When should a climber consider switching from a fixed bar rack to a figure 8 device?

Switching from a fixed bar rack to a figure 8 device is primarily considered when a climber needs more control over descent speed. The figure 8 device allows for greater modulation of descent, enabling the climber to adjust the speed more precisely compared to a fixed bar rack, which is designed for more continuous and rapid descent with less friction control. This becomes particularly important in situations where precision in lowering a climber is necessary, such as during a rescue operation or when descending a challenging route.

In contrast, carrying extra gear, securing a lead climber, or only ascending do not directly relate to the need for enhanced descent control. These factors are more about the specifics of the climbing situation and gear management, which do not influence the choice of device based on descent speed. Thus, the choice to use a figure 8 device for its superior control during descents is pivotal for both safety and efficiency in climbing scenarios.