Bit configuration and acting rein forces play a crucial role in oral health and comfort of ridden horses. Although it is a big animal welfare issue, dynamic response of horses to different bits has yet not been thoroughly investigated. This convenience sample experimental study describes a model to overcome the almost uncontrollable influence of riders on rein tension and evaluates self-controlled maximum side rein tension of ten sound horses randomly bitted with a double-jointed (DJS) and a version of a Mullen mouth snaffle-bit under unridden conditions. Horses were exercised at walk and trot on a horizontal treadmill wearing custom made force-sensing resistors (FSR) equipped to side reins. FSR were synchronized with a camera-based motion analysis system providing information on amplitudes and temporal occurrence of self-controlled maximum side rein tensile forces during different phases of separated motion cycles. The DJS exhibited larger side rein tension, indicating higher bit contact. Constant temporal occurrence of monophasic maxima at walk and biphasic maxima at trot could be observed in both bits. Within the limitations of this study, application of FSR linked to side reins in unridden horses may provide a promising tool when studying subjective response of horses to different bits.