Proper Tires and Pressures Are Required for Fire Apparatus

Proper Tires and Pressures Are Required for Fire Apparatus

DEPARTMENTS

Fire Apparatus Maintenance

Hubert Walker

CHECKING tire pressures is important on every monthly inspection. This is not a “hit or miss” operation. First, the inflation pressure should be checked only when the tires are at approximately station temperature, not immediately following a run. The tire temperature increases with the vehicle speed, length of the run. and is affected somewhat by the atmospheric temperature. As the tire temperature increases, so does the inflation pressure. On a day when the atmospheric temperature is 80 to 85°F. the tire temperature may reach 200 to 200°E. with an increase of 15-psi inflation pressure. If the tire is underinflated for the imposed load, the temperature and pressure rise will Ire greater. A maximum of mileage and normal tire life will only be realized when the tire is maintained consistently at the correct inflation pressure.

What is the correct inflation pressure?

For each size and ply rating of a tire there is a maximum load rating at a specified inflation pressure. For example, the 9 x 20 tire, most used in the fire service, has a recommended maximum load capacity rating of 3960 pounds for the 10-ply rated tire at 70-psi inflation pressure. The 12-ply rated tire has a recommended maximum load capacity rating of 4480 pounds at 85-psi inflation pressure. There are reduced load capacity ratings at lower inflation pressures.

To inflate the tire at the proper pressure we must therefore know the actual load imposed on each tire.

A record of the tires on each vehicle in the department should be maintained. This record should give the actual scale weight with full service load for each vehicle with the actual scale weight on the front tires and on the rear tires, so that the actual load on each tire is known. Some fire departments enlist the aid of the state police. Their scales are usually portable and weigh one tire load at a time. The proper inflation pressure is obtained from the table of load and inflation pressures published by the 1 ire and Kim Association or most of the tire manufacturers.

In the fire service the load carried on each vehicle is fairly constant, so that the correct inflation can be consistently maintained. When the tire load is appreciably less than the recommended maximum rating, the inflation pressure can be reduced according to the schedule and a better ride results without sacrificing tire life.

When the tire is overloaded beyond the 10 percent permitted in NFPA Specification 19, the life of the tire is shortened. Overinflation will not compensate for overloading. Overinflation is a common cause of tire failure as it weakens the cord body. It reduces traction, skid resistance and braking ability. It also causes rapid wear in the center of the tires.

Increased load capacity is available in other sizes of tires without affecting road performance such as acceleration and hill climbing ability. No change in the speedometer is required. When the capacity is increased, the tire size increases. As a result, new rims and speedometer drives are required and there is a reduction in performance characteristics.

The 10.3 x 20 tire is available as a 9 x 20 replacement (10.3 x 22.5 if you have a tubeless 10 x 22.5). It has a 14ply rating with a maximum load capacity rating of 5350 pounds for single tires or 4750 pounds for dual tires. The rim size is 7.5 inches wide. Usually the 9 x 20 tire is mounted on a 6.5 or 7-inch rim. This rim size is inadequate for the tire with the increased load capacity.

The second alternate which is being used by quite a number of fire departments is the steel cord tire. Not only does this tire construction provide increased load capacity, but it provides a 20 percent increase in tire mileage and a freedom from punctures that is phenomenal. The tire has, like the 10.3 x 20 tire, a 14-ply rating, but it only has about four body plies plus two tread plies. This reduces the ply friction giving a longer life to the tire.

The steel cord tire’s recommended maximum load capacity varies with the different manufacturers. For the 9 x 20 tire the capacity rating for the Goodyear Unisteel is 4480 pounds; for Michelin Metallic two-ply it is 4500 pounds; and for Firestone Steelcord W3 it is 5350 pounds for single and 4750 pounds for dual mounting.

If you now have 7-inch-wide rims, no rim change is required to mount these steel cord tires.

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