“…permanent structural damage was only visible when an impact speed between 14 and 15 km/h was experienced by the vehicle. Very Uttle frame deformation occurs for impact speeds below this value. Below this threshold, the vehicle frame can be considered rigid; vehicle response being dominated by the compliance of the bumper and suspension systems as well as sliding of the locked wheels. The accompanying occupant response was a differential rebound of the head and shoulders off the seatback and head restraint. This relative motion between the head and torso was evident in eachtest and increases the potential for injury. Typical occupant response observed consisted of an initial loading and deflection of the seatback due to the occupant’s inertia followed by the release of this stored spring energy as the occupant was catapulted forward. It is this elastic behaviour of the seatback which is the likely cause of whiplash injury. Resulting head velocities were found to be in the order of 1.5 – 2 times the resulting vehicle speed. Initial occupant postures which increased the distance between the torso and seatback tended to increase the dynamic loading experienced by the passenger.”
“Velocity change is meaningless if the time over which it occurs is omitted. Tarriere et al described this shortcoming with examples of accidents with similar AV’s but varying severity due to the different acceleration histories experienced by the vehicle and the occupant.”
From Navin & Thomson, 1988:
“There is a difference between the vehicle velocity attained and the original impact velocity. This reductionis 38% at the higher speed where structural crush takes place and 22% at the lower speed where a greater portion of the energy is elastic and is translated to the occupant compartment”
“The effect of an improperly positioned headrestraint and initial occupantposturewas shown to affect the maximum deflections of the head. The occupant experienced lower accelerations with increased deflections when the headrest is not present.The head also experienced larger deflections relative to the shoulder when the occupant’s initial position was moved farther from the seat. This latter effect was seen to produce effects comparable to responses at twice the impact speeds with a normally seated occupant.” (Ask me why this is important)
From Brach Engineering:
“…the presence of struck vehicle braking should heighten the effect of suspension dynamics particularly at very low speeds”.
“…braking of the struck vehicle can increase the contact duration significantly and significantly reduce the coefficient of restitution to the point that both vehicles come to rest.” (Meaning that the struck vehicle that applies its brakes will cause the struck vehicle to absorb 100% of the impact energy of both vehicles… which is then tranferred almost entirely to the passenger compartment)
From Bostrom, et al:
“…according to statistics and the mathematical simulations, no conventional seat-back design adequately prevented neck injuries or keeps the NIC50 values below the injury threshold.”
From Croft & Freeman (Croft Text):
“Property damage is neither a valid predictor of acute injury risk nor of symptom duration. Other factors, such as head restraint geometry, awareness of the impending crash, sex, and prior injury are likely to impose competitive or stronger outcome effects, particularly as regards long-term outcome. Based upon our best evidence synthesis, the level of vehicle property damage appears to be an invalid construct for injury presence, severity, or duration. The MIST protocol for prediction of injury does not appear to be valid.”
“We compared male and female subjects in crash tests in which each subject experienced both frontal and rear impacts. Crash speed and other crash parameters were held constant. We believe this was the first experiment using an independent variable of crash vector and dependent variables of head linear acceleration and volunteer qualitative tolerance. Analysis of data revealed that the rear impact vector crash resulted in 2.8 times greater head linear acceleration than frontal crashes. Rear impact crashes resulted in biphasic, complex kinematics compared to the monophasic, less complex frontal crashes. Rear impact crashes were rated markedly less tolerable.”
“We found that football impacts caused higher brain strains than most of the rear-end crash tests we examined, but there were some instances where both types of impacts produced similar amounts of strain in the brain. In one crash involving a 15 km/h speed change and a low head restraint, the dummy’s head wrapped onto the top of the head restraint and caused similar brain strains to head impacts that were more severe than the two responsible for concussions in NFL players.”
Actual citation: Elkin BS, Elliott JM, Siegmund GP (2016). Whiplash injury or concussion? A possible explanation for concussion symptoms in some individuals following a rear-end collision. Journal of Orthopaedic & Sports Physical Therapy 46(10):874-85.
From Masory & Poncet:
“Experimental results from low speed rear end collisions, which involved live human subjects, have shown that the peak head acceleration is at least two and a half times larger than peak acceleration of the struck vehicle. This assessment is correct for impact speed below 10 [km/h] (6.8 mph).”
Video of Four Low Speed impacts (5, 7, 9 and 11 MPH), with no brakes and proper headrests:
“Closing velocity was 3.4 m/s (7.6mph), delta V was 2.7 m/s, and
the subject’s head linear (x) acceleration was 11.8 g.”
(Yes, 11.8 gs. that’s not a typo. The Space Shuttle only pulls 3 gs at launch. Whiplash is considered a foregone conclusion diagnosis at 7.8 gs)
“Plaintiff’s expert must have reviewed the articles cited by defense counsel and be able to explain that the forces in daily living do not produce comparable force vectors to the incident in question, since the mechanism of injury is different; and the injured person’s ability to perceive the event and to react is different. The expert must be able to establish that it is not a valid comparison.”
“Insurance defense attorneys hire a biomechanics engineer or another “expert” to express an opinion that the physical forces involved in the accident couldn’t have caused the victim’s injuries. Here, we explore some of the “science” used by these defense experts and detail 10 major ways that their “expert” opinions may be flawed.”
From Counter Argument:
“The study concluded that, the “limit of harmlessness” for stresses arising from rear-end impacts with regard to the velocity changes lies between 10 and 15 km/h. For everyday practice, photographs of the damage to cars involved in a rear-end impact are essential to determine this velocity change. The stress occurring in vehicle rear-end collisions can be compared to the stress in bumper car collisions.”
RESPONSE: The bumper cars had an average velocity/energy that was 15%-22% less than an the auto accidents they compared them to, they made no provisions for the difference in mass of the vehicles (a factor from Bostrom), nor did they account for the “twice the speed” effect of improper headrest positions from Navin & Thomson. This paper doesn’t apply to this situation. In addition, the people in the bumpercars always have their hands on the steering wheels, which has a substantial effect on the energy absorption of the neck and head.
Compare it to a more appropriate rollercoaster injury, because of the fluid motion of the body’s response to the changes acceleration, then provide this work, looking at injuries from The Rattler roller coaster in San Antonio, Texas:
“there were a total of 656 neck and back injuries during the (19 month) study period, and 39 were considered significant by the study inclusion criteria. Seventy-two percent (28/39) of the injured subjects sustained a cervical disk injury; 71% of these injuries were at C5-6 (15 disk herniations, 5 symptomatic disk bulges) and 54% were at C6-7 (11 disk herniations, 4 symptomatic disk bulges). In the lumbar spine, the most frequent injury was a symptomatic disk bulge (20% of the cohort), followed by vertebral body compression fracture (18%), and L4-5 or L5-S1 disk herniation (13%). Accelerometry testing of passengers and train cars indicated a peak of 4.5 to 5g of vertical or axial acceleration and 1.5g of lateral acceleration over approximately 100ms (0.1s) on both. The results of this study suggest that there is no established minimum threshold of significant spine injury. The greatest explanation for injury from traumatic loading of the spine is individual susceptibility to injury, an unpredictable variable.”
NOTE: 5G is within the range of acceleration experienced during a 10KM rear end collison. In some cases, an 11.8 g force has been measured in a 7.6 mph collision.