Post Race/Crash Inspection of Unibody Type Vehicles

Bob Turner - Philadelphia Region SCCA

[During the week, Bob Turner inspects crash-damaged cars for State Farm Insurance; on weekends, he inspects crash-damaged cars at Race Tracks as an SCCA Scrutineer.
Article Copyright 1996 by Bob Turner; All rights reserved. This article is made available via the "Unofficial SCCA Pages" with permission from Bob.]


The advent of mass produced unibody sports cars and sports sedans has resulted in a number of racing series that stress minimal preparation and an adherence to "factory type" construction. The SCCA has classes such as "Showroom Stock" and the "IT" or Improved Touring cars as prim examples of this philosophy.

The intent of this crash inspection seminar is to inform our tech inspectors of possible trouble areas. GCR issues and specific rules or preparation techniques are not my area of expertise and therefore will be discounted.

Simply put, unibody construction refers to a manufacturing technique that utilizes preformed metal panels welded together to form an inner structure designed to support an outer cosmetic sheet metal shell. For the most part, each individual part of the unibody has little strength. Combined as a unit, the individual panels can withstand significant loads. The roof and quarter panels (rear fenders) are for the most part stressed panels that contribute strentgh and add style to the body. Front Fenders, usually bolt-on items, are rarely seen as stressed members in modern designs. Cars that have welded fenders are usually not unibody construction.

The car can be divided into 3 separate sections: The front section, the center section, and the rear section. Each of these show different and distinct damage patterns depending on where the initial impact occured

The Front Section

The front section holds most of the car's vital organs and therefore has the most built-in strength and structural integrity. The front bumper assembly is usually attached to the front "Frame Ends". Serving as both a cosmetic and a structural component, the bumper ties the "frame ends" together and provides a visual reference for potential damage. The grille and fenders are there simply for cosmetics and serve only as reference points for our purposes. The real meat of the matter lies below the fenders and behind the grille. The radiator support and the panels that support it are the true structural strength of the car. The square tubes that originalte from below the firewall or cowl and have panels that support fenders and the rad support are refered to as "frame rails". The panels that connect the "rails" to the radiator support and the fenders are commonly known as "aprons". The complete assembly, including the radiator support, the aprons and the rails are known as the inner structure. Usually you will find an engine "cradle" or some other substructure that will support the en gine and drive train.

The Center Section

The center section is measured from the cowel and door hinge pillars back to the "dogleg" area of the quarter panels. The cowl area is considered the most heavily reinforced part of the car. The rocker panels and floor can consist of numerous welded in panels. The windshield pillars and quarter panels support the roof and tie it into the whole structure.

The Rear Section

The rear structure, similar to the front, has separate rear "frame rails" connected to a floor or trunk panel. The rear body panel connects both quarter panels and the trunk floor, completing the unibody.

Impacts and Race Cars

The typical car we see in "Showroom Stock" or "IT" will have a unibody as the underlying form of construction. As a car gets developed as a race car, the addition of a roll cage moves the car more towards perimiter frame type construction. Somewhere in between we have the Showroom Stocker or the IT car. The added structure of a full rollcage, either welded-in or bolted-in, changes the way damage will travel through the body in the event of an on track incident.

Some very interesting things happen to a vehicle that has had a sudden encounter with a fixed object. The laws of physics apply to all moving objects and race cars are no exception. When a race car strikes an object, either fixed or moving at some dissimlar velocity, an amount of inertia is expended as energy. The energey tha tis dissipated by the crash will carry back into the vehicle until the energy is used up. Bent sheet metal, broken bones, and seriously damaged property are usually the result.

When a race car comes off the track "on the hook", the car becomes a piece of evidence that will tell the inspector most of the facts surrounding an on track incident. Our inspections often become the only time an "official" really sees the damage until the car makes its next appearance, either at the track or as the car makes its way to the "boneyard". There are subtle differences between a "rebuildable" unibody car and scrap. A good post race inspection will reveal most of the differences.

An inspection starts with a good "walk around". Your initial impressions of what may have happened may in fact not be proven valid after a complete inspection. Try to see the primary impact points and visualize where the damage carried through the car. Secondary damage may be visible in places where you might not expect. The key here is to fully inspect each individual section until you're satisified you've seen all the damage.

Damage at the Front

Most race car incidents have some sort of front section damage as the prime component. Before you "inspect" anything, understand the "First Rule of the Junkyard": don't touch anything. Hot metal parts and sharp edges will cut you. The "Second rule of the Junkyard" is do not feed the dogs! Now that you're safe, a complete inspection starts where you think the initial impact occured.

