Coyotes jump fences by grabbing the top of the fence with their fore-paws and then pull their hind legs up to the same point and bound over by pushing off with their hind legs. The barbed wire bending in THEIR direction also prevents them from grabbing the top of the fence. Roller bars prevent them from pulling themselves up — they just roll right off. An 8′ fence works because a coyote cannot jump that high.
Below is excellent fencing information from the State of Washington, click on the image to enlarge it, or go to the State of Washington link, below: