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Friday, 12 July 2019 20:13

Truck Topics: Hey Buddy … Got a Millisecond?

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During the 2nd annual HD Repair Forum event held in Fort Worth, Texas, in March 2019, John Spoto, National Heavy Duty Truck Commercial Fleet manager for the 3M Company gave a fascinating presentation on the effects of a crash and how so much can happen in a very short amount of time.

The following is a brief synopsis of that presentation. It is not meant to be scientifically precise, but only meant to give some perspective on what happens during a crash and what must be done to return the vehicle to road-worthiness.

 

To begin with, there are many variables that determine the severity of a crash including:

 

  • The speed of the moving vehicle. Did you know the faster you drive, the greater the impact or striking power of your vehicle? The laws of physics determine that the force of impact increases with the square of the increase in speed. So, if you double the speed of a vehicle, you increase its force of impact four times.
  • The weight of the moving vehicle. This is especially crucial with trucks. Is the truck traveling empty or loaded? If it is loaded, how much does it weigh and how secure is the load?
  • What is the nature of the object the truck will come into contact with? Is it completely stationary and immovable, like a reinforced brick or block wall, a bridge abutment, or a large tree? If so, the impact energy of the truck will be pushed back in an equal amount and the truck will sustain the entire force of the crash. If the truck hits something that will move, like a parked vehicle, for instance, the other vehicle will absorb some of the energy. The deceleration of the truck would not be so abrupt and thereby mitigating some of the damage to the truck and occupants.
  • If truck “A” is going 60 mph and hits truck “B” of equal size and weight in a head-on manner, also going 60 mph, it would be the same as truck “A” hitting an immovable object at 60 mph.
    The identical speeds and mass cancel out each other or, better said, the two objects push back on each other with equal force. But this rarely happens. Chances are if a head-on collision is to occur with a semi-truck, it will more likely be with a car of considerably smaller mass and weight. From a pure physics point of view, this would help mitigate the damage to the truck, but could be catastrophic for the car.
  • What is the direction of impact? In other words, was it a head-on crash or a glancing blow which could have dissipated much of the crash energy?

 

What isn’t a variable is a law defined by Sir Isaac Newton – the law of inertia. It says an object in motion tends to stay in motion until acted upon by an outside force. Think in terms of the payload being transported by the truck in question. When the tractor hits the brick wall, Newton’s law of an outside force taking over to stop motion takes over to stop the tractor – but his law of inertia says the load and trailer want to keep going!

 

And just think … what is going to take days or maybe weeks to repair, happened in a matter of milliseconds. How long is a millisecond? It is one-thousandth of a second – the time it takes a camera flash to go off. A housefly’s wings flap every three milliseconds. Eight milliseconds is equal to 1/125th of a second or a common camera shutter speed. Researchers at MIT determined that the human eye can interpret images exposed for as little as 13 milliseconds. Anything faster goes undetected.

 

Zero Milliseconds

 

This is the point where the front bumper first makes contact with a barrier or some other object – what you might call the “point of no return.” Using specific materials, parts and fastening protocols, engineers have designed the truck to collapse in a predictable manner to protect the truck’s occupants. This is where all that engineering pays off. And if the truck has been in a prior accident, this will tell if the repair was done in a safe and complete manner—or not.


 

Five Milliseconds

 

The truck’s body structure is already absorbing and managing crash energy. Each section area within the cab has a specific function in the event of a collision to channel the impact energy around the occupants.

 

Ten Milliseconds

 

The front bumper is fully collapsed and crash forces are being channeled through upper and lower members and body panels. Panels are designed to collapse to a certain point to not only absorb the energy, but to keep the occupants from becoming trapped inside. This is also called “Controlled Deceleration.” Starting in January 1965, Ford Motor Company crashed over 175 cars into a concrete barrier at 30 mph. The reason, to build a vehicle frame and structural parts that deform in a uniform manner upon impact to absorb the energy and mitigate cabin deformation, thereby saving the car’s occupants. The culmination of this testing would be introduced in all 1968 model Ford cars with what was being called a “Controlled Crush” front end. Other car and truck makers would follow.

 

15 Milliseconds

 

The engine has been contacted and the subframe is being deformed. Different strengths of metal are used in the truck’s construction to either “break away” or transfer collision energy to other parts of the truck. (Before the days of “Controlled Deceleration” the engine may have penetrated the cab at this point injuring the driver.)

 

20 Milliseconds

 

The structure forward of the engine is now fully deformed and the crash energy is being channeled into the roof rails, rocker and rear portion of the engine subframe.

 

30 Milliseconds

 

The cab continues to deform. Crush-zones crumple and redirect the crash energy around the truck’s occupants. (Crush Zones can be seen as dimples, slots drilled in body parts or different types and thicknesses of metals. Ribs or stamped areas across the width of a part are also called convolutions and designed to aid in a Controlled Deceleration event.) In a repair bulletin, Volvo notes, “The cab frame and body panels form a cage that protects the driver and passenger. The cab exceeds protection safety standards in case of collisions or rollovers. Unlike conventional cab structures, where a load carrying frame supports the outer body panels, both the VN and VHD cab frame and body panels are designed to be part of the load carrying structure.”


 

40 Milliseconds

 

As the crush zones deform, the crash energy is transferred to the dash, front cowl, floor pan and rockers. At this point, some sheet metal may have been bent, some kinked. Bent metal, depending on its strength and hardness may be bent back into shape. Kinked metal generally has to be replaced, especially if it is high-strength steel. (Straightening kinked metal could weaken it making it collapse in a subsequent crash.)

 

50 Milliseconds

 

The engine assembly contacts the dash. The “A” pillar, roof, door pillar, rockers and floor pan carry the balance for the crash load.

 

67 Milliseconds

 

The truck has reached maximum deformation. The penetration into the occupant area was controlled and limited due to the construction of the truck and materials used. The crash load was directed around and under the truck occupants. But even in the moment, the truck has completely stopped, momentum continues possibly forcing the payload through the back of the cab.

 

100 Milliseconds

 Event is complete

 

Now, the damage estimating and repair process begins. Referring again to a Volvo collision repair bulletin, they specifically point out, “When major body damage occurs, you should replace entire sections instead of changing parts within a section. Replacing an entire section preserves the structural integrity of the cab and generally takes less time.”

 

When reviewing or estimating a crash, the primary point of impact, probably the front of the cab, will no doubt get the most attention as it is the most obvious place to look. However, there could be considerable “Indirect Damage” which could include the frame or any part of the truck as the force of the collision was dissipated through the entire vehicle. This is why it is so important to look over the entire truck, not just the point of impact.


 

Indirect Damage could have occurred on mechanical parts, electrical parts, engine brackets, seat brackets, air valves, dashboard and door trim panels just to name a few. Indirect Damage could have also loosened, deformed or separated seam sealer, cavity foams or those parts secured with adhesives.

 

Some repairers may overlook foams and sealants, yet they perform a vital role in the drivability, and performance of the truck as well as driver comfort. Foam and sealants can be used to reduce noise, vibration and harshness in the cab, stiffen the body structure, as well as seal out dust, rainwater or unwanted fumes. And while you’re at it, don’t forget to apply corrosion protection.

 

When making collision repairs, remember to never compromise safety. Return the truck to the driver/owner with the same structural integrity it had prior to the crash because in a subsequent accident … milliseconds still count!

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