The higher functional requirements in the new BMW X5 are met by the implementation of innovative light-weight designs.
BMW created the Sports Activity Vehicle (SAV) segment with the introduction of the BMW X5 in 1999. Approximately 580,000 BMW X5s were delivered to customers since then. BMW now introduces the second edition of its successful SAV model. To give the car a new level of functionality and versatility, the car is 18.7 centimetres (or 7.4”) longer, and 6.1 centimetres (or 2.4”) wider than its predecessor. However, the body designers have maintained the overall sporty impression of the vehicle. The extra size only becomes apparent when initially looking at the larger interior of the new BMW X5 with the seven seat option. Despite the larger dimensions of the new X5 (reference the former model), BMW engineers and designers have succeeded in maintaining the weight of the new BMW X5 (compared to the initial one) and making the body even stiffer in the process.
The new BMW X5 starts by setting new standards of driving dynamics and premium character in the SAV segment. To achieve these goals, a double wishbone front axle that optimises tyre contact on the road was implemented. The transmission of lateral forces allows particularly fast and dynamic lateral acceleration. An innovative integral rear axle was also added in combination with run flat tires. These features simultaneously provide excellent road holding capabilities and a comfortable ride. A lot of efforts and money were invested to improve these capabilities and to achieve a 50 percent balance on each axle, in combination with a low centre of gravity. But how can a Body-in-white provide these results?
To achieve a 50 percent balance on each axle, there was a need for weight savings in the front-end. Thus, the decision was made for an aluminium hood, a magnesium IP carrier for the instrument panel, and a high-pressure cast aluminium front shock tower (Figure 1). To provide additional space and extra rigidity, without a significant increase in weight, the engineers have continually concentrated on intelligent lightweight technology. Both the choice of materials and the geometry arrangement of the load path are based on an overall concept designed for maximum passive safety in crash, combined with superior agility while driving. To optimise passive safety to the highest standard, BMW developed the Body-in-white with maximum stability of the passenger compartment from its inception. This was achieved via the intensive use of advanced high strength steel and ultra high strength steel. The stiffness itself cannot be increased by the strength of the materials; it can only be achieved by the E-modulus of the material increasing in the structure of the Body-in-White (i.e. optimising the structure of beams yields high torsion stiffness).
The stiffness contributes to better driving dynamics (i.e. a Body-in-White with high torsion stiffness is the backbone for the innovative axle components). Figure 2 shows three different approaches to increase stiffness through engineering solutions. A significant improvement in stiffness can be achieved by locating the support wheelhouse carrier in a relatively low position and giving it a curved shape towards the vehicle centre, reinforced by a stiff connection to the engine carrier. The torsion stiffness can be further increased by connecting the carrier support wheelhouse to the bumper cross-member via the diagonal struts. By these new designs of the carrier structures and load paths, the entire front end of the car can be optimised in terms of torsion stiffness, without the need for more components, and without an increase in weight. Additionally, the stiffness can be increased by the use of a high-pressure casted aluminum shock tower.
The implementation of a closed torsion ring in the D-pillar area also contributes significantly to the increase in stiffness. Essentially, the torsion ring consists of the rear roof cross-member, C-pillar, lateral longitudinal carrier, and the cross-members in the floor panel area. The jointly-wedged trunk lid represents another feature to increase the torsion stiffness. By bracing the trunk lid between the D-pillars, the profiles of the trunk lid area inherit a strength-increasing function. Compared to a conventional solution (i.e. increasing the wall thickness of parts), a weight savings of 6kg can be achieved. By implementing all the individual design solutions and optimising the beam structures, the torsion stiffness increased from 23500Nm/degree to 27000Nm/degree, compared to the previous X5.
The BMW X5 body of the predecessor largely contributed to the achievement of functional crash requirements by the implementation of innovative light-weight design and new material technologies (Figure 3). High-strength and ultra-high-strength steel are used in the rear end of the new BMW X5 to achieve an optimum load resistance in a rear-end collision, despite minimal use of material. Specific complex phase steel grades with a minimum yield strength of 680MPa have been used for the longitudinal rear member. To meet the requirements of any side crash test, ultra-high-strength steel grades have also been used in the side frames. The loads occurring here show the highest values in the centre of the B-pillar. In order to cope with this fact simultaneously with the light-weight design, the B-pillar reinforcement was made of hot formed boron steel. A tailor rolled blank with a distinctly higher wall thickness in the center area than in the upper and lower ends was also used. Compared to a B-pillar Reinforcement, made from the same hot formed boron steel in a continuous wall thickness application, 2kg of weight can be saved for each B-pillar reinforcement.
