Elsevier

Journal of Biomechanics

Volume 47, Issue 8, 3 June 2014, Pages 1894-1898
Journal of Biomechanics

Optimal cycling time trial position models: Aerodynamics versus power output and metabolic energy

https://doi.org/10.1016/j.jbiomech.2014.02.029Get rights and content

Abstract

The aerodynamic drag of a cyclist in time trial (TT) position is strongly influenced by the torso angle. While decreasing the torso angle reduces the drag, it limits the physiological functioning of the cyclist. Therefore the aims of this study were to predict the optimal TT cycling position as function of the cycling speed and to determine at which speed the aerodynamic power losses start to dominate. Two models were developed to determine the optimal torso angle: a ‘Metabolic Energy Model’ and a ‘Power Output Model’. The Metabolic Energy Model minimised the required cycling energy expenditure, while the Power Output Model maximised the cyclists׳ power output. The input parameters were experimentally collected from 19 TT cyclists at different torso angle positions (0–24°). The results showed that for both models, the optimal torso angle depends strongly on the cycling speed, with decreasing torso angles at increasing speeds. The aerodynamic losses outweigh the power losses at cycling speeds above 46 km/h. However, a fully horizontal torso is not optimal. For speeds below 30 km/h, it is beneficial to ride in a more upright TT position. The two model outputs were not completely similar, due to the different model approaches. The Metabolic Energy Model could be applied for endurance events, while the Power Output Model is more suitable in sprinting or in variable conditions (wind, undulating course, etc.). It is suggested that despite some limitations, the models give valuable information about improving the cycling performance by optimising the TT cycling position.

Introduction

In order to minimise the aerodynamic drag, cyclists adopt a time trial (TT) position (often called the ‘aerodynamic position’). The TT handlebars allow the rider to adopt this aerodynamic position, resulting in a decreased frontal area and hence aerodynamic drag experienced by the rider. A reduction in aerodynamic drag of approximately 35% is found between an upright position and a TT position (Hennekam, 1990). In addition, Underwood et al. (2011) showed with wind tunnel experiments that in a TT position the total aerodynamic drag is strongly influenced by the torso angle. A difference in drag area of approximately 16% was found for torso angles between 2 and 20°. Moreover, Garcia-Lopez et al. (2009) showed a significant decrease in aerodynamic drag of about 14% when the height of the TT handlebars was lowered. Kyle (2003) also stated that in general the aerodynamic drag is minimal with an almost flat back. From these findings it can be concluded that cyclists should adopt an almost flat (0°) torso angle position to minimise the aerodynamic drag.

However, along with the drag the cyclists peak power output decreases with lower torso angle (Fintelman et al., 2013, Gnehm et al., 1997, Grappe et al., 1997, Jobson et al., 2008). For instance a reduction of 14% peak power output was recorded between an upright (24°) and flat (0°) torso angle TT position (Fintelman et al., 2013). It is suggested by Gnehm et al. (1997) that this peak power output reduction could be related to: (1) muscles not working in their optimal range, (2) a difference in muscle recruitment, (3) greater muscular fatigue, (4) increased pressure on shoulder griddle, neck and arms, and (5) increased adductor activation to keep the leg movement in the sagittal plane due to the extreme hip angles, or a combination of these factors. These experiments imply that cyclists should not adopt an almost flat position.

Clearly there are two conflicting constraints, with aerodynamics requiring a flat position and biomechanics favouring a more upright position. Therefore it can be inferred that combining the results obtained for aerodynamic drag and peak power in different TT positions, a trade-off can be found between the loss in power output and drag as function of cycling speed. This is supported by the energy expenditure (IE) which is a function of the workload divided by the gross efficiency (ɳ). The workload to overcome drag decreases with smaller torso angles, while the ɳ also decreases.

In previous literature (Gnehm et al., 1997, Jeukendrup and Martin, 2001, Lukes et al., 2005), suggestions have been made that the aerodynamic gains outweigh the loss in peak power output for TT cyclists. However, these statements are based on elite TT cycling speeds, e.g., >45 km/h (Gnehm et al., 1997). Contrary, Underwood et al. (2011) have estimated the optimal cycling position for a relative wind speed of 40 km/h in terms of power output performance and aerodynamic losses. They introduced a new method to analyse the optimal cycling position, the so-called ‘surplus power’. The surplus power was defined as the maximal power output of the cyclist minus the aerodynamic power losses and rolling resistance of the tires with the road. In their study, no consistent results about the optimal position were found, which could be due to the limited number of participants (n=3). Nevertheless, they have demonstrated the existence of an optimal torso angle at cycling speeds of 40 km/h.

To the best knowledge of the authors, the speed of the cyclist at which the aerodynamic power loss starts to dominate has not been defined. Therefore the aims of this investigation were to predict the optimal TT cycling position as function of the cycling speed and to determine at which cycling speed the aerodynamic power losses starts to dominate. It has been hypothesised that an optimal torso angle exists for each cycling speed and type of event. In the presented work, the torso angle of non-elite cyclists is optimised by using two mathematical models.

Section snippets

Method

Two models were developed to determine the optimal torso angle cycling position for a certain cycling speed: the ‘Metabolic Energy Model’ and the ‘Power Output Model’. The Metabolic Energy Model minimised the required cycling energy, while the Power Output Model maximised the surplus power. The inputs for the models came from experimental data of 19 participants in different torso angle positions, β, from 0° to 24° relative to the ground (Fig. 1). Main input parameters were the cycling speeds

Models outcome

Two different models were used to predict the optimal torso angle, βopt. The optimal torso angle was determined for torso angles between 0° and 24°. In Fig. 4 the results of both models and the corresponding confidence intervals (significance level p=0.05) for non-elite TT cyclists are shown for speeds between 28 and 40 km/h. Outside this range, the torso angle was predicted based on extrapolation of the experimental data. It could be seen that the optimal torso angle is dependent on the cycling

Discussion

Two models were developed to predict the optimal torso angle position at different cycling speeds: the Metabolic Energy Model and the Power Output Model. The results showed that the optimal torso angle depends strongly on the cycling speed, with decreasing torso angles at increasing speeds. However, a fully horizontal back is not optimal. At speeds above 46 km/h, the aerodynamic losses outweigh the power output losses, which is in line with previous literature (Gnehm et al., 1997, Jeukendrup and

Conflict of interest statement

None.

Acknowledgements

The study was not externally funded. The authors would like to thank P. Highton and T. Adams for their assistance with the data collection.

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