A right crank arm assembly for a bicycle, comprises a right crank arm and at least one front sprocket coupled with the crank arm at least one coupling portion of the crank arm. The front sprocket has an inner side adapted, in use, to face towards the frame of the bicycle and an outer side opposite the inner side. The assembly comprises at least one first crank arm element which acts in contact with the front sprocket on one of said sides and at least one second crank arm element which acts in contact with the front sprocket on the other of said sides. The contact action of the crank arm elements on the two opposite sides of the front sprocket prevents the twisting deformation of the front sprocket during pedaling. Such a deformation would cause a reduction in the transmission efficiency of the right crank arm assembly.