Extreme Case of Bumper Over-Ride Highlights A Common Safety Issue

An extreme case of bumper over-ride was published on the OPP’s West Region twitter account today. The front end of a tall pick-up truck was shown on top of the hood and windshield of a small passenger car and the incident reportedly occurred in the area of Norfolk County, Ontario. OPP reported no one was injured from the incident and that is good news. More importantly these views may have caught the attention of many persons who would ordinarily be too busy with seemingly more important matters in their lives – until they are caught up in a major collision that threatens them.

Views from this present incident indicate that it was more eye-catching than life-threatening. There is minimal crush visible on both vehicles. Even the interior of the car shows minimal potential for intrusion into the driver’s space and the air bag did not deploy. That is the goods news. But this scenario outlines much more serious concerns.

Even though the left front wheel of the pick-up truck is partly onto the car’s windshield there was minimal structural intrusion into the driver’s space and the air bag did not deploy.

Even a quick glance at my reference library shows a copy of a Society of Automotive Engineers (SAE) manual from 2005 entitled “Vehicle Aggressivity and Compatibility in Automotive Crashes”. This is a compendium of research papers related to collisions similar  to what is shown in the above photographs. Even early in my career, in the 1980s while I was conducting investigations of over 100 major collisions a year, I had developed a theory from studying the results of hundreds of head-on collisions including a specific study of Light Truck and Van (LTV) collisions that was conducted over a period of over 3 years. The theory I developed was that, if one wanted to survive a major head-on impact at relatively low cost, one should buy a four-wheel-drive, Chevrolet Suburban. The Suburban was stiff, it had a wide stance and its bumper was taller than most passenger cars. My studies demonstrated that, in a head-on impact, the vehicle that rode over top of the other vehicle’s front end resulted in a better chance of survival for the over-riding driver. That observation was true in many ways for many years.

Not long after the owners of pick-up trucks began placing lifters on their suspensions and the bodies of their trucks began to rise higher. Even manufacturers began building light trucks with higher suspensions. It can be seen of the Chevrolet Silverado in the above example that its body is well above the typical height of an old-style truck of the 1980s. The advantage of a higher body was often seen in Jeeps which also had a stiff bumper/frame that was higher than typical passenger cars. The drawback of Jeeps however was that they had a smaller wheelbase and track width than a Suburban and were more prone to instability, particularly after the initial head-on impact. And this leads to another issue regarding survivability, height and instability that few would expect.

As I originally observed “height wins”, But not in all facets. Height wins during the initial 110 milli-seconds as two vehicles reach they point of maximum engagement (maximum crush) and the stiff bumper area of the taller vehicle drives through the softer upper regions (hood, fenders, etc) of the vehicle beneath. But often the vehicle collision does not end there. In most instances a head-on collision involves 60 percent or less of a vehicle’s front end. This means the vehicles’ centre-of-gravity do not line up and that the collision is not “central” but is off-set. This means that, although much of a vehicle’s velocity could be lost in the initial impact, the vehicle may also have additional, residual velocity after the initial impact. This is more true of taller vehicles because they are also likely to be more massive and they are less likely to be stopped by the lower, less-massive, collision partner. The bottom line is that this taller vehicle which rides over the top of the lower vehicle now becomes destabilized. That destabilization often involves the lifting of only the struck half of the vehicle such that, upon leaving the area of impact this taller vehicle has a greater tendency to begin  to rollover rather than slide on its wheels to final rest.

So, having survived the initial impact because of the vehicle’s superior body height and mass, this driver is now faced with a possible substantial, post-impact velocity while commencing a rollover. This may not sound threatening because we know that, as long as the driver is properly restrained in a seat-belt, and stays within the safe confines of his/her vehicle, the deceleration rate of about 0.5 g will be of minimal consequence. But that is not the only issue.

In a major head-on impact there is a higher probability that the direct contact damage and crush is to the left front and thus a varying amount of deformation can occur which can exist as far back as the driver’s door without producing any serious structural intrusion. Meanwhile, near the end of this same impact, after the air bag starts to deflate and the driver is out of position, the relationship between the restraint system may not be ideal and the various components such as the door, side window or window sill may not be of the same shape as it was pre-crash. There is no guarantee therefore that a driver will stay comfortably confined by the restrain system, in a properly seated position, while the vehicle goes through a post-impact rollover with a substantial post-impact velocity. Partial ejection of the upper torso and head during these occasions have resulted in deadly consequences.

Too often the public has seen TV commercials demonstrating how a vehicle occupant is ejected clearly out of their vehicle and sustained their fatal injuries from striking the exterior environment. While this is one mechanism by which fatal injuries occur it is by no means the only mechanism. Whether it is full ejection or just partial ejection the body of the ejected occupant is in the vicinity of the vehicle when that vehicle is rolling over. The result is that the rolling vehicle may actually  crush the ejected occupant’s body several times during the rollover before both come to their rest positions. Thus the lucky driver whose  taller and heavier vehicle allows he/her to survive an initial impact may sustain fatal injuries from the seemingly less dangerous post-impact motion to rest.

Another important issue that is illustrated by the above photographs is air bag deployment. Note that the air bag in the small passenger car in the above photos did not deploy. Certainly this could be for a reasonable and foreseen eventuality programmed in the algorithm of the air bag control module. But one really knows. The triggering of air bags and the algorithms of how that is done is considered proprietary information that is only available to the manufacturer. I belong to several internet chat group of over 1000 international reconstructionists and there are many occasions where such investigators pose the question: ” Should the air bag have deployed in this described case?” or conversely “Should the air bag have not deployed considering the the low severity of this described case?” While a number of experts are willing to propose various reasoning for why the deployment should nor should not occur, the reality is that, without the details of the decision-making continued in the control module, it is only conjecture.

When vehicles of very different heights come into a head-on collision it becomes problematic, not only for the manufacturers to detect when an certain scenario requires air bag deployment, but investigators outside of the manufacturers also have a greater difficulty in making a correct determination of what should have happened. The reality is that the decision to deploy an air bag has to be made in a very short of time of 25 to 50 milli-seconds and that is well before there is enough information to know how the complete collision will eventually unfold. Much of the decision-making is made about the rate at which the velocity is changing. In fact the decision is made partly on the rate at which the acceleration is changing, which is referred to as Jerk. But no one can be sure what additional factors are taken into account or how. So the above photos help to bring attention to this fact that vehicles of different heights cause problems when they collide because the sensors that are positioned to capture the typical collision may not sense a collision in sufficient time to make a proper decision about air bag deployment.

In the end it does not create much harm when the OPP place photos on their Twitter account of a seemingly harmless and meaningless collision that catches the public’s attention. It gives me the opportunity to catch the public’s attention and discuss some consequences that could be major factors in their lives or of those near to them.