Safe repair of vehicle frames

Despite the fact that road transport is the basis for most logistics activities concerning both the transportation of goods and passengers, the issues related to the repair of frames of commercial vehicles are very poorly described in the technical literature. When looking for information in scientific studies or even on the Internet, you may get the impression that the topic actually does not exist. You can find detailed analyzes of the strength of welded joints and descriptions of tests of the strength parameters of frame structures depending on the welding methods used. But does this matter for the post-accident repair services sector for commercial vehicles? Definitely not.

The publication of the article is related to the 1st Safe Vehicle Repair Conference organized by the Bydgoszcz University of Science and Technology and the Institute of Car Body Technology

Vehicle frames

The frame is the most important load-bearing part of a truck. It is not, as some say, part of the running gear. The frame is a load-bearing element to which the remaining parts of the vehicle, including the suspension, are attached. This might challenge the understanding many readers have had so far, but I ask for your attention even more. The purpose of the frame is to provide the vehicle with appropriate stiffness depending on its purpose. There are vehicles for transporting low-weight goods, but also those that must be prepared for heavy loads and work in very difficult conditions. Examples include large dump trucks or truck cranes. It is worth noting that commercial vehicles are exposed to deformations of the supporting structure not only in the event of a road collision but also during operation.
Thus, the concept of operational straightening emerged when specialits developed an innovative repair line for commercial vehicles. Frame designs vary significantly, primarily based on the vehicle’s load capacity and purpose. The most common design is the longitudinal structure, widely used in tractor units and semi-trailers. Typically, a longitudinal frame consists of two longitudinal load-bearing beams connected by multiple crossbars. Research and extensive experience have demonstrated that this design performs exceptionally well under the most challenging conditions, maintaining appropriate mechanical properties with relatively low weight. For clarity, it is beneficial to categorize the main types of frames based on their structure:

• cruciferous,
• stringer,
• spatial.

Additionally, it should be noted that there are also frames integrated with the vehicle chassis, partial frames, auxiliary frames, and truss frames (e.g., in buses). An interesting example of a car frame is the so-called intermediate frame, designed for use with interchangeable bodies depending on the vehicle’s purpose. This type of frame is mainly found in the construction of semi-trailers.

Due to the mechanical loads transferred, the frames are made of high-quality steel. These are mainly low-alloy steels with increased strength, such as 18G2 and 18G2A. To increase the yield strength, currently used steel grades have reduced phosphorus, sulfur, and carbon content. However, they contain small refining additives, allowing them to achieve a yield strength of 570 MPa. An interesting addition to the alloys used in frame construction is copper. A slight addition of this element (max. 0.45%) significantly reduces the susceptibility of steel to corrosion caused by harsh weather conditions and salt contamination.

Can the frames be repaired?

You can proceed, but it must be done according to the regulations. This means that if, during the repair activities, the structure of the materials is not compromised (e.g., by overheating), the elements that are damaged beyond repair are replaced, and the geometry of the entire structure is restored, it can be concluded that there are no other contraindications apart from economic considerations.
It all starts with assessing the extent of the damage. Properly distinguishing between elements that can be repaired and those that need replacement is a crucial step in the claims settlement process. Due to technological challenges, repairing the riveted frame may pose the most significant difficulties. This involves reconstructing riveted joints to match the technical parameters achieved during production. Riveting requires specialized devices and high-quality rivets, with the forces needed to rivet the frame reaching several hundred kilonewtons.
Welded joints might seem easier to handle in post-accident repair services and companies providing special bodies. However, this is not necessarily the case. Creating a correct welded connection that adheres to the manufacturer’s specifications, ensuring the original strength and expected mechanical properties of the frame, is not straightforward. The method and technique of the connection are crucial. Additionally, manufacturers often specify how to prepare frame elements for welding and the potential use of reinforcements.
Some frames cannot be welded during the repair process but only in cases of modification or adaptation to a special body, and even then, only in accordance with the vehicle manufacturers’ documentation.

 

Repairing bolted frames seems to be the easiest. It is worth noting that most repairs, whether post-accident or operational, involve the process of straightening frame elements. Local heating often occurs during straightening. Within certain limits, heating the straightening area is desirable because it facilitates gentle deformation of the material. However, it is crucial to follow the basic rule: never heat a steel element above the limit temperature of 650°C.

To reduce the vehicle’s weight, most structural parts are made of high-quality metal alloys, primarily steel. Overheating these high-quality alloys, which have enhanced mechanical properties, results in irreversible loss of those properties. Unfortunately, during training and consultations with service workers repairing commercial vehicles, it becomes evident that practically no one follows this basic rule. However, if there is someone who uses heating without exceeding the limit temperature, I respect that. Unfortunately, this will only be the exception that proves the rule.

Measurements and diagnostics

An essential and integral part of the vehicle repair process is its measurements. This applies equally to the subframe and the chassis. Most important for the process of vehicle operation are the correct parameters of chassis geometry, which are sometimes related to the shape of the substructure. There is a noticeable lack of thorough knowledge regarding the principles of both frame and chassis geometry of vehicles. Ubiquitous misleading theories, myths, as well as the process of constant replacement of personnel in repair services are not conducive to raising the level of familiarity with these issues among service technicians. During a post-accident repair, it always starts with restoring the frame geometry, and for the whole process to run smoothly it is necessary to use a measuring system.

 

Measurements are a crucial and integral part of the vehicle repair process, encompassing both the supporting frame and the running gear. The most critical aspect for vehicle operation is ensuring the correct wheel alignment, which are often linked to the shape of the supporting structure. There is a noticeable lack of comprehensive knowledge regarding the principles of both frame geometry and wheel alignment. The prevalence of erroneous theories, myths, and the frequent turnover of staff in repair services hinder the improvement of knowledge on these topics among service technicians. In post-accident repairs, the process always begins with restoring the frame geometry. To ensure the entire process runs smoothly, it is essential to use a precise measuring system. Measuring the shape of the frame seems logical and relatively straightforward using various available methods. However, special attention should be paid to shape defects, particularly twisting. Without appropriate measuring equipment, it is very difficult to check for this defect, especially when vehicle accessories are attached to the frame.
Axles and wheels are attached to the frame, and the wheel alignment directly impacts both the safety and economy of moving vehicles. This is equally true for commercial vehicles, where the economic aspects are significant, affecting tire costs, fuel consumption, and suspension components. For these vehicles, the situation is often more complex due to the presence of multiple axles. The wheel alignment is defined as a set of angular relationships describing the alignment and mutual location of all the vehicle’s road wheels. These parameters include the wheel position relative to the plane on which the vehicle is standing (wheel camber) and the positioning of steering system elements (King Pin). Additionally, the measurements encompass wheel steering angles. By checking all the road wheels, the axes of the running gear (e.g., geometric driving axis) are determined, allowing for the measurement of axle deviations and the subsequent correct adjustment of the wheel alignment for all axles.

When measuring frame vehicles, the measurement is usually made in relation to the vehicle’s supporting frame. It is important to note that the most crucial aspect is ensuring the entire axle and wheel alignment of the vehicle is properly adjusted. The shape of the frame is a separate issue.
Before starting repairs, especially those involving welding technology, it is necessary to perform a thorough economic and technological analysis. The correctness of the repair decision and the proper use of repair methods depend on this analysis. To ensure a safe repair of the frame, you must follow all the vehicle manufacturer’s recommendations and general best practices. It is beneficial to be inquisitive and consult sources of information developed by vehicle manufacturers. Many helpful materials can be found on the Internet, and while they may not be extensive, they will certainly be useful.