like criggster mentioned a 1.5 coil spring is your best bet, but don't be cheap when you buy your traps. You will be glad you spent a little extra when your first coyote gets pinched. Trap preperation is key you need a clean dyed trap alot of people wax them, I never have and i do allright. Once the trap is clean and scent free you need to keep it that way. I hang mine out side away from foriegn odors untill needed. Also when making sets use a clean pair of gloves to set and bed your trap and a different pair to apply lure and bait. But the biggest thing is to make sure your trap is bedded solid. Practice this at home. It is easiest to dig a deeper hole that you can fill in with loose dirt and press the trap down into and then pack dirt in around and under the jaws and springs. test by pushing down on a spot on the trap if it is done right it shouldn't move at all.