Golf clubs according to at least some example aspects of this disclosure may include a golf club head and a shaft configured to engage with the golf club head which includes a grip engaged with the shaft. The first and second reference voltage generating circuits output first and second reference voltages to first and second output terminals, respectively. The differential gear device and the countershaft are connected by a first pair of gears. The inflatable chamber portions are defined at least partly by a common flexible membrane that is constrained near a middle region to define a narrow waist between the two inflatable chamber portions.