A brake group for a transmission between a drive shaft and a driven shaft, comprising: a frame; an input shaft, connectable to the drive shaft; elastic means; a first friction surface which is solidly constrained to the frame; a second friction surface; a first member, rotatable with respect to a first rotation axis and connectable to a driven shaft such as to draw the driven shaft in rotation; a second member, connected to the input shaft such as to be drawn in rotation thereby about the first rotation axis; the first member and the second member being further couplable by a clutch, such that: the second member can transmit drive to the first member; and a variation in relative angular velocity thereof corresponds to a variation in relative distance thereof; wherein the first member can slide along a parallel direction to the first rotation axis; the elastic means act on the first member such as to guarantee clutching coupling of the first member with the second member; the first member bears the second friction surface; the brake group is configured such that when the angular velocity of the second member diminishes with respect to the angular velocity of the first member, the first member slides along the parallel direction to the first rotation axis until the second friction surface presses against the first friction surface in order to determine the braking of the first member, the first member comprises a first clutching surface, and the second member comprises a second clutching surface, the first clutching surface and the second clutching surface being conformed such as to engage to one another and comprising coupling profiles which make a relative sliding possible, such that the first member can rotate relatively with respect to the second member and at the same time slide along the parallel direction to the first rotation axis; wherein the coupling profiles are inclined with respect to the direction identified by the first rotation axis; characterised in that the second member is connected to the input shaft via a free-wheel mechanism, which free-wheel mechanism is configured such as to make the second member freely rotatable with respect to the input shaft when the angular velocity of the second member diminishes with respect to the angular velocity of the first member until the second friction surface contacts the first friction surface and the first member is braked. a frame; an input shaft, connectable to the drive shaft; elastic means; a first friction surface which is solidly constrained to the frame; a second friction surface; a first member, rotatable with respect to a first rotation axis and connectable to a driven shaft such as to draw the driven shaft in rotation; a second member, connected to the input shaft such as to be drawn in rotation thereby about the first rotation axis; the first member and the second member being further couplable by a clutch, such that: the second member can transmit drive to the first member; and a variation in relative angular velocity thereof corresponds to a variation in relative distance thereof; wherein the first member can slide along a parallel direction to the first rotation axis; the elastic means act on the first member such as to guarantee clutching coupling of the first member with the second member; the first member bears the second friction surface; the brake group is configured such that when the angular velocity of the second member diminishes with respect to the angular velocity of the first member, the first member slides along the parallel direction to the first rotation axis until the second friction surface presses against the first friction surface in order to determine the braking of the first member, the first member comprises a first clutching surface, and the second member comprises a second clutching surface, the first clutching surface and the second clutching surface being conformed such as to engage to one another and comprising coupling profiles which make a relative sliding possible, such that the first member can rotate relatively with respect to the second member and at the same time slide along the parallel direction to the first rotation axis; wherein the coupling profiles are inclined with respect to the direction identified by the first rotation axis; characterised in that the second member is connected to the input shaft via a free-wheel mechanism, which free-wheel mechanism is configured such as to make the second member freely rotatable with respect to the input shaft when the angular velocity of the second member diminishes with respect to the angular velocity of the first member until the second friction surface contacts the first friction surface and the first member is braked.