In this article, heavy vehicle engineer Rick Barrett provides an introduction to heavy truck crash reconstruction, including an explanation of why the Conservation of Energy (COE) method is crucial for accurate and reliable reconstructions.
Heavy Truck Crash Reconstruction - Expert Overview
In reconstructing vehicle crashes involving heavy trucks, several important factors must be considered. These issues may be less relevant in automobile to automobile crashes.
Size: Trucks are bigger and heavier than automobiles, up to 40 tons, compared to 1-2 tons. As the difference in mass between colliding vehicles increases, some calculation models may become less reliable.
Configuration: There are many different configurations of heavy trucks, from a single unit straight truck to 3 trailer triples. Trailer types and payload conditions of these truck configurations affect vehicle maneuvering and stopping characteristics.
Damage: Damage to the truck in a truck-automobile crash is usually minimal compared to the auto. This may require alternate approaches to calculating forces.
Most automobile crash reconstructions are performed using the Conservation of Momentum (COM) method. This method is acceptable and appropriate for collisions involving vehicles of similar mass; however, as the difference in mass between the colliding vehicles grows (such as heavy truck-automobile collisions), the COM method becomes increasingly sensitive to uncertainties in impact and departure angles. Using the Conservation of Energy (COE) method may be more reliable in heavy truck-automobile crashes as it does not depend on the angles of impact and departure.
Energy calculations require a quantification of the total energy dissipated in the crush of the vehicles and any post impact travel. Energy being conserved means the total energy after the collision is equal to the total energy prior to the collision. There is usually little damage done to the heavy components of the truck; the plastic bumper and hood may be damaged, but the energy used for this is relatively low. The energy to crush the automobile can be calculated accurately by measuring crush on the passenger/light truck or SUV vehicle. Most energy formulas for automobiles are developed from crash tests run at or below 35 mph into a solid barrier. Barrier tests commonly provide an acceptable model for measuring automobile crush against a heavy truck.
Determining Pre-Crash Speeds
Once the crash speed is established, the vehicle movements before the crash should be analyzed to establish pre-crash speeds. The calculations for the auto are no different than in an auto to auto crash, but a heavy truck brings more issues that need to be investigated to calculate a proper brake factor for the truck.
Skid marks are often used to estimate critical speeds and deceleration rates, but with 5 or more axles on a heavy truck determining what type of tire mark and what axle made the tire marks, it is more complicated.
- First it must be determined what tires made which marks. Was it the steer or drive axles of the truck or the trailer axles?
- Did the truck stop straight or did the trailer not follow the tractor?
- Was the truck and/or trailer equipped with ABS?
- Was the truck equipped with disc brakes on the tractor and trailer?
- How was the weight distributed on the truck, and how did that change during braking due to the center of gravity?
- Does the more complex calculation for drag at each wheel need to be done, or can the truck be treated as a single unit? How does that affect the accuracy?
- What is the braking drag coefficient for the truck? Drag coefficients are frequently done with autos, but how do you adjust for a heavy truck?
- Did all brakes operate properly or were some out of adjustment?
- How long did it take for the air brakes at the back of the truck to actuate after the brake pedal was applied?
HEAVY TRUCK CRASH INVESTIGATIONS
In order to provide a reliable analysis, an expert should have training, education, and experience in physics as well as heavy truck dynamics/systems to understand the forces and energy involved and accurately reconcile those forces with the crashed vehicles.
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Automotive Engineer & Heavy Truck Expert
Rick Barrett is a vehicle engineering expert with 20+ years of industry experience involving passenger vehicles, heavy trucks, and specialty vehicles utilized by the military and emergency responders. His career within the automotive industry spans nearly twenty years and touches virtually every major vehicle system. Rick applies his expertise to forensic casework involving vehicle failures, vehicle performance, dynamic handling, lifecycle durability, and intellectual property disputes.