Air Pressure On and Off Road
© Copyright 2000 - 2007 Chuck Kopelson 01/08/2005
Updated September 22, 2007
How to Get the Correct On-road Tire Pressure
Your best bet is to start at the manufacturer's recomendations usually on a sticker in the vehicle. For Hummers pressures between 30 and 40 cold usually work fine. I like 37 with my GSA's and MT/R's. At too high pressures a tire tends to balloon out Instead of having a flat area touching the road. The tire will wear in the center. Too little pressure and the tread wears the edges. I've owned both of these tires and had very even tread wear at these pressures.
One way to get the proper inflation on big bias ply tires is to chalk them . Make a heavy mark all the way across the tire. Then drive it a few blocks and check it. If the chalk is gone in the center then let air out. If it's gone on the edges then add air. If it's just lighter all the way across then you have it just perfect.
Tire Footprint When Aired Down; Radials vs. Bias ply
Take two identical tires in size, width, height, tread width, tread style, air pressure, and load rating and ink them up like a rubber stamp. Put them on a truck and lower them onto a piece of white paper and you'll get the footprint of the tires. That footprint would look somewhat oval in appearance, but would cover an area that you could measure in square inches.
The footprint that you just took would show that at the same pressure the radial would have more square inches on the ground in a slightly different shape than the bias ply tire. The radial tire has a longer footprint with the same width as a bias ply tire which in essence gives the radial tire more of a track type tread like on a tank. That's one, but certainly not all of the reasons a radial tire performs much better on the road than bias ply tires. The longer tread track adds substantially to the handling characteristics of the vehicle and with the typical, but not always, steel belts, the tire will make very little noise in lateral "G" cornering tests. Compared to a bias tire, a radial will hold with almost no squeal until it gets close to breaking loose. A bias tire will start squealing and just keep getting louder all the way to the break loose point.
The radial tire provides a lot more off-road flotation at higher pressures compared to the bias ply tire. That's one of the reasons you don't need to lower the pressure on a radial tire as much as you would on a bias tire to get the same performance. I'm an instructor for the LA County Sheriff's Department and have shown this fact in many demonstrations to the Deputies in each class. Below is a tread contact patch example to demonstrate what happens with the footprint when you lower the tire pressure on each type tire.
The radial tire starts out with a longer tread patch and more square inches on the ground than the bias ply tire. Lowering the air pressure the radial tire gains a little in width, and a little in tread patch length, where the bias ply tire gains substantially more in width than the radial tire, but not a lot more in tread length. The radial tire still ends up with more square inches on the ground and "generally" performs better in similar situations.
There are, obviously, exceptions to every rule. One example would be rock crawling. When the pressure is lowered in a bias ply tire it will be a lot more extensible and have a tendency to literally wrap around and grip the terrain more than the Radial tire will.
In the rather limited testing that I have done with bias ply tires, we had to deflate them to the point that the sidewall easily folded over before there was much increase in footprint (measured subjectively). The tests we ran were not scientific as would be done at a tire company. This test data is hard to come by, as you typically will not have identical tires available in both radial and bias ply. We've done these tests in years past where we had some bias ply tires made up from our radial tire molds. You cannot get accurate results when you compare 2 different tires.
Generally speaking the bias ply tires need to be aired down more than the radial tires for a good footprint. For example, if you were to use identical 33/12.50R-15 tires, in bias ply and in radial ply both with a street pressure of 28 psi on the same piece of terrain there are situations where you would need to air down the bias ply tire to gain the same amount of performance as the radial. What you are really trying to accomplish with air reduction, is to get the perfect amount of flotation and traction. Too much air will not provide the flotation, and too little air not enough traction. Using the above example, you may never have to go below 18-20 psi on the radial tire where you may have to go as low as 10 psi on the bias.
Another consideration is the terrain you are traversing. If you are in an area of sharp rocks you don't want to be aired down too much or you will cut your sidewalls. Generally, sidewalls in radials are not as tough as bias tires because radial sidewalls have to flex.
In a different test, I measured the tire axle height (changed by reducing tire pressure) and compared that with the effective radius (circumference, actually) of the tire and was surprised by the result. Lowering the tire pressure in the radial tires did not significantly affect the distance traveled per tire revolution, while with the bias ply tires it had a direct and proportional effect (the lower the axle, the shorter the distance the truck moved per tire revolution). This is because the bias tire, tread and all blows up and shrinks like a balloon. The radial tire has a fixed belted radial tread that always remains the same length.
Doesn't more footprint give more traction and flotation?
I'm assuming that traction is the amount of friction between the tire and the road. There are all kinds of things that will happen with lower air pressure, depending on the terrain. A couple examples would be mud, snow, and sand. In mud and snow, too little pressure and you get a lot of flotation where the tires will have a tendency to keep the vehicle on top the surface where it will slip and slide all over the place. You may get from point A to point B, but not in a very straight line. When the pressure is too low you may not get stuck, but you will have a problem keeping up your forward momentum which may result in you wandering all over the place. In this case, you'd be better off with a bit more air pressure.
The same thing happens in the snow. There are some instances in snow and off-road where a tall, skinny tire is much better. In areas where you have permafrost conditions a skinny tire allows the tire to cut down through the loose unpacked snow to the hard ground, and get traction. A narrow tire doesn't plow the snow like wide high flotation tires. That's why trucks with snow plows usually have tall narrow tires. In the sand, too little air, and the tire doesn't work well either, since the tire tread area will start to fold over itself and create less flotation and traction. In the case of fairly round tires with little or no tread, it can, in some situations actually help, since the folding and rolling of the tread area of the tire starts to act like a paddle tire. In this situation, too little air pressure can actually help.
The amount of air pressure you use is not an exact science and does not always fall into a convenient category. You need to have experience with your specific setup and be able to feel and constantly adjust for varying conditions.