Off-road tires are engineered and built with massive strength to support today's mining equipment. This equipment is called upon to carry heavier loads at higher speeds and over longer distances than ever before. They are the driving force behind tons-per-hour productivity, and the wrong one could mean losing valuable profits and competitive advantage.

Off-road tires are expensive pieces of equipment. "Next to fuel, they can be the biggest expense in a company's operation, sometimes accounting for 20% of operating costs." said Bob Ochsenhirt, manager of off-road tires and sales engineering at Goodyear.

It can be difficult to pick the right tires for off-road wheel loader and haul truck applications. Bias to radial, small to giant, smooth to ribbed, there are numerous models, makes, strengths and varieties to choose from.

Which tire is right? Some off-road applications with long haul cycles need more heat-resistant tread rubber. Sharp rock environments demand extra tread depth to minimize cutting. Soft underfoot or muddy conditions call for better-than- average traction.

According to tire manufacturers, key factors influencing off-road tire selection include:

* tire size and loads carried;

* haul road surface, condition and profile;

* type and condition of loading and dumping areas;

* length of the haul cycle;

* maximum number of cycles per work shift;

* length of work shift;

* machine/tire combination behavior in traction;

* tire wear and reasons for replacement;

* fitment possibilities offered by the machine manufacturer; and

* on-site service and maintenance conditions.

All off-road tires must provide:

* high carcass strength to carry heavy loads and resist impact damage;

* tread and sidewall rubber compounds formulated to resist cuts and bruises;

* tread patterns designed for high traction to transmit high engine horsepower; and

* flotation capabilities in soft underfoot conditions.

Bias vs. radial Both off-road and on-road tires are available in bias and radial construction. Bias tires were first introduced in 1934 and radials came out in the 1970s. Radial tires have slowly been taking over the industrial marketplace as the tire of choice.

"Twenty-five years ago we saw the transition from bias to radial in cars, 10 years ago we saw that transition in on-road vehicles like semi-trucks and now there's been that change in earth-moving tires," Ochsenhirt said.

Bias tires have a thick wall casing to ward off penetrations in the sidewall. This casing consists of layers of rubber-coated plies composed of textile cords placed upon each other at approximately 30degrees angles and wrapped around the bead wires that form the inner radius of the tire. The casing is covered with rubber that is formed into the tread.

The only way to increase bias-ply tire strength is to increase the number of plies. The greater the number of plies, the larger the circumference and bulk of the tire. These changes to the tire increase centrifugal force and can distort the tread when the tire is in use.

The bias ply's rigid, multi-piece construction can often cause unwanted interaction between the tread and the sidewall. This interaction produces high heat buildup and the potential for premature wear. Excessive heat is the main cause of tire failure (see page 18).

The bias-ply tire walls and tread form a single working unit. When the sidewalls deflect, the tread squeezes in and distorts. This makes the tire slide, reduces traction and accelerates tread wear, according to Michelin. On a wet surface, bias-ply tires are subject to sliding because they have a smaller footprint.

Radial tire casings are formed by rubber-coated cables that run from one bead to the other. Rubber-coated steel belts are placed one over the other to form a stabilizing unit. These belts are placed in the crown of the tire under the tread to stabilize the tread area.

The combination of steel stabilizing belts and the radial casing allows the tread and sidewall to act independently. The sidewall flexes under the weight of the vehicle and its cargo, while the tread provides contact with the road surface.

A radial tire's stabilizing steel belts offer better protection from tread punctures, cuts and blowouts, major causes of earthmover tire downtime. These belts decrease tread distortion and hold the tire on the road better. This minimizes tread scrub, which causes the tire to wear rapidly and unevenly. It also saves fuel and yields less heat buildup.

"Even though the market is converting to radial, there will still be applications where bias is better, such as in shot-rock applications, underground mining and extremely soft sand applications," Ochsenhirt said. "Also, bias tires cost less and certain markets like city and county municipalities, will almost always buy at the lowest price. Bias tires aren't just going to go away."

Ton-mile-per-hour factor "An important tool to aid in selecting the proper tire is the "ton-mile-perhour" (TMPH) formula," said Barry Rexroad, manager of original equipment engineering and product quality at Bridgestone/Firestone Off-Road Tire Co. "It's the grading point for which pattern and which compound are suitable for the tire."

Loads and speeds are major factors in determining life expectancy of earthmover tires. Both affect performance and either, if excessive, can lead to premature tire failure from heat degradation.

TMPH is an expression of the working capacity of a tire. Use of the TMPH formula permits estimation, in advance, of the job requirements and selection of the tire that will perform within safe operating temperature limits. The two factors determining the TMPH requirement for the job are the average tire load and the average speed for the day.

