The Importance of Tire Pressure in Preventing Apparatus Rollovers
BY JASON ESTEP
Today’s fire apparatus are built to the strictest safety standards of any industry, yet we still continue to experience major apparatus incidents.
We see them in our news feeds on nearly a daily basis. This serves as a constant reminder that our first task in any response is to arrive safely. Why does this continue to occur?
While there are many contributing factors, such as training, speed, and apparatus deficiencies, one simple factor is often taken for granted: tire pressure. National Fire Protection Association 1901, Standard for Automotive Fire Apparatus, general requirement 4.13.4 requires that each tire on the apparatus must be equipped with a visual indicator or monitoring system that indicates tire pressure. This makes it easier than ever to check air pressure, but are we correcting it?
My experience is that we are not, which is putting our firefighters and apparatus at a much greater risk of accidents. A tire depends on proper air pressure to give it a structurally sound shape. Improper tire pressure can increase braking distance, create less responsive steering, cause increased tire wear, and influence poor fuel economy. Underinflation also allows the sidewalls to excessively flex, which generates heat. High heat can cause tread separation and blowouts, often leading to apparatus rollovers. This problem is compounded on large apparatus like aerial devices and tankers (tenders). The tire’s air pressure is what determines the gross vehicle weight rating (GVWR) of an apparatus, not the axles. Tire manufacturers are required to brand the maximum weight-carrying capacity of a tire at its maximum safe air pressure into the sidewall of the tire. In addition to this, they often will publish load and inflation charts that will correlate the weight capacity of a tire as tire pressures decrease. Charts 1 and 2 are for an 11R22.5 tire, common to many apparatus, including a front-line engine on my department.
My department’s engine is a commercial chassis apparatus built by a major apparatus builder. It has a 12,500-pound front axle, with tires rated at 6,175 pounds at 105 pounds per square inch (psi) of maximum air pressure, meaning that both tires combined can carry 12,350 pounds. If you reference the inflation chart, a mere 5 psi of air pressure loss puts the apparatus below the GVWR of the front axle! We recently upgraded the front tires to 120-psi tires, which if you reference Chart 2, increased the front axle rating to 13,220 pounds at 120 psi, meaning that it is much more forgiving to a couple psi of pressure loss.
Tire Inflation Best Practices
You can use the same chart for rear dual tires. It is not uncommon to find apparatus that have rear tires with 80 to 90 psi in them, which means that you can be losing nearly 5,000 pounds of weight carrying capacity. This is a major contributing factor to apparatus rollover. The less pressure in the tire, the softer the sidewalls, allowing the apparatus to have more side-to-side roll. Factor this into the increased chance of blowout, and it becomes a recipe for disaster.
Want to take a step to eliminate potential rollovers? Take the time to correct all of your tire pressures. The first time can be time consuming, but once all the tires are at the correct pressure, only the exceptions need to be corrected. Do not allow your firefighters to tell you that the tire looks OK with a visual inspection or by thumping the tire with a bat or hammer. This is an age-old myth. The only accurate way to check a tire is with a pressure checking device on the valve stem. Research what works best for your department and implement it.
JASON ESTEP is chief engineer of the Morrisvale (WV) Volunteer Fire Department with 21 years of service. He has a mechanical engineering degree from Fairmont State University and is s a senior maintenance planner for a major coal company.