It’s been common practice in the trucking industry to equip Class 5 and larger
vehicles with different tire designs on steer, drive and tag or trailer axles. The primary reasons for this are performance and economics.
No surprises there, but we need to take a closer look to understand some of the underlying forces that have led to this application-specific refinement. After all, axle-specific tires are not usually found on passenger cars, SUVs, pickups, or even on many large earthmoving vehicles today.
Several decades ago, it was quite common to fit aggressive, deep tread tires on drive axles of cars and light trucks in the U.S. other than in the in the extreme Southwest and Southeast. These were traditional "snow" tires that had many discontinuous lug or button-type tread elements designed to self-clean and deliver traction on soft, deformable surfaces ®“ snow, slush, mud and so forth.
Since these tires tended to wear rapidly in warm weather, were noisy, and usually didn’t have the higher speed capabilities of shallow rib tread designs, they were mounted and removed seasonally. Often owners kept them on wheels year-round to avoid the expense and trouble of annual mounting and balancing.
With few exceptions, such as use in remote mountainous areas and on law enforcement and emergency service vehicles, modern "all-season" radials eliminated the seasonal tire change.
Advancements in rubber compounding and polymer technology, and the use of fine blading and siping to create additional biting edges, provided these newer tires with traction far superior to those old snow designs in nearly all winter driving situations. And they performed equally well all year long.
Of course, road building has also improved. Today there are far fewer miles of unimproved road surfaces. Now all-season radials are fitted to both axles of passenger vehicles whether they are front-, rear- or all-wheel drive.
The Short of It
Modern commercial trucks, including over-the-road and mixed service (on/off road) designs, have evolved to become very reliable, fuel efficient and straightforward to repair. And, in many ways, they have become standardized in their basic chassis layouts.
Nearly all commercial trucks differ from lighter duty vehicles in that they are rear drive only, have forged beam (non-independent) steer axles, very simplified steering systems, and have relatively small brakes.
Another major difference is the use of dual tire sets on drive and trail axles, which doubles the number of tire sidewalls available at each axle end. Since all of the necessary driving/braking torque and cornering forces are transmitted through the tires, chassis and suspension choices influence the wear, durability, handling, and, ultimately, the engineering design of these tires.
Steer axle tires must generate the cornering forces needed to maintain vehicle direction against side winds, road crown, torque thrust angles, and, most importantly, to make the truck chassis change direction in turning maneuvers.
Relatively small forces are required to steer single-tire drive axle units. Larger forces must be generated to force tandem drive tires to slide laterally in turning, since these tend to fight each other when traveling in any direction other than straight ahead.
Likewise, steer tires must generate higher cornering forces to turn a short (vs. long) wheelbase truck, because these forces are working through a shorter moment arm (think torque wrench) to coax the drive axles from their straight direction.
Over time, the abrasive wear rates of steer tires will be higher on short wheel-base tandem axle trucks. These tires are also less likely to face the development of irregular wear patterns, since the faster abrasive wear tends to retard the onset of free rolling wear.
The Long of It
So, with that bit of history of passenger tire applications and the tire requirements for smaller trucks behind us, you get a sense why tire engineers have attempted to optimize certain performance characteristics on axle-specific tires.
Steer tires fitted to longer wheel-base trucks must be designed with high resistance to irregular wear, since they may be fitted to a wide variety of truck wheel-base and drive axle configurations. High-speed highway service, where there is less turning, is one of the most challenging applications for tire engineers.
Drive axle tires must transfer high levels of torque, especially on single-drive units where four tires often deliver the same torque levels as eight tires on tandem drives. Multiple trailer configurations and soft surface on/off road applications present the most challenging drive tire requirements.
Resistance to abrasive wear from normal acceleration or the torque transfer required to overcome hills and wind. Irregular wear caused by lug distortion from high torque transfer. Irregular wear characteristic of softer sprung/lower dampened drive axle suspensions. The impact of aggressive traction. The ability to transfer torque without wheelspin or loss of lateral stability. All of these are design issues taken into account in the development of today’s commercial drive axle tires.
Trailer tires and other free-rolling tires ®“ must be highly resistant to the onset of irregular wear since they don’t face the same abrasive cornering or torque transfer forces that help drive and steer axle tires retard the development of free-rolling wear.
Shallow skid depths and non-aggressive "see through" grooves are commonly used for trailer tires. These features also benefit fuel efficiency (lower rolling resistance), and provide high lateral stability for tall trailers and loads with high centers of gravity.
One other differentiating characteristic that applies to each of the axles is that the tires must perform properly under much wider variations of loading than their passenger car or SUV counterparts. Trucks can go out in the morning fully loaded, be half full by noon, and be totally empty by nightfall.
The axle-specific truck tire designs and rubber compounds that are popular today have evolved over time to provide optimized service for the truck configurations we currently use.
Some engineers and fleet managers point out that while today’s trucks are reliable and cost effective, they are really of very old, basic designs.
Perhaps the ultimate power unit configuration to pull double or triple trailers, for example, would have a short wheelbase, be all-wheel drive, have single tire on all axles, and include independent front and rear suspensions.
If we ever see the "perfect" power unit and the perfect driver, weather conditions, loads and tire maintenance practices ®“ you should get to know your favorite tire engineers better.