Crash Dynamics and Accident Reconstruction Q & A's

By: Patrick Sundby, Accident Investigator

Specializing in Low Speed and Catastrophic Crashes

1. “How do airbags work and why do they deploy in some cases and not others.”

Almost all airbag equipped vehicles contain an airbag control module.  The module monitors various vehicle systems and has a predetermined threshold for deployment; in simpler terms, this means the collision has to meet certain settings to deploy an airbag.  While each car brand’s system is specifically different from the next the concept is the same.

The module constantly monitors a vehicles speed and when a collision occurs the module can tell the change in speed is happening faster than if the car was slowing by brakes alone.  IF the collision, as calculated by the module, is extreme enough it will deploy the appropriate airbag(s).  The module has the final say in why an airbag is deployed, this is truly vehicle specific as well as module software & hardware dependent.

The module can knows, via onboard accelerometers, of changes in the vehicles direction and speed.  The module constantly calculates these changes and when it “sees” a change beyond preset thresholds it begins to monitor, very closely, the changes (this is called algorithm enablement).  If it determines the changes meet the criteria for airbag deployment it will deploy the appropriate airbag(s).

Many vehicles also have failsafe sensors mounted in the vehicle which are designed as a secondary mechanical and/or electrical triggering system.  These sensors are mounted on the front of the vehicle, usually under the radiator, when crushed or damaged they force an airbag deployment.    

Occasionally, someone will ask if how a vehicle knows if a seat is occupied.  The driver’s seat is obvious, beyond this, the front passenger seat has a pressure sensor in it which can tell when a predetermined amount of weight is on it, and the rest of the seats use the seatbelt latch (vehicle specific).  When you are driving a vehicle the module also monitors the status of seatbelts and the pressure sensors, it uses this data to make the best decision possible about which airbags to deploy and when.

2. I’m often asked about a specialists report, but the most common subset questions are about the lack of support for findings in the report.  I have chosen to address this question because it’s of personal & professional interest to me.

“I got this collision expert’s report but there doesn’t appear to be any explanation for his findings, is this normal?”

Yes and No.  Yes, this happens; no, it’s not acceptable standard.  One of the reasons I have chosen to work with Dr. Studin is his tenacious commitment to research.  If you have seen Mark present you know he has scholarly research to back up his points.  Mark and his colleagues have been through accredited and standardized training based on a lot of scholarly research.  All professional fields of post primary education are all based in accredited & scholarly formal standards. 

Collison reconstruction specialists are no different.  While not necessary part of an undergraduate or graduate program, the training and education they have is based on the same accredited & scholarly formal training and education - because of this correlation, the same standard should be applied to collision reconstruction specialists.  Scholarly research is based on objective methods of testing and investigation, peer review, and rigorous scrutiny before being accepted. 

When an expert offers an opinion without citing supporting scholarly documentation it’s not worthless, but rather it stands alone; it is only his opinion.  Conversely, when an expert offers and opinion with appropriate supporting scholarly documentation, all the work, expertise, and research is offered with his opinion.   

3. Often times an appraisal for repairs is used to justify “low speed” by citing minimal costs.  There are a few points regarding them to consider so the question is: 

“Is the listed cost on the appraisal an accurate reflection of damage?”

The short easy answer is “no”.  The long answer starts with understanding who did the appraisal and what is there background?  Usually, appraisers are trained by the insurance company – as such, minimizing the costs and expenses of repair is in the insurance company’s interests.  Secondly, most appraisers do not disassemble a vehicle to determine if there is any hidden damage, particularly in low speed collisions.

The next problem is when replacement parts are needed where do they come from?  Original Equipment Manufacturer (OEM) parts cost substantially more than Equal or Like Quality (ELQ) parts, as such, ELQ parts are the preferred choice of insurance companies.  It would cost the industry millions more to use OEM parts instead of ELQ parts when making repairs.  Along this same line, the quality of paint also varies.  Paint manufacturers offer paint systems which will meet the OEM specifications and are very durable paints, however, they also offer more economically friendly paint which is not as durable or closely color matched to the original, and as expected, it costs less.

The last problem to discuss is job downtime.  The longer a vehicle is in for repairs the more it costs the insurance company in rental fees.  While a shop can, and will, have a minimum amount of time to fix the vehicle the insurance company is going to keep them on this timeframe and constantly press for the vehicle to be completed.  Sometimes this drive can create an environment where the repair facility will sacrifice quality of workmanship to complete the job faster for a better profit margin.

The above variables greatly dictate the final number making it too subjective for a reliable point to support the threshold of injury; in other terms, the use of “low cost” as a justification for no injury is not appropriate as no causality relationship exists.  If a breakdown of the repair bill is provided, you could objectively price the repair parts and effectively show the bias towards reducing the cost of the repair.  

Patrick Sundby has decades of experience in the automotive industry including several years in law enforcement collision investigation. He has also been a driver training and firearms instructor in law enforcement and a police officer for 9 years before specializing in accident investigations. He has had the privilege of participating in both learning and teaching at Prince William County Criminal Justice Training Academy in Virginia and studied at the Federal Law Enforcement Training Center in Georgia. His specialty is low speed and catastrophic crashes and has testified over 500 times at various level. He can be reached at 571-265-8076 or patrick.sundby@gmail.com

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Conservation of Momentum:

Where does it go, Part II.

By: Patrick Sundby, Accident Investigator

Specializing in Low Speed and Catastrophic Crashes

Mark Studin DC, FASBE(C), DAAPM, DAAMLP

In the previous writing we explored the standards for vehicle integrity during low speed collisions.  In this writing we will expand on Conservation of Momentum.  If you have not read the previous article you are encouraged to do so.  While it’s not necessary to read it doing so will assist you as this writing will build on concepts contained within.  If you do not have it, please contact us and we will make it available to you.

