Trailers of today have inherited the traditional chassis design from the first antique carts that was pulled by oxen or horses about 4.000 years ago. It is natural to think that new production techniques and more advanced materials would have promoted new design concepts for a product that has virtually remained unchanged since then, but it is only in the last 150 years steel was used in the vehicle structures and only about 30 years since advanced high strength steel (AHSS) was introduced. For modern vehicles invented during the 20th century, like the automobile and the airplane, a much faster development has occurred to improve design and reduce the weight promoted by mass production, demand for better performance or very high operational cost.
Demand for lighter and stronger vehicles
Today heavy road vehicles like trucks and trailers are facing very tough demands on transport efficiency and operational cost. There has been a natural increase in the use of AHSS for these applications; primarily in the yield strength range of 500 to 700 MPa. Taking this evolutionary step is quite easy and the 4,000 years old conceptual design still used for the trailer chassis can be upgraded to develop a stronger and lighter chassis by reducing the thickness and implementing local design improvements.
New stronger steel grades
However, we at SSAB have always been developing new stronger steel grades. Already today there are grades avail- able with yield strength of 960 and 1100 MPa, which offers new opportunities and challenges in further product development. Although there are new, stronger grades available it does not necessarily mean that they can easily be adopted to the old design concept. Dimensioning of trailer chassis have traditionally been confirmed by years of experience, trial and error and nowadays some FEA-simulations and controlled testing. With the introduction of AHSS the material is permitted to work under higher stresses and larger elastic deflections if possible, but the wish to truly utilize the material employed in the sections finds its limit at any welded joint in the structures. Welded joints are one of the most restricting factors to keep cutting weight of the chassis due to their limited fatigue resistance. Therefore the introduction of post-treatment methods like TIG-dressing or UIT-treatment and alternative joining methods like bolting, riveting, clinching, adhesive bonding and other technologies can help to boost the use of higher steel grades. Development of even stronger and lighter chassis by introducing higher steel grades need a new design approach in which welding is kept to a minimum and where the load introduction permits a good material utilization to be achieved.
Exploring new design concepts
Introduction of thinner gauges and new manufacturing techniques enables completely new concepts to be explored. A self-carrying monocoque chassis, similar to the concepts used in the car industry, is an interesting solution. Such continuous structures of thin gauges also implies a need for a new approach on forming of the material, by introducing process- es that today are uncommon in trailer production workshops, like roll-forming, and pressing. More advanced methods are also possible for these new materials, like 3D roll-forming and stamping. Continuous structures in thin gauges are often not only lighter, but also stiffer and offer the possibility of integrating components and functions in the same structure.
However, adopting new designs need a lot of effort to be put into understanding the structural behavior of the chassis under all load cases, and it is also necessary to investigate the stiffness requirements, fatigue resistance and crashworthiness of components like rear bumper or bulkhead.
Taking the next step in developing lighter and stronger trailers requires that all the experience of chassis design acquired over the years is combined with new knowledge in a number of areas, where material knowledge and material selection play an important role. We at SSAB are happy to share our knowledge in this area and are always eager to assist our customers.
Fuel consumption and greenhouse gas emissions per one-ton mile.
The information in this report is only applicable to SSAB’s products and should not be applied to any other products than original SSAB products.
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