A lot of you probably know to equalize your tire pressure without really thinking about exactly why it is important. Consider the tire as a big balloon: The more air you put into it, the bigger it gets; less air, and it gets smaller. If you put more air in the tire on one side of the vehicle than on the other side, it’s easy to see that one tire is going to have what is referred to as a rolling diameter that’s larger than the other tire.
What happens when you go down the road in a straight line with these tires at different rolling diameters? The bigger tire is going to cover more ground at a given speed than the smaller one. It’s like two people walking side by side, except one person has a 30-inch stride and the other a 36-inch stride. The person with the longer stride is going to walk further than the one with the shorter stride over a given timeframe. In this case, the two tires are connected by the axle, and luckily, with a thing called a differential. It does exactly what its name implies: it allows for a difference in wheel/tire speed through a special set of gears. Without this differential, one tire would always have to slip and skid to keep up with the other. Left-turn-only race cars (i.e., circle trackers) often use a locked rearend which has no differential action. These vehicles use what’s called “tire stagger.” This refers to a setup with an outside tire that’s larger in circumference than the one on the inside. The tighter the track (or circle), the larger the difference between tire sizes. With the outside tire having to go a greater distance than the inside tire, overall tire speed is balanced.
It’s pretty easy to get tire stagger when you don’t really want it by not having equal tire pressure. That brings up the subject of using a spool or “welded” differential on an off-road vehicle. Yes, it is often referred to as the “poor man’s locker.” The same problem exists with a locking differential like an ARB Air Locker or other similar types, but fortunately these can be turned off when not needed. With no differential action, both tires have to turn at the same speed. Tire speed changes whenever the vehicle is turned. Even a slight variation in tire diameter, be it from air pressure or tread wear, can greatly affect the way the vehicle handles and can be quite hard on axleshafts. Now we also have to deal with just not turning but straight-line driving.
With locking differentials that utilize special gearing or clutches, they may sense this unequal tire size as a “stuck” condition and figure they need to help out and pull both tires equally, so we end up with rapid tire wear, as well as extra load being placed on the axleshafts. On clutch-type differentials, the clutch plates themselves can wear out in a relatively short number of miles. Unequal rolling radius can also cause some very strange and potentially dangerous driving and handling problems in your vehicle, especially for those who remember running a ’70s-style Detroit Locker.
Besides these problems, we have something else called “slip angle.” This happens when you turn a corner. It’s the angle of the tread surface as compared to the direction of vehicle travel. Because of sidewall flex and distortion, the tire’s tread will not be directly in line with the way the tire is pointed. Slip angle is very much affected by tire pressure as well as vehicle weight, tread design, rim width, and tire construction. That’s why it’s important to always make sure to run the same make and designed tires on the same axle—and in reality, on both axles.
Varying the air pressure on front or rear tires changes the slip angle, which will affect over or understeer conditions. This is used as a method to “tune” the handling on race vehicles. Generally speaking, air pressure on a street-driven vehicle is adjusted to a pressure that meets the compromises of tread wear, ride quality, handling and load capacity.
Just because a tire’s sidewall says “45 psi maximum inflation pressure” doesn’t mean you have to run 45 psi. There are also some other numbers in front of the air pressure. They might be in the form of something like “load capacity 2,500 pounds.” This means that the tire will safely support 2,500 pounds at, in this instance, 45 psi. Since not many 4x4 vehicles weigh 10,000 pounds (2,500 x four tires), it’s easy to assume that we can run considerably less air in each tire. Most tire stores have a book available that shows load capacity at different air pressures for a given tire. How much inflation pressure can be reduced is again determined by vehicle weight, handling, ride quality and the other factors mentioned above. In fact, on some vehicles like pickup trucks, you may not want to run equal air pressures, front to rear, depending on what’s carried or not carried in the bed. (Figuring the variations to use here is like advanced calculus. The easiest method would be to accurately measure the tire’s loaded height and then adjust the air pressure accordingly.)
Let’s say if the front tires have a lower air pressure than the rears, or have a heavier load on them at equal pressure, then it’s just like having a different tire size altogether. Generally speaking, when in four-wheel drive, power is being delivered equally to the rear and the front, and that due to a direct connection through the transfer case, they’ve driven at the same speeds. However, with tires at a different rolling diameter, all sorts of things can happen. Not only are the tires, differential, and axleshafts now under excessive loads, but so are the front and rear driveshafts, U-joints, transmission, and transfer case. Usually the U-joint goes first, but I’ve seen twisted driveshafts and broken transfer cases—and I don’t mean just the gears within, but the case itself.
Something else I should bring up about tires: Just because they’re marked a certain size doesn’t mean that they are the same exact size. This is especially true between brands and/or tread designs. I know of a guy who was running Gleason differentials in both ends of his Quadra-Trac Wagoneer. The handling was terrible, as it was darting and jumping to the point of almost being out of control. Finally I got him to measure the circumference of each tire, and, to his surprise, there was a ¾-inch difference between the tires.
Keep a close eye on air pressure. It can be the solution to better tire tread life and longer drivetrain life, as well as better handling.