Predicting Time Trial Performance Using Field Measures Of The Bicycle-rider Aerodynamic Drag

Abstract

PURPOSE To assess the actual aerodynamic drag of the bicycle-rider system in the field and to evaluate its role along with body mass in predicting outdoor level and uphill time trial performance. METHODS Nineteen competitive cyclists completed laboratory testing designed to determine selected physiological predictors of time trial performance. Within three days of the laboratory testing, uphill (9.1 km) and level (22.1 km) time trials were randomly performed. These events were separated by one week. Field determination of aerodynamic drag and rolling resistance involved the use of a rear hub based power meter (Cycleops Power Tap, Madison, WI) and a flat segment of road. The aerodynamic resistance per velocity squared (k) and rolling resistance (R r) were calculated for each subject from the slope and intercept, respectively, of tractive resistance vs. velocity squared. RESULTS (mean ± SD): VO2 peak, lactate threshold (LT), and economy (econ) were 4.67 ± 0.40 l·min−1 (362 ± 30 watts), 76 ± 4.5% of VO2 peak (271 ± 29 watts), and 73.5 ± 3.1 watts·1.02–1, respectively. Body weight, k, and Rr were 70.0 ± 8.0 kg, 0.17 ± 0.03 N·V-2, and 4.9 ± 1.3 N, respectively. Mean uphill time was 31:27 ± 3:18 (min:sec) at a power output of 324 ± 29 watts, while mean level time was 31:24 ± 2:15 at 303 ± 26 watts. The correlation between uphill and level power (r = 0.90, P <0.01) was significantly greater (P <0.001) than the correlation between uphill and level time (r = 0.57, p = 0.01). The ability to predict uphill performance time improved significantly (P <0.05) when uphill power (r = −.42 vs. −0.94, p = 0.08 vs. <0.001), VO2 peak (r = −0.31 vs. −0.82, p = 0.19 vs. <0.001), and power at LT (r = − 0.46 vs. −0.82, p = .05 vs. <0.001 were normalized to body weight. Likewise, the ability to predict level performance time improved significantly (P <0.05) when level power (r = − 0.59 vs. −0.92, p = 0.01 vs. <0.001), power at VO2 peak (r = −0.42 vs. 0.92, p = 0.08 vs. <0.001), and power at LT (r = −0.45 vs. −0.85, p = 0.06 vs. <0.001) were normalized to aerodynamic resistance represented as k. Economy and rolling resistance were not related to field time or power. It is notable that alone, k was better correlated to level time (r = 0.85, P <0.001) than any single or group of physiological measures. CONCLUSIONS Predicting level time trial performance requires an accurate determination of the aerodynamic resistance of the bicycle-rider system, which can be determined in the field by measuring tractive resistance and velocity squared with a cycle mounted power meter. Likewise, body mass is required to accurately predict uphill time trial performance.