R. Scott King, BSME, CFEI, Principal Automotive/Mechanical Engineer
Because commercial tractor-trailer combinations possess a wide variety of electrical, mechanical, and hydraulic systems and components, investigating fires that occur on such a vehicle requires not only an understanding of fire science, but knowledge of the systems unique to commercial trucks. Experience has shown that one of the most common causes of commercial tractor-trailer fires is related to the braking system.
In particular, commercial tractor-trailers utilize what are commonly referred to as “spring brakes.” Spring brakes, or perhaps better described as parking brakes, engage when air pressure is removed from the parking brake actuator. Normally, this is what happens when a driver stops the truck and applies the parking brake. By activating the parking brake valve, air pressure is vented from the brake chamber and a powerful spring within the chamber extends. The brake shoes expand to hold the brake drum and wheel from rotating.
Failures within the system can occur that cause the spring to partially extend while the truck is moving without the parking brake applied. As a result, the brake shoes contact the rotating brake drum with just enough force to cause rubbing friction between the shoes and drum. Over time, this friction creates heat, which in the extreme case, can result in what the industry calls a “wheel fire.”
Wheel fires can often be confirmed through witness testimony and physical evidence. In addition to exterior fire, heat, and smoke patterns, wheel fires resulting from dragging brakes can be confirmed by removing wheel and brake drums for a detailed examination. During this removal, other potential causes, such as improperly lubricated wheel bearings, can be examined. With this information, fire investigators can identify whether factors such as manufacturing or vehicle maintenance are in any way relevant.
R. Scott King, BSME, CFEI, Principal Automotive/Mechanical Engineer with DJS Associates, Inc., can be reached via email at experts@forensicDJS.com or via phone at 215-659-2010.