There's lots of "art" in spec'ing tires for mixed service

With the advent of more versatile radial tire technology, the guidelines between on-highway and off-highway tires have blurred a bit. (Remember that in the early 1970s nearly all radials were strictly on-highway.) Also, many trucks in vocations once considered strictly off-road today travel significant over-the-road distances between job sites. This has led to an evolution of mixed-service (on-/off-road) tires that must perform double duty.

Since spec'ing a tire that's not quite right can be a costly mistake, I'd like to clear up some of the confusion that surrounds this mixed-service area.

When deciding where your application falls on the on-/off-road continuum, a number of factors should be taken into consideration: 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; and lateral traction/stability requirements.

Surface conditions and vehicle configuration should be analyzed with these performance properties in mind.

Since off-road surface conditions will be the limiting factor, one useful guideline is the amount of time your equipment operates off-road vs. on-road. Trucks that spend the majority of their time working on unimproved surface conditions will generally benefit if they're spec'd with tires that are closer to off-road designs.

If load and unload areas are relatively smooth and well maintained, on the other hand, some dump trucks, transit mix vehicles and construction trucks can use highway tires just like their 18-wheel linehaul counterparts. Advantages to spec'ing on-highway tires are that they may be less expensive initially, have longer tread life and deliver better fuel-efficiency than off-road tires.

Caution is in order, however, since an off-road percentage of as little as 5% can destroy casings on highway tires if you're running a high-torque operation on sharp rock or crushed stone.

Traction and tread self-cleaning requirements also vary with local conditions; all mud is not created equal. Traction can also be a major consideration for steer and trail position tires. For example, steer tires for use on steep, narrow logging roads need aggressive lateral grips.

Speed/load conditions, often referred to as duty-cycle severity, are also important when selecting tires for a mixed-service operation. Sustained highway speeds with heavy loads can result in excessive heat buildup in off-road tires. Tread depth, internal design features, and tread compound all play a role in high-speed performance. Tires with deeper treads generally provide better off-road traction, while shallow tread depths are a better choice for lengthy, high-speed hauls.

If your operation is general for-hire service, rather than dedicated to repeat hauls over the same or similar routes, you're better off choosing tires geared for the expected off-road conditions to reduce casing loss.

Vehicle configuration can also affect tire selection for mixed-service operations. Loads with high centers of gravity prompt stability and sway concerns. Wheelbase and axle load distribution may create a need for flotation or extra-load tires and, in rare instances, some off-road suspensions may not work well with certain tires.

You should periodically calculate the percentage of casings lost to damage prior to the first and second retread. Casing represents a substantial portion of the cost of a new tire, and retreading can extend its life.

It's always a good idea to get advice from experienced local operators and experiment cautiously with new tire designs before making a commitment for the entire fleet.