The aerodynamic drag force is proportional to the density of the air though which a cyclist is pushing both body and bike. When riding fast, your effort goes principally into overcoming drag: maintaining a speed of 50kpm requires almost double the power of riding at 40kpm (50 3/40 3 = 125/64 = 1.95). The power required to offset the mechanical forces, of gravity and rolling resistance, increases in proportion to speed, but the power needed to overcome aerodynamic drag rises with the cube of velocity. This blog investigates how atmospheric conditions affect the density of air, which, in turn, determines the aerodynamic drag that a cyclist needs to overcome. So far we have established that the fastest times have tended to be in the summer, with a decent wind blowing from the East.
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This is the third in a series of articles investigating factors that determine the best times on Strava leaderboards, using the popular Tour de Richmond Park segment as a case study. Gradients are from 21% downhill to 32% uphill (Stanwix Bank Climb).
Distances range from 93m to 93km, with an average of 2.3km. The following chart shows the gradient and length of 1364 popular Strava segments from around Britain. Sprinters have more fast-twitch muscle fibres, producing extremely high power for short periods, while pedalling at a rapid cadence. Time trialers generate high absolute power for long periods, on smoother terrain, while maintaining an aerodynamic tuck. You can experiment with this interactive chart.Ĭlimbers are able to sustain high force on the pedals, taking advantage of their ability to accelerate quickly on the steepest slopes. Reaching a 5% hill, they both increase power by 50%, but now Nairo is riding at 27kph, dropping Fabian, whose extra weight slows him to 26kph. Under reasonable assumptions, Fabian rides at 276 Watts or 3.4 Watts/kg, while Nairo benefits from a smaller frontal area and lower rolling resistance, requiring 230 Watts, though this equates to 3.8 Watts/kg. Suppose they are cruising along side by side at 40kph. However, on a climb, where speed is lower, the power required to do work against gravity quickly becomes important, especially for heavier riders.Ĭonsider a rider weighing 60kg, call him Nairo, and another weighing 80kg, say Fabian. Since the power needed to overcome aerodynamic drag scales with the cube of velocity, it is the dominant factor when riding fast on flat or downhill segments. Power = Drag Factors * V 3 + Mechanical Factors * V Additional mechanical factors are due to gravity, the rolling resistance of the tyres on the road surface and drive chain loss. Drag can be decreased by reducing frontal area and by adopting a streamlined shape, while wearing a skinsuit. Aerodynamic drag is due to the resistance of pushing the rider and bike frame through the air, with some additional drag coming from the rotating wheels. The power required to maintain a constant speed, V, needs to balance the forces acting on a rider. It can help with training, racing or selecting Strava segments where you have the best chance of moving up the leaderboard. If you have a power meter or access to a Wattbike, it is well worth gathering this data for yourself. For a heavier rider this profile would be consistent with a time trialer, who can generate a high absolute number of Watts, whereas a light rider with this profile may be a better climber, due to a good sustainable power to weight ratio.
In the chart above, the 5 minute and functional threshold (1 hour) Watts/kg rank more highly than 5 second and 1 minute figures, indicating that this cyclist can generate fairly high power for long periods, but has a relatively weaker sprint. Dr Andrew Coggan explains how this information can be used to define a cyclist’s individual power profile. The shape of the power curve reveals a lot about the characteristics of the cyclist. Power can be expressed in Watts or in Watts per kilogram, as in the example below.
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Alternatively, Strava Premium or software such as Training Peaks or Golden Cheetah can generate power curves from a history of power data files.
Ideally, the curve is plotted from the results of a series of maximal effort tests performed over times ranging from 5 seconds to an hour. Your best chance of picking up a KOM is to target a segment that matches your strengths as a cyclist.Ī power curve plots the maximal power a cyclist can sustain over a range of time periods. The most important components are power, bodyweight and aerodynamic drag area or CdA. This blog considers how the attributes of the cyclist affect the time to complete a segment. So far this series of article has explored to the time of year, wind and weather conditions when riders have set their best times on the Strava leaderboard, using the popular Tour of Richmond Park segment as a case study.
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