A car’s coil spring gets shorter on every bump, yet its steel wire barely bends: it twists.
The load on the spring acts on every section of the wire at the coil’s radius, like a wrench, so the wire carries a torque and twists about its own centre line (torsion). A coil spring works like a long twisting bar wound into a spiral: the twist is shared along the whole wire, so a short spring can travel far and spring back. Push a straight bar end to end instead and nothing twists: it buckles sideways and, past its yield point, stays bent.
Sources: US Air Force Stress Analysis Manual, “Helical Springs” (“The primary stresses in the wire of a helical spring are due to torsion”; the wire as a straight round beam under a torsional load P x r); Shigley’s Mechanical Engineering Design ch. 10 (internal reactions: a direct shear and a torque T = FD/2); Wikipedia, “Coil spring” (“Under tension or compression, the material (wire) of a coil spring undergoes torsion”); University of Illinois, Introductory Solid Mechanics, “Torsion”; Wikipedia, “Buckling”.
The painted stripe’s twist and the tick fan are drawn far larger than real (a real stripe would lean under half a degree); the bumps are in slow motion; the spring, strut and car corner are generic (no maker). The wire also carries a little direct shear and bending; torsion is the main load. 3D models: everything was modelled for this film (no third-party models).
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