If you’re a car enthusiast, or at least spend some time reading the promotion-
al material selling high performance passenger vehicles, it’s hard to miss the importance placed on tires and wheels in achieving outstanding cornering and crisp response.
The tire contribution in what truly is a multi-faceted handling equation is further underscored weekly at racetracks around the world. From grassroots modified and sprint car classes all the way up to NASCAR, IRL and Formula 1 competition, race tires strain against the reality of this equation.
Race tire engineers routinely deal with fractional differences in reinforcement cord angles, rubber hardness, lateral and vertical spring rates, cornering force generation at different slip angles, and countless other variables in attempts to extract even the smallest improvement in traction, cornering ability and speed.
While some fruits of these efforts only appear in tires labeled "For Racing Purposes Only ®“ Not For Highway Use", other developments have truly improved consumer tires over the years. Much of what is learned on the racetrack is transferred to the chemistry and engineering efforts for UHP tires.
Certainly, the traction and braking characteristics of even the broadline all-season tire have gained from racing experience. And no one questions that tires play a big part in car handling.
Change is Slow
The influence that tires can have on the handling characteristics of medium and heavy-duty highway trucks, however, has traditionally been limited to maintenance issues. But with changes in the trucking industry and the advent of newer Class 4-8 truck designs, tire performance characteristics like handling are getting a much closer look.
Traditionally, commercial truck tire performance issues came down to tire maintenance. Proper inflation pressure has always been most important. Fleet maintenance managers have been nagged endlessly for decades about the importance of regular inflation checks.
On the consumer side, inflation maintenance remains something everyone wants to see improve. However, unless there is some real education out there, federal mandates for tire pressure monitoring systems in passenger vehicles will not likely contribute to a surge in consumer tire care.
After standard inflation pressure maintenance, fleets can enhance tire handling with proper pressure and diameter matching in dual tire sets, and matching tread patterns across axles.
Unlike P-metric and LT-metric tires, where handling issues can often be addressed by an almost bewildering array of alternative sizes, profiles and tread types available, medium and heavy truck tire sizes and types are few in number. And they have evolved slowly.
New commercial truck tire offerings are typically based on advances in materials, processing or design technology, and advantages are promoted based on their positive impact on fleet operating costs, not tire performance.
Axle-specific tires, for instance, became popular for those reasons. Still, because the alternatives were few and far between, the tire selection process has remained fairly simple.
Several trends, especially in medium duty trucks, may change the role tires play in truck handling. In the past, vocational applications have been addressed by a broad array of alternatives, with the same chassis potentially being used in multiple markets. Often, fleets would over buy, getting more truck than needed because what they bought was the best long-term option.
Today, the quest for performance and cost efficiency means vocational vehicles are far more specific, and while there are still alternatives offered, even these are still quite specific to the task at hand. As more variations of Class 4-7 trucks hit the market, vocational fleets are cautiously reviewing these alternatives to avoid purchasing vehicles that are any larger, heavier or more expensive than necessary to perform anticipated tasks.
Additionally, truck manufacturers, both domestic and off-shore, are adopting more car-like cab environments, steering and suspensions that improve ride comfort, and computer matched components ®“ all in an effort to please drivers and contain costs that can result from overspecing.
Tire/Truck Design Integration
This move to tighter application-specific vehicles has impacted the tire industry. Tire manufacturers are responding to requests from truck engineers to design smaller heavy-duty commercial tires tailored to particular chassis types and GVW ratings. This has been common practice in the passenger car industry for years, but is a recent phenomenon for Class 3 and larger trucks.
Overall, the concept of integrating tire and truck design efforts at an early stage is a very good thing. A number of potential headaches can be averted, for the benefit of fleets.
‘New’ Truck Considerations
But fleet truck purchasers and maintenance personnel should be aware that, in some cases, all tires of a given size and load range may not be as generically interchangeable as they have been in the past ®“ or, at least, appeared to be. Today, in fact, it is vital that fleets pay greater attention to tire application.
Some basic tire/vehicle characteristics ®“ beyond the traditional maintenance issues mentioned earlier ®“ should be kept in mind, especially when selecting tires for these new efficient truck designs.
®′ Deeper tread depth tires tend to "feel" less stable laterally than comparable shallow tread tires. In extreme cases, stability complaints can result, especially on single drive axle units. Obviously, it follows that worn tires sometimes feel more stable than new tires.
®′ Lug-type tires, with large lateral grooves, sometimes feel less stable laterally than solid circumferential rib tread designs. An intermediate design with traction elements in the center of the pattern, but having solid shoulder ribs, similar to the drive tires developed for tandem air-ride highway tractors, will normally fall in between.
®′ Newer tire designs rely heavily on sophisticated rubber compounds to achieve good wear, fuel efficiency and solid handling. Therefore, it is more difficult and less reliable today to predict tire performance based on appearance or tread depth alone.
®′ In some cases, new trucks may have a different track width than the vehicle being replaced. This can result in tires running outside the normal "ruts" or worn pavement sections of older roads, which can create complaints of wander or excessive sway.
®′ Lower profile (height/width ratio) tires tend to have more lateral stability than higher profile tires. This difference diminishes with higher inflation pressures, but can be a factor on lower load range, lower inflation pressure rated tires. Some vocational trucks that may be used in high-center-of-gravity applications, such as utility service buckets, may benefit from low profile tires.
These are some examples of complexities that operators may discover when adding new generation vehicles to fleets. The best insurance to avoid potential issues is staying abreast of the latest developments.
Well-versed truck sales personnel and experienced tire field engineers can answer many questions and provide guidance tailored to individual service requirements.