How many of us have gone to a business meeting or social gathering only to discover that everyone else was dressed similar to one another, but very different than we had chosen? This could result from not reading dress guidelines on the invitation, not knowing established practices, or simply not paying attention to the circumstances.
If it’s an innocent oversight and we’re among friends, there’s probably no ill harm done and we can contribute to the business discussion or converse comfortably with no further consequences. If, however, our intended function is dress-dependent and we show up, for example, to play in a football game dressed for a tennis match, we can expect to suffer some otherwise avoidable damage.
This is the situation some truck fleets face when deciding which tires are best suited for their operations.
Most major truck tire manufacturers offer over-the-road and off-the-road tire types. In recent years, and with the advent of more versatile radial tire technology (remember, in the early 1970s nearly all radial tires were strictly on-highway), the guidelines have blurred a bit.
Also, many trucks in vocations once considered purely off-the-road today travel significant distance over-the-road between job sites or duties. This has led to an evolution of mixed service, or on-/off-road, tires that perform double duty, a difficult task from the tire designer’s standpoint.
Since pure over-the-highway service and pure off-the-road service tires, along with respective tire designs for those applications, are clearly explained in readily available literature from most manufacturers, let’s concentrate on exploring that cloudy mixed-service area.
Key Performance Needs
Here is a checklist of some major tire performance properties that should be considered in deciding where your (or your customer’s) requirements fall in the continuum between pure over- vs. off-the-road.
®€′ Abrasion Resistance
®€′ Spin Cut Resistance (drive tires)
®€′ Flotation Required on Soft Surfaces
®€′ Speed vs. Load Requirements
®€′ Sidewall Cut Resistance
®€′ Tread Self-Cleaning Needs
®€′ Drive Tire Traction
®€′ Lateral Traction/Stability Requirements
The two most critical variables that should be analyzed with the performance properties above in mind ®€“ are surface conditions and vehicle configuration. Since off-the-road surface conditions will be the limiting factor, one useful guideline is the percentage of off-the-road vs. on-highway operation.
Generally, trucks with a higher percentage of unimproved -vs. improved-surface operation will benefit from the selection of tires closer to off-the-road designs. Some dump trucks, transit mix vehicles and construction material delivery trucks are able to utilize highway tires just like their 18-wheel line haul counterparts, if their load and unload areas are relatively smooth and well-maintained. Here highway tires may be less expensive initially, deliver long treadlife and be more fuel- efficient than off-the-road tires.
Local Conditions Vary
However, caution is in order, since even a small percentage 5% or less ®€“ of off-the-road use can destroy highway tire casings if that percentage consists of high torque operation on sharp rock or crushed stone, such as a loaded dump truck working in a quarry.
Rock types, road building materials, terrain and elevation vary significantly in North America. An extreme example is found in parts of western Texas where even some on-highway trucks are most economically operated with mixed-service tires due to the type of aggregate used in local road building mixes.
It’s always wise to seek advice from experienced operators in your local area and experiment cautiously with new or different tire designs prior to making a commitment for the entire fleet.
These local conditions can also create varying traction and tread self-cleaning requirements. All mud is not created equal! Traction can also be a major consideration for steer and trailer position tires. The aggressive lateral grip needed for steer tires on steep and narrow logging roads is a good example.
Duty Cycle Impacts Choices
Speed/load conditions, often referred to as duty cycle severity, are also important in mixed-service tire selection. Sustained highway speeds with heavy loads can result in excessive heat buildup in off-the-road tires. Tread depth, internal design features, and tread compound affect the tire’s high-speed performance. Tires with deeper treads will generally provide better off-road traction, while shallow tread depths would be wiser choices for lengthy high-speed hauls.
Operators of trucks in general for-hire service, instead of being dedicated to repeat hauls over the same or similar routes, should tend to select tires more appropriate for the expected off-the-road service to avoid unnecessary casing loss.
Mixed-service vehicle configuration can also impact tire selection. Loads with high centers of gravity prompt stability and sway concerns. Wheel base and axle load distribution may create a need for flotation or extra-load tires. And, in rare instances, some off-the-road suspension systems may not work well with certain tire types.
As with surface conditions, it is usually wise to seek advice from experienced operators and/or tire industry experts.
Consider Retreadability of Casings
An important statistic to quantify and review periodically is the percentage of casings lost to damage prior to the first and second retreading. Since the casing represents a substantial portion of the total new tire cost, and successful retreading can delay additional new tire purchases, life cycle economics of different tire selections should be compared.
Most experienced industry advisers agree there is no fail-safe method for selecting the optimum tire for all mixed-service conditions. It’s often helpful to think of these service requirements as a continuum between pure on-highway and off-the-road conditions to home in on the proper tire choice. ®€′