Of all the components that make up a fire apparatus, tires could be considered the most critical. Without them, moving the vehicle would be extremely difficult.
They bear the weight of the entire vehicle. Not that anyone can tell it in states where road conditions are poor, but they do provide some cushion for little cracks or rises in the road. Tires may seem like relatively simple components on a fire apparatus. However, consider them one of the most complex components and arguably one of the strongest.
People may not realize it, but there is critical information imprinted on the sidewall of each tire on a fire apparatus. If certain information is not adhered to, such as tire inflation pressure and load carrying capacity, there could be detrimental effects to the tires. Speed is another factor that can significantly impact tires. And, in certain cases, tire speed limitations may not be on the sidewall.
Tire Inflation
Tire inflation pressure is related to the load-carrying capacity of the tire and may vary depending on the tire as well as the total weight of the apparatus. The units of measurement are given in pounds per square inch (psi) or Kilopascals (KpA). Operating an apparatus with a tire having more than the required inflation pressure (overinflated) or not having enough air in the tire (underinflated) can cause damage to the tire.
An underinflated tire may deflect when driving, thus causing heat to be generated. Prolonged driving under this condition could lead to tread separation and other damage to the tire including a complete blowout. Telltale signs of an underinflated tire are tire bulge, the apparatus pulling to one side, or the tire feeling "loose" in the rear of the apparatus. The best way to eliminate tire pressure issues is to check the pressure frequently.
Overinflating a tire may result in the tire having less contact with the road than designed. On a steer axle, this could reduce the traction when cornering and result in an increased turn radius. Damage could occur to the tire when hitting a pothole or other objects on the road. An overinflated tire is not as easily detected as an underinflated tire. National Fire Protection Association (NFPA) 1901, Standard for Automotive Fire Apparatus (2009 ed.), requires a tire pressure monitoring system to be installed on new compliant apparatus. This provides a means of easily checking tire pressures, which should be done on a frequent basis.
Weight Capacity
Weight capacity is also indicated on the side wall of the tire. The units of measurement are given in both pounds (lbs.) and kilograms (kg.). Overloading a tire can cause premature wear as well as heat buildup in the tire similar to that of an underinflated tire and yield similar results. It is absolutely critical when considering replacement tires for the apparatus to install a tire of the same load rating. When considering a new apparatus, consider using the Fire Apparatus Weight and Cube Calculator found on the Fire Apparatus Manufacturers' Association's (FAMA) Web site at http://www.fama.org/fireServiceResources.cfm. By using this tool, the estimated amount of weight being applied to the apparatus will allow your apparatus manufacturer to determine the tire size the apparatus will require.
Intermittent Service Rating
Within the fire apparatus industry, apparatus manufacturers may receive a special capacity rating for tires that differs from the rating identified on the sidewall. This is referred to as the "Intermittent Service Rating" and mean