The zip-line trolley is the frame or assembly together with the pulley(s) also known as sheave(s) inside that run along the cable. (The term "trolley" is more often used when this assembly consists of more than a single pulley with simple hanger and bearing.) Often more than one pulley is used to spread the load over more than one spot on the cable, to reduce cable bending stresses that may lead to metal fatigue and cable breakage. This also reduces any tendency of a pulley to twist sideways and run off the cable, with disastrous results. In addition, the trolley is usually shaped with guards to hold the cable in the groove(s) of the pulley(s).
Departure zip SuperFly in Whistler, BC, Canada
A pivoting link, such as a carabiner, is used to secure the load to the trolley so that the trolley does not have a tendency to rock and thus fall off the cable if the load should sway. Load carriers ranging from enclosed cabins to gondolas to harnesses are attached to the link. Occasionally the load carrier is just a handhold or handlebar, although there is the danger of the rider's losing his grip and falling. Such a simple carrier should be used only on zip lines that are near the ground or over water. The rider losing his grip due to the use of the simple carrier has led to ten deaths in two years alone (2010–2012).[citation needed]