COURTESY OF I-CAR
Full-frames have seen their fair share of changes throughout the years. Understanding the details of full-frame repair will help you approach them with confidence.
Full frames have come full circle...well, almost. When Henry Ford first started mass-producing the Model T in an assembly plant, the frame served a single purpose—as a platform to attach the wheels, body and drivetrain. Now a frame serves multiple functions, including absorbing collision energy and protecting vehicle occupants during a collision.
Full frames have been around since automobile production began. With the increased number of light trucks and SUVs on the road today, the knowledge and skills required to repair collision damage to these vehicles is a necessity. Although full frames are nothing new, a great number of collision repair technicians have little training on repairs to full frames. When the modern unibody came of age, career and technical schools and colleges placed a heavy emphasis on repairing collision damage to unibody vehicles, and spent less time on repairs to full-frame vehicles. Although the majority of passenger cars are made using a unibody-type construction, some still use a full frame. The majority of full-frame passenger cars also use a unitized body structure mounted to a full frame. Some examples include the Ford Crown Victoria/Mercury Marquis and the Lincoln Town Car.
What makes the full frames of today so different from previous generations? For starters, the full frames of the modern era must have the ability to absorb collision energy to meet National Highway Traffic Safety Administration (NHTSA) requirements. Combine this with fuel economy requirements and it is evident that vehicle makers are required to make a full frame lighter than previous generations, yet strong enough to hold up to day-to-day stresses, and to absorb collision energy to protect vehicle occupants during a collision.
A medium wavelength noncontact thermometer will range high enough to monitor the temperatures required when straightening a steel frame.
A thermal image of heating a steel frame demonstrates how much area is affected.
Vehicle engineers use several different construction methods to accomplish this, including the use of welded crossmembers, hydroformed metal, and higher-strength, lighter-weight steel (Fig. 1). Hydroforming is a process that forms a tubular piece of metal into a desired shape with the use of dies and a pressurized liquid. This process has been around for a long time, although the ability to form metal thick enough to be used for automotive structural parts is only about 15 years old. Vehicle makers are using the hydroforming process to make parts that require strength and durability, as well as parts with complex shapes.
Frames are an area where the hydroforming process has been used quite frequently. The advantage to using this process is primarily weight savings. A thinner wall material can be used without compromising the strength when compared to a welded box or C-channel. Another advantage is that a complex shape can be made with one piece of metal without having to weld multiple pieces of metal together. This enables the vehicle maker to build a frame with a modular design that is welded together.
Previously, areas with a change in shape required joining pre-bent frame members together with GMA (MIG) welding. With hydroforming, this step is eliminated. Additional advantages include improved corrosion resistance due to fewer welded seams and greater predictability of how collision energy will be absorbed and transferred. When collapse zones are incorporated into the hydroformed part, engineers can better predict the rate the vehicle will collapse during a collision.
The first high-volume vehicle that used a hydroformed structure was the 1997 Chevrolet Corvette. Along with this vehicle came several sectioning procedures where a portion of the frame could be replaced in the area of damage without replacing a complete frame. Since the introduction of the 1997 Corvette, General Motors, along with DaimlerChrysler and Ford, have used hydroformed steel on the frames of trucks and SUVs, as well as for fender rails, roof rail supports and pillars.
Collision repair technicians are concerned that the hydroformed frames may have special considerations for repair. Although the rigidity and strength of these frames may create a challenge when straightening, vehicle makers have made an effort to make replacement parts available for repairs to the areas that are frequently damaged in a collision. Repair options offered by vehicle makers include sectioning and replacement of frame parts at factory seams. These replacement part kits generally include a procedure for the repair.
What makes a full frame repair so different from repairing a unibody vehicle? For starters, a frame is far more rigid than a unibody. Compound this even further by placing a unitized structure on top of the frame and you have an even stronger vehicle (Fig. 2). When the frame and body are both damaged, multiple pulls may be necessary to remove the damage (Fig. 3). Can the frame be repaired, or should it be replaced? The kink vs. bend rule may be used as a guide, although the area of damage also must be considered.
The kink vs. bend rule states that if a part is kinked, it should be replaced and if it is bent, it may be repairable using straightening techniques. A part is considered kinked if it has a sharp bend of small radius over a short distance. A part is also considered kinked if, after straightening, there is a permanent area of deformation that will not return to its original state and shape without the use of excessive heat. A bend is defined as a change in the shape of the part between the damaged and undamaged area that is smooth and continuous. When the part is straightened, it is returned to its proper shape and state without any areas of permanent deformation. The kink vs. bend rule does not always provide a definite repair vs. replace answer. If a kink is near the end of a part in a non-crush zone, straightening may be an option. Conversely, a bent part may be very difficult to straighten because of the thickness of the steel. Therefore replacement, either partial or complete, may be necessary.
