[0016] The bumper bar that is shown in FIG. 1 is to be mounted horizontally at the front end of a vehicle, at the front ends of the side bars of the vehicle. The mounting of the bumper at the side bars takes place with a freely chosen method. When the bumper bar has been mounted onto the vehicle, its main task is to meet, distribute and dissipate the energy that is absorbed during a collision between the vehicle and another object. The bar should mainly take care of those forces that act on the vehicle during a collision directly from the front or obliquely from the front. In the following description, the front end of the vehicle will provide a direction indicator. The word “forwards” will denote the direction in which the vehicle normally travels, and “backwards” will denote the opposite direction.

[0017] The bar is formed from sheet metal, for example, steel. Boron steel gives a good result and is economically advantageous. The bar is extended in one dimension and shaped as a tube with a closed cross-section (see FIG. 1 ). The bar includes a front flange 1 , a rear flange 2 , a top side 3 and a bottom side 4 (see FIG. 2 ). The flanges and the top and bottom sides comprise sections of one and the same piece of material and form the main limiting surfaces of the bar. The front flange 1 points forwards in the direction of the vehicle when the bar is mounted, and it comprises the first contact area between the bar and the collision object. The rear flange 2 makes it possible to fasten the bar at the front ends of the side bars of a vehicle. The flanges 1 and 2 and the top and bottom sides 3 and 4 are furnished with bends 5 , grooves 6 and 7 , or other similar shapings of the material, longitudinal and/or transverse relative to the long axis of the bar, affecting the energy-absorbing properties of the bar.

[0018] The bar has an undivided open inner at its central part, and it is here that it has its maximum volume, distributed around the main axis of the bar (see FIGS. 1 and 2 ). The central part of the bar has a large volume and a large amount of material in order to offer the best possible energy absorption during a collision.

[0019] The shape of the bar changes at the ends in order to make the mounting of the bar onto the vehicle easier (see FIGS. 1 and 3 ). The change of shape has taken place in a controlled manner, for example in a forming tool. A longitudinal part 2 . 1 of the rear flange 2 is pressed in towards a longitudinal part 1 . 1 of the front flange 1 . The longitudinal parts 1 . 1 and 2 . 1 constitute each a longitudinal central part of the two flanges 1 and 2 . It is also possible to carry out the change of shape based on two or more longitudinal parts on each flange. Most of the rear flange 2 , including a boundary region between the top side 3 and the bottom side 4 and the rear flange 2 , are pressed into the bar. Through this pressing together, the bar obtains two tube-formed parts 8 with an open inner part along a length X at both of its ends (see FIGS. 1 and 3 ). The bar in the boundary region between the top side 3 and the bottom side 4 and the rear flange 2 is furnished with longitudinal bends 5 providing guides for and facilitating the pressing. The rear flange 2 has longitudinal grooves 6 that act as pliable bellows during the pressing. The sheet metal material in the grooves 6 is drawn out during the pressing and contributes to reducing the occurrence of inner tensions in the material. In this way, the formation of cracks and weaknesses can be reduced.

[0020] The rear flange 2 is depressed to such an extent that the inner surface of the longitudinal part 2 . 1 is brought into contact with the inner surface of the longitudinal part 1 . 1 of the front flange 1 (see FIG. 3 ). The two flanges 1 and 2 are welded together along the stretch X. The front flange 1 has not had its shape altered by the pressing together, but has the same appearance along the full length of the bar. The front flange 1 is furnished in its longitudinal central part 1 . 1 with a longitudinal trench-shaped indentation 9 . The rear flange also has a longitudinal trench-shaped indentation 10 in its longitudinal central part 2 . 1 . The two indentations 9 and 10 are directed inwards into the central cavity of the bar, and both have a bottom part 9 . 1 and 10 . 1 , respectively. The bottom part 9 . 1 of the indentation 9 in the front flange is, at the ends of the bar, in contact with the bottom part 10 . 1 of the indentation 10 in the rear flange. The welded joint is in principle as long as the stretch X, and the bar shows the two tube formations 8 along the stretch X. This pressing together and welding implies that the bar has different energy-absorbing properties in this region than it has in its central part. The region of the welded joint constitutes a clear rotational guide for the bar during a collision. The bar will be more easily broken in this distinct region and in this way absorb large amounts of energy.

[0021] The bar has its maximum volume at its central part and its minimum volume at the ends, to be more precise, in the region where the flanges are united by the welded joint. The volume of the bar increases from the point where the welded joints terminate to its maximum volume at the centre of the bar.

