I made a wood balance beam for my daughter out of a fir 4x4 and some compression joints. Which direction should the grain be? Should the grain run parallel or perpendicular to the top of the beam?
You want the grain/growth lines to be oriented vertically to maximize strength. Most 4x4 posts will have curved annual growth lines. Choose the orientation that is most vertical. As a caution, note that professional balance beams are made from laminated wood and are much deeper than the 3 1/2" in a 4x4 post. If your daughter is going to do any gymnastics on the beam a pine 4x4 may not be rigid enough especially if you make it any longer than 8'.