By Chris Daly
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%|
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 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 Magazine and a regular presenter at FDIC. He has a master’s degree in safety from Johns Hopkins University. Chris can be contacted with any questions regarding his “Drive to Survive” seminar or assisting with emergency vehicle crash reconstructions at [email protected]