“Fire Apparatus Rollovers, Part 14: Tires” (November 2020), on tire-related vehicle crashes, dealt with tire age. Tire age is an important and sometimes controversial topic in the fire service because the National Fire Protection Association requires apparatus tires to be replaced every seven years because an old and brittle tire is much more susceptible to failure.
There have been countless emergency vehicle crashes that were the direct result of a catastrophic tire failure caused by an old and brittle tire (photo 1).
Although age is a common cause of tire failure, new tires may also fail. Some of these failures are caused by damage to the tire, but many are the result of underinflation. To better understand the danger of an underinflated tire, let’s examine how a tire works.
Walk out to the apparatus bay and look at a tire. The tire is round at the top and compressed at the bottom. Now imagine the tire rotating around the axle as the vehicle drives down the road. As the tire rotates, it will constantly change shape from round at the top to compressed at the bottom. The tire changes its shape by flexing the sidewalls of the tire. Depending on the speed of the vehicle, the tire sidewalls may flex hundreds of times a minute, which generates a tremendous amount of heat. The tire is designed to withstand this heat, but it can only take so much of it.
This was a relatively new tire that blew out at highway speed. Fortunately, the driver was able to maintain control and bring the rig to a safe stop. (Photos by author)
In an underinflated tire, the sidewalls of the tire will not be as stiff as they should be because of the reduced internal pressure. As a result, the tire sidewalls flex more than usual as the tire changes shape from round at the top to compressed at the bottom. As the tire sidewalls “overflex,” they generate excess heat. If the tire generates too much heat, the rubber tire could overheat and fail.
Driving a vehicle with a flat tire involves safety issues that are plain to see. However, underinflated tires are not as obvious as flat tires. Fire apparatus operators must understand that while an underinflated tire is not as obvious as a flat tire, it is just as dangerous. Many manufacturers consider a tire to have been run flat if it has been driven on the road while more than 20% underinflated. This is why fire apparatus operators and fleet managers must strive to keep a close eye on the air pressure of the tires.
Few people realize that a tire is not completely airtight. Instead, air can migrate through the rubber at a rate of 1 to 2 pounds per square inch (psi) per month. Over the course of several months, the tire can become substantially underinflated from air migration.
Outside temperature will also affect the air pressure of a tire. A decrease in temperature of 10 degrees can result in a 1 psi loss of tire pressure. I live in Pennsylvania, where it is not unusual to go to bed on a 65°F day and wake up to a 30°F morning. If a vehicle is parked outside, the tire pressures may have been substantially reduced by the cold weather. Keep an especially close eye on tire pressures during cold weather and change of seasons.
When checking tire pressures, it is important to remember to check the inside dual tires. Studies have shown that the inside dual tires are often overlooked or neglected during tire inspections. Members often don’t feel like climbing under the apparatus or getting dirty by sticking their arm between two tires to get an air pressure reading. To ensure that the inside duals are checked on a regular basis, talk to your tire dealer and see about having valve stem extensions installed on the inside tires. This will help make it easier to check the air pressure on the inside duals and ensure that they are not being operated in an underinflated and unsafe condition.
The tire pressure printed on the sidewall is the maximum pressure the tire can safely handle. This does not necessarily mean that it is the recommended tire pressure. The tire pressure, as recommended by the manufacturer, is printed on a label or plate somewhere on the vehicle (photo 3). Keep in mind that if the weight of an apparatus or the type of tires should change, you will need to revisit the recommended tire pressures by consulting with the tire or apparatus manufacturer.
Recommended tire pressure from the manufacturer. Notice that this does not correspond to the maximum tire pressure, which is printed on the tire sidewall.
How do you know what the proper tire inflation pressure should be? A common misconception is that the recommended tire inflation pressure is printed on the sidewall of the tire. This is not the case. The air pressure printed on the side of the tire is the MAXIMUM air pressure for the tire (photo 2). The recommended air pressure may be quite different. To determine the recommended air pressure for a tire, check the door jamb, fuel tank cover, or trunk lid for an informational sticker (photo 3). This sticker will provide you with the recommended air pressure as well as other important information related to the tires. In a custom apparatus, this information is usually provided on a metal plate near the driver’s legs. When in doubt, call the manufacturer to determine the proper tire pressure. Just keep in mind that if the weight of the truck should change, or if the department purchases new tires, it is important to consult with the tire or vehicle manufacturer to ensure that the recommended tire pressure has not changed as well.
Remember that the recommended tire pressures are COLD TIRE PRESSURES, meaning the tires have not been used for a prolonged period prior to checking the air pressure. It is best to check air pressures when the vehicle has not been driven for a while. If you have to drive somewhere to have the air pressure checked, limit the drive to under three miles. This will ensure that the use of the tire has not warmed the air inside, which will result in an artificially high air pressure reading.
While tire age is something easy to keep track of, air pressure is much more difficult. Time and temperature can have a significant effect on tire pressure. For this reason, tire pressures must be constantly monitored so the tires are not run in an unsafe condition. Running an underinflated tire could result in a catastrophic tire failure. When checking tire pressures, also don’t forget to check the rarely used specialized vehicles and trailers that are often parked outside the firehouse!
Chris Daly is a 23-year police veteran currently serving as a patrol supervisor in West Chester, Pennsylvania. He is an accredited crash reconstructionist (ACTAR #1863) and a lead investigator for the Chester County Serious Crash Assistance Team. In addition to his police duties, he has served 29 years as both a career and volunteer firefighter, holding numerous positions including the rank of assistant chief. He has a master’s degree in environmental health engineering from Johns Hopkins University and is a contributing author to numerous fire service publications including Fire Apparatus & Emergency Equipment and Fire Engineering. He has also developed an emergency vehicle driver training program entitled “Drive to Survive,” which has been presented to more than 22,000 firefighters and police officers across the United States. He is the author of the book Drive to Survive–The Art of Wheeling the Rig (Fire Engineering). Additional resources concerning emergency vehicle crash reconstruction and seminar scheduling can be found at his Web site www.drivetosurvive.org.
