Construction: Polyester Cover w/ a Rubber (Elastic) Core The best shock cord we've found. It features a tough polyester cover that protects the elastic core from abrasion and sun exposure. In general, you'll want to keep the shock cord stretched between 30% and 80% of its length. (Below 30% stretch, return force is too low, and above 80%, you're approaching maximum stretch.) A good rule is to buy a length of shock cord twice as long as the "throw" in your system, then set it up so the shock cord is 30% stretched in its most-eased position. The largest sizes exert a return force of approximately 10 lbs. at 100% stretch. Available Colours: Black and Blue