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Modern Trucking Techniques
Tires: Your greatest maintenance expense
Don’t they deserve more respect?

by Paul Abelson, technical editor

Tires are arguably the most important maintenance item on your truck. Think about what your tires have to do:

  • They support the entire weight of your truck.
  • They cushion the ride and help protect you and your load.
  • They provide lateral stability.
  • They transfer braking force to the road.
  • They transfer acceleration to the road.
  • They provide traction in a variety of weather conditions.

Tires represent the single greatest maintenance expense you have, taking more than 20 cents of every maintenance dollar, according to studies by the Technology & Maintenance Council (TMC). 

With tires being so important, you’d think they’d get some respect. But what do we do? We look at them, and if one is not obviously flat, we walk around and pound them with a blunt instrument trying to detect low pressure with our ears. Occasionally, we’ll pry some large stones from between the treads. Until one goes flat or wears out its treads, that’s about the only attention tires get.

The more conscientious among us may gauge the tires every few weeks or so. For many, it’s most often the “or so.”

Tires should have their pressure adjusted for load and speed. Tires should be checked for irregular wear. Just those simple steps could extend tire life substantially and save you thousands of dollars annually.

Air pressure’s the key
Whenever I write about tires, I start and end with air pressure. That’s because air is a structural part of every tire, as much as the cords and the rubber. In fact, the cords are there to shape the rubber casing that holds the air in, while the air supports the tread that provides the traction. If you think of air as part of a tire’s structure, you can understand that running with less air pressure than the tire maker specifies is like running with less steel in your chassis or with fewer leaves in your springs. It will work for a while, but it’s a recipe for overstress and premature failure. 

Check all your tires with an air gauge at least once a week. 

While you’re there, look for uneven wear. It’s a sure sign of imbalance or misalignment. (More on that later.)

Without enough air in your tires, they flex too much. They heat up. Rolling resistance increases, and fuel economy drops. Tires are more susceptible to damage. Treads wear unevenly. All these cost you money. Here’s how things happen.

Flexing: The heat is on
As tires roll under load, they flatten where the tread first contacts the road, then they spring back past their original shape and eventually return to equilibrium. To better understand, visualize a point on the tread. As the tire rolls, this point travels in an arc about the rolling center of the tire (nominally at a radius of 20 inches or so). The point comes into contact with the road, then, as the tread flattens, the point moves closer to the axis. When its radius is perpendicular to the road, the point starts to move away from the axis until it is no longer in contact with the road. But the tire is not a point. The tread has mass and inertia, so the point keeps moving further from the axis until it is restrained by the tire cords. The point then moves back up the radius, finding its original distance from the axis. Then it rotates back to the road, and the cycle starts again. Each individual point on the tire’s circumference goes through this. 

If you were to travel alongside the tire as the truck was moving down the road, the tire behind the tread contact area would look like a wave that never changed position. Tire engineers call it a standing wave because it doesn’t seem to move, although the tire’s components are in constant motion. The standing wave is actually the tire structure flexing. 

Flexing causes heat. Take a paper clip and fold it open. Gently but quickly flex the clip, then feel the center after a few seconds. Be careful you don’t burn your fingers. Soon the wire will break. 

At 60 mph, a typical truck tire turns 500 rotations a minute, or more than eight per second. That’s 16 back-and-forth flexes for the steel belt in your radial tire, and 16 flexes for the side cords. At 75 mph, it’s 20 per second.

How far the point moves, how deeply the tread flexes, depends on load, speed and air pressure. Load pushes down on the tread contact point. The heavier the load, the longer the contact patch and the deeper the flex. The faster the rotation, the greater the inertia (a function of velocity squared) and the deeper the flex. The greater the air pressure, the more structural support inside the tire and the less the flex. 

Of course, when pressure decreases, load and speed intensify flexing. Deeper flexing means more heat is generated (internal friction). If internal tire temperatures exceed 265 degrees F, rubber liquefies. It doesn’t melt, but the vulcanization process is reversed and the rubber converts to oil and carbon. Normally, tires run at 180 to 200 degrees. For every 2 psi a tire is down, internal temperature rises 5 degrees. If a tire is down by 30 psi, its temperature could be 275 degrees. If pressure should be 110 psi to handle higher speeds, and a tire is at 80, its internal temperature will exceed the point at which the tire starts to weaken.

