is slidably affixed in the plate 100 which is secured to the fender of the automobile by a cable 103 and heavy extension spring 104 a fluid cylinder may be substituted for the cable and spring the plate 100 is held in tension between its bearing on the spindle 7 and fender of the vehicle the outer end of the pin 15 is in sliding engagement with an axially moveable collar 106 a latch 111 tensioned by spring 112 attached to collar 110 which is axially movable inwardly on the spindle 7 and locked in place by latch 111 by engaging one of the two holes in the spindle 7 thereby locking the weights in their extended position and pin 15 is then engaged in the hole of disc 5 thereby locking the traction members 6 in their non operating position this mechanism uses the effect of centrifugal force which causes the levers 109 to push collar 106 inwardly said collar pushing pin 15 into the hole in the disc 5 non operating traction members this arrangement can be used as a strictly manually engaged mechanism that will operate automatically thereafter once engaged for example as a driver starts to work on a snowy morning he will engage the latch 111 into the hole furthest from the collar 107 thus allowing free movement of the weights 108 which effects the movement of the collar 106 and pin 15 in response to the varying speed of the rotation of the automobile.