It happens a lot to tire people, sales people and engineers alike. It can happen at the end of a training class, in the middle of a presentation to a maintenance group, even in discussions with engineers of other truck components. And it especially happens face-to-face with a potential customer when trying to sell a new type of tire.
It’s The Big Question, and it comes out of the blue, just as it has for decades: "How many miles should my tires really get?"
In my experience, most industry veterans are resigned to giving a pretty vague answer and avoiding specific mileage claims. I’m often tempted to answer that same question with another: "How many miles would you like them to get?"
The reasons for large variations in tire wear are many but tend to fall into two categories. First, users themselves can actually have considerable influence on tire longevity, and secondly, there are truly many hardware and environmental variables that impact removal mileage.
Since it’s well established that driving behavior can affect tire wear considerably, let’s summarize that influence by stating that driving practices that maximize fuel economy also tend to extend tire life. Some professional drivers note that minimizing the frequency and severity of brake applications also conserves both brakes and tires. The secret here is taking advantage of momentum and avoiding rapid deceleration.
Except for high-speed cornering caused by lack of proper brake application and intentional overinflation of tires to minimize rolling resistance, the correlations above are reasonably valid. Let’s turn to some of the equipment characteristics and variations that can create wear differences.
Load/Inflation and Tread Wear
Radial truck tires are designed to operate at specific load/inflation relationships for optimum wear.
Therefore, tires that are either lightly loaded or overloaded (for their operating inflation) can be expected to wear faster. This is because the designed pressure distribution in the footprint area at the tire/road interface is disturbed under these conditions.
Lightly loaded tires tend to suffer most for treadwear. Overloaded conditions bring diminished wear concerns, but casing durability becomes a major concern. This begs the question of wear effects caused by vocational service conditions that include diminishing or varying loads. Regional or local delivery service or low tare weight tanker applications come to mind. Assuming that tires are properly inflated to carry the full GVWR of these trucks safely when loaded, it is likely that more tire wear occurs on empty return runs than on the outbound, heavily loaded legs.
Other Causes of Wear
Cornering forces (experienced by steer tires) and torque applied through drive tires cause wear faster than those experienced by non-steering, free-rolling tires (trailer, tag, and pusher axles). Wear rates on a typical tandem drive 18-wheeler would be fastest on the drive axles, moderate on the steer axle, and slowest on the trailer axles. The onset of irregular wear would normally be in the reverse order.
Since tires with deeper tread depths tend to develop irregular wear sooner than those with shallow treads, axle-specific treads for high-speed rigs have emerged. Typically, new steer tires would have 18/32nds or 19/32nds tread depth, drive tires would have 24/32nds to 30/32nds depth and trailer tires would have 12/32nds to 14/32nds. Consider, too, that tire rolling resistance, and therefore fuel economy, is improved with shallower treads.
Wheelbase can also be a tire wear factor, especially on tandem drive trucks. Steer tires must generate sufficient cornering force to slide the tandem drive tires sideways in turns. Therefore, shorter wheelbase trucks tend to wear steer tires faster, as greater cornering forces are required.
Trucks having steer axles with higher angle wheel cut capability ®“ considered more driver-friendly ®“ tend to wear steer tires faster. Also, steer tires on any tandem drive axle vehicle can be expected to wear faster than those on single drive axles.
On light-duty vehicles with idler arm type steering linkages, right-side steer tires usually wear faster than left-side tires due to the more abrasive texture of the road surface near the shoulder. However, on medium- and heavy-duty trucks with solid beam axles and drag link mechanisms that connect the pitman arm directly to the left steering knuckle, this road surface effect is overcome and the left tire typically wears faster, often with a differential of as much as 5% to 10%. The left tire, in essence, provides more of the steering side force effort to maintain direction in straight or nearly straight highway driving.
Overall, newer road surfaces tend to wear tires faster than older, more polished surfaces, and certain aggregate materials in the road can also significantly change tire wear rates. Rounded aggregate is kinder to tires. It is also well documented that certain types of seashells, used as road surface reinforcement in some geographic areas, accelerate tire wear.
Drive tires on single-drive vehicles wear faster than their tandem drive counterparts because of the difference in torque being transferred by each tire. Higher torque engines tend to wear drive tires faster, and diesel-engine-equipped, small- and medium-duty trucks wear drive tires faster than comparable horsepower-rated gasoline engine models.
Because tire wear responds more to torque than horsepower, tire wear also tends to parallel clutch and universal joint life expectancies. In tandem drive sets, rear axle tires usually wear slightly faster than the forward axle tires, since the latter act as pivot points in turning maneuvers, with the rear tires sliding sideways. Fifth wheel position and tire load distribution obviously affect wear.
The moral of all this is that many tire wear differences can occur "normally," given the nature of your customer’s equipment specifics and operating conditions. Ultimately, the benchmarks for removal mileage expectations are often based on previous customer experiences or the performance realized by similar fleet operations in the same geographic region.
Answers to wear rate differences or sudden negative changes can sometimes be brought to light by methodically asking the question: "What changed?" Are the drivers driving differently? Was there a component change? Weather? Routing? Loads?
Technicians from the major tire manufacturers can often be of great assistance in analyzing tire wear differences. This is a valuable resource and a key benefit of a good supplier/dealer relationship. Forward-thinking tire dealers make use of these resources.
So the next time a customer asks The Big Question, they’ll be ready with a realistic answer.