Ring size was proved to impact bicycle drivetrain efficiency as larger rings are used to improve this efficiency. However, results from studies focused on sprockets create a gain expectation that is not met when considering ring size at a fixed gear ratio. This study was designed to understand this discrepancy better. Three chainring sizes (58, 60, 62 teeth) and 3 sprocket sizes (13, 16, 24 teeth) were tested on a dedicated testing rig in conditions that were representative for professional time-trial racing (450 W, 100 rpm). Four derailleur pulley combinations (11 teeth for the upper pulley and 11 for the lower, 14–14, 14–18 and 14–19 teeth) were tested as well. Bigger sprockets allow an increased efficiency, as expected based on the literature. However, a slight trend toward a decrease in efficiency was observed when increasing chainring size. If it was to be confirmed, this trend could be explained by the fact that, at a fixed torque, chainring size governs chain tension, on which efficiency depends. Indeed, a bigger chainring lowers chain tension and therefore could decrease efficiency. This finding could explain the moderate gains in efficiency measured for a fixed gear ratio when compared to studies focused on sprocket size. This study did not find any effect from derailleur pulley size. Overall, bigger rings offer better efficiency, as efficiency is mainly governed by the sprocket effect, but this effect could well be reduced by an opposed chainring effect at a fixed gear ratio and torque.
Aubert et al. (Thu,) studied this question.