Two examples of collapse of a Neimark–Sacker torus in the Rössler system are studied. Their stability is evaluated by measuring the rotation number, defined as the long-term average value of the angular phase shift that occurs in the Poincaré map on each pass, and by theoretically calculating the same angular shift within the context of a normalized cubic model. The parameters of the cubic model are evaluated using perturbation theory applied to a limit cycle, which involves first evaluating the response after a fixed time, the known period of the limit cycle, and then calculating the additional effect due to variations in time of flight to reach a fixed plane perpendicular to velocity.For the first example of collapse, the above two methods yield the same result, namely that the collapse is characterized by a constant rotation number as we move away from the bifurcation, leading to a torus that usually coalesces into discrete branches and then collapses due to turning points. This mechanism allows us to define a simple criterion that the parameters of the cubic model must satisfy, which is related to stability. The second example of collapse is more complex: the normalized cubic model suggests correctly that the collapse will be associated with the calculated radius of the torus becoming complex, which leads to a second criterion for the cubic parameters. However, the predicted point of collapse is significantly premature. The actual torus continues to survive until the rotation number approaches zero, leading to a single wave packet of infinite duration that travels between the two known equilibrium points of the system.