Back in September, 2005, the "Workbench" subject was "Tire Tech: Load Capacity and Tread Wear." At the end of that "Workbench," I promised that we would cover how to figure trail pressures the following month. For some reason, that kind of got pushed out due to space limitations, but here it is finally.
First, let me start by saying that there is no set value for a given tire on a certain vehicle under particular terrain. Multiple factors determine the proper air pressure. We can start with the tire itself. Size is naturally one of the main influencing factors, as well as type of construction, which is bias or radial or even bias-belted. Then there's the material used in the tire's construction; such as polyester, nylon, or steel; the number of plies; and even the tread design. For instance, a heavy outside lug on a tire will keep the sidewall more rigid than if the tire tread had a more rounded shoulder. This heavy outside lug can also cause the center of the tread to cup upward at extremely low pressures.
The sidewall height-to-overall-tire-diameter aspect ratio plays a very important part. It's easy to predict a lot more sidewall flex out of a 36-inch tire on a 15-inch rim as compared to a 36-inch tire on a 17-inch rim. Rim width also plays an important role in determining air pressure. An 8-inch rim with a 12.50 tire will offer better sidewall and rim flange protection, as well as maintaining a better chance of staying seated on the bead than, say, a 10-inch-wide rim. However, the 10-incher offers less chance of pinching the sidewall between a rock and the rim, plus one gains much better tire stability and a bit more footprint.
Vehicle weight is another important aspect to take into consideration. Remember, up to a point, it's the air in the tire that supports the vehicle's weight. More air = more weight. Just because you and your buddy both have the same tire doesn't mean you can run your fullsize truck at the same air pressure he runs on his CJ-5. OK, that's an extreme example, but you get the idea.
Now we come into driving conditions and driving styles. Take for instance, Four Wheeler contributor Jimmy Nylund and myself. We both own Jeeps of about the same weight and run very similar tires. However, he will run anywhere from 5 to 10 psi less air pressure than I do. Why this discrepancy? He drives slow and sure, no matter if it's uphill, downhill, rocks, or a sandy wash. I like to "get it on" wherever I can, allowing the racer part of me to come out. High speeds and extremely low air pressure just don't mix well.
Now, how do you pick what is the best air pressure? Trial and error play a big part. But remember, vehicle weight, tire size, tire construction, rim width, tread design, driving style, and driving conditions all come into play.
Find some place that you can load one tire with the majority of the vehicle's weight. Put one tire on something like a wheel-travel indexing ramp, or-OK-a big boulder, a tree stump, a hillside, or some steps. Get as much weight transferred to one corner as possible. Let the air out of that tire until it's got a really scary bulge in the sidewall (but keep it a bulge that you feel comfortable with, one that won't destroy the tire, but will offer maximum traction). This is a good starting point for air pressure for all the tires. This way, you know what the tire is going to look like under extreme conditions. Put your vehicle back on level ground and let a matching amount of air out on the other three tires. Measure all four tire's rolling radius-that is, from the ground to the center of the wheel hub. Adjust the air pressure until all four tires have this same rolling radius. It may be a different amount in each tire due to variations in vehicle weight from corner to corner. Now write this down in your log book (you do keep a log book, right?). You now have a good starting point to go up or down in air pressure as the situation demands or your driving habits necessitate. For those who don't have a way to air the tire back up other than a friendly service station, keep the air pressure at a higher level so you can drive on the pavement at a reasonable speed. Low air pressure and highway speeds cause a tremendous amount of heat buildup as the tread and sidewall flex, which can definitely lead to tire failure.
Better traction isn't the only benefit of lowered air pressure. The soft, flexible tires now act as a spring or shock absorber to not only offer a much smoother ride, but to also absorb impacts that may cause damage to mechanical components. They are able to flex instead of trying to remain rigid spreading the load of say a small sharp rock over more tire area, thereby not damaging the tire. The additional traction also prevents tire slippage, which makes us all much more responsible in keeping with the "Tread Lightly" program.
Proper tire pressure on the trail is just as important as it is on the highway. The benefits gained are many-just remember to match the pressure with the load, terrain, and driving style.
