Unlike our other staff-compilation stories in this issue, we're really not debating the pros and cons of airing down. Every Jp staffer agrees that you gotta drop your pressure to get the most out of your tires. But that's not to say we all practice the same techniques. Airing down is the best thing you can do for off-road performance, second only to running lockers in your axles. The difference between aired-up and aired-down on the trail can be staggering. Without exaggeration, we've watched a monster Jeep on 50-plus-inch-tires fail miserably at an obstacle because the tire pressure was too high. Then, a nearly-stock Jeep with mild tires at low pressure came and walked right up like nothing. So grab a gauge and check out how the staff airs down.
Generally speaking, 15-inch...
Generally speaking, 15-inch tires mounted on most Wrangler and CJs will easily tolerate trail pressures of 10 psi up front and 8 psi out back. You'll want a bit more pressure up front to prevent tire bead slipping when turning.
For starters, I understand why some people don't want to air down for the trail. Airing down means you've got to air up at some point. And not a lot of people can afford an onboard air or CO2 inflation setup. However, I'm always floored when I see a guy running $1,500 beadlock wheels with his tires at 15 psi. What a waste. No matter what wheel/tire you're running, you'll need to experiment a bit to find the sweet spot for your setup. There generally comes a point of diminishing return when dropping your tire pressures at which the increase in traction is overshadowed by the tire's propensity to slip off the rim bead. Once you find the lowest pressure you can get without the tire coming off all the time, jot it down under the hood with a Sharpie pen so you don't forget.
I run beadlocks on most of my more serious trail rigs. Vehicle weight and terrain play a small part in my final tire pressure selection, but generally I tend to go a lot lower than most people think is possible. In my 3,000lb flattie, I'll drop the front and rear pressure to 2-4 psi for hardcore rockcrawling, Moab slickrock, and alluvial hillclimbs like for the TDS Desert Safari event. If I'm hitting the dunes like in Glamis, I'll put the tires at 1-2 psi. That's right-just enough pressure to kinda keep the inner beads on the rims. I consider 7-8 psi high pressure for the 35x13.50R15 tires on that vehicle. Heck, it only requires 12-15 psi to get a proper contact patch for street driving. Despite living "dangerously" like this, I've never slipped an inner bead off a beadlock wheel. I'll apply similar tire pressures to other vehicles running beadlocks, but if I'm in a heavier vehicle I'll tend to up the pressures listed above as needed, usually by 2-4 psi.
If you're a serious wheeler...
If you're a serious wheeler this is gonna happen. Experimenting to determine how much you can lower your tire pressure only works if you find the bottom end. And that happens when the tire slips off the bead. Run some sort of onboard air, CO2 inflation setup, or just bring along a full-size spare to keep you going down the trail.
Running regular rims is a different story. I'm usually traveling with a Power Tank, so I have the ability to air up a slipped bead, but I rarely have the inclination. As such, I'll tend to run a bit more pressure than I'd like to. For a normal C load-range tire like the 31x11.50-15s on my Wrangler, I'll run about 9-10 psi in the front and maybe 8psi out back. The higher front tire pressure helps prevent bead slippage when turning. The rears will tolerate a bit lower pressure without coming unseated and the lower pressure in the back helps when climbing. For LT metric 16- and 17-inch tires with higher load ratings, I've found I need to drop the pressure both on the street and on the trail to get the right performance. Too high on the street and the tires will crown and wear out the center treads. Too high on the trail and they just don't work that well. The downside is that the heavier sidewalls and increased load capacity of these 16- and 17-inch tires doesn't always translate into increased bead retention. It's for these reasons I'll always chose a 15-inch tire whenever possible for use on a lightweight Jeep.