Under Pressure: Proper Tire Pressure for the TrailPosted in How To on May 1, 2011 Comment (0)
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.
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.
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.
Almost everyone I meet runs too much air pressure in their Jeep's tires both on- and off-road. Like Christian, I cringe when I see beadlocks on a Jeep where the tire barely has a sidewall bulge. The owner generally considers 25 psi aired down since he runs the max sidewall-listed pressure of 35-50 psi on the street. Not only is he prematurely wearing out his tires, but he's rattling his teeth loose and giving up a ton of off-road performance. This is even more widespread now that 16-, 17-, and even 18-inch wheels have become common on small Jeeps, whereas before these wheels and higher-pressure tires were typically only found on 3/4- and 1-ton trucks.
Running the correct tire pressure starts by picking the correct tires. Most Jeeps only need a load range C tire. The D- and E-rated tires are for heavy-hauling trucks. With a C range radial tire most Jeeps can run about 20-25 psi on the street. Most bias-ply tires can hold up a Jeep with about half that. Off-road I consider 10-12 psi a good starting point for a radial tire and I'll often go lower than that if the terrain is really soft and there are few tire hazards. On a bias-ply tire you really don't get a good traction bulge until you hit the single digits, so I'll start at around 5-10 psi and go down from there if need be. To some people these pressures sound unreasonably low, but they increase traction significantly and improve the ride as well. You can either adjust your driving style to keep the tires on the beads or step into beadlock wheels if you decide to run at the lower end of these pressures. Either way, invest in an on-board air system so you can reseat a popped bead and reinflate for the road.
I don't have a ton of Jeeps with beadlocks and I end up out on my own a lot more than I should, so I am a bit more conservative with airing down than Cappa or Hazel. To add to the mess I rarely take the same Jeep out twice in a row, and rarely am I on the same kind of trip.
My Jeep trips break up into three main kinds: "camping," "expedition," and "down-and-back." On the camping trips I'm either in a tent or a hotel, so I'm able to take all my junk out of the Jeep and then take the Jeep out to play. On these trips I'm able to drop the pressure the lowest. For example, on my four-cylinder YJ with a soft top I often drop my 33x12.50R15s down to 7 or 8 psi, even without beadlocks. Even though it is heavier, I'm running beadlocks on my Cherokee and I'll normally go down to about the same 7 or 8 psi- and thanks to the weight, that provides a huge sidewall bulge.
I hate the term "expedition" but I find it is the quickest way to get the point across. These are the trips where I've got everything in the Jeep while wheeling. Tools, spare parts and fluids, camping gear, cooking gear, the cooler, food, and water are all along for the ride. It depends on what Jeep I'm out with how much stuff I take but my normal airing-down theory here is I want to go low enough to gain traction but not so low that I can't drive on a road if I need to. You see, I don't have a CO2 or engine-driven air source, so I like to keep enough air in the tires so I can make it to the nearest gas station without damaging or delaminating the tires. This normally translates to about 9 or 10 psi in the light Wrangler and about 12 psi in the Cherokee.
The "down-and-backs" are the ones where I leave my house in the morning, drive the Jeep to the trail, wheel all day, and plan to drive the Jeep home that night. If I'm not wheeling rocks, I'll normally just leave the tires at street pressure until I lose traction and can't make something. I'd say about 75 percent of the time I get away with it, and drive home afterwards without ever having to mess with it. The other 25 percent of the time, I'll pull the valve stems and drop it right down to the "expedition" level. If that doesn't work after an attempt or two, it goes down to the "camping" level. After all, I'm usually travelling pretty light on these jaunts and the only downside to running single digits is that I'll have to keep the on-road speed low until I can fill the tires back up.