Vibrational analysis of a flexible bicycle stem during indoor in-vivo cycling on a two rollers servohydraulic test bench

Introduction. Comfort is an important parameter correlated to bicycle usability and depends deeply on vibrations and human perception. The most of vibrations is generated by the interaction between road and wheels and sensed at hands and seat [1][2]. Different approaches were used to evaluate the in-vivo behaviour of different bycicle postures and components, including vibrational excitation applied to the wheels [2],[3]. The present work presents results obtained from a full scale roller bench test [3] on a flexible stem supposed to improve the riding comfort. External vibrations were applied to both wheels using a two rollers servohydraulic test bench [3] with a random load function generator corresponding to road types defined by the ISO 8608: 2016. Methods. A specific flexible stem (Shockstop produced by Redshift), designed to reduce accelerations transmitted to the hands by selectable elastomeric shock-absorbing inserts (SOFT, MEDIUM, HARD), was compared to a standard rigid stem (Deda zero 100 Alluminum). Three road profiles (A= Airport runways and super highways; B= Normal pavements; C= Unpaved and damaged roads, ISO 8608: 2016) were applied to the cyclist in two postures (Posture1-Hands on upper handlebar brakes, Posture2-Hands on handlebar drops). Results. Table 1 reports the Comfort index and Transmission index percent variations respect to the RIGID stem (Red cells represent lower performance). As it can be seen, based on CI the Flexible stems seems to improve comfort in both postures and stem hardness with lower roas harshness, but gives not favourable results in Posture 1 with road C. The transmission index TI seems to give opposite results than the CI in Posture 1, whereas shows better agreement in posture 2. Conclusions. The study shows the suitability of the roller test bench for the stationary analysis of comfort related component in bicycle. The significance of the two parameters CI and TI deserve comparison with a larger number of subjects and their subjective evaluation.