Safety is the first priority when designing robots that are intended to physically interact with humans. New robotics standards state as a condition for collaboration that the robot should be designed so that it cannot exert forces larger than 150 N at its tool center point. An effective and reliable way of guaranteeing that this force cannot be exceeded is to place a torque limiter in series with each actuator, thus forming series clutch actuators (SCAs). Since the relationship between the joint limit torques and the achievable end-effector forces is configuration dependent, it is preferable to use adjustable torque limiters. This paper presents a method to optimally control the limit torques of a serial manipulator equipped with adjustable series clutch actuators. It also introduces two performance indices to evaluate the quality of the relationship between the joint limit torques and the achievable end-effector forces. The first one is the ratio of the minimum and maximum force thresholds. Even if it has a strong physical meaning, it is not differentiable everywhere in the workspace and is thus difficult to use in an optimization process based on its gradient. A second index, smooth, and expressed in a closed-form, is therefore introduced which is the determinant of the normalized Jacobian matrix postmultiplied by its transposed. Examples of redundant manipulator motion optimization and of collaborative robot architecture optimization using the second index are shown. The limitations of the proposed approach are that it is based on a static model—which is nevertheless valid under the current safety standards—and that gravity is neglected.