Observation of operation
Timing belt drive - Operating characteristics

Relative to an angular reference θ _i0 on each pulley i at an initial instant noted 0, the actual angular position of the pulleys at a given instant j is noted θ _ij . Due to various effects, mainly geometric, there is a discrepancy between the theoretical position and the actual position; the transmission error between pulleys i and i + n is introduced: $\begin{array}{l}E{T}_{i,i+n}=\left({\theta }_{ij}-{\theta }_{i0}\right)-Z_{i+n}\text{/}{Z}_i\left({\theta }_{i+n,0}-{\theta }_{i+n,j}\right)\\ E{T}_{i,i+1}+E{T}_{i+\mathrm{1,}i+2}=E{T}_{i,i+2}\end{array}$