Look to see if the frame rail ends were deformed. Was the bumper not involved? As you develop a sequence of inspection, try not to skip areas of damage by moving around the car too fast. Concentrate on looking for damage in very specific and confined areas. From the front bumper assembly you should start to look at the radiator support. That's the panel where the radiator and fan shroud are attached. You may notice the factory designed "convolutions" on the upper cross bar or upper tie bar. They were designed in to allow the bar to deform in a predictable manner. You may also notice the support bar that runs from the upper tie bar, back toward the cowl, on top of the apron. This upper reinforcement is designed to deform and should not be reinforced in any way. A common "stock car" trick is to "sister" this piece from below (so the tech inspector can't see it) and therefore have a stronger front end. In Showroom Stock type classes, this kind of reinforcement only serves to defeat the designed in safety of the vehicle and tends to compromise the whole structure.

The front section is also supported by the front suspension. Depending on what make of car is involved you may have to look for engine cradle or crossmember damage. Most front drive vehicles have some sort of "frame-within-a-frame" structure that is very prone to minor damage. Outwardly the car may appear to have minor damage until you bend over and see the whole cradle assembly bent up. These parts are usually not readily reparable and would require some frame straightening techniques not available at most race tracks. While you're looking at the engine/transaxle assembly, notice if the lower control arms and steering knuckles are damaged. Stock type strut shock absorbers are notoriously weak and very rarely rebuildable. If someone just wants to "bend them back" or "turn them around on their mountings", just "say no".

Damage in the Center

The center section of a car is the true heart and soul of the beast. A good repair shop can "clip" a new front or rear section on, but if the cowl section is compromised to any extent, the whole car is scrap. The floor and rocker panels roughly define the raw dimensions of the vehicle. At various intervals along the floor and rockers the manufacturer has provided holes, originally used to "jig" the floor into a standard measurement. Once the measurement holes are compromised, the whole structure cannot be reassembled properly. In cases where roll cages are bolted through the floor panels, careful attention must be paid to the attachment points.

At this point the concept of deflection must be introduced. In all impacts, the energy pushes structural parts past their original placement. This movement is called "deflection". Deflection on a vertical plane is called "sag". Deflection on a horizontal plane is called "sway". IN a typical front end impact, the car might be under extreme braking with the front spoiler dragging the ground. On impact, the front rails start to crush and the designed-in convolutions deform as impact energy is dissipated by the inner structure. As all this happens, in less time than it takes to describe it, the rail ends bend downward. Further back along the rail, usually right over the axle, the rail actually bends upward, sort of like bending a piece of paper. The force of the impact is still coming back into the structure and the strucutre itself is "folding up". Assuming this is a straight hit, you'll see sag. If the striking vehicle or the object being hit is traveling at an angle, sway or sideways damage will be seen. Because the entire structure is tied together at the cowl area, damage will sometimes travel into the center section. The place to look for this secondary damage would be right at the top of the "B" pillar area. Look for a dent in teh roof just about where the rear of the front door frame meets the roof. If a bolt-in roll cage is in place, look for broken or stretched bolts at the floor mounts and also check the cage joints. Welded-in cages will withstand much more impact damage, but you should continue your search for damage beyond the center section.

Damage in the Rear

The rear section has far fewer built in safety devices than the front does. The rear section has far fewer built in safety devices than the front does. The real area of concern here is the fuel tank or cell. Stock gas tank placements are usually "OK" and are protected from most impacts short of an atomic blast. Fuel cells, on the other hand, are often subject to creative engineering. Look for fuel cells that are just bolted to the floor pan with little or no protective frame work surrounding the can. Typical installations might be seen with just bolts running through the can flange and no "cage type" surrounding structure. In the event of a rear hit, this can will lleak. Remember, the floor has structural strength and loses most of it just by cutting the hole for the cell can. Rear rails also have the factory convolutions bui8lt in. Common areas for rear damage to appear would be in the "kick-up" area of the rail just over the axle. A good solid rear hit may also bend up some rear suspension parts.


By following just a few basic rules and understanding where and how damage travels, your efforts to fully inspect a post race incident will be made easier and more complete. Remember to make a "walk around". Note where the initial impact was made. Try to visualize where that damage traveled. Try to fully document the extent and location of the primary and secondary damage.

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