Mechanical bulls can be a lot of fun and they are not hard to make yourself for parties and home entertainment. Homemade mechanical bulls are actually harder to ride than commercial bulls. Remember that when you are making this and planning where to put the homemade bull.
Make a rectangle 12 feet long by 6 feet wide using 4 fence posts at the corners. Use 6 foot posts, and put 2 feet of them in the ground. Secure the posts with concrete.
Place the barrel in the middle of the rectangle with the length of the barrel parallel to the long side of the rectangle. Attach the four large springs, using the nuts and bolts, to the barrel with one spring pointing in the direction of each post.
Insert the screw eyes into the four posts, pointing in the direction of the barrel in the middle of the rectangle. Be sure that these screw eyes are securely set into the posts approximately a foot or less below the top of the post.
Attach rope from each spring to the screw eyes in the post. Leave enough excess rope to wrap around the post a few times for leverage. Do not pull the rope tight on the springs, but allow enough sag so that the barrel is approximately 2 1/2 feet to 3 feet off the ground.
Insert a long rope on either end of the barrel. You can use a screw eye to do this or knot the rope. Make it long enough to go outside of the rectangle made by the posts for someone to start rocking the bull and making it buck.
Spread the straw around underneath the barrel, making there be enough cushion for a light landing when the riders fall off of the bull. Constantly refurbish the straw as it gets old and matted.
Children should wear helmets when riding the bull. A bicycle helmet will suffice.