Technical field of the invention

The invention relates to a roll formed bumper beam with a, along the length, 5 constant width of the original strip and a cross section with at least two closed

chambers separated by an essentially horizontal intermediate wall, wherein each chamber has a side turned away from the vehicle with at least one, along the entire length, longitudinal inwards curved portion. The invention also relates to a method for manufacturing a beam.

10 Background of the invention

US 2012/0074420A1 shows a roll formed beam of this kind with a constant cross section along the entire length. JP P2001-62532A shows a beam with quadratic cross section and ends folded together in order to reduce the horizontal

dimensions of the ends.

15 The purpose of the invention

It is an object of the invention to be able to provide in an economical way a bumper beam with desired properties. Another object is to enable manufacturing of bumper beams adapted to different vehicles in one and the same manufacturing unit without large investments.

20 Short description of the invention

The object of the invention is fulfilled when roll forming so that the chambers are provided with an outwards curved side and an inwards curved side and, after the roll forming, changing a certain portion of the cross section by pressing it together and thereby changing the properties of the beam. The invention is defined by the 25 claims.

Short description of the drawings

Figure 1 shows as an example a front bumper in accordance with the invention seen from above.

Figure 2 shows the bumper according to figure 1 seen from the front. Figure 3 is a section taken along the line 3-3 in figure 2.

Figure 4 is a section taken along the line 4-4 in figure 2.

Figures 5a-h show cross sections which are alternatives to the cross section shown in figure 3. Description of shown examples of the invention

Figure 1 shows a bumper beam 11 which has two crash boxes 12, 13, welded thereon. The bumper beam 1 1-13 is intended as the front bumper of the vehicle and the crash boxes are adapted to be attached to the side beams of the vehicle. From a steel strip with constant width a profile is roll formed in a continuous process, the profile having a constant cross section along the length, which is then cut in suitable length. It is possible to roll form so that the profile becomes convex as is evident from figure 1. Alternatively, it is possible to bend each length after the cutting.

As is evident from figure 3 the profile is roll formed to have a horizontal web 14, i.e., an essentially horizontal partition wall 14 which divides the profile in an upper and a lower closed chamber 15, 16. Integrated with the roll forming the longitudinal edges of the profile are continuously laser welded to the sides of the profile at 17, 18, so that a rigid profile is achieved. The web increases the torsional rigidity as well as the bending rigidity. The web is also important for the deformation process and increases the energy absorption during a crash. The side 19 which is directed outwards from the vehicle forwards, i.e., forwards in the example with a front bumper, has for each one of the chambers a concave (inwards curved) portion which in the shown example consists of two essentially plane side portions 20, 21 ; 22, 23 at an angle to each other. The back side 24, i.e., the side which is facing the vehicle, has a corresponding convex (outwards curved) portion, which in the shown example consists of essentially plane side portions 25, 26; 27, 28 at an angle to each other. The side 19 facing forward has in the shown example its impact surfaces essentially along a straight line which is essentially vertical. A portion 32, in the example shown as a central portion, of the length of the bumper has been pressed together vertically so that it has got the cross section shown in figure 4. The front surface 19 on each chamber 15, 16, is convex and the back side 24 is concave and for that reason the width of the cross section will increase during compression which increases the bending rigidity locally. The increased bending rigidity at the middle of the bumper beam which is achieved decreases the risk for a fold to form at crash load from straight ahead, which leads to the bumper beam collapsing locally. By horizontal pressing instead of vertical pressing the width of the cross section is decreased and may in this way decrease the bending rigidity locally. By using horizontal pressing it is also possible to change the radius of the bumper at the same time at the width of the profile is decreased, which is often desirable at the ends of the bumper.

By in this way pressing together parts of the length of the bumper beam, in the shown example less than half the length, it is possible to control, at a very low cost, the behaviour of the bumper beam during crash load despite the fact that the width of the material and the basic profile of the beam are constant. It is also made possible to adapt one and the same basic profile to different vehicles. The profile is roll formed in continuous equipment and is cut in suitable lengths and desired compressions of the basic profile are made for adaption to different vehicles. The use of a beam manufactured according to the invention as a bumper is only one example on a use.