The invention comprises a tensioner having a pulley (1) journaled to a housing (3). The housing further comprises a guide (4). The guide slidingly engages rails (7, 17) on a base (6). The guide and rails constrain the housing to move in a predetermined linear path. The rails are horizontally offset from two axis defined by the pulley. The pulley load is also vertically offset from the axis of movements of the guide. The guide and rails have a predetermined frictional coefficient so that the cumulative result is an asymmetric damping effect. Further, a linkage (8) is connected between the housing and a camming body (9). The camming body is rotatably connected to the base. A biasing member such as a torsion spring (11) biases the camming boy against the belt load through the linkage. The radius of the camming body is variable to maintain a constant belt load as the tensioner pulley moves in response to a load change.