Tires Wearing Out Faster, Fire Departments Report
In your annual report you may notice maintenance items with a high total cost or high rate of replacement. One such item is frequently vehicle tires.
Gone, apparently, are the days when the fire department used tires for three or four years and replaced them because some surface checking of the rubber was visible, even though considerable tread rubber was left. It is difficult to reconcile the low tire tread mileage experienced by the fire service with the 40,000 to 50,000-mile tread life claimed for tires on commercial trucks.
During World War II, it was necessary to use a high percentage of synthetic rubber in tires, and this is still standard practice.
Research by the tire industry had made an important improvement in tire strength and tire safety. After the cotton cord came rayon cord, followed by nylon cord and now Polyester cord. The steel cord also gave the fire service a stronger tire with longer tread life because it runs cooler.
Synthetic rubber criticized: The tire designer is making every effort to provide a tire that will run cooler to defeat the No. 1 enemy of tires, heat. But research has not yet produced a synthetic rubber tread compound that is the equal of natural rubber. The synthetic rubber compound lacks in some measure the homogeneous properties of natural rubber, being a more heterogeneous compound. This is also indicated by the recommendation of the Tire Industry Safety Council in its booklet, “Consumer Tire Guide,” which recommends a break-in period for all new tires. Speed should be limited to 60 mph for the first 50 miles of driving to enable “the many complex elements in a tire to adjust gradually to each other and function as an integral unit.”
An examination of several fire department annu,al reports disclosed that 70 percent of the tread life is used in one year. This means an average tread life of less than two years for busy companies in larger fire departments. A personal survey of 15 smaller departments showed the same tread life experience of the larger departments. The average tread life before replacement by smaller departments was 7000 to 8000 miles, and this mileage was attained in two years. On some fire apparatus, tread life did not exceed 2000 miles for the same period.
The survey also showed some interesting attempts to improve tread life. One fire department is applying retreads to new tires before replacing tires in service. The fire chief advises that while this practice does give added tread life, he does not feel it is the final answer to the problem.
Radial and steel cord options: Some fire departments have adopted the radial tire, steel cord, for vehicles showing the greatest tire wear with regular tires. There is little question about the better mileage provided by the radial tire with steel cords, but the higher cost of the tire leaves the cost per mile of tread life about a standoff with a regular tire.
There are many factors affecting tire tread life, and it is just good management to check out these factors to be sure that part of the short tire life is due to conditions over which the fire department has no control rather than lack of maintenance.
Nothing is assumed to be correct in an investigation of this type. As heat is the No. 1 enemy of tires, a listing of the usual causes indicate areas for investigation:
- Overloading of tires
- Underinflation of tires
- Overinflation of tires
- High speed turns
- Front or rear axle out of alignment
- Defective front shock absorbers
- Too much toe-in or toe-out of front wheels
- Excessive caster
- Incorrect camber
- Rims too small (narrow)
- Quick stops and starts
- Atmospheric temperature
- Chassis frame sprung or out-ofsquare
- Brakes out of adjustment or drums out-of-round
- Worn or loose wheel bearings, unbalanced wheels, wobbly wheels
- Loose chassis spring U-bolts
Determine load: The first step is to know the actual in-service load on each axle. If the actual weight is not known, weigh the vehicle so that the weight on the front axle and rear axle is known. Correct inflation pressure cannot be determined until the weight on each tire is known.
The fact that a truck has 9.00-20 tires does not mean the size is correct or that the normal published load rating is the actual safe maximum. For example, the following tables show the variation in load limits for different tire pressures, single or dual wheel use:
9.00-20 Tire Loads and Inflation Pressures Single Tire
PSI 65 70 75 80 85 90 95
10 pr 4000421044104610 12 pr 4000421044104610479049705150
10 pr 3870 4040
12 pr 3870 4040 4200 4360 4520
If the tire is mounted on a 6.5-inch rim, the load ratings are as follows:
10 pr 3790 3960
12 pr 3790 3960 4120 4280 4480
From these tables, it can be seen that the tire load rating and inflation pressure will depend on whether the tire is used as a single or dual tire. Also, if the wheel rim is 6.5 inches, the load capacity at any pressure is less than for a dual tire on 7-inch or 7.5inch rims.
Extra load leeway: The above is quite understandable if it stopped there, but the tire manufacturers give leeway to these ratings by approval of a 9 percent increase in load if the maximum sustained speeds do not exceed 50 mph. Actually, most fire apparatus are operated within this 50-mph speed limit. This changes the load and inflation tables to read as follows for 9.00-20 tires:
9.00-20 Tire Loads and Inflation Pressures Single Tire (+9%)
PSI 65 70 75 80 85 90 95
10 pr 4360 4589 4807 5025 12 pr 43604589 4807 5025 5221 5417 5614
Dual Tire (+9%)
10 pr 4218 4404
12 pr 4218 4404 4578 4752 4927
If the load per tire is greater than shown in the above table, the tire is overloaded and the next larger size tire should be used. You also can go to an increased ply rating (load range F in place of load range E) or select one of two options, a steel cord tire, or a larger tire, such as 10.3-20, which will not affect turning or tire chain clearances. Also, this size will not affect the speedometer reading.
Changing to a larger size, such as 11.00-20 can bring several problems, such as fender clearance, tire chain clearance, turning radius, overall width and speedometer reading.
Summing up, the problem resolves to making sure the tire is not overloaded and the correct inflation pressure is used for the load on the tire.
To be continued next month