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What's the deal with Cable Sag?
I recently had a customer ask me why ziplines have to have cable sag and aren't guitar string tight, and I'd like to provide an answer to those who might be asking themselves the same thing!
Zipline cable has sag in to because first, it is nearly impossible to pull a cable completely tight, you would need some serious tools to accomplish this. Even then gravity would ultimately pull towards the center of the cable. Secondly, and the main reason, is because it creates inertia for the rider. What is inertia you ask?
Inertia in·er·tia (ĭ-nûr'shə) n. The tendency of a body to resist acceleration; the tendency of a body at rest to remain at rest or of a body in motion to stay in motion in a straight line unless acted on by an outside force. Resistance or disinclination to motion, action, or change.
In a nut shell this means that as the rider declines along the slope of the zipline it creates a tensioning effect further along the cable that creates a slight acceleration and allows the rider to pick up speed along the line. Although too much sag can cause the rider to decelerate so don't over do it.
Zipline sag also releases some of the pressure on the cable. This is important because every zip line cable has a pound per square inch limit that it can withstand, cable sag increases the flexibility of the line and therefore releases the pressure from the point at where the riders weight is being distributed on the line.
If you're looking for more answers on this subject and you're into the science behind it, check out this Physics Forum on Newtonian Mechanics.
Are you looking to set up you own zipline and need calculations?
Click Here: How To Determine Zipline Cable Slope