[0022] The bar is slightly curved (see FIG. 1 ). The ends of the bar point partially or wholly backwards and in this way the bar more easily follows the rounded frontal shape of the front of a vehicle. The top side and the bottom side of the bar have transverse grooves 7 in order to facilitate the bending of the bar. The grooves 7 act as bellows and are drawn out during the bending of the bar. The grooves 7 can also function as bending guides. The surplus sheet metal material in the grooves 7 is drawn out during the bending and this contributes to reducing the occurrence of inner tensions in the material. The formation of cracks and weaknesses can be minimised. The pressing together of the rear flange 2 against the front flange 1 facilitates the bending of the ends of the bar since the width B of the bar is reduced and the bar thus acquires a reduced resistance to bending.

[0023] The bar has a constant height H and a varying width B along its complete length (see FIG. 1 ). The bar has its maximal width at the central part and its minimum width at the ends. It is, of course, possible to form the bar with its construction of bends and grooves in a similar way along its complete length. The external dimensions of the bar after the pressing are always equal to or less than they were in the initial state of the bar. The bar becomes neither longer, wider or higher at any part of its extent. Nor is the bar exposed to any reduction of material that would lead to weakening.

[0024] The bumper bar satisfies the demands for durability that are posed on a bar of this type and the demands that exist concerning appearance, adaptability and, not least, cost-efficiency. The bumper bar makes it possible to manufacture vehicles that are safe in traffic within the strict technical and economic demands that exist in the vehicle industry.

[0025] The method for manufacturing the bumper bar is commenced by feeding a flat sheet into a roll-forming device. The sheet is shaped into a tubular extended element with a closed cross-section in the roll-forming device. The closed cross-section is obtained by welding together by spot welding, seam welding, or a similar method, the edges of the plate, which after the roll-forming meet and make contact with each other. The roll-forming device can shape bends 5 , longitudinal grooves 6 and transverse grooves 7 in the sheet in order to make the further shaping of the bar possible, and also to improve the final torsional rigidity, flexural rigidity and other properties of the bar that take up forces during a collision. Before the roll-forming is commenced, the sheet can be cut into suitable lengths, which ensures that the final length of the bumper bar is as desired. The sheet can also be cut after the roll-forming operation.

[0026] After this, the bar is placed in an oven and heated to a suitable austenitizing temperature for the material. A suitable temperature for boron steel lies around 900° C. Once the bar has reached the correct temperature, it is placed in a combined forming- and quenching tool.

[0027] The longitudinal central part 2 . 1 of the rear flange is pushed in towards the longitudinal central part 1 . 1 of the front flange in the forming tool, whereby the width B, the so-called building height, of the bumper bar is reduced. This inwards pressing mainly takes place at the ends of the bumper bar, from that region of the bar that is to be fixed to the side bars of the vehicle and outwards, approximately along the stretch X shown in FIG. 1 . During the roll-forming, the bar has received bends 5 that act as guides to facilitate the pressing together. The rear flange 2 is also furnished with longitudinal grooves 6 which are drawn out as a bellow during this pressing in of the rear flange, and facilitate the shaping. The risk for extension of the material and tension that arises from it is minimised. It is an advantage if the pressing together is complete in a centred part of the cross-section of the bar at the very end of the furthest part of the bar, so that the two flanges 1 and 2 come into contact with each other. It is an advantage if the flanges 1 and 2 of the bumper bar are welded together where the inner surfaces of the flanges meet each other after the pressing together, along the stretch X. The welded joint will constitute a clear rotation- and breakage guide during a collision, to take care of the collision forces.

[0028] In addition to the pressing together, the bumper bar is shaped along its horizontal extension by bending. The bar is completely straight when it comes out from the roll-forming device. The front of a vehicle is often bent backwards out towards the sides of the vehicle in order to reduce air resistance, to improve the properties of the vehicle for absorbing collision forces and to make the appearance attractive. The bumper bar should have an equivalent bending in order to optimise the use of available space. The ends of the bar are bent backwards with the same forming tool that carries out the pressing together. During the roll-forming, the bar has been furnished with transverse grooves 7 on the top side 3 and the bottom side 4 , which facilitate this bending. The grooves 7 are drawn out as a bellow and the risk for extension of the material and tension that arises from it is thus minimised. The pressing together of the rear flange 2 against the front flange 1 facilitates the bending of the bar since the width B of the bar and thus its flexural rigidity have been reduced at the end parts of the bar.

[0029] After the shaping, the bumper bar is quenched, still located in the forming tool. This method gives a bumper bar that is constructed in one piece and that only requires one roll-forming device and only one further forming device, which shapes the details and at the same time provides the bar with stability during quenching. The manufacture of vehicles that are safe in traffic becomes cost-efficient and of high quality.

[0030] It is fully possible within the scope of the invention to have other solutions and detailed designs of certain parts of the bar. The number, appearance and construction of grooves and bends can be varied and complemented in order to control and vary the collision properties of the bar. This description is not to be seen as a limitation of the invention; it should rather be seen as a guide to full understanding of the invention in all of its parts.