a tensioning device is provided for selecting and imparting torsional bias to a shaft and simultaneously providing a shock absorbing mechanism , the device including a first hub member selectively connectable to the shaft , a second hub member rotatable wiht respect to the first hub member , a torsion coupling of elastomeric material connected at a first end to the first hub and at the other end to the second hub , and a locking mechanism to lock the second hub to a stationary frame member when the desired torque has been reached .