AT FIRST glance, tires mounted in steer positions don't seem to deserve a lot of attention, says Joe Zekoski, director of commercial tire technology, The Goodyear Tire & Rubber Company. “After all, they make up a small percentage of the total tire costs on most vehicles,” he says.
“For example, on an 18-wheel tractor-trailer, a carrier would typically have around $600 invested in steer tires, $2,400 in drive tires (when new), and around another $1,040 in retreads on the trailer (using $130 per tread as a benchmark). That puts steer tires at under 15 percent of the total tire investment.”
But the numbers can be deceiving, Zekoski points out. Although steer tires represent a small portion of the initial investment, they are subject to more wear and abuse than tires in other positions, making them the most expensive tires to run on a cost-per-mile basis.
Steer tires usually carry more load than any other tire on the vehicle. A typical drive or trailer tire will carry less than 4,500 pounds, while a steer tire is usually asked to carry 5,000 to 6,000 pounds.
Side forces
But the biggest pressure on the steer positions comes from the side forces. Every time the driver turns the wheel, the steer tires encounter significant lateral forces as they fight the tendency of the truck's tandem rear axles to go straight ahead. These forces are at their greatest during hard cornering or in an emergency turning maneuver.
Side-forces away from the direction of the turn cause scrubbing across the tread surface, which leads to rapid tread wear, particularly on the outer rib. Since drivers tend to turn more sharply to the left (sight side) rather than to the right (blind side), the right steer tire tends to get scrubbed the most.
Setback front axles, characteristically 13 to 15 inches back from the standard position, are designed to improve weight distribution as well as the truck's turning circle. But that also increases the side forces on tires in turns.
“For example, steer tires on a tractor with a 140-inch wheelbase must generate about 65 percent more cornering force to slide the tandem drive axles around a corner than steers on a 210-inch wheelbase tractor,” says Zekoski.
The setback axles also incorporate increased wheel cut angles, which add to steer-tire scrubbing when turning. For many years, the standard industry wheel cut angles have been between 32 and 34 degrees. Setback axles are typically at 42 to 44 degrees.
While side forces on steer tires can accelerate tread wear, higher loads on steer tires can actually help even tread wear by providing a larger, squarer footprint where the tire contacts the road surface.
Testing by engineers at Goodyear has proven that vehicles with lightly-loaded steer axles (10,000 to 10,500 pounds or less) are more prone to irregular steer-tire wear than those with heavily-loaded front ends.
“With light loads, the tire's contact area is quite long in the area of the center tread rib and much shorter toward the shoulder rib,” says Zekoski. “Since the footprint is uneven, there is more scrubbing action and wearing away of the shoulder rib.
“It should come as no surprise that the most frequent steer-tire problem is fast-shoulder wear, which can lead to cupping and early removal of the tire. Instead of running steer tires down to 6/32 remaining nonskid and over 100,000 miles before removal, you might get only 70,000 miles out of them before you have to remove them for worn shoulders, while the center portion of the tread may have over 10/32 of remaining nonskid.”
Significant threats
Two other significant threats to steer-tire performance are misalignment, which results in uneven wear and underinflations, which causes excessive casing heat, thus reducing the tire's retreadability or possibly leading to something far more serious: an on-road failure. Unlike a failure on one of a pair of dual tires, there is no support when a steer tire fails, and the driver may struggle to keep control of the vehicle.
“With all the other pressures on truckers these days, the fewer worries, the better,” says Zekoski. “By investing in advances in steer-engineering, truck operators can strike steer tires off their worry list. More importantly, they'll be able to feel more confident about lowering the total cost per mile of their tires.”
