Last month, I talked a bit about bias, bias-belted, and radial tires. This month, I want to continue a bit more on tires-or tire pressure, to be exact.
If you remember, I said it's the air that supports a vehicle's weight, not the tire. The tire is, in some ways, merely the container. Let's take a 31x10.50x15LT with a C load rating for example. The Tire and Rim Association lists it as capable of supporting 1,400 pounds at 20 psi. Add 5 psi at a time, and capacity jumps up to 1,595, 1,775, 1,945, 2,100, and finally, its maximum rating of 2,250 pounds at 45 psi. This is for a bias-ply, as radials need 5 psi more at each weight step.
Load capacities don't go below 20 psi because they're based on highway travel, and generally speaking, tire size is matched to vehicle weight. But according to the Association, just one of these tires could pretty much support the weight of a stock CJ-2A. This is why, on a 4,000-pound vehicle, we can get away with running only 10 psi in the tire. That is, if we limit the speed. Remember, the higher the speed, the more tire revolutions per mile, which with lowered tire pressures means a lot more sidewall flex, which produces dangerous heat buildup, which in turn can lead to tire failure.
Just how much air to run? Well, on the street it's a pretty good bet as a starting point to follow the Tire and Rim Association's guidelines. Any tire shop should have a book you can look at that lists the various size tire's capacity at a given psi. Naturally, you're going to have to know your vehicle's actual going-down-the-highway weight. However, remember this is just a guideline, as tire design and rim width will cause some variations, as will driving style.
To get exact pressure, you're going to have to do a bit of work yourself. Start out with a given pressure: For instance, the minimum that meets the vehicle's weight requirement. Let's say it's 25 psi. You're going to need a couple of tools, such as a tread depth indicator, a piece of chalk, and a notebook to keep track of the data. Measure the tread depth in several locations across the face of the tire. Let's say the outside edges read (in increments of 11/432 inch) 15 and 15, but 2 measurements near the center read 13 and 12. This means there is less tread in the center than on the outside edges and is a pretty good indication that you're overinflating the tires, causing them to "crown." If the tread depth goes from a high to a low across the face from edge to edge, it's an indicator that the tire is not running straight down the road, so an alignment is necessary.
Make a note of these measurements and where they were taken. Now, with the chalk, make a wide line across the tire's tread. Drive the vehicle in as straight a line as possible until the chalk starts to wear off. As the chalk wears off, it indicates where the tire is making the most contact. Too much air pressure, and the center wears. Too little pressure, and the outside wears. If the chalk wears off in an uneven line, there is an alignment problem. Keep adjusting the air pressure until the chalk wears off evenly.
You are doing this with all four tires, right? Front-to-rear weight differences are almost always different. To meet your ride quality or handling criteria, you may have to adjust the air pressure up or down a few pounds.
Every 1,000 miles or so, run your tread depth indicator across the face of the tires. This is where your notebook comes in. You've got a record of past wear patterns. Now you can make any necessary adjustments in air pressure. Naturally, don't forget to make necessary air pressure adjustments when you rotate the tires or change the vehicle's overall weight.
Remember, we are only talking highway pressures here, but next month we'll continue with how to figure off-highway pressures.
