Abstract: This invention relates to a transmission between a drive element and a drive shaft (1) of the wheels of a self-propelled tractor vehicle, in particular with an accompanying driver walking behind the vehicle, such as a lawn mower, whereby the transmission comprises—generally housed at least partially inside a box (40)—a progressive friction clutch of the type that comprises at least one rotary driving element (30) and two driven elements (5A, 5B) that are carried by the drive shaft (1) and are integral in rotation with the latter, whereby these driving elements (30) and driven elements (5A, 5B) are, under the action of a control element (20), brought into variable-tightening support contact to obtain a variation of the torque that is transmitted to the shaft (1).