thats really a good question. I air down off highway in general to about 2o lbs but have not wanted to go below this for fear of popping a bead.
Serious rock crawlers air way down to 8 psi or so. But they carry big tanks of CO2 to reset their beads or use beadlocker rims.
It reduces risk of puncture, increases the ability of the tire to smooth the ride over rough terrain, and gives the tire better "flotation" in sand.
I would like to see a reference called Tire Pressure Offroad but I did find this:
Technical Information: Tires
excerpt
Off-Road:
Just like on-road, there are several schools of thought on choosing the correct off-road tire pressure. Off-road, there are many more variables, such as the type of terrain, the tire and wheel construction which determine the type of problem you are trying to solve. The following solutions should work for 15 and 16" rims with safety beads. Note many 16.5" rims lack safety beads and running lowered pressures is risky. Rims with beadlocks are an entirely different issue.
Anyway, why do you want to lower your tire pressure off-road? Several reasons come to mind:
It softens the tire, making it more flexible to absorb bumps and less likely to get punctured due to sharp objects
It increases the contact patch with the ground, increasing traction and flotation
So, how do you go about picking a pressure to run off-road?
One rule of thumb is 1/4 of the tires maximum inflation pressure. In the above case of a 50 psi max, 12.5 should be a safe lower limit of off-road pressure.
A more sophisticated method is to measure the height of the sidewall (from ground to rim) at normal street pressure, then lower the air pressure until your sidewall height drops to 75% of the street height. I tried this with my 33x9.50 BFG M/T tires and found this occurred at 12 psi, which is very close to the 1/4 of max pressure rule.
A third method is to pick a pressure where the tires start to bulge out (assuming radial construction) and beginning to get soft. For me, this happens in the 15-18 psi range.
In any event, you want to pick a pressure that is low enough to handle the terrain, but high enough to protect the wheel and tire as well as preventing the loss of the bead.
Now for some terrain-specific observations:
On hard surfaces, lower is not always better:
I notice my BFGs begin to get slippery below about 14 psi. I think this is because the tread starts to buckle (33x9.50 mounted on 6" rims) and squirm, reducing traction. Also, below about 15 psi, if a tire is on a rock, it will deflect nearly to the rim, which can lead to sidewall punctures. If you'll be going any distance at higher speeds, then keep the pressures higher. I'll choose 18 or 20 psi if pavement driving is anticipated, otherwise 15. Also, you should correctly observe that as the pressure drops, so does your ground clearance. If clearance is more important, keep the pressure on the high side.
I notice my Swamper bias ply tires below about 8-10 psi loose their sidewall stiffness and begin to slip off of rocks when the sidewall buckles instead of grabbing on and climbing up the rock.
On soft surfaces like sand and snow, lower is better.
In fact, I follow these rules in the deep stuff:
Air down, way down, then air down some more. Then,
if your engine bogs down or overheats, air down
if your tires dig in, air down
if you get stuck, air down
After running for a while, check the air pressure
Aired-down tires tend to heat up and raise the internal pressure, if this happens, air down
If your tire looses a bead, you've aired down just a bit too much :-)
So how low is low?
I've been to 6 psi in my 33x9.50 BFGs in the snow. At 10 psi, I was digging in going downhill, at 6 psi, they just floated on top.
I've been really low in my 33x15.50 Swampers. How low? I don't know, my digital gauge reads down to 2.5 psi, then it says 0.0 (its supposed to go to 1.0, but I guess the mfg. lied). So, I estimate the tires are running 1-2 psi. They get so low that you don't hear air escaping when the valve stem is depressed. In fact, one time, I jacked a wheel up in the air, pulled the valve stem allowing all the air to escape, then I reinstalled the valve stem, lowered the jack and there was enough pressure built up due to the collapsing sidewall to support the vehicle.
Anyway, I think of tire pressure kind of like cross country ski wax. You have to know your tires and vehicle, read the terrain and then choose an air pressure to run. If you err on the high side, you can always go lower if needed. Of course if you have on-board air, its no problem either way.
More information:
Goodyear has an excellent engineering data section with lots of inflation vs. load information.
Harry Lewellyn's article on "Tire Pressures for 4-Wheeling" is an excellent primer on the subject.
Tire Pressures for 4-Wheeling
Linear Mode
