The battle of the metals
Twistbeam design
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HSS and ultra HSS sheet, tube, bar, and forgings were made with manufacturing technologies such as hydroforming to result in the ULSAS twistbeam design achieving a mass reduction of up to 32% at no cost penalty. The ULSAS twistbeam differs from conventional twistbeams because it features a U-shaped swept section that provides continuity of structure from hub to hub. The HSS tube employed is a constant section thin-wall tube, bent through a tight radius at each corner. The center section of the tube has a plasma-cut profile to reduce torsional stiffness, allowing the twistbeam to twist. The two forward facing arms are hydroformed to achieve appropriate geometry with the main tube. The forged wheel bearing mountings are metal inert gas (MIG) welded onto the main tube, but the hub units are detachable for ease of service.
The twistbeam's transverse beam is made of 600 MPa (87 ksi) ultra HSS tube with a 2.8 to 4.1 mm (0.11 to 0.16 in) wall thickness, depending on the vehicle class, with a plasma-trimmed cutout. The trailing arm is a 70 by 2-mm (2.8 by 0.08-in), 400-MPa (58-ksi) tube that can be MIG welded to the twistbeam. FEA of this component geometry demonstrated that a satisfactory part might be produced by hydroforming; however, to produce the full design geometry, an end flaring or punch point operation would be required. The spring pan is stamped from a 3.2 to 4.0-mm (0.13 to 0.16-in), 500-MPa (73-ksi) ultra HSS blank. The springs are made from a 10.04 to 11.43-mm (0.39 to 0.45-in) diameter, 1300-MPa (188-ksi) ultra HSS rod. The hub mounting plate is forged from 600-MPa (87-ksi) ultra HSS.
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