In London’s system of metrical cycles, the beat cycle is “the level of the meter that carries the tactus. This may be the N cycle but more often is a subcycle. IOIs for the beat cycle strongly tend toward the 500-700ms range” (London, 2012: 97).
In this cycle, we cannot determine the beat cycle without knowing the tempo of the metrical cycle, which will determine which subcycle (or the N cycle) carries a frequency close to 500-700 ms, which we would experience as the tactus.
image from: http://www.people.carleton.edu/~jlondon/hierarchical_representatio.htm