Section 4: Airing Down/Up
“Airing Down is essential when offroading.”
Why we Air Down
When we are just driving around town, on paved streets, our normal tire pressure is approximately 36 psi, give or take a few pounds depending on our specific tires and vehicle weight.
But as soon as we leave the asphalt and begin our offroad adventure on less civilized surfaces -- such as dirt, gravel, sand, stones, rock – we deflate our AT/MT tires to approximately 15-20 psi. Most of us aim to get closer to 15-16, but the main thing is to get below 20.
There are three main reasons for airing down.
1) It will soften the ride and greatly reduce getting bounced around in the passenger compartment. Imagine dribbling a basketball. Fully inflated, the ball has a lot of bounce. Let half of the air out, and the ball simply goes splat when it hits the ground.
2) Deflating the tire increases the tread footprint for much greater traction. Think of a tank or bulldozer caterpillar tread wrapping itself over obstacles, maximizing the contact area even over oddly shaped hard obstacles. In the case of sand, mud, or snow – this increased and flexible traction helps prevent getting stuck.
3) Less internal air pressure will reduce the risk of a puncture. Picture a balloon, fully inflated. Touch it slightly with a sharp object, and it pops. But grab a semi deflated balloon, and you could poke a pencil inches into it without it bursting. The balloon wall simply envelops a sharp object.
Many of you are nervous about the concept of driving around on “flat” tires. The low tire pressure warning light is flashing in your dashboard, and you remember your high school driving instructor telling you about all the bad things that could happen to you. Well, your instructor was right… it is not good to be driving around at high speed on a flat “street” tire. However, these are heavy duty AT/MT offroad tires and are built a lot stronger. Also, you are not cruising at 55-75 mph on the freeway but crawling at 5 or 10 mph along nature’s trail.
Sometimes we do drive along paved highways a brief distance in between adjoining trails. It is okay to remain aired down on these hard roads, just so long as you keep your speed within the 35-45 range. 55+ is not a good idea. If you were in 4wd on the dirt, don’t forget to shift back into 2wd while on hard roads.
Tools for Airing Down
Sure, you could just poke the end of a key against the springy valve stem core and let the air hiss out, stopping now and then check it with a tire pressure gauge. But there are much easier ways.
Types of DEFLATORS
The least expensive way to air down is with a deflator tool that is available at all auto parts shops and online. This consists of a gauge attached to a short hose. The end of the hose screws onto your valve stem. The rod that protrudes is then pressed inward and is used to unscrew and remove the internal valve core, which releases air quickly. Use the gauge to monitor tire pressure until you reach the desired psi. Replace the valve core. Then do the other tires. Be sure to keep a package of spare valve cores with you; sometimes one may spring loose into oblivion.
There are more expensive devices that can be set to stop at a preset psi.
Some people have invested in a quad hose system that connects to all four tires simultaneously with a central gauge/compressor that can be used to deflate or inflate all four tires at once.
However, the most popular system for deflating is to use a set of small, adjustable nodules that you simply screw onto each valve stem. These devices have a ring that can be set or calibrated to stop at a specific psi. You just walk around the car, attach to each valve stem, and then just wait for air to stop hissing out. Remove the devices, and you are done.
To calibrate them, you manually get the psi in one tire to where you want it, using a manual tire gauge. Then, attach the deflator, and turn the ring until you feel some air hissing. Back off the ring a partial twist until the hissing stops – and lock the ring in place.
Some brands of these deflators (such as J.T. Brooks) are laser engraved with psi; other brands (such as Staun) are not. In either case, I use a paint pen to denote my setting, just in case the ring gets loose.
The high-quality deflator sets, such as Staun and J.T. Brooks, run close to $100/set. There are cheaper imitation products as low as $35, but I think that the premium sets are worth the extra cost.
There are many tire deflator systems out there, so shop around and see what works for your budget. I would recommend going out on a trail run with a local offroading club so that you can see and try out different brands.
Don’t forget to purchase a good tire gauge. There are some excellent digital ones on Amazon for under ten dollars.
Another useful tip: Most of your modern SUV’s have a dashboard setting to display individual tire pressures. Use it to monitor the progress of your air down and air up. In my Sahara, my TPM (tire pressure monitor) is accurate enough that I almost never need my handheld gauge.
Airing Up
At the conclusion of the trail run, we will air our tires back up to normal street pressure, which is usually around 36 psi (but could be 33-38 psi). However, it depends on your specific tires and vehicle weight.
Short of driving to a nearby gas station (not really practical), here are your sources of air for re-inflating the tires.
The most common source is to use a portable 12v air compressor that you remove from the vehicle, hook up to your battery (engine running), and connect a hose.
Some offroaders have on-board mounted air compressors, so that they only need to connect their air hose.
Hardcore offroaders may carry a compressed air tank. The tank is high pressure, so it fills the tire very quickly; but it is bulky.
Sometimes you will see offroaders equipped with quad-hose systems. Instead of a single air hose that does one tire at a time, quad-hose systems octopus out into all four tires simultaneously.
Smittybilt #2781 air compressor
Here is an example of a Smittybilt (model #2781) 12v, 5.6 cfm air compressor. Note that it has a thicker power cord with clip-on battery leads for connecting directly to your vehicle battery. Besides Smittybilt, there are several major manufacturers such as ARB, Viair, Moreflate, and others – so ask and shop around. Make sure that the compressor is rated at least 5.6 cfm or higher, or else it will take a long time to do all four tires.
Here is an example of what NOT to get. This is for emergency roadside use: putting a few pounds of air into a street tire. It is neither powerful nor heavy duty, and will take you a long time to reinflate four AT/MT big tires. The unit, besides being very slow, will overheat if you do not allow it to rest every 10 minutes or so.
The first giveaway that this is a light duty air compressor (maybe 1 cfm?) is the thinly corded 12v CIG plug, which means that it does not draw very much current.
A stronger air compressor will draw more amps, and needs to be directly connected to your battery by clipping onto the battery terminals. Make sure to keep the engine running, or you will suck your battery dry.
Useful time-saver
• When airing up or even airing down – have your passenger monitor your tire pressures using your dashboard display.
• Ask them to signal you or honk the horn when each tire achieves the desired psi.
• Some jeeps come equipped with Tire Fill Assist, under Settings, which honks when each tire reaches 36 psi
No worries…
we got you covered!
When you join a Beginners Trail Run, don’t be concerned that you might not have proper deflators or an air compressor, or know how to use them.
Offroaders are a friendly, helpful community.
We will bring over our equipment and teach you how to air down or air up.
No one gets left behind. Trail Leaders and Trail Staff will not leave the Trail finish area until everyone is aired up and good to go.
