The first design step is getting a basic clue about the parameters of the zip line ride. Here are the basic questions you have to answer before starting:
- How long is the zip line?
- What are you attaching each end to?
- Does your design have a reasonable pitch?
- Roughly how high are the start and finish points?
How long is the zip line?
Measure the distance along the ground between the start and finish. add some cable to allow for sag (about 3% or so), cable terminations per your design (at least 2 feet on each side, more if wrapping around a tree), and a little more to make the installation easier (enough to reach from the attachment point to the ground). Aircraft cable is relatively cheap, so make sure you have plenty.
What are you attaching each end to?
Zip lines are most commonly attached to trees or poles. Depending on the design, most poles and some small or questionable trees will need guy wires to counter the tension forces placed by the weight of the cable and the rider which pulls strongly inward due to the vector forces involved. Some home zip lines have been attached to tree houses, ground houses, playsets, rock outcrops, or steel towers. If you are designing a zip line and you don't know if your attachment method is strong enough, you should get the opinion of a qualified professional such as an arborist, engineer, or carpenter.
Does your design have a reasonable grade?
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. Your 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.
Roughly how high are the start and finish points?
The cable should not be installed taut. This makes the tension way too high when weight is applied in the center. Professional zip line builders will typically build in about 5% sag as a rule of thumb - the percentage changes with each zip line design! The cable size, length of run, and maximum rider weight all affect the minimum required sag. So to use the rule of thumb on an unrealistically simple design, a 200' zip line that is 7' high in the middle (when loaded with a rider), should sag 5% or 10 feet. This means that the end attachments are 17' high each. At this point, plan that the longer the zip line, the higher in the air you'll need to be to start. Make sure that you have a way to get that high in the air in order to ride. Furthermore, make sure that you are comfortable with climbing up to that height and being that high above the ground. Designing & building a zip line at your home can put you and others at risk of a fall.
Sometimes a starting point is known, and in these cases, the ending attachment is the plug factor during trial and error test runs (always test first with a weight in case your guess is too fast a ride to control).