Relationship between Twist, Jerk, and Speed: Twist-Tolerance Values and Measuring Chords

Twist is a track defect that affects comfort by inducing jerk and rolling and endangers the derailment safety of a vehicle by wheel off-loading. Currently, there is no formula in the literature that relates speed and jerk with twist. In this paper, a relationship between jerk, speed, and twist is developed. The relationship was used to assess the tolerance values of twist for design, construction, and maintenance of the track. The relation was also used to assess slow speed on a twisted track beyond the maintenance limit. The advantage of the formulas is that they are speed specific. The relation between twist, jerk, and speed was theoretically validated. The derived equations from the aforementioned relation to estimate tolerance values of twist were validated by comparing with the values given by US and European specifications. A literature review and analysis was performed on chord length to measure the twist. A chord equal to the wheel base is suggested to measure twist on a newly constructed or renewed track. On a revenue track, chord lengths equal to both wheel base and truck center to center distance are recommended to capture both short- and long-wave twist defects. The suggested chord lengths would integrate vehicle parameters, track defects, comfort, and safety.