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The battle of the metals
Strut and link design
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The ULSAS twistbeam design features a U-shaped swept section that provides continuity of structure from hub to hub.
Lotus Engineering reduced the mass of the ULSAS strut and link system up to 25% in two alternative design approaches, one using a hybrid stamped knuckle and the other using a hydroformed tubing knuckle.
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Strut and link systems are normally used in low-cost designs with some compromises in ride, handling, and vehicle refinement. Compared to the benchmark, Lotus reduced the mass of its strut and link system up to 25% in two alternative design approaches, one using a stamped hybrid knuckle and one using a hydroformed tubing knuckle. The hybrid is appropriate for D-, E-, and PNGV-class vehicles, while the hydroformed tubular knuckle is for B- and C-class vehicles, though Lotus engineers said both knuckles could be developed across the entire range. Most other components for both designs were similar; all used HSS or ultra HSS. The hybrid knuckle is made from 4.0-mm (0.16-in), 500-MPa (73-ksi) HSS. It was stamped in two halves and laser welded together. Two parts completed the knuckle design, a lower housing forged from 500-MPa (73-ksi) HSS, and a lower bracket, blanked and folded from 250-MPa (36-ksi), 4.0-mm (0.16-in) thick HSS.
The hydroformed knuckle is made from 3.5-mm (0.14-in) wall, 500-MPa (73-ksi) HSS tubing. Only one additional part was required to complete the knuckle-a lower housing forged from 500-MPa (73-ksi) HSS. The forward and rear lateral links were tubular components with a 2.0-mm (0.08-in) wall thickness, 250-MPa (36-ksi) HSS. The springs were made of 1300-MPa (188-ksi) ultra HSS, with a diameter ranging from 10.6 to 12.3 mm (0.42 to 0.48 in), according to vehicle class.
A unique feature of the structure is that the knuckle is integrally welded onto both the hub-bearing unit and directly to the base of the shock absorber through a process called "through-wall" laser welding. During the process, a laser beam penetrates directly through the relatively low carbon content of the hydroformed knuckle onto the body of the hub-bearing unit underneath
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