and not have to worry about rolling a bead off.
 
XJ_ranger said:
I think last time i was at JV i was at 4? and i needed less....

however the tires im running have a 3/4" thick sidewall...
Click to expand...


mmmmmmm, Creepies!
 
I can run less tire presure in my CJ than I can in my XJ with the same tire. The CJ is perfect at around 9-10 with BFG MT's.

My XJ can't go lower than 15 with those tires. Part of the problem I see is with the Steering on the XJ. XJ's steering cuts very sharply. Turning sharply while moving forward pushes hard against the sidewall and tries to push the bead off the rim. I have watched an xj blow beads all day long at 10 psi. with MTs.

Alot of it has to do with the type tires and rims!
 
i like the fact that the author of that 'link' knows the relation between volume and pressure... and as mentioned above, loosing a bead is a possability even as high a pressure as 15.
Personally, with non-bead locks, I'd not run less then 18psi unless you are skilled at seating beads on the trail.
 
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