Dangers of Overweight Vehicles – Part 2


Editor's note: Be sure to check out part 1 of Chris Daly's series on the dangers of overweight vehicles.

Overweight vehicles not only increase the potential for brake fade, they may also lead to a tire blowout. A review of NIOSH Firefighter Fatality Reports provides several tragic examples of fatal crashes that were the direct result of a tire blowout.

While tires play one of the most important roles in the safe operation of a vehicle, they are often the most neglected part of it. Proper tire pressures are necessary to ensure that the weight of the vehicle can be properly supported as it travels down the road.

A common misconception is that the weight of a vehicle is completely supported by the rubber tire structure. In reality, it's supported by the air inside the tires. This concept is no different than that of a rescue air bag used to lift heavy loads. The air bag itself is not lifting the load; the air inside the bag is. This is why proper tire pressures must be constantly monitored and maintained.

Safety issues arise when there is not enough air in the tire, or if the vehicle is overweight. When this occurs, there is no longer enough air in the tire to properly support the full weight of the vehicle. As a result, the rubber sidewalls of the tire will begin to support the weight of the vehicle. Then, as the sidewall begins to bear the vehicle's weight, it will begin to over-flex, or "squish." You can imagine what this looks like when you think of a flat tire.

Compressed bottom
When you stand next to a vehicle and look at the tire, you will notice that while the top of it is rounded, the bottom appears compressed between the axle and the road surface. Now consider that as the tire rotates around the axle, it is constantly changing its shape from rounded on the top to compressed on the bottom. This change in shape causes the sidewalls of the tire to flex, which in turn causes the tire sidewall to heat up. This heat is normal and tire manufacturers account for heat build-up from sidewall flexion when designing a tire.

When a tire is under-inflated, the over-flexing of the sidewall as it rotates around the axle will create more heat than the tire was designed to handle. The tire may overheat to the point that it fails and a blowout occurs. This situation can lead to total loss of vehicle control to the untrained operator.

A tire blowout is a scary experience that often catches a vehicle operator off guard. You may hear a loud noise, followed by a severe shaking of the steering wheel or shaking through the driver's seat. When this happens, the reaction of most drivers is to forcefully apply the brakes. This is actually the worst thing you can do. Believe it or not, the correct response is to apply acceleration.

Immediately following a tire blowout, you should apply enough acceleration to regain control of the vehicle. This does not mean to floor the accelerator, but rather to provide the vehicle with enough forward momentum to allow you to maintain control of your rig. Once you have control of the vehicle, gently decrease your speed and come to a controlled stop in an area of safe refuge. While a tire blowout is a serious event, a trained operator will be able to safely mitigate the situation.

Most people believe that tire blowouts are the result of having too much air in a tire. While this type of blowout can occur, under-inflated tires are usually our worst enemy. It is important to ensure that tire pressures are checked on a weekly basis and are within the range of pressure that is recommended by the manufacturer for the tire that you are using. It is also important to ensure that vehicles are not overweight, as this could also lead to a tire blowout.

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