Heat builds up right below the tread plies. Look closely at the tire debris along the highway. Most pieces have wires sticking out from the sides and ends of the rubber. Those are not recapped tires that have thrown their treads. If they were, there would be only rubber, because there is no wire cord in a tread section. The cause of most thrown treads is heat buildup due to underinflation.

Speed and pressure 
Every tire manufacturer has inflation tables for each of its tires for each position. Steer tires take a heavier load, so they should have more air. If you’re running retreads, use the table for the casing maker, not the retreader. Fully loaded (80,000 pounds) out west at 75 mph, you should be using load range H tires, probably at 110 to 120 psi to handle the speed. If you’re limited to 60 to 65 or cubing out before you hit gross weight, you could probably run a bit softer, maybe as low as 90 psi. Check the tables. You could always go up to 10 percent higher as a safety margin, but don’t exceed the maximum pressure for that tire. It’s molded into the sidewall. 

Energy from your fuel gets the rig moving and keeps it rolling. It also overcomes aerodynamic drag and rolling resistance. Up to about 60 mph, the three are about equal. After 60, because drag increases with square of vehicle speed, it becomes a greater factor in proportion to the speed. 

Rolling resistance includes internal friction in the engine and drive train plus the friction of the wheels and tires. Modern lubricants and component design have reduced internal losses over the years, as have improvements in tire contours and materials. But no matter how designs improve, soft tires will still flex more and generate heat from internal friction. 

Rib-type tires flex less than lug-type, and because there is less void area, they wear as well or better than deeper tread lug tires. Remember, flexing consumes fuel. It takes energy to flex a tire, and that comes from fuel. That’s why tires contribute significantly to fuel economy.

If you can, use rib tires for on-highway use. If you run through snow and need lug tires, consider modified lug patterns or use those tires only in winter. 

Deep lug patterns help keep traction off-highway and in snow, and deep treads help channel rain from under the tire. The law requires at least 2/32-inch tread (4/32-inch for steer tires) to be legal. Wise operators remove their tires from service at 4/32 or 5/32 tread depth for two reasons. First, tire casings with more tread on them are worth more to retreaders. If you have your own tires retreaded (that way you know their history), the retreader will have a better casing to work with. Also, when tread gets shallow, it cannot pass water away fast enough in heavy rainstorms. When water accumulates under the tire, the tire rides onto the water and all traction is lost. To avoid this hydroplaning, change tires at 4/32.

Other things to watch for
Earlier, I mentioned inspecting your tires when you gauge them weekly. Tire surface is a good indicator of alignment, and misaligned tires hurt fuel economy and reduce each tire’s grip on the road. Misalignment of all five axles can reduce fuel economy by more than 2 percent. Out of line wheels are pushed or pulled slightly to the side, scrubbing off rubber. That’s how they wear unevenly. The effort it takes is from energy (fuel) that is wasted. 

A misaligned tire is already in a partial skid. Just 1/4-inch out of line results in 10 feet of side travel every mile. It also takes less effort to break traction than if the tire were rolling normally. 

Another condition that causes tires to scrub is mismatching. Keep tires matched as closely as possible, not just by nominal size but by actual measured dimensions. 

It may be impossible to match tires exactly, especially after a tire is replaced. As soon as you can, try to match tire diameters within 1/4 inch, or 3/4 inch circumference. With a 1/4-inch mismatch, the larger tire can carry 600 pounds more load than the smaller. Since they are bolted together, they turn together even though their circumferences differ. The larger tire will wear out faster. Always mount the larger tire outboard to take advantage of road crowning. 

Dual tires can be perfectly matched by size, but if pressures differ, the higher-pressure tire will carry hundreds of pounds more than its mate.

Next issue We’ll continue our discussion of tires with a look at some worthwhile accessory components, a discussion of road service and a review of TMC’s newest edition of its perennial best seller, “The Radial Tire Conditions Analysis Guide.”

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