There are two distinct benefits to airing down (letting air out of your tires): mechanical advantage and personal comfort. Both of which can drastically impact your experience on the trail.
Disclaimer – BackroadVentures does not advocate the use of your vehicle, tire, or equipment in a way the manufacturer has not intended and doing so is at the full risk of the consumer/owner.
Let’s cover mechanical first. There are two advantages to be gained here: 1) Increased contact patch (where the surface of the tire meets the ground) and 2) more pliable contact patch. Reducing tire pressure allows the “footprint” of the tire to expand. A common misconception is that “airing down makes the tire wider.” While yes, it appears that the tire is wider because of the “sidewall squat,” the actual mechanical advantage is the tire contact patch is longer. This elongated contact patch provides more traction by increasing the amount tire tread that is in contact with the road/trail surface. Additionally, reducing the pressure allows the contact patch to mold to changes in the road/trail surface, like rocks and such. The contact patch being able to mold to the contour of the terrain allows the power to transfer to from the vehicle to the terrain with less slipping. This pliability also helps to reduce the likelihood of tread-face punctures.
Anyone who has spent time on washboard roads knows that having full, stiff tires does not help to alleviate the tooth chattering, dash shaking, “oh no my coffee is coming out the sip hole” experience. Reducing the tire pressure can help to absorb some of that by acting as the initial “shock absorber” in the suspension system. This is obviously subjective, and everyone will have different comfort levels, but the overall experience is generally improved.
Can You Run Them Low All The Time?
There are a couple of things to be aware of. Running tires at lower pressures makes the sidewall work harder, which is alright at slower speeds, but not good when driving faster on pavement. If you drive at higher speeds on pavement with soft tires, you may experience erratic behavior when cornering. More importantly, the friction generated by the sidewall changing shape will cause heat buildup and eventually delamination or failure of the tire.
All that said, the benefits are there. The biggest debate is how low you should go.
How Low Should You Go?
The pressure you choose is really a combination of personal preference and risk mitigation. The lower you go, the more the benefits are noticeable, but the likelihood of popping your tire bead off the wheel increases. Most people are confident going down to 15-20 psi without bead-lock wheels (if you aren’t familiar with bead-locks, they are wheels that use a ring rather than tire pressure to secure the tire bead to the wheel). With bead-locks, people will run in the single digits and often be just fine, but when is that really needed? Pressures that low are extremely beneficial in rock-crawling; not so much in general overlanding. For overlanding, and if you are new to airing down, test out different pressures to see what you like. Start in the mid-20’s, if you like it, try the low-20’s. For the majority of what/where/how I drive, I typically run my tires at 20-22 psi. I find it to be nice blend of steering responsiveness, traction, and comfort, considering the weight of my vehicle with expedition load.
KEEP IN MIND – if you air down, you must have a way to air back up.