Author: Chris Daly of www.fireapparatusmagazine.com. Source: Part 1. Part 2.
While many may think that the catastrophic failure of a tire is a rare event, they are actually much more common than we might think. The fire apparatus operator must be trained and prepared to handle the sudden and violent loss of control associated with a tire blowout. Failure to train our drivers on this important aspect of apparatus operation can result in tragedy, as evidenced by a review of National Institute for Occupational Safety and Health (NIOSH) Firefighter Fatality Reports.
On August 19, 2001, an Oregon firefighter lost his life when the tanker he was driving experienced a catastrophic failure (blowout) of the front right tire (NIOSH Report #F2001-36). As a result, the vehicle left the roadway and struck a large boulder and a tree. The firefighter victim became heavily entrapped in the tanker; emergency medical personnel pronounced him dead at the scene (photo 1).
Investigation revealed that the victim was traveling on an interstate highway after picking the truck up from a repair shop. As he traveled down the highway, he attempted to move into the left lane and pass a tractor trailer. After passing the tractor trailer, he was attempting to change back into the right lane when the right front tire blew out. After the blowout, the vehicle traveled approximately 535 feet before striking the boulder and tree.
State Police investigators inspected the vehicle after the crash and found that the tire that failed was “a 1979 model and that the outer shell fragment of the tire revealed a brittle and obviously aged material.” In addition, “pieces of the steel cords showed signs of rust from years of moisture exposure due to openings in the tread.” (1) This fatality occurred in 2001, which makes the tire in question approximately 22 years old at the time of the crash (photo 2).
On March 3, 2004, a Florida firefighter was killed when the right front tire of the brush truck he was driving blew out (NIOSH Report #F2004-15). The vehicle left the road, struck a culvert, flipped over, and came to rest upside down in approximately two feet of water (photo 3). The victim firefighter was trapped in the cab of the vehicle and subsequently drowned.
Investigation of this crash revealed that the victim was traveling approximately 55 miles per hour along a straight road when the blowout occurred. A crash reconstructionist who investigated the incident noted that the tire damage “resembled similar damage documentation patterns resulting from a previous impact that may have compromised the inner tire radial ply and liner.” In other words, the tire had sustained an impact on an earlier date that resulted in damage to the inside of the tire. As a result of this existing damage and other possible factors, a tire blowout resulted (see photo 4). The reconstructionist noted that this damage is “nearly impossible to detect because the tires may still hold air and show no outward signs of deformation.”(2)
A number of factors can lead to a tire blowout. Some believe that a tire blowout is the result of too much air in a tire, but this is usually not the case. The two most common causes of tire blowout are tires that are underinflated or overloaded.
The structure of a tire does not support the weight of the vehicle—the air inside the tire supports the weight of the vehicle. This is the same theory behind lifting air bags used for rescue. The lifting bag does not lift the load; the AIR in the bag lifts the load.
An underinflated tire does not have enough air pressure inside of it to support the weight of the vehicle. When this occurs, the structure of the tire begins to support the weight of the vehicle. In this situation, the sidewall of the tire begins to bulge out as it takes over the job of supporting the vehicle’s weight. The lower the air pressure, the more the tire bulges, just as we’ve all seen with a flat tire.
If you picture a tire attached to a parked vehicle, you can plainly see how the top of the tire is rounded and the bottom of the tire bulges between the axle and the roadway (photo 5).
A tire mounted on an axle is asymmetrical. The top of the tire is rounded while the bottom of the tire is “squished.” As the tire rotates, it changes shape from “round” at the top to “squished” at the bottom. The tire changes shape by flexing the sidewalls of the tire. This constant flexing of the sidewalls produces heat.
Now imagine this tire driving down the road at 60 miles per hour, rotating a few hundred times a minute. As the tire rotates, the sidewalls constantly flex and change between being rounded at the top of the rotation and becomes “squished” as it comes in contact with the roadway. This constant flexing causes the sidewalls of the tire to heat up. Under normal circumstances—in a properly inflated and loaded tire—the tire can handle the heat generated by the flexing sidewalls. However, if the tire is low on air and therefore not able to properly support the weight of the vehicle, the sidewalls may overheat as they rotate around the axle. This excess heat is caused by the constant overflexing of the sidewalls. The more the sidewalls have to flex, the more heat that will build up. If the tire heats up too much, it may fail and cause a blowout.
A similar situation may occur if the vehicle is overweight. In this case, the tire may have the proper air pressure inside for the weight the vehicle was designed to carry. However, if too much equipment or an oversized load is placed on the vehicle, the recommended air pressure will no longer be able to support the weight of the vehicle. As a result, the tire will begin to bulge at the bottom, just as it would if it were underinflated. Once again, the constant overflexing of the tire as the vehicle drives down the road may cause excess heat to build up and the tire to suddenly fail without warning. Combine an underinflated tire with an overloaded vehicle, and disaster is likely.
Bridgestone/Firestone conducted a study of emergency medical service vehicles to examine the inflation pressures of dual tire assemblies. The results of this study were quite startling. For starters, 39 percent of the tires couldn’t be checked because there was no access to the valve stems. (3) Of those tires that could be checked, two-thirds were found to be underinflated by at least 20 psi—or 25 percent capacity. (3) According to the tire industry, a tire that is 20 percent underinflated is considered to have been “run flat.” A tire that has been run flat may result in damage to the tire, which can result in an unexpected and catastrophic blowout.
Bridgestone/Firestone EMS Vehicle Tire Survey (3)
Inside Dual Over 20 psi Underinflated 33%
Outside Dual over 20 psi Underinflated 15%
Couldn’t Check 39%
Checked OK 13%
Key points to remember are:
• Tire pressures must be checked regularly to ensure that tires are properly inflated.
• NFPA 1901 D.4.3 states that fire apparatus tires shall not be more than seven years old.
1. NIOSH Fatality Assessment and Control Evaluation Investigative Report #F2001-36.
2. NIOSH Fatality Assessment and Control Evaluation Investigative Report #F2004-15.
3. “Ready to Roll: The Shocking Truth!” Bridgestone/Firestone, publication BF50919, July 2001.
CHRIS DALY is a 24-year veteran of the fire service and a full-time police officer who specializes in the reconstruction of serious vehicle crashes and emergency vehicle crashes. He is a Fire Apparatus & Emergency Equipment editorial advisory board member. He developed the “Drive to Survive” training program (www.drivetosurvive.org) which has been presented to over 14,000 emergency responders across the country and lectures nationally on the prevention of emergency vehicle crashes. Chris has been a contributing author to Fire Engineering and a regular presenter at FDIC International.
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