14 AUTOMOTIVE DESIGN
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Interestingly, the Jaguar XF shuns aluminium in favour of the latest generation steels, including high-carbon steels, dual-phase, hot-formed boron steels, and bake-hardened steels to form a vertical safety ‘ring’ around the occupant cell (Fig. 3).
high-strength or ultra-high-strength steel. Much of the lower section is formed from tailored blanks in which the high-strength steel panels have a thickness that varies between 0.75 and 1.5 mm. This ensures strength is sufficient in areas subject to stress, while avoiding unnecessary weight. Compared with the previous generation of Zafira,
the new model benefits from a 30 per cent increase in torsional stiffness. Flexural strength, which is important for occupant comfort, is 74 per cent higher. This remarkable increase was achieved by optimising the tailgate opening, reinforcing the door sills, and careful design of weld points and specification of sheet-metal thicknesses. In terms of passenger protection, the new Zafira is
Fig. 3. The Jaguar XF uses high-carbon steels, dual-phase, hot-formed boron steels, and bake-hardened steels to form a vertical safety ‘ring’ around the occupant cell.
A further advantage is that these steels enable the
XF’s A- and B-pillars to be slimmer, which benefits visibility and accessibility. The lower sills on the XF are the first component on any Jaguar to use high-strength dual-phase DP600 steel. High-strength and ultra-high-strength steels
are becoming more popular among higher-volume manufacturers too. For example, GM’s second-generation Opel and Vauxhall Zafira is based on the new Astra’s architecture, albeit modified and reinforced for the larger and heavier Zafira. As well as completely redesigning the front end and body, the engineers have optimised rigidity, strength, crash behaviour and pedestrian protection. To achieve this, up to 51 per cent of the body is manufactured from
said to be particularly safe in side-impact collisions, since the force of the impact is channelled over a wide surface area into the body structure by reinforced A- and B-pillars, sills and doors with a reinforced, bending- resistant door well, and wide, ultra-high-strength steel impact protection guards. The energy absorption zone for rear-end collisions is restricted as far as possible to the area behind the third row of seats, with the high-strength occupant cell located in front of this deformation zone (Fig. 4).
Magnesium components
When the Saab 9-3 Convertible was launched in 2007, it was claimed to have a level of structural integrity that placed it at the top of its class. High-strength and ultra-high-strength steels have been used extensively, accounting for about 60 per cent of the Convertible’s body weight (Fig. 5). Another notable weight-saving feature on this car is the use of magnesium for the soft- top frame, swivelling braces and front header rail. Existing dual-phase cold rolled steels offer
carmakers weight reduction benefits through down- gauging (reducing the material thicknesses) for certain applications. However, cold rolled products can only be used for vehicle applications with a gauge range of 0.6 to 2 mm. At the end of 2008 Corus announced that production-ready coils of Dual-Phase 600 hot-rolled steel (DP600HR) in gauges above 2 mm were available. This will help carmakers to reduce weight in areas other than the vehicle body structure. According to Corus, DP600HR offers potential weight savings of 10-20 per cent in component assemblies requiring exceptional durability and strength performance, such as sub-frames, lower and upper control arms and wheel centres.
Fig. 4. GM’s second-generation Zafira features high-strength and ultra-high-strength steel for 51 per cent of the body, creating a high-strength occupant cell.
Moving on from structural components and assemblies, there are plenty of opportunities to reduce fuel consumption by improving the engine. So many different routes are being pursued that they cannot all be covered here. However, there are two projects that can be used to illustrate the type of work going on. The first of these is the lightweight balancer shaft and rolling bearing assembly that the Schaeffler Group has started
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