Expand on Conservation of Momentum.

Remember we previously said “The momentum going into a collision can be accounted for in the outcome” when we discussed the concept of Conservation of Momentum.  Here we will introduce the formula and walk through its components; we will need to understand this in order to explore how different size vehicles affect each other in a collision.

The full formula:

Let’s walk through this, on the left side of the equation we have  which is the weight of the first vehicle before the collision multiplied by  which is the velocity (in feet per second) of the first vehicle before the collision.   is the weight of the second vehicle before the collision times  which is the velocity (in feet per second) of the second vehicle before the collision.  On the right side of the equation we have  which is the weight of the first vehicle after the collision multiplied by  which is the velocity (in feet per second) of the first vehicle after the collision.   is the weight of the second vehicle after the collision times  which is the velocity (in feet per second) of the second vehicle after the collision.

Ok, I know this seems very complex and the explanation is not jumping off the page so let’s write with a little more ease of understanding.  Let’s take the National Highway Transportation Safety Administration (NHTSA) standards for testing and put two of the same mass vehicles in this.  Let’s use a 2012 Toyota Corolla, and because we need two of them we will say one is red and the other is blue.

Red Corolla * 5 mph + Blue Corolla * 0 mph = Red Corolla * 0 mph + Blue Corolla * 5 mph

The 2012 Toyota Corolla has a curb weight of 2,734 pounds, substituted in the formula it looks like this:

2,734 lbs * 5 mph + 2,734 lbs * 0 mph = 2,734 lbs * 0 mph + 2,734 lbs * 5 mph

We need the speeds in feet per second, to do this we will multiply by 1.47 times the miles per hour.  This gives us 7.35 feet per second.

2,734 lbs * 7.35 fps + 2,734 lbs * 0 fps = 2,734 lbs * 0 fps + 2,734 lbs * 7.35 fps

Now when we do the math to show the conservation of momentum we end up with the following:

20,094.9 + 0 = 0 + 20,094.9

20,094.9 = 20,094.9

Momentum conserved.

Now we have proved the concept so we are going to apply it to a collision involving two different vehicles.  We will substitute the 2012 red Toyota Corolla for a 2012 red Chevrolet Tahoe.  The 2012 Chevrolet Tahoe weighs 5,448 lbs.  Now the formula looks like this:

Red Tahoe * 5 mph + Blue Corolla * 0 mph = Red Tahoe * 0 mph + Blue Corolla * 9.96 mph

5,448 lbs * 5 mph + 2,734 lbs * 0 mph = 5,448 lbs * 0 mph + 2,734 lbs * 9.96 mph (speed after impact)

We need speeds in feet per second, to do this we will multiply by 1.47.  This gives us 7.35 (5mph) and 14.64 (9.96mph).

5,448 lbs * 7.35 fps + 2,734 lbs * 0 fps = 5,448 lbs * 0 fps + 2,734 lbs * 14.64 fps

Now when we do the math to show the conservation of momentum we end up with the following:

40,042.8 + 0 = 0 + 40,042.8[1]

40,042.8 = 40,042.8

Momentum conserved.

Three important points can be observed in this demonstration. 

First, when testing is done note the change in speed in the Tahoe is 5 mph (5 to 0).  This is less than the speeds used by the Insurance Institute for Highway Safety which we have previously discussed and we would expect the Tahoe to have no structural deformation and minimal cosmetic damage.

The second point to note is the change in speed the Corolla experiences, 9.96 mph (0 to 9.96).  This change in speed is four times the minimum needed to induce whiplash injury.

Finally, neither vehicle exceeds the speed of 10 mph, which the auto manufactures and insrunace institute for highway safety often consider threshold for injury. This confirms that cars can easily deform and occupants get injured in low speed crashes once you begin to look at the conservation of energy (momentum) and coefficient of forces transferred to the target car.

Should you want a further explanation or to discuss a case, please contact me 571-265-8076

References

Edmunds.com. (2012). 2012 Chevrolet Tahoe Specifications. Retrieved from Edmunds.com: www.edmunds.com

Edmunds.com. (2012). 2012 Toyota Corolla Sedan Specifications. Retrieved from Edmunds.com: www.edmunds.com

Brault J., Wheeler J., Gunter S., Brault E., (1998) Clinical Response of Human Subjects to Rear End Automobile Collisions. Archives of Physical Medicine and Rehabilitation, 72-80.

Patrick Sundbyhas decades of experience in the automotive industry including several years in law enforcement collision investigation. He has also been a driver training and firearms instructor in law enforcement and a police officer for 9 years before specializing in accident investigations. He has had the privilege of participating in both learning and teaching at Prince William County Criminal Justice Training Academy in Virginia and studied at the Federal Law Enforcement Training Center in Georgia. His specialty is low speed and catastrophic crashes and has testified over 500 times at various level. He can be reached at 571-265-8076 or patrick.sundby@gmail.com

Dr. Mark Studinis an adjunct associate professor of chiropractic at the University of Bridgeport College of Chiropractic, an Adjunct Professor of Clinical Sciences at Texas Chiropractic College and a clinical presenter for the State of New York at Buffalo, School of Medicine and Biomedical Sciences for postdoctoral education, teaching MRI spine interpretation and triaging trauma cases. He is also the president of the Academy of Chiropractic, teaching doctors how to interface with the legal community (www.DoctorsPIProgram.com). He teaches MRI interpretation and triaging trauma cases to doctors of all disciplines nationally, and studies trends in health care on a national scale (www.TeachDoctors.com). He can be reached at DrMark@AcademyofChiropractic.com or at 631-786-4253.

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