by Paul Abelson, technical editor
In our February issue, we talked about the importance of maintaining good tire pressure. With new tires running at least $300 each, your investment could be $3,000 or more just for your tractor. Add in $2,400 more for your trailer. Even with recaps on drive and trailer tires, you could still have the better part of $3,000 invested in rubber. If that isn't reason enough to pay attention to your tires, think of what they are called on to do.
Tires support the vehicle, no matter if it is standing still or traveling 75 mph. Steer tires generate the forces that cause 80,000 pounds of loaded tractor-trailer to change direction, often while also carrying an undue share of braking forces due to inertia causing weight to transfer forward during braking. Drive tires provide traction for starting and stopping, often combined with cornering forces that won't throw the truck into a skid. Trailer tires must maintain stability while free-wheeling or braking.
Flexing and heat
The tire must do all this with the lowest internal friction (rolling resistance) possible so that fuel economy won't be compromised. The forces carried by the steel, Kevlar, carbon and rubber (the major ingredients of tires) are enormous. Air under pressure must also be considered as part of any tire's structure, because it is the right amount of air that gives the tire the ability to resist excess flexing.
If you've ever bent a paper-clip back and forth, you know that flexing generates heat. The paper clip gets hotter and finally breaks. Tire cords, also made of steel, flex with the tread. A tire does not stay perfectly round as it rolls under the load of the truck. If it did, we'd have only a thin line of contact between the rubber and the road. Instead, the tread flattens to bring about 50 to 60 square inches in contact with the road. As the tire rolls, a given spot on its tread comes into contact with the road surface, transmits any tractive or braking forces, then lifts up to follow the tire around. As it goes back to a circular path after being flattened, the spot springs up. Like
a spring rebounding until it reaches stability, the point on the tire's circumference flexes until the tire structure causes an end to this motion. When the point on the tire again contacts the road, the cycle starts again.
Heat is generated by this flexing. When air pressure is too low, tire temperatures can exceed 265 degrees F. That much heat can separate rubber into its original components. The rubber will melt at the point of flex, breaking the bond between the rubber and the steel cord, allowing the tread to separate.
Inflation and fuel economy
Maintaining proper pressure is the single most important thing you can do to protect your investment. Tire pressure also affects fuel economy, because the energy consumed by any excessive flexing is initially created by burning fuel in the engine. Therefore, tire expense (which is the second greatest operational cost in trucking) also has an effect on the most costly - fuel. Tire problems can decrease fuel economy up to 10 percent. A truck getting 6.5 mpg will burn almost 15,400 gallons in 100,000 miles. Increase that by 10 percent, and at $1.20 per gallon, you have added $1,846 to your annual operating cost.
Are you sufficiently motivated by these numbers? Consider instituting a tire program, just as the fleets do. A tire program starts with the selection of the right tires for your specific application. It involves maintenance beyond just checking tire pressure. It also involves preparation and good record keeping to track the actual results you're getting. Let's start by looking at tire selection.
We won't attempt to select between the major suppliers. They all have good products, and they tend to leapfrog the latest technology. If one gets a lead in traction or fuel economy, the others will close the gap quickly. We can, however, offer some suggestions on tread selection, with help from Al Cohn, marketing manager of Commercial Systems Engineering at Goodyear.
Al starts by reminding us that, "20 cents of every maintenance dollar is spent on tires. When selecting tires, you want to minimize uneven wear, maximize tread life, maximize retreadability and maximize fuel economy." Service conditions have a great affect on tire selection. Requirements differ for line-haul and P&D operations and even for a combination of the two. Types of loads, such as bulk and palletized loads, are as important as vocational differences. Vehicle parameters, such as wheelbase, horsepower, suspension type and steer axle location (forward or set-back) must be considered.
Tread designs vary, because they are optimized for the conditions of use. Smooth treads, of the type used on NASCAR and Championship racecars, would be OK if the tire is operated only on a smooth, hard, dry surface. "One way to look at a tread pattern," Al told us, "is to think of it as the division or separation of a smooth tread into smaller elements, arranged in a repetitive pattern of voids, ribs, lugs, slots and grooves."
