The present invention relates to a speed reducer comprising a shaft (11) extending along a longitudinal axis, two rolling bearings (16, 17) arranged around the shaft (11), the two rolling bearings (16, 17) each comprising a radially outer ring (19), a radially inner ring (18) and rolling elements (20) arranged between the radially outer and inner rings (19, 18), characterized in that it comprises a preload system positioned in the space between the two rolling bearings (16, 17), this preload system comprising: - two spacer elements (40) each one resting against one of the rings (18; 19) of the rolling bearings (16, 17), - at least one elastically deformable element (41) interposed between the two spacing elements (40), and configured to be able to occupy a preloaded first state and an least partially relaxed second state, the preload system being arranged in such a way that the transition of the said at least one elastically deformable element (41) from the first state to the second state causes a unidirectio