After calculating the job TMPH requirement, contact tire manufacturers and request established TMPH ratings for tire sizes and types applicable for the equipment involved. The best application is a size, ply rating and type which would not be overloaded at maximum recommended operating pressures and would have a tire TMPH rating slightly in excess of job TMPH requirements.

If tires currently in use are rated less than the job TMPH: reduce speed, reduce load or change to tires with a higher TMPH rating.

According to Goodyear, tests have shown that the TMPH formula does not apply when tires are loaded 20% above their capacity or on hauls of more than 20 miles.

Tire types, treads and depths The Tire and Rim Association is a technical group that establishes standards for tire and rim manufacturers. This organization of tire and rim equipment manufacturer representatives helps prevent confusion in the sizes and types of equipment available. The association's industry codes identify the specific application for which an earthmover tire is designed.

Virtually all manufacturers of off-road tires use the industry codes in their literature and on their tires (Table 1). The codes identify the tire's type of service, tread type and relative tread depth.

According to the association's codes, E-coded tires are designed for truck and scraper haulage applications at maximum speeds of 30 mph. L-coded tires are designed for loader and dozer applications at maximum speeds of 5 mph.

According to Ochsenhirt, "It's not that these machines can't go faster than these speeds, but heat generation is affected by speed. Off-the-road tires don't dissipate heat as quickly because they are so thick. Some are 1-ft thick. Heat builds up in the carcass under the casing. On-highway tires are thinner and dissipate heat better."

A corresponding number to each letter identifies relative tread depths. E -1, L-1, E-2, L-2, E-3, and L-3 tires have different treads and applications, but the tread depth is generally the same. The E-4 and the L-4 have 50% more tread depth than the 1, 2 and 3 series. An E-5 and L-5 have 250% more tread depth. The E-7 is a specialized tire for haul trucks offering good flotation in soft terrain and fine-grained sand.

Matching the correct tread depth with projected service life is key to this selection process. For instance, an L-5 tire is not always the most economical tire. If the loader is used infrequently or in relatively light environments or is used in a load-and-carry operation, the L-5 may fail from old age or from excessive heat long before the tread depth wears off.

Michelin North America, while still using this industry coding for clarification and translation, has developed its own rating system. Michelin does not include industry codes on its tire sidewalls. It uses a maximum ply rating and corresponding star marking system.

"We make comparisons to [the industry codes], but the 50-year-old tire industry code is based on a cotton-cord rating system," said Deems Nelson, product manager of the earthmover group at Michelin. "To have a steel cable be equal to so many plies of cotton seemed ludicrous. Also, our system helps us evolve into ISO-certification."

Ask the experts Customized tread chemistries and reinforced casings are other variables that demand consideration when selecting tires. With so many options, varieties and choices present with such a crucial investment, there is no substitute for expert advice from a tire supplier or manufacturer.

"A local tire dealer or representative is the best source of information," said Rexroad. "We can act as a consultant and look at the environment, machinery and all aspects of the operation. We can make house calls with our field sales/service force."

Proper tire selection must then be complemented with a sound maintenance program and conscientious vehicle operation. Tires must be regularly inspected for inflation pressure, repairable cut damage or other unusual conditions that could contribute to early failure.

Bridgestone/Firestone Off Road Tire Co. (BFOR) assembled a portable scale system to help its customers improve tire application and wear on haul trucks, loaders and other off-road equipment. According to the company, the new scales can weigh dynamically (while vehicles are moving). The unit can reportedly weigh off-road equipment as heavy as 1.7 million lb per axle. The 8- Yen 14-ft scales are transported by a Peterbilt truck with a Caterpillar diesel engine and MAPCO crane.

Traditional portable scales require vehicles to stop each axle on the scales and remain there for 2 to 5 minutes while the weight is measured. BFOR's scales, manufactured by Transcale Pty. of Australia, can reportedly weigh a vehicle at up to 3 mph and within seconds display axle weight, total vehicle weight and payload. They can weigh each individual position, not only the right rear, but also right rear inside and right rear outside. With this data, BFOR can provide a variety of reports regarding weight distribution and loading practices.

One way purchasing managers are finding the appropriate off-road tire is through tire-tracking software. Many tire companies sell computer programs for less than $500 that not only give prepurchase information like cost and brand criteria but also track tire inventory and product performance.

These software packages are designed for use by tire dealers, off-road end users and fleets. Users can follow a tire through its entire life cycle, and can even create personalized wheel configurations and then view them graphically.

Continental General Tire has a new package called the QVP monitoring system. It's IBM compatible with a point-and-click interface for Windows 3.11 through NT users. Users can track costs by wheel position, profit center and application type. Important warnings give reminders about low air pressure, damage and minimum tread depths. "It acts as an inventory-control and product-performance system to evaluate how any given brand performs on a vehicle," said Jerry Starnes, commercial field engineering manager at Continental. "A large company with ten quarries can maintain these multiple locations and offer comparisons between them in terms of tire performance."