In this article, heavy vehicle engineer, Erin Shipp, P.E. 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
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. Truck configurations effect 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 Inertia (COI) method. This method is acceptable and appropriate for similarly sized vehicles; however, as the difference in mass between the colliding vehicles grows (such as heavy truck-automobile collisions), the COI 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 energy used in the crush of the vehicles. 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 vehicle. Most energy formulas for automobiles are developed for crashes below 35 mph into a solid barrier. Barrier tests commonly provide an acceptable model for measuring automobile crush against a heavy truck. As the relative speed of the truck increases beyond 35 mph, the calculations become less accurate, but a reasonable answer can be achieved at normal highway speeds.
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 the truck brings more issues.
Skid marks are often used to estimate speeds, but with 5 or more axles on a heavy truck it is more complicated.
- First it must be determined what tires made which marks. Was it the drive axle of the truck or the trailer axles?
- Did the truck stop straight or did the trailer not follow the tractor?
- Were the truck/trailer/both equipped with ABS?
- How was the weight distributed on the truck and how did that change during braking due to the high 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?
Not everyone can properly analyze a heavy truck crash. In order to provide a reliable analysis, an expert should have training, education, and experience in physics, to understand the forces and energy involved in the crash, as well as heavy truck dynamics/systems to accurately reconcile those forces with the crashed vehicles.
Erin is a Bus and Heavy Truck expert with more than 30 years of engineering experience in product design and development of heavy trucks, motor homes, buses, and passenger cars.
Erin is no longer accepting new case assignments, but Robson Forensic offers other vehicle engineers with specific expertise reconstruction heavy truck collisions.
Please contact the head of our vehicle crash group, Peter J Leiss, P.E. to discuss your case, or submit an inquiry through our website.