We all know, or if you’re a newbie you have at least heard, that lowering the air pressure in your tires can help improve traction on dirt, sand, and even rocks. However, it’s a concept that not everyone understands. Why does it aid traction when off-road? How does it all work? We set out to explain and graphically represent the how and why of something many of us have long accepted but may have not fully understood.
There are upsides and downsides to almost everything, and lowering tire pressure is not excluded from this axiom. Lowering the air pressure (pounds per square inch, or psi) in your tires is only recommended for off-road travel. When tires are filled to their maximum cold pressure, only then do they attain their maximum load-carrying capacity for highway use. When you lower the pressure, you are also lowering load capacity. Dropping from 35 to 20 psi can cut the load capacity by as much as 500 pounds.
At maximum inflation pressure a tire has a mild bulge in the sidewall. That bulge becomes larger as the air pressure is dropped for off-road use, exposing the sidewall to hazards such as sharp rocks and sticks. The sidewall also flexes more at lower pressures as the tire rotates, and that flexing creates heat. This increased heat is not a problem when driving slowly off-road, but increased flexing and the associated heat at highway speeds is one of the reasons under-inflated tires fail on the highway.
Once back on the highway after an off-road adventure, you should always air up your tires (still warm from a day’s drive) to a few notches below the cold pressure maximum. You don’t want to exceed the tire’s maximum pressure rating, as over-inflation can also lead to failure.
Lowering the inflation level for off-road use can also make tires easier to roll or spin off the wheel. How low can you go without encountering a problem? That can depend upon the tire and the wheel, but in general, we have found that beadlock wheels are a good idea with most tires to avoid the tire bead/wheel rim separation blues if you run pressures of 10 psi or lower.
So how and why does lowering air pressure help traction off-road? We used a fairly common tire size and a well-known brand. From its maximum cold inflation pressure of 35 pounds per square inch (psi) to 10 psi in steps of 5 psi, we created footprints (using black paint) on sheets of rigid plastic of a BFGoodrich 33×12.50R15LT tire. The tire was on the front driver’s corner of a ’79 Jeep Cherokee with a base weight of 4,390 pounds. Each footprint was measured (length and width). Although this isn’t perfect science, the measurements taken were used to calculate a footprint size in square inches. Then each footprint was compared visually and numerically (in psi). What we learned was interesting, to say the least.
A key to accurate monitoring of tire inflation pressure is a high-quality pressure gauge. Get one. That pen-style gauge with the numbered plastic stick isn’t a whole lot better than the highly inaccurate gauges in the gas station air hoses.
The dramatic difference seen here between the contact patches of the tire at 10 psi and 35 psi graphically illustrates just how much inflation levels can improve the traction capability of a tire. Not only is the weight per square inch half of what it was at 35 psi (offering greater flotation on sand), but the hugely increased size of the contact patch also offers improved traction and flexibility on rocky surfaces.