Airing down—and up—the pro way

One of the most frequent questions people ask me is, “What’s the the best thing I can do to improve my vehicle’s off-pavement performance?” Many of them seem distinctly disappointed when I answer, “Air down your tires.” I’m sure they’re hoping I’ll facilitate some expensive and impressive modification—diff locks, external-bypass shocks, three-piece titanium wheels, something that would justify putting a stylish brand sticker in a window. But the fact is that nothing is easier to do or more effective at providing several instant benefits than reducing your normal street tire pressures to suit the immediate conditions. I’ll repeat: Nothing.

First, lower pressure increases traction by increasing the contact area of the tire and allowing it to better deform around obstacles and grip them. Flotation in sand is enormously enhanced with the longer footprint provided by lower pressure (not so much greater width as many suppose).

But the advantages don’t end there. Lowering pressure alleviates stress on the vehicle by effectively reducing the spring rate—the tires flex enough to absorb impacts that would otherwise have to be dealt with by the springs and shocks. That translates to much greater comfort for the driver and passengers.

Finally, the above characteristics contribute to reduced impact on the trail. We saw first-hand evidence of this on a recent crossing of the Simpson Desert in Australia, via the Madigan Line—so-called after Dr. Cecil Madigan, who led the first scientific expedition across the area in 1939. The Madigan Line cuts directly across the huge field of parallel sand dunes that characterizes this part of the Simpson—1,130 sand dunes to be more or less precise. The dunes themselves are stabilized and well-vegetated, but the bare track still needs to ascend and descend each dune, and despite the very sparse traffic on this route the final approaches and crests are often cratered with “hoon holes,” where those who disdain airing down—or even engaging four wheel drive—have left huge divots from futilely spinning tires or frantic, lunging ascents.

Graham and I actually aired down our Land Cruisers prior to hitting the dunes, on the stretch leading to the old Andado Station, a fine track but well-known for its long stretches of corrugations (or washboard as we refer to them in the U.S.). I reduced our pressure from 40 psi, which we’d run on the paved Stuart Highway to Alice Springs, down to 32, and Graham did likewise (he’d found his tubed tires on split rims at a harsh 50 psi). It took much of the sting out of the sharp undulations and eliminated the skip-fishtailing that can occur with higher pressures. (An Australian writer described those corrugations as “brutal.” All I can say is he needs to see the corrugations on the seven-mile dirt track to our house in Arizona. Or those on the road from Namanga to Amboseli. But that’s another story.)

Once in the dunes, we further reduced pressure to around 22 psi. This is well above the 14 we might run in very soft sand, but it worked perfectly on the combination of flat, compacted inter-dune track and the chewed-up ascents and descents. We even summited the famous Big Red dune outside Birdsville with no drama.

Once past Birdsville, on the high-speed gravel Birdsville Developmental Road, we re-inflated—and that’s where the catch is for many people who grasp the concept of airing down, appreciate its advantages, but rarely if ever do it.

Why? Because a lot of those people carry a compressor that cost them as much as a couple of pizzas and is about as effective at actually adding air to a tire, despite the “150 PSI!” claims on the box. A single 45-minute session laboriously moving four tires from 25 psi to, say, 28, while their $29.95 compressor buzzes and vibrates in circles like an enraged chihuahua, and that’s it. The thing gets tossed in the bottom of the tools, to be used in the event of an actual flat, if at all. The same people likely used the point of their Swiss Army knife to depress the valve to deflate each tire, another laborious procedure. It’s little wonder they inflate the tires on their new truck to 40 psi and never budge from that.

We knew better. And we were, after all, in Australia, home of some of the best expedition equipment manufacturers on the planet. In Sydney we had picked up a pair of ARB E-Z deflators, and two ARB portable Twin compressor kits (CKMTP12). The single-cylinder ARB High Output compressor on our Tacoma has been working perfectly for several years operating a locker and inflating tires, so I was eager to compare the more powerful Twin in field use. 

Airing down takes less than a minute per tire with the E-Z deflator, which unscrews and captures the tire’s valve core, allowing a much greater volume of air to exit the valve stem, and gives you precise control with its sliding actuator. The only faster way I know to air down four tires is with a full set of the superb set-and-forget CB Developments Mil-Spec automatic deflators—but that full set will cost you $400, versus $40 for the ARB unit. An E-Z choice, if you will.

