1. Underframe for rail freight cars, a) without a car body which is rigid against torsion, comprising at least b) external longitudinal bearers (1 or 8) and c) transverse bearers (4) arranged fixed between these and d) at least one additional hollow bearer (2 or 9), arranged between the external longitudinal bearers (1 or 8) in the transverse direction of the car and e) connected in a manner rigid against torsion to the external longitudinal bearers (1 or 8), characterised in that f) the system not yet containing the hollow bearers, and comprising external longitudinal bearers (1 or 8) and the transverse bearers (4) arranged fixed between these, has a rigidity against torsion which is so low that this rigidity against torsion is too low, taken by itself, for the underframe, g) the desired and required torsional rigidity of the underframe is achieved by providing each of the hollow bearers (2 or 9) with a specially established torsional rigidity such that this torsional rigidity determines the torsional rigidity of the entire underframe and thus of the rail freight car, and h) openings (3) on the underside of the hollow bearers determine the magnitude of the torsional rigidity of the hollow bearers (2 or 9) by the shape and size or the cross-sections of the hollow bearers or the wall thickness of the hollow bearers or several of these parameters together.
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