Abstract: A mounting plate is provided for removable rigid support from and in overlapped engagement with the lower marginal portion of a mount flange carried by the rear swing arm of a motorcycle adjacent to which the motorcycle chain is trained. A pivot shaft has one end portion removably rigidly secured through the plate and one pair of corresponding ends of parallel arms are mounted on the pivot shaft for oscillation relative thereto. A roller is journaled between the other pair of corresponding ends of the parallel arms and spring structure is operatively connected between one of the arms and the mounting plate for yieldingly biasing the arms in one direction of angular displacement about the pivot shaft, the chain of the associated motorcycle being receivable between the end portions of the arms between which the roller is journaled and with the chain passing over the roller.