Here are some basic guidelines for selecting the Shaft flex, which is directly related to trajectory, distance and feel. The proper shaft flex for your swing speed helps produce the optimal trajectory for your shots. The best way to determine the proper shaft flex is to use a swing speed device to measure your swing. If a device is not available, the following chart may be used to approximate swing speed based on the carry distance of your Driver or the Club used to approach the greeen from 150 yeards out.