On The Straight And Narrow

From the early days of the automobile until the 1980s, passenger cars were built from two major components, a body and a frame. The body was a complete structure that was bolted and secured to the frame. The wheels, drivetrain and engine were also mounted to the frame. Then, the unibody came on the scene.

The unibody is a vehicle design in which the body and frame are combined into one assembled unit. The only bolt-on body components are the hood, trunk lid, doors and front fenders. A unibody is an inherently stronger vehicle than a body-on-frame design car. Also, unibodies are designed to collapse in an accident, absorbing much of the energy of collision. This feature makes a unibody vehicle safer for passengers in the event of an accident.

The biggest downside to the unibody is that it must remain intact to keep its strength. If a certain section is damaged, the strength of the entire structure is compromised. Rust can severely weaken a unibody vehicle. Another downside, though not that important regarding passenger vehicles, is the weight limit. Body-on-frame vehicles, in general, can carry more weight and withstand severe-duty operating conditions.

Because body-on-frame vehicles can better handle the heavy loads, these vehicles still exist. In the Chrysler Group LLC lineup we see this design in the Ram pickup trucks, the Dodge Dakota and the venerable off-road veteran, the Jeep® Wrangler.

FRAME BASICS

The frame is the structural center of a body-on-frame vehicle. In addition to supporting the body and the payload, the frame provides a platform for the engine and drivetrain. Most frames are ladder-type featuring parallel channels that are often box-channel rails (Figure 1). Some vehicles, such as the Ram pickup, have front box channel rails and rear open channel rails. Crossmembers are attached with rivets, welds and/or bolts.

The frame is designed to absorb and dissipate flexing and twisting due to acceleration, braking, cornering and road surface irregularities without bending when subjected to normal driving conditions. Sometimes, however, these limits are exceeded, causing a bent frame. Collisions can leave a frame cracked and crumpled. So how do you repair a frame when it is damaged?

Frames are often repaired by straightening and welding. Straightening involves mounting the vehicle on a fixture and, basically, pulling on it to restore it to its original dimensions. Do not use heat to realign the frame of a newer model Jeep or Ram truck (more on that later). Welding would involve attaching a reinforcing plate on the frame to fix it.

FRAME REPAIR PRECAUTIONS

First of all, do not reuse any damaged fasteners. This will compromise the quality of the repair job. When using fasteners (for example, to install a reinforcement section on the frame), Grade 5 bolts are adequate. Grade 3 or softer hardware should not be used. Conical-type washers are preferred over split-ring type lock washers. And always tighten bolts and nuts to the proper torque specification.

Do not drill holes in top or bottom frame rail flanges. Such holes can cause frame failure. Holes drilled in the side of the frame rail must be at least 1½" from the top and bottom flanges. Finally, additional holes should be located away from existing holes.

To prepare the frame for repair, position the vehicle on a level work surface. Using screw or bottle jacks, adjust the vehicle PLP (Principal Locating Point) heights to the specified dimension above the level work surface. All dimensions are from the center-to-center of a PLP, or from center-to-center of a PLP and fastener location.

FRAME STRAIGHTENING

Chrysler Group LLC recommends the “cold straightening” (no heat) method when straightening a Chrysler, Jeep, Dodge or Ram vehicle frame (see sidebar). While any frame can be cold straightened, not every frame can be straightened with heat. Heat should never be used to straighten a frame made from new, high-strength materials.

The use of high strength and advanced high strength steels for frames began in 2007. The service manuals for Jeep Wranglers, Dodge Dakotas and Ram trucks explicitly state this warning from that year to the current model year. High-strength materials can be substantially and negatively affected from heat input. This applies to all frame components that will be repaired and reused. These frames must be repaired using the cold straightening method.

During rough straightening prior to panel replacement, damaged frame components can be heated to assist in body/frame realignment. The use of heat must be restricted to parts that will be replaced.

FRAME WELDING

Welding (MIG, TIG or arc) can be used to repair frame components and to reinforce a section of the frame. Let’s take a look at common repair.

During the course of inspecting the damaged frame, a crack is found. This crack needs to be repaired before the vehicle is returned to service. If the crack is an accessible location, drill a hole 
at each end of the crack with an 1/8" diameter drill bit (be sure the holes are at the ends of the crack). Using a suitable die grinder, grind a V-shaped groove the entire length of the crack. The groove should be deep enough to allow 100 percent weld penetration. Then, weld the crack.

If you are repairing a side rail, it is common practice to grind the weld smooth and install a reinforcement channel over the repaired area. Such a channel is shown in Figure 2. The reinforcement should completely cover the repaired area. Also, it should overlap the top and bottom of the frame by more than 50 percent of its width.

This reinforcement should never be used on the front section of the frame. This is the section forward of the suspension mounts. This area contains energy management holes (these holes allow the frame to be crushed in the event of a collision, absorbing some of the collision energy). Reinforcing this area can affect energy management.

One final word on welding is in order. Before any welding is performed on a Chrysler Group product, refer to the Welding Parameters application chart found in most Chrysler Group LLC Body Repair Manuals. It covers all the welding parameters for arc, MIG and TIG welding including, but not limited to, material thickness, electrode type, amperage and welding direction. If you have any questions about the parameters for the particular welding method you are using, refer to the chart.

USE OF HEAT DURING REPAIR

Chrysler Group LLC Service Engineering’s position on the use of heat during collision repair is as follows:

• Any damaged body panel or frame component, which is to be repaired, must be repaired using the “cold straightening” method. No heat may be used during the straightening process.
• During rough straightening prior to replacement, damaged panels or frame components may be heated to assist in body/frame realignment. This application of heat, if absolutely necessary, must be constrained to the parts which will be replaced and not allowed to affect any other components.

This “no heat” recommendation is due to the extensive use of high-strength and advanced high-strength steels in Chrysler Group LLC vehicles. High-strength materials can be substantially and negatively affected from heat input which will not be obviously known to the repairer or consumer. Additionally, application of heat will alter or destroy material coatings utilized for corrosion protection and which may not be restorable.

Ignoring these recommendations may lead to serious compromises in the ability to protect occupants in a future collision event, reduce the engineered qualities and attributes, or decrease the durability and reliability of the vehicle.

This statement supersedes any previously released information by Chrysler Group LLC.
Chrysler Group LLC
Service Engineering
August 11, 2010