Coyotes jump fences by grabbing the top of the fence with their forepaws and then pull their hind legs up to the same point and bound over by pushing off with their hind legs. Roller bars prevent them from pulling themselves up — they just roll right off. The barbed wire bending in THEIR direction also prevents them from grabbing the top of the fence. An 8′ fence works because a coyote cannot jump that high.
For the complete brochure from the State of Washington, please see this link: https://stoptrapping101.files.wordpress.com/2014/10/coyotes-living-with-wildlife-_-washington-department-of-fish-wildlife.pdf