In the front and floor panel, advanced high strength steel grades have been applied to fulfill the requirements of several crash tests, and to build-up the required load paths. The development of the Body-in-White focussed on an extremely rigid occupant compartment to ensure passive safety objectives and offer the highest degree of safety to the passengers in case of a high-speed crash. This was achieved by distributing the forces transferred into the structure via the engine carrier, along several load paths across the driver/passenger compartment, and hence limiting load peaks in specific support structures. This requires the use of micro-alloyed steel grades with a minimum yield strength of 380 - 420 MPa for the load path. Irrespective of the type of the front crash, the objective was to move the wheel backwards in a straight line directly onto the rocker panel. This generates a significant load path from the barrier/crash partner via the wheel into the rocker panel, which was integrated with a stiff profile to meet this purpose. Again, to save weight, a dual phase steel grade with a minimum yield strength of 550Mpa was used. In the front, the pressure cast aluminium spring support serves to reduce weight while optimising stiffness at the same time. Additionally, an optimised packaging for the double wishbone front axle—unique in its class—was possible.
To demonstrate in a simple way, the value of the average minimum yield strength was introduced. Figure 4 shows the weight-based value for a number of cars starting with a production start in 1994. The value for the previous X5 is shown as approximately 200Mpa. Due to the intensive use of advanced high strength steel, the average minimum yield strength increases to around 300MPa in the new X5. To get an indication about what that value means, the use of a bake-hardened steel with a minimum yield strength of 280MPa is not an innovative choice, it is just the standard of the X5. The dotted lines show two possibilities for future material concepts, raising a basic question: Is the material concept of the new BMW X5, the maximum which can be achieved or will there be further increases in the minimum yield strength? The answer to that question mainly depends on how much money can be allocated for weight savings by cost intensive ultra high strength steel grades.
In the pre-development phase of the BMW X5, various front end concepts have been examined. The objective was to meet the specifications of the styling department in terms of more freedom of vehicle differentiation and styling. However, taking into account all functions of the front-end including pedestrian protection, this led to the concept shown in figure 5. The structural Body-in-White frame has been lowered and filled with a plastic support structure which acts as a module carrier with a plastic side panel. In this way, the “soft” styling requirements and pedestrian protection were separated from the “hard” requirements for stiffness, strength, and high/low-speed crash.
This module carrier increased the degree of modularisation and could therefore be shifted to pre-assembly. The most important advantages are high freedom in styling, less gaps with reduced gap dimensions, and less weight. Lowering the carrier support wheelhouse creates a free space that is filled by a plastic module carrier that serves to support components: The front fender is made of a plastic that belongs to the same material family as the bumper finishers (PP-EPDM). This allows the application of the same paint process for both components. In simulated tests for pedestrian protection, a test body (3.5kg) is fired onto the front fender at a speed of 35-40 km/h. In most cases, the resulting deformation proved to be reversible. No shattering or fracturing of the component (including paint) occurs.
Optical measurement processes at the assembly line continuously compare the exact match of the paint of the front fenders and of the other components and trigger paint remix even in the slightest event of deviation. Despite the large front fender, the number of lifters in the tool could be reduced in order to minimise the risk of impressions left by the lifters. Furthermore, the module carrier of the front fender assembly allows the fulfillment of the pedestrian protection requirements by a passive system, since the module carrier acts mainly as a spring element due to its rib structure. The results are achieved in the overall system via:
The BMW X5 is the first model of the X-series that is fitted with an aluminum die-cast shock tower. The aluminum die-cast shock tower absorbs the suspension forces and transfers these into the body. The strut and the upper control arm are mounted to the die-cast shock tower which necessitates a high stiffness of this component. This is achieved by an optimal material distribution (i.e. material is accumulated only in areas where this is necessary). In this way, the shock tower contributes significantly to the driving characteristics since it absorbs static and dynamic wheel forces.
By means of the die-cast design, many individual functions and components can be integrated into one component. Therefore, this design is much more compact compared to the conventional shell design, and it contributes significantly to weight reduction. By the use of the high pressure casted aluminum shock tower, a weight reduction of approximately 50 percent was achieved in comparison to a conventional steel design. Referring to packaging, which is always a severe problem in a front end, a space of 80mm was saved (i.e. the front end was shortened by 80mm). By integrating several brackets into the aluminum shock tower, the assembly of components was avoided. The aluminum shock tower is connected to the neighbouring steel components with self-pierce riveting and adhesive bonding. To prevent the contact of dissimilar metals, the aluminum shock tower is e-coated prior to the front end assembly process. The design results in a lower weight and a reduced number of parts. However, the body is more rigid in torsion stiffness and the local stiffness has also been increased. Consequently, this leads to better driving dynamics.
The higher functional requirements in the new BMW X5 are met by the implementation of innovative light-weight designs. Process suitable design of structural components allow for the use of multi-phase steels, hot-formed materials, and aluminum castings. The new design of the carrier structure resulted in a torsion stiffness increase from 23500Nm/° to 27000Nm/°. In the future, there will be increased demand for thin gauged, wider coil width, zink coated ultra high strength steel grades.
