Sunday, January 21st, 2018

Tank Design Reduces Post Collision Fires


Motorcycle accidents are often angled collisions, a type of accident which occurs when a motorcycle is impacted by another vehicle on the side or obliquely. Such accidents occur at road intersections or when a vehicle is joining the main flow of traffic, e.g., a vehicle leaving a gas station. In some of these collisions, the motorcycle gas tank is dented, decreasing the internal volume and dramatically increasing the internal pressure. If this happens when the gas tank is full or nearly so, the pressure build-up can blow the tank cap off, drenching the rider with gasoline. The gasoline often ignites resulting in severe injuries or death. There are four questions you need to know to determine if improper tank design is a factor in your post-collision motorcycle fire.


The pressure build-up can be reduced considerably by a simple device which traps air in the tank. This device consists of a short tube inserted in the tank filler hole projecting downward a short distance. As gasoline fills the tank, it displaces the air until the gasoline reaches the level of the bottom of the filler tube. At this point, any additional gasoline added to the tank will simply flow out the tank filler hole, not into the air trapped in the voids left in the tank.

The device works on the principle of the compressibility of gases versus the near-incompressibility of liquids. For a full tank without air trapped inside, even an indentation as little as 1% of its volume will increase the pressure inside the tank approximately 225 PSI. It is unlikely that a gas cap or tank seam will be able to withstand this pressure and prevent gasoline from blowing up out of the tank during a collision. Think of squeezing a nearly full plastic gallon milk jug with the cap on; even a small squeeze will cause the cap to fly off. However, by allowing air to occupy just 10% of the tank’s total volume, this same indentation will only increase the pressure by approximately 30 PSI; this is an internal pressure which the tank and cap can be designed to withstand.

The tradeoff for this margin of safety is a decrease in usable tank capacity with a corresponding decrease in the mileage the rider gets from one tank of gasoline. One way to defeat this safety device and restore the tank’s volume capacity is by drilling a small hole in the side of the tube. This allows the air to escape and the fluid level to rise to the level of the drilled hole. Internet bulletin board chats are filled with discussions and “advice” to drill this hole and restore tank capacity, although no one seems to know the reason for the tank filler tube being there in the first place or the dramatic loss in safety traded for being able to ride 40 miles further before filling up.

In addition to the user defeating this safety device by drilling a hole in it, custom built motorcycles or motorcycles with customized tanks on them may not even have the device to begin with. Older motorcycles (prior to the mid-1980s) do not have this device. Unfortunately, some manufacturers have forgotten the original intent of the device and have defeated it by producing new motorcycles whose tank filler necks have holes already drilled in the side.

Questions to Consider

The next time you have a case involving a motorcycle collision with an ensuing fire remember the following: Is there any deformation of the tank? How full was the tank when the accident occurred?/When did you last fill up? Check inside the gas cap and look to see if there is a neck filler tube. Does it have holes in the side of it? Do they appear to be drilled by the end user or were they put there during manufacturing?

Knowing the answer to these questions will help you and your experts in determining the root cause of a post-collision motorcycle fire.

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