Regarding portable 12V compressors, as with so many other products it’s been my experience that you get what you pay for. The $30 units that plug into a cigarette lighter simply won’t cut it for field use. I know people who’ve been happy with the ubiquitous Masterflow MT50 and its variations, which are available for less than $100; these clip directly to the battery, meaning they can draw more amperage, but they are still achingly slow, and I can recall at least three failures related to me by users. Simply put, if you’re going to go the pro route for airing down (and repairing) tires, you need a pro-level compressor. You do not want to get caught after a section of soft sand with all your tires at 14 psi, no way to inflate them, and 30 miles of rocks ahead. (Especially when it's been six days since you've seen another human.)

Ignore the psi rating—virtually any pump will produce more than enough theoretical pressure. It’s the cfm (cubic feet per minute) rating and duty cycle you need to evaluate. The cfm is self-explanatory. Duty cycle refers to how long the unit can run before it needs to shut down and cool off. A 25-percent duty cycle means the pump can run for 15 minutes out of an hour. It’s easy to see the relationship: A high cfm rating means little if the duty cycle is poor, and a 100-percent duty cycle means little if the cfm rating is below standard. Furthermore, some compressors display an impressive cfm rating at zero psi, but will fall off significantly with higher pressures. Look for factory specs that list both. 

The Australian-built ARB Twin boasts a 100-percent duty cycle, produces 6.16 cfm at zero psi and an impressive 4.68 cfm at 29 psi. How does this relate to the real world? Back on the Birdsville Developmental Road to air up, I hooked up the Twin’s leads to our Land Cruiser’s battery, flipped the rocker switch, connected the chuck to the first tire (still at 22 psi), settled back on my heels to wait a couple minutes, and, er, what? The tire seemed to rise awfully quickly. I disconnected and checked the pressure: 42 psi. I was only aiming for 38 . . . 

I bled a bit out and moved to the other three tires. I didn’t set a watch to any of them, but it certainly took no more than a minute per tire to reinflate from 22 psi to 38. That rivals my benchmark for powerful compressors, the Extreme Outback ExtremeAir Magnum. And the ARB doesn't have a single "Extreme" in its name.

As a reliable and durable tool for remote use it would be hard to imagine a better configuration than the ARB Twin, given the extensive redundancy: Two all-ball-bearing motors, two cylinders, two inline fuses, and internal thermal protection for each motor. Twin air filters are washable sintered bronze, not paper. It’s highly moisture and dust resistant (the cooling fan is actually sealed to IP55 specs); the cylinder bores are hard anodized and the piston seals are Teflon-impregnated carbon fiber. In addition, the portable kit incorporates a four-liter aluminum air tank, which enables the system to run most air tools (it’s regulated to 150 psi). For the distinctly premium price, I wish the kit came with the ARB inflator that incorporates a dial gauge, rather than the simple clip-on chuck that is standard. But the waterproof case is strong, and the organizer pockets keep hoses and accessories neat—an underappreciated feature on expeditions where entropy nudges things toward clutter. The battery clamps are sturdy, the inline fuses easy to access if necessary, and there's a solidly mounted quick-release fitting for the air line next to the (lighted) power switch. You’ll know you’re dealing with a substantial piece of equipment when you pick it up—the whole package weighs 33 pounds, about as much as a Hi-Lift jack. The compressor alone weighs 19.4. 

If you prefer a built-in compressor (I normally do, but we haven’t yet decided on the final configuration in the Troopy), the air filters of the Twin can be relocated, rendering the entire unit submersible. ARB was not messing around when they designed this compressor.

The Twin is a significant investment ($830 for the kit; $520 for the compressor alone). But consider these two facts: 1) As stressed above, varying your tires’ pressures to suit conditions will do more than anything else to enhance your vehicle’s off-pavement prowess, your comfort, and the condition of the trail, and 2) Tire failure is by a significant margin the number one reason for breakdowns in the bush. With a high-quality compressor such as the ARB Twin you have both scenarios covered with professional-level ease.