Aerodynamics of time trial bicycle helmets (P226)

During a time-trial (TT) stage on flat track at 50 Km.h(-1), aerodynamic drag force represents 90% of the power developed by the runner (Belluye and Cid, 2001). Given the drag resistance impact on TT performances, aerodynamics optimization Of Posture and equipment is thus an essential point (McLean and al. 1994, Martin 1996). The purpose of this study concerns the aerodynamic comparison of six TT helmets which the aim is to minimize the aerodynamic drag. The aerodynamics drag resistances (R-D) of nine professional cyclists using these different TT helmets allow to quantify their aerodynamic performances, the visor and the frontal vents influences according to head and trunk tilts assumed during stages. Experiments were carried out in a wind tunnel at a free-stream velocity of 13.9 m.s(-1). A 3D motion analysis system SIMI MOTION measured cyclists postural angles (three head and two trunk tilts). Statistical analysis shows that drag resistance and frontal area of a TT posture is significantly lower than the classical road Posture (-14.9%). Coefficients of drag in road and TT posture are not significantly different (p>0.05). Besides, interaction between the global posture and the helmet inclination is significant (p<0.05). In TT posture, drag resistance connected with the natural inclination of the helmet is significantly lower (p<0.05) than high (-3.4%) and low inclination (-1.5%). Usual inclination of the helmet provides a drag coefficient reduction of 2.2% compared with the other inclinations (not significantly different together). In high inclination, frontal area is significantly higher (2.4%). Without changing the frontal area, the visor allows a significant reduction of the drag coefficient for low and high inclination (-1.5%), and thus of the drag resistance. This reduction is not significant for the natural inclination. Whatever the helmet orientation, frontal vents have no significant influence on drag coefficient and on frontal area.