An improved rheometer and rotatable sensing shaft sub-assembly therefor with flexural pivot support at two ends of the shaft, and means for allowing limited axial movement of the shaft while maintaining rigid radial support, with transducer means for extracting and signaling rotational position of the shaft based on shear stress of a fluent material contacting a spindle, cone/plate or other extension of the shaft and/or a related motor-driven member such as a cylinder.