Publication White Paper:
Motorcycle Accident Reconstruction Techniques
Forward
Vehicle accident reconstruction is founded upon the scientific principles of conservation of momentum and conservation of energy. Most medium to large size police forces employ individuals who have been trained in the basic reconstruction of vehicle accidents for the primary purpose of determining the speeds of the vehicles involved. Automobile accidents, generally speaking, are relatively simple to reconstruct due to the vast information available related to a vehicle’s crush characteristic as well as the relative ease in understanding the dynamic motion and interactions of automobiles. By contrast, the reconstruction of accident involving motorcycles can be quite complex and challenging to many accident reconstructionists. Proper motorcycle accident reconstruction requires an intimate knowledge of motorcycle dynamics and a strong understanding of how motorcycles react to rider inputs.
Introduction to Motorcycle Reconstruction
The reconstruction of a motorcycle accident typically progresses in reverse of the chronological events of the accident. Specifically, the reconstruction begins at the point of rest of the motorcycle and/or rider and then works backwards to some point in time prior to the beginning of the accident sequence, such as to when appropriate actions could have prevented the accident. Typically there are up to 5 distinct phases of a motorcycle accident sequence.
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1) Perception-Reaction: The first phase of accident analysis actually begins before any impact or accident avoidance occurs and is often referred to as the perception-reaction phase; this is the phase where the rider perceives a hazard in front of him and decides what his response to the hazard will be. Published values for the time it takes a typical rider to perceive a hazard, decide an appropriate course of action and then begin to implement his chosen reaction is on the order of 1.1 to 1.5 seconds.
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2) Avoidance - Braking/Steering: After completing the perception-reaction process, the rider typically enters an avoidance phase. During this portion of the accident the rider may decide to steer or brake. If braking is chosen, the rider has the option of applying the front brake alone, the rear brake alone or a combination of both front and rear brakes. The physical evidence at the scene, combined with witness statements, will often give clues as to which course of avoidance was implemented.
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3) Pre-impact Sliding: During the braking phase occasionally riders will overuse the vehicle’s brakes which can result in locking the front and/or rear wheel. If the rear wheel locks, with the motorcycle traveling on a straight trajectory, most riders can maintain control of the vehicle for a significant distance. However, if the front wheel locks, it is almost guaranteed that the rider will lose control of the vehicle and crash, usually very quickly. A front wheel lockup can be a fairly common occurrence, especially when executing an emergency braking maneuver. If the rider loses control while braking, the vehicle and rider typically separate and slide along the roadway. The trajectory of the bike and rider usually follows the same trajectory that they were on prior to the loss of control, and can direct the rider right into the impact zone.
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4) Impact: During the impact phase, the bike and/or rider may impact some other object, such as a vehicle pulling out in front of them or a stationary object such as a guardrail. Impact damage may be evaluated and combined with the sliding distance to help determine a vehicle’s speed during the accident sequence.
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5) Post-impact Motion: After a vehicle and/or rider impacts an object there may be additional post impact movement to the point of final rest. It is common that the rider separates from the motorcycle during the accident sequence and travels independently to rest. Analysis of the post-impact travel distance of both the rider and the vehicle can often yield independent, yet similar results as to speeds associated with the accident.
The basis of motorcycle accident reconstructions is similar to that of other accident reconstruction techniques in that it relies upon the basic principles of conservation energy and momentum. However, there are two distinct issues which make motorcycle accident reconstruction more challenging than a typical automobile accident reconstruction. 1) Motorcycle reconstruction often lacks the availability of crash test information, which is vital for performing a crush energy analysis, and 2) understanding the dynamics associated with motorcycle accidents requires an understanding of how a rider interacts with the motorcycle and how a motorcycle responds to rider inputs.
Application of “Conservation of Energy” to motorcycle accident reconstruction: Crush Energy
One common basis of automobile accident reconstruction is the publication of a vehicle’s “crush stiffness”. In simple terms, crush stiffness is the amount of energy required to cause a specific amount of permanent deformation to a vehicle’s body. The crush stiffness values are typically obtained, for passenger cars, through crash testing of the vehicle into a barrier. The testing is performed by “driving” the vehicle into a barrier at a fixed speed and measuring the permanent deformation of the vehicle’s body. With the availability of the published values for deformation vs. impact speed, a Reconstructionist can then measure the deformation of a similar car and calculate the likely speed which the car was going at impact. This technique is sometimes referred to as “crush energy analysis” and employs the concept of the conservation of energy.
By comparison, there is very little information published related to similar crash testing of motorcycles. There have been attempts in the past to crash test motorcycles by running them into fixed objects such as passenger cars or concrete barriers. The SAE publication “17 Motorcycle Crash Tests into Vehicles and a Barrier” reports information and results of crash testing several decommissioned police motorcycles. While the information obtained during these tests is valuable, it is unfortunately limited to a specific type of motorcycle. If one is investigating an accident involving a motorcycle which is similar to the motorcycles tested, then the published information and results are useful. However, if one is attempting to reconstruct an accident involving a sportbike, for example, then relying upon the results reported in the SAE publication is questionable for many reasons. One of the primary reasons that applying the published results to sportbike accidents is that the published crash testing was performed on motorcycles with conventional style forks while most sportbikes employ a cartridge style (or upside down) fork. This distinction is important because the speed estimation equations derived from the crash testing rely upon the vehicle’s wheelbase reduction as a result of fork deformation but the forks on a sportbike do not typically deform like the test motorcycles. The “upside down” style of fork utilized on most sportbikes results in a significantly stiffer fork housing and mounting style. As a result, sportbikes typically experience an entirely different failure mode when the motorcycle is involved in a frontal collision. In many cases, when a sportbike experiences a frontal impact with another object, the failure mode is the fracture or deformation of the mainframe near the steering head while the forks often exhibit minimal deformation damage. By contrast, the failure mode of the tested vehicles was the bending of the fork tubes which resulted in a rearward displacement of the front wheel.
Therefore, currently the technique of “crush energy analysis” based upon wheelbase reduction is limited to motorcycles which utilize a fork and mounting structure similar to that of the vehicles involved in the crash testing; i.e. conventional style forks. While the information published in the SAE paper is valuable, like any test results they must be utilized and applied in an appropriate manner for the analysis to be considered accurate.
Application of “Conservation of Energy” to motorcycle accident reconstruction: Slide Energy
While the analysis method utilizing crush energy may have limited applications for motorcycle accidents, the concept of conservation of energy related to sliding can be applied not only to the vehicle, but also to the rider. In the large majority of motorcycle accidents, the rider separates from the motorcycle at some point during the crash sequence. As a result, a Reconstructionist is able to evaluate not only the motorcycle’s path and travel distance, but the rider’s path and distance can be analyzed, often as a second data point for the purposes of speed estimation.
The technique of utilizing slide energy is facilitated by an adequate amount of published information related to the deceleration rates for various motorcycles as well as riders during the sliding phases of a motorcycle accident sequence. Published test results consistently show that motorcycles slide easier than riders on most surfaces. The result of this physical property is that typically the motorcycle will slide further than its rider after the two separate assuming that the vehicle and rider slide independently and do not strike any objects during the slide.
Summary
In summary, while motorcycle accident reconstruction relies on the basic principles of conservation of energy and momentum, it is the proper application of these principles along with an understanding of motorcycle dynamics and control which result in an accurate reconstruction of motorcycle accidents.
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