Spec Smart: How to “steer” clear of trouble in selecting tires
At first glance, tires mounted in steer positions don’t deserve a lot of attention. After all, they make up a small percentage of the total tire costs on most vehicles.
For example, on an 18-wheel tractor-trailer, you 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 retread as a benchmark). That puts steer tires at under 15% of the total initial tire investment.
But, according to Joe Zekoski, Goodyear’s director of commercial tire technology, these numbers can be deceiving. Although steer tires represent a small portion of the initial investment, he says they are subject to more wear and abuse than other tire positions, which actually makes them the most expensive tires to run on a cost-per-mile basis. (Goodyear has done extensive testing on steer tires and a few years ago introduced its Enhanced Casing Design to addresses some of the challenges.)
Zekoski explains that steer tires typically 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.
But the biggest pressure on the steer positions comes from the side. 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, typically 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% more cornering force to “slide” the tandem drive axles around a corner than steers on a 210-inch wheelbase tractor.
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.
Extensive testing by engineers at The Goodyear Tire & Rubber Company, for example, 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.”
Two other significant threats to steer tire performance are misalignment which results in uneven wear, and underinflation 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.
Goodyear and other tire manufacturers have evolved steer tire design in recent years by adding features such as cooler running tread compounds; a barrel footprint to place more tread on the ground, which improves traction; and shorter and larger pressure distribution grooves on the outer tire edge to reduce pressure build-up on the outside ribs and cut down on shoulder wear.
“By investing in these advances in steer tire 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,” concludes Zekoski.
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