Increasing influence of lightweight construction

Lightweight Construction in the BMW 6 Series GT

| Author / Editor: Claus-Peter Köth / Janina Seit

Thanks to intelligent lightweight construction, BMW was able to reduce the weight of the new 6 Series GT by around 150 kg compared to its predecessor. In order to connect the aluminium rear longitudinal member to other body parts in accordance with the mixed construction principle, the OEM took a new interdisciplinary approach.

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Compared to its predecessor, the BMW 6 Series GT is 150 kilograms lighter on average; 44 kilograms of this amount are used for the body alone.
( Source: BMW )

The bodywork of the BMW 5 Series Sedan and Touring, 6 Series Gran Turismo and 7 Series has an innovative design comprising a die-cast aluminium rear longitudinal member. According to the company, it not only sets new standards in terms of precision and crash safety, but also significantly reduces the weight of the car.

In the latest development stage, the rear longitudinal member of the BMW 6 Series GT consists of a single aluminium die-cast part. The predecessor 5 Series GT model still comprised 25 individual components per side. In this way the weight of the two members could be reduced by 15 kg altogether. This represents the most effective individual measure in the body-in-white of the new 6 Series GT. Furthermore, this design incorporates a particularly high dimensional accuracy. Manual straightening of the components with a "massage" device, which eliminates deviations from the nominal dimensions in a targeted and automatic manner, is no longer necessary.

Aluminium Die Casting

Another advantage is that the aluminium die casting process offers a high degree of flexibility with regard to geometry. The reinforcing joints designed for this component increase the stiffness of the vehicle. The construction principle in accordance with an intelligent mix of materials also ensures that each body part meets the respective design requirements, e.g. in terms of weight and stiffness. The rear longitudinal member is connected with body parts made of different materials at numerous connecting points. This places high demands on the joining technology. In a design with mixed construction materials, glued seams ensure that two body parts made of different materials do not come into direct contact with each other. Otherwise the parts would corrode. The glued seams also ensure high rigidity and durability.

A single aluminium die-cast part: the rear longitudinal member of the BMW 6 Series GT
A single aluminium die-cast part: the rear longitudinal member of the BMW 6 Series GT
( Source: BMW )

Highly Accurate Joining

he gluing process requires maximum precision: The joint flanges must be completely filled in order to create a tight connection. A lack of adhesive would impair the quality of the joint. Too much adhesive, would in turn lead to leakage at the joints and contaminate body construction systems and paint baths. Extensive monitoring ensures that the exact amount of adhesive is applied. Cameras check the adhesive pattern on the flange; the final inspection is done by a visual control for adhesive leakage that might have occurred after the joining process, supplemented by a visual control system. In addition, the glued joints are reinforced by means of mechanical joining technology such as self-tapping screws. A special feature: While bolting the screw into the die-cast aluminium part, it creates a thread. This happens in an automatic process that is fully integrated into the production cycle.

Self-tapping screws also increase the load-bearing capacity and fix the joining flanges until the adhesives have cured. Only then has the joint reached its final strength. Introduced worldwide at the end of 2017, the BMW 6 Series Gran Turismo is a further development of the 5 Series GT. Intelligent lightweight construction and the increased use of aluminium and high-strength steels in body and chassis components helped to reduce the weight of the 6 Series GT by an average of around 150 kg compared to its predecessor. Of this amount, 44 kg are accounted for by the bodywork. In addition, aerodynamics could be significantly improved.

Related to: Lightweight Construction in the BMW 6 Series GT

Euro-Car-Body-Award 2017

At the end of October last year, the BMW 6 Series GT won the Euro Car Body Award 2017. The jurors emphasized the smart constructive solutions developed by BMW for the rear of the car: Based on a large, monolithic die-cast aluminium tailgate frame and a new rear structure that does not require a water drainage channel, a very large, comfortable rear opening and maximum visibility through the rear window was achieved — combined with a trunk volume that has been increased by 110 liters compared to the predecessor model," as stated in the explanatory statement at the award ceremony.

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