Spec'ing the right truck tries

Next to fuel, tires are your biggest vehicle operating cost. Higher costs are often associated with poor choices when selecting tires. Tires are designed for specific applications and uses. Each has advantages and disadvantages depending on vehicle design and the expected duty cycle.
     To keep your overall tire costs down, tires need to be matched as closely as possible to your application. In this article, we will look at factors you need to consider.

Tire construction
    While most new tires sold today are radials, you can still purchase bias ply tires. A bias ply tire is so named because it has overlapping crossed layers of cord material typically made with nylon, polyester, or other materials. The crossed plies run diagonally all the way from one tire bead to the other.
     Extra plies are sometimes used under the tread area to stiffen the crown area and make the tire more resistant to punctures. Bias ply tires also tend to have stiffer sidewalls and can handle more punishment from hitting objects.
     If you are expecting to spend a lot of time off-road with your trucks in areas where there are a lot of rocks and other obstacles, bias ply tires may be the best choice. Bias ply tires also give drivers better feel and handling and they're cheaper than radials. However, radial tires have become more durable and can be used in a lot of mixed service applications. Radial tires are so-named because they have cord material running in a radial or direct line from the bead. They typically have one steel body ply or multiple plies of other materials.
     While radial sidewalls are not as stiff as bias ply sidewalls, the tread area of a radial has more rigidity. This gives you longer tread life, more resistance to tread area punctures, and better fuel-efficiency than bias ply. Radials tend to run cooler than bias ply, so if you will be driving extended periods on-highway, radials will be the best choice. "Heat is the tire's enemy," says Al Cohn, manager, training and technology for Goodyear Tire Co. "The more you can do to keep a tire running cooler, the longer the life of the tire."

Tubed vs. tubeless
    Running a tire with a tube would seem to provide you a little more insurance against punctures, but there are a lot of advantages to going tubeless in most applications. A tubeless tire uses a single-piece wheel while a tube-type tire needs a special multi-piece wheel assembly. Using a single-piece rim reduces the risk of injury during tire mounting.
     Tubeless tires also offer better bead durability because there is less brake drum heat from the higher wheel clearance. Crown and sidewall durability is better because the casing runs cooler. Lateral stability is improved with the lower section height.
     The simpler wheel assembly also is lighter, saving valuable pounds if you are loading your trucks to the limit. And when you do have punctures, downtime is lower because it's easier and quicker to change the tire.

Keeping a low profile
     If you are loading and unloading a lot, it's in your best interest to get the deck height down. It's easier on the equipment operator and reduces the risk of injuries from climbing up or climbing down.
     "You can get the deck height down with low profile tires," says Goodyear's Cohn. "These are tires which are basically wider than their section height. The ratio of section height to section width is usually between 70 per cent and 80 per cent."
     As well as getting your deck height down, the lower section height also reduces the amount of irregular tread wear on steer tires and gives you better stability and handling. Low profiles are also lighter and you don't have to pay as much federal excise tax.
     The downside is you are at greater risk of sidewall damage from curbs and other solid objects. "You also need to be aware of the impact on your truck's drivetrain and gearing," says Cohn. "A smaller tire will rotate faster than a standard size tire at the same speed. To stay fuel-efficient, you may need to adjust the transmission and rear axle ratios so the engine RPM isn't too high at highway speeds. As a general rule, you should make a gearing change if the tire RPM changes 3 per cent or more."

Load limits
     Tires play a crucial role in supporting the vehicle's load, so it's important that you consider maximum axle loading in the tire spec'ing process. All tires have specific load limits at defined speeds and inflation pressures. The Tire and Rim Association has established these limits and publishes them in tables. They are also available through Goodyear and other tire manufacturers.
     To figure out what tire you need, you first determine the maximum expected loads at the wheel position. Then look at the tables, find your tire size, and look up the load that is close to, but slightly more than, the maximum anticipated load. Loads are shown for both single and dual applications. If you run duals on the rear, the allowable load will be less than with singles. This is to reduce the risk of overloading when one tire in the dual assembly is underinflated or flat.
     Since duals are designed to carry the load evenly, it's important not to mismatch tires when replacing a dual tire. "If one dual is larger than the other, it will obviously carry more load and wear faster and unevenly," says Cohn. "The smaller dual will also wear unevenly because it's forced to scuff over the road surface."

     Cohn says if your tire size is 8.25R20 or smaller, the overall diameter of the replacement tire should be within 6.3 mm of the original. If the tire size is 9.00R20 and higher, it should be within 12.7 mm. "The closer you can get to the diameter of the matching tire, the better," Cohn stresses.

Watch air pressure
    Without air, tires have no load-bearing capacity. "It's air which actually supports the load, not the tire itself," says Cohn. "That's why it's important to maintain the inflation pressures shown in the load tables."
     Each tire is rated at a specific load based on inflation pressure. The lower the pressure, the lower the weight it can carry. Underinflation also leads to higher heat, greater risk of premature tire failure, and reduced mileage. "A tire that is 20 per cent underinflated will result in 16 per cent fewer miles to removal," Cohn says.
     Overinflating is not recommended either. It will give drivers an uncomfortable, bouncy ride and the tires will develop excessive centerline wear, resulting in early removal.

Tread selection
    The type of tread you select can have a significant bearing on your overall tire costs. A block-type tread is good for traction on drive wheels, but it will increase the vehicle's rolling resistance and increase fuel usage. "The deeper the tread, the more flex there will be," says Cohn. "More flex generates more heat and that's what hurts fuel mileage."
     You should choose a tread which best suits the terrain you will be operating in and your mileage goals, says Cohn.

Retreading
    Retreading tires is a great way to cut overall tire costs. Tire casings which can be recycled two, three, or more times provide savings in a number of ways.
"The key to successful retreading is protecting the casing," says Cohn. "The casing's two biggest enemies are heat buildup and moisture penetration. Heat buildup is caused mostly by running underinflated or overloading the vehicle, or both.
     "When moisture gets into the tire, the steel belt package rusts and the tire's structural integrity goes. Goodyear helps reduce this with its Enhanced Casing Design (ECD) which is included in all of the company's mixed service radial tires."
     According to Cohn, ECD includes a non-metallic "polyamide" top belt which provides a protective barrier against moisture. It also incorporates special compounds which can lower running temperature by 25 to 40 degrees F and extend tread life by up to 24 per cent. "All this adds up to casings that can run more miles to removal and maintain integrity," says Cohn. "And that equates to lower overall tire costs."

September/October 2002 issue

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