variable torque capacity clutch 21 of a forward and reverse drive means the clutch 21 comprises a drum 22 connected by a median web 23 for rotation with the shaft 19 and slidably mounted within the drum 22 are a pair of pistons 24a 24b each piston 24a 24b is adapted respectively to act on sets of clutch plates 25a 25b and 26a 26b respectively the clutch plates 25a are keyed to the drum 22 whilst the clutch plates 25b are keyed to a gear 27 the clutch plates 26a are keyed to the drum 22 and the clutch plates 26b to a gear 28 in use the piston 24a can be urged by hydraulic fluid to the right in fig1 to compress the clutch plates 25a and b together and against a stop member 25c so as to connect the gear 27 to the drum 22 alternatively the piston 24b can be operated to urge the clutch plates 26a and b together and against a stop member 26c to connect the gear 28 to the drum 22 in both cases the torque capacity of the clutch can be varied by clutch torque capacity control 55 which operates under the manual control of an operator in any conventional manner e g by varying the pressure of the hydraulic fluid which actuates clutch plates 25 or.