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Understanding Structure

Your customers cherish their automobiles — giving them names and personalities. And while you can dispute whether cars have souls, you can’t dispute that they have dimension. In fact, looking at cars as 3-D objects is the heart and soul of understanding structure.

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This past June, I exhibited my show car at the Los Angeles Concours d’Elegance. I’ve been showing the vehicle since 1994, and it’s a consistent winner. But I have to say that my heart isn’t really in it anymore.

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Because of my lack of interest, I became a people watcher, and I noticed that most of the people in attendance were car people — people who have a love affair with cars. These men and women, yes women, talked about these inanimate hunks of iron, plastic, rubber and glass as if they had personalities. They caressed the cars as if they were real and even gave them names.

While I find it hard to relate to what I consider "nonsense," our society revolves around the car. Think about how much our daily lives are touched by the automobile. It’s mind boggling.

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With that in mind, we, as repairers, need to look at cars in a different light. This doesn’t mean we need to believe cars have souls, but it does mean we need to understand they have dimension. In fact, looking at cars as 3-D objects is the heart and soul of understanding structure.

A Multi-Dimensional Look
Every point on a vehicle will have three things in common: length, width and height. When a vehicle has been involved in a collision, it’s the job of the collision industry professional to restore the vehicle to its pre-loss condition. In other words, the vehicle will have the similar corrosion protection and paint warranty, and it will behave in the same manner structurally if it’s involved in another accident. To accomplish this, the vehicle must be repaired to its original dimensions, which is why repair facilities must have the equipment to measure three dimensionally.

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Stop! Put down this magazine and get three dimes. Place them in front of you. Don’t look at one dime as 10 cents or 1/10 of a dollar. Instead, look at the thickness of the dime; it’s approximately l mm thick. Now, put the three dimes together; that thickness is the tolerance for most vehicles (the three-dime test is a good way to check most door gaps).


Learning Blocks
Before we can get a better understanding of structure, we must first look at theoretical forces involved in a collision and vehicle design.

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A front, side or rear collision involves two objects coming in contact with each other. The first object in the collision is a vehicle; the second can be either another vehicle or a stationary object (pole, wall, etc.). We can represent each of these objects as separate forces.

As a result of a collision, a vehicle receives damage from two forces: the external force exerted by the other vehicle or object and the internal force from within the vehicle.

Let’s stop a minute and review high school physics. The subject: inertia.

Newton’s third law of motion states that the velocity of a body is a constant in the absence of external force. In simple terms, matter (a vehicle, in our case) has resistance to any change in motion. In a collision, you have two forces opposing each other: the collision force and the inertial force. The greater the collision force, the greater the inertial force. For example, let’s say a vehicle is moving from right to left and hits a wall. The wall becomes a force and the energy will be exerted from left to right. The amount of force exerted by the wall will depend upon the force (in this case, the speed of the vehicle) that’s being expended by the car. The front of the car stops, but the back of the vehicle is still in motion and will continue to move even though the front has stopped. The force that propels the rear to move is called inertial force.

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Before we proceed any further into what happens during a collision, we should discuss vehicle design.

When you look at an unitized-body vehicle, you need to look at it as three distinct parts or sections: front, passenger and rear sections. In actuality, these three sections are all tied together, with the passenger section being the strongest. It’s been demonstrated that if a vehicle traveling 30 mph hits a wall head-on, the car will have an overall 30 to 40 percent reduction in length, but the passenger section will have only a 1 to 2 percent reduction.

Another aspect of unitized body construction is built-in crush zones. These zones are slots, holes and convolutions that are built into the structure to absorb and dissipate energy as a collision force moves through the structure. Once a crush zone has been destroyed, the part that houses the crush zone should be replaced.

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Just how do these crush zones work? In this example, a vehicle is impacted above the bumper and destroys the headlamp, fender and hood (we’ll call this Point A). The force will travel along the apron and top reinforcement. At this juncture, the force encounters a series of crush zones (Points B and C). They collapse, and part of the force is dissipated. When the force comes in contact with the windshield door pillar, the force is split in two, with some of the force traveling down toward the bottom of the car and the rest moving into the roof area. In most front-end impacts, the force at this juncture has been used up.

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If you need more clarification, here’s a simple experiment you can try that demonstrates the above principle: Get two raw eggs and a partner. Stand about 20 feet apart and tell your partner to catch the egg — keeping his elbows locked — when you toss it. I don’t need tell you the outcome. If your partner is still talking to you, ask him to catch the second egg, bending his elbows and cushioning the impact by drawing his hands toward his body. If he isn’t a total klutz, the egg will mostly likely survive.

The same principle holds true with today’s cars, but on a more sophisticated level.

Let’s take another look at what happens during a front, rear or side impact. In a frontal collision, the bumper is pushed toward the rear of the vehicle, and the reinforcement will collapse, followed by the frame rails, which will deflect upward (due to the shape of the rails). The fenders will deflect outward, the core support will collapse and the hood will bend at the crush zones. (Once a crush zone in the hood has been disturbed, the hood, in most instances, should be replaced because it’s been weakened). If the force of the collision is still moving, deflection will occur at the doorpost, causing the door to drop. Energy will move into the roof, causing it to deflect, and down the rockers. Under severe conditions, the rear rail will move upward at the kick-up section of the frame.

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Don’t Learn the Hard Way
Eric Streton of the Automobile Club of Southern California is the lead I-CAR instructor of the South-Pacific Region. He teaches all his students on a severe front impact to look at the rear of the vehicle first and pay particular attention to any disturbance of the seam sealer, alignment and fit of the deck lid. His reasoning for going to the rear first is quite simple: It gives you an idea of how far the energy has traveled through the vehicle. With a better understanding of secondary damage, you can assess overall damage more thoroughly, and you’ll write a more accurate estimate, which will reduce supplements and shop cycle time— not to mention, your customer will get a better repair.

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Case in point: About three years ago, we repaired a particular vehicle with severe front-impact damage. I wrote the estimate without paying attention to the secondary damage (the rear frame rail had kicked up), and repaired the vehicle for my customer. That winter, Los Angeles had a lot of rain, and the vehicle owner came back to the shop with a flood in his trunk. My first reaction was that the car was probably involved in a prior accident, but I told the customer to leave his vehicle and I’d check it out. I put the vehicle on the hoist and, to my surprise, there was a gap of about a quarter-inch between the inner wheelhouse and the trunk floor that had resulted from the frontal impact I’d repaired. Needless to say, if I’d had a better understanding of the collision force at the time, I would’ve saved the owner — and the shop — and lot of time and money.

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One final note on repairing structure: There are two words you should add to your collision vocabulary: form and function. Form refers to having the structure look the same after the accident as it did before the accident. Function means the repaired structure will behave in the same manner as a pre-accident structure if it’s involved in another accident. Take a look at the photo on page 49. This is a bumper reinforcement from a Lexus. The technician welded, heated and added internal brackets to repair the part. Though the repaired reinforcement looks similar to a new part, would it function the same as a new one in an accident? No way.

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The Moral of the Story
The bottom line is, as a collision repair professional, you’re responsible for the safety of the occupants in any vehicle you repair. That’s why taking the time to learn as much as you can about vehicle structure is so important. If you think I’m exaggerating, look into the eyes of a customer whose life was just saved thanks to crush zones and OE vehicle design — and then think again.

Writer Toby Chess is director of training for Caliber Collision Centers. He’s also the Los Angeles I-CAR chairman, an I-CAR instructor and a certified ASE Master Technician.

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