Access to the inside of a bent hydroformed frame is limited due to the tubular design of hydroformed parts. Pushing the damage outward from the backside is typically not an option unless the damage is located at the end of the part. If a slightly collapsed area needs to be raised from the backside of the rail assembly, a small hydraulic ram may be used if there are no inner obstructions. In areas where a ram cannot be used, removing the damage is often limited to using welded-on plates. One concern with weld-on attachments is that heat from the welding process may affect the integrity of the part.
To date, many hydroformed parts on full-frame assemblies have been made from mild steel, but applications of high-strength hydroformed steel (HSS) are becoming more common. HSS provides a stronger, more rigid vehicle while reducing the weight. The 2004 Ford F-150 and the 2004 Dodge Durango are two vehicles that are constructed with frame parts made with HSS. Also, some vehicle makers are using external welded-on reinforcement plates on portions of the frame to strengthen the frame assembly. The 2004 Ford F-150 is an example of a vehicle with a frame that has welded-on reinforcement plates.
Little documentation exists in regard to heating hydroformed parts. Many vehicle makers publish heating recommendations that apply to a specific vehicle, but there are typically no specific recommendations given for the hydroformed portions of the vehicle structure. Vehicle makers that allow heat generally provide temperature limits, maximum cumulative heat times and recommended number of applications. Monitoring the temperature can be a challenge. Heat-detection paints and crayons can be effective, although heat from the heating device can melt the markings before the metal reaches the recommended temperature. Noncontact thermometers also can be effective, but many of these will not have a high enough range for the temperatures required. A medium wavelength noncontact thermometer has a high enough range to allow an accurate temperature reading. Heat should be confined to the damaged area to prevent changing the properties of the adjacent metal.
Although a frame is pulled to be dimensionally correct, the visible deformation also must be removed for the frame to be considered in the correct shape and state. Accessing the damage on a hydroformed or fully boxed frame is of particular concern. The temptation exists to cut a window in the boxed section of frame to access the deformed area. After the deformation is straightened, the window is re-welded to the frame. There is no documentation available to support this being an acceptable method to remove this type of damage.
The location of damage also will be a concern. Mounting brackets for suspension or other mechanical parts are often damaged during a collision. If these parts are available as a service part, they can be reinstalled on the frame following the vehicle maker’s recommendations. Most frame service brackets are installed by replacing the part using GMA (MIG) welding, using bolts, or replacing factory rivets with bolts. Follow the vehicle maker’s replacement recommendations.
Repair or Replace?
Full frame replacement is becoming quite common. When a frame is damaged beyond repair, the frame can be replaced as a complete assembly. Although there are a lot of mechanical details to replacing a complete frame, the complete process is less intimidating than it looks (Fig. 4). Some items to consider when replacing the complete frame assembly include state titling and registration laws, the possible need for retrofitting the frame due to model year changes, and the inspection process of a recycled frame. Some states, including Texas, Louisiana, Arizona and Oregon have laws requiring branding of the vehicle title when the frame assembly is replaced. If the frame assembly is replaced with a new frame assembly, note that there may be model year changes or changes from the supplier and vendor. Although not real common, consider that some retrofitting may be required. If a recycled frame will be used, the frame must first be inspected for damage, corrosion, or improper previous repairs. Although the frame is being replaced, some straightening will most likely be required to remove indirect damage to the body structure before the body is removed.
Where to Get Training
Training is one area where a technician can obtain the skills required to repair collision damage to full-frame vehicles. Many of the frame repair equipment makers provide detailed training on analyzing damage, measuring and straightening full frames using their equipment. I-CAR also has two training programs dedicated to repairs on full frame vehicles, Steel Full-Frame Sectioning (SPS03) and Full-Frame Replacement (SPS06). Other I-CAR programs that may benefit a collision repair technician when repairing full frames include Measuring (MEA01) and Structural Straightening Steel (SSS01).
Vehicle production has come a long way since the first mass-produced vehicle rolled off Ford’s assembly line, but the idea of using a full frame to provide a platform for the vehicle has withstood the test of time. With some training and experience, repairs to full frames can be less of a concern.
This article was submitted by I-CAR, the Inter-Industry Conference on Auto Collision Repair. I-CAR is a collision repair training organization dedicated to improving the quality and safety of collision repairs for the ultimate benefit of consumers.
