You can't zip line on a flat or uphill pitch and expect to reach the end of the ride due to friction and gravity. Neither can you zip down a steep cliff and expect to stop right at the bottom. Suicide Home Zip LineYour design must take into account the pitch between the starting attachment and the ending attachment. Ideally, this will be on a slightly downhill pitch. This all changes starting with longer zip lines over 250' or so (the longer the zip line, the greater the tolerable pitch). If you are going down a cliff or off a high point to a low point (i.e. treehouse to ground), then just make sure that the line is long enough for friction to slow the rider and/or that the tree at the bottom is tall enough to raise your estimated attachment point if necessary to reduce the pitch. Don't worry about the precise math at this point because you can make adjustments to the design while building, but make sure you are in the ballpark.