The influence of speed, grade and mass during simulated off road bicycling

Abstract

The purpose of this investigation was to examine the effects of bicycle mass, speed, and grade on oxygen consumption ($\text{V}\text{̇}\text{O}{}_2$), heart rate (HR), and ratings of perceived exertion (RPE) during a simulated off-road riding paradigm. Nine adult subjects with mean±SD age, mass, and $\text{V}\text{̇}\text{O}{}_2$ max of 26.1±5.6 years, 71.7±7.5 kg, 56.6±5.2 ml·kg⁻¹·min⁻¹, respectively, were trained to ride a fully suspended Trek Y-22 mountain bike on a treadmill with a 3.8 cm bump affixed to the belt. Riders completed a maximum of nine separate trials encompassing three different bike masses (11.6, 12.6 and 13.6 kg), 3 speeds (2.7, 3.6 and 4.5 m·s⁻¹), and 3 grades (0, 2.5, and 5%). Throughout a trial, bike mass and speed remained constant while riding grade was increased every 5 min. During simulated off-road riding on a fully suspended mountain bike, increases in speed and grade significantly increased $\text{V}\text{̇}\text{O}{}_2$, heart rate, and RPE. Increases in bike mass had no significant effects on $\text{V}\text{̇}\text{O}{}_2$, heart rate or RPE. In addition, speed and grade changes interacted to differentially affect $\text{V}\text{̇}\text{O}{}_2$, heart rate, and RPE at all speeds and grades.

Introduction

The factors that affect energy expenditure during road cycling have been well characterized. They include: (1) rolling resistance, or the resistance due to deformation of the tire, wheel and road; and bearing and chain drag, (2) air resistance, which is the drag caused by the bike–rider system moving through the air and (3) gravitational effects of ascents or descents on the bike–rider system. The effect of these factors on energy expenditure was originally expressed mathematically by di Prampero et al. (1979), and more recently by Swain (1994) as

$$\text{factor}\text{1}\text{factor}\text{2}\text{factor}\text{3}$$

$$\text{Energy}\text{cost}\text{=(k}{}_{\mathrm{<mtext>r</mtext>}}\text{Ps)+(k}{}_{\mathrm{<mtext>a</mtext>}}\text{Av}{}^2\text{s)+(gPIs)}$$

where k_r is the rolling resistance coefficient, P the combined mass of cyclist and bicycle, s the bicycle road speed, k_a the air resistance coefficient, A the cyclist's surface area, v the bicycle speed in air, g the acceleration of gravity and I the road incline.

The force required to overcome rolling resistance is influenced by the mass of the bike–rider system such that as mass increases so does the force required to overcome this resistance. Kyle suggested that at a given force output the addition of as little as 1 kg of mass to a road bicycle could decrease speed as a result of an increase in rolling resistance (1991).

The force required to overcome air resistance is the greatest contributor to the energy cost of level road cycling and increases rapidly with speed. Hagberg and McCole (1996) reported that air resistance accounts for less than 20% of the total resistance to movement at speeds less than 10 km/h. However, at speeds of 20 and 40 km/h, air resistance accounts for 54 and 82% of the total resistance, respectively.

The force required to overcome gravity is the third factor and is also influenced by the mass of the bike–rider system such that as mass increases so does the force required to overcome this resistance. Howe (1995) estimated that during uphill road cycling gravity may account for more than 90% of the resistance to motion. Additionally, he estimated that reducing the weight of the bicycle by using lightweight parts could significantly improve performance during uphill road cycling.

From the above discussion, it is clear that air resistance and mass are determinants of the energy cost of road cycling. How these factors affect off road cycling performance has not been well documented. Typically, the speeds reached during off road cycling are much less than those achieved on the road, and, as such, the effects of air resistance do not add significantly to the energy cost. Furthermore, strategies used by road cyclists to obviate the effects of speed (i.e. drafting) are not applicable during off road cycling. Therefore, one of the strategies adopted by off road cyclists to improve performance is to decrease the mass of the bike as much as possible.

Previously, it was demonstrated that the use of a well-designed suspension system would decrease the energy cost during simulated off road cycling (Berry et al., 1993). However, a potential problem with the use of such systems is that they usually add additional mass to the bike potentially increasing the force required to overcome the increased rolling resistance and the increased resistance due to gravity. Thus, it appears that the two strategies used by off road cyclists to decrease energy expenditure, use of a suspension system and the decrease in bike mass, are diametrical. Therefore, the purpose of this investigation was to determine the effects of bike mass on the energy cost of simulated off road cycling at different speeds and grades.