Rib designs have the elements oriented in the circumferential direction. They provide a good, overall tire design that can be used most applications. Rib tires are used on steer axles due to good lateral traction and because they tend to wear uniformly. They are also used on trailer wheels because of those same wear characteristics.
Tire selection for drive axles becomes more complex. For on-highway use, many fleets prefer rib tires because they provide better fuel economy. Rib designs for drive tires have what engineers call higher groove amplitude. This means that the lateral displacement of the grooves is greater than straight rib tires. Increased amplitude improves traction by providing more lateral edges. But reduced amplitude improves wet skid and hydroplaning performance by providing a more direct path to channel water away.
Highway lug designs are used where forward traction is critical and fast wear occurs due to torque-induced slip. These tires have more void area than rib designs. They provide excellent traction in mud and snow. To maintain lateral stability, they often have closely spaced lugs, or ribs, in the center of the tread.
On/Off Highway tires have large, deep grooves with a great deal of groove amplitude. They provide even greater traction than other lug designs, but they have a higher ratio of voids to tread area, may wear quickly and provide poor mileage. Because of their deep treads, fuel economy may suffer. Look for angled grooves to promote self-cleaning.
Off-highway treads are strictly that. Everything takes a back seat to traction. Their net-to-gross area, the measure of the amount of surface rubber against the amount of void, is 55 to 65 percent, compared to almost 80 percent in a steer or trailer tire.
To increase traction, some manufacturers add small, individual voids called "sipes." In severe applications, they may contribute to chunking (sections tearing out of the tread) if the sipes are too close together.
All position tires strike a balance between traction, stability and fuel economy. They are most often found in larger fleets, where they simplify inventory. In drive positions, they do not do well in snow and other demanding situations.
Trailer tires must provide braking traction and lateral stability, and must not hurt fuel economy. Rib-type tires with low groove amplitude and a high net-to-gross area percentage are usually best for trailers.
Another variable - tread compound - cannot be seen by the buyer or measured without specialized instruments. Harder compounds resist wear, but give up some of the superior traction softer compounds provide. Tire company chemists and engineers are continuously working to develop compounds that improve both grip and tread life.
The inner structure of the tire casing consists of a number of wire cords running radially from one bead to the other. Over this, around the circumference of the tire, are several belts of steel cord. The belts keep the tread as flat to the ground as possible, and prevent squirm, or tread distortion. The radial cords provide support for the tread with enough flexibility to allow the tread to stay in its design position. The bead keeps the tire seated on the wheel rim and provides anchor points for the radial cords. The final structural element - air gives the tire its ultimate shape and firmness. Without it, tires would be ... flat.
Some operators run only retreaded or re-capped tires, citing lower initial cost and almost as much durability. Some take their own used casings in to be retreaded, since they know how the tire was used and (with any luck) not abused. Others avoid retreads, feeling they are avoiding risks of catastrophic failures. According to both the Tire Retread Information Bureau and The Maintenance Council's Tire Debris Prevention Task Force, properly processed and maintained recaps are as safe and durable as new tires. They travel millions of miles a day without incident. Examination of roadside tire debris shows the major causes of tire failure to be heat buildup due to low inflation pressures, and to physical damage from foreign objects. If there were no tire retreading industry, we would be short about 50 percent of the supply needed to fulfill truck tire demand. Retreading can also be looked at as recycling.
Retreaders provide the same choice of tread compounds and designs, with structures based on tire makers' casings.
Armed with thisknowledge, you should be able to sit down with your choice of suppliers (new or retreaded) to determine which tires are best for your applications. All manufacturers have guidelines based on tread pattern, structure and compound. You can use the Internet to get selection help, as all major manufacturers have sites.
In our February issue, we'll talk about what's involved in a good maintenance program. We'll also look at some of the latest technologies that promise to reduce tire costs, and we'll discuss some old-fashioned ways to detect tire problems while they're still correctable.