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Rider Aerodynamics

 John Cobb has done studies of the aerodynamic drag on riders. The data from one of his studies, dated January 11, 2000, is analyzed here.

Conclusions:
  • Keep the jersey zipper zipped.
  • Ride on the drops.
  • Keep your drink bottles on the frame.
  • Carrying a drink bottle is better than not.
  • Wear an Aeropak under the jersey but better yet, keep the bottles on the frame.
  • Placement of aerobars is highly dependent on the rider.

    In this analysis, we model performance by first defining a simple flat, point-to-point 10k course. The rider on this course is subject to wind at various speeds and from various directions. Then using Cobb's drag measurements in the model, we estimate the time it takes for the rider to ride the 10k distance.

    The rider in the model weights 160 pounds and rides a 22 pound bike. Wheels are spoked and the manufacturer's marketing people describe them as "aero". The rider can sustain a 185 watt effort. This is a very typical rider, one that you would fine on most local club rides.

    Cobb measured drag on this rider and bike in a wind-tunnel. He recorded measurements at wind yaw angles of {0, 5, 10, 15} degrees and at 30 mph. Can't ride 30 mph? The data as used in the models applies as well at lower rider and wind speeds.

    In the analysis we estimate a time for the base-case and a time for an alternative and then compare the differences in time for various wind speeds and directions. We then plot the results. In the base case the rider's hands are on the top of the bars.

    The analysis uses private models similar to the ones available at the Wind on Rider topic at www.AnalyticCycling.com

    Many people have improved their performance by using rough estimates to select amongst alternatives. However when estimating the benefit from small changes to equipment or position like the ones shown here, these rough estimates are not good enough. When we try to measure these small differences on the road or in a velodrome, small gusts of wind or small variations in slope or small changes in position can completely mask the effect being measured. This is the reason wind tunnel testing is necessary.

    Also, as can be seen from the plots, the effects are dependent on the wind direction and speed. We used a flat point-to-point course into a headwind here. However, in most competitive situations course are more complicated. If one wants to estimate the effect on performance over such courses one has to integrate all the dynamic forces on the rider over the complete path.

    A rider can reduce drag and cover the distance faster or a rider can increase power and cover the distance faster. The plot below shows, for the rider reported here, how much faster the rider would cover the distance if the rider had more power. You can use this plot in conjunction with others reported here to compare drag reductions to equivalent increases in power.

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    The base case uses an Air Density of 1.226 kg/m^2, a Coefficient of Rolling Resistance of 0.005, Power of 185 watts, and mass of (160 + 22)/2.2 kg for bike and rider.

    The results from the model for various wind speeds and directions are plotted to the right and estimates of time to complete the 10k distance at Wind Speed = 0 and Wind Direction = 0 are listed below:


    Seconds 1209.66
    Min:sec 20:09.66
    m/s 8.26679
    mph 18.4349
    In the absence of wind tunnel measurements, it's common to estimate effect of wind speed and direction on the drag on a rider and bike by approximating the rider and bike as a cylinder. In such approach, the area and coefficient of drag are treated as constants as wind speed and direction change. Brandt uses this approach in his paper ["Headwinds, Crosswinds, and Tailwinds, a Practical Analysis of Aerodynamic Drag", Jobst Brandt, Bike Tech, pp4-6, Aug 1988]. Wind tunnel measurements show that the area and coefficient of drag do depend on the direction and speed of the wind. A comparison plot of the differences between the estimated times for these two approaches is plotted at top right.

    The bottom plot shows this as a percentage. One can see that the two approaches are consistent well within 3% of each other. In the absence of wind tunnel data it is reasonable to use the approach in Brandt's paper as an estimate. However, if you want to improve the analysis, wind tunnel data is necessary along with models that can model the details properly.


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    The top plot compares riding on the brake hoods with the jersey zipper open vs. the base case (hands on top of bars). The bottom plot is with the hands on the brake hoods and the jersey zipper closed. Keep the zipper closed.

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    Plot of riding on the drops vs riding on the tops of the bars. There is less drag riding on the drops.

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    Aero bars vs. riding on tops. From Top to bottom, aero bars with elbows wide and high, med, and low and aero bars with elbows narrow and high, med, and low. These results are highly dependent on the rider and bike. One should not draw any general conclusions unless one is looking at one's own wind tunnel data. Cobb commented that one must do the measurement; setting up a rider so that "the rider looks right" does not give good results.

    High, Med, and Low correspond to about 2 cm increments in height.

    Aero Bars High




    Aero Bars Low
    Aero Bars, Elbows Wide
    (High, Med, Low)




    Aero Bars, Elbows Narrow
    (High, Med, Low)





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    Where should you carry your drink bottle? In the following cases, the base case is the rider riding on the brake hoods. It appears that a drink bottle provides an advantage particularly with strong side winds. One explanation is that the drink bottle provides a "sail effect" that acts to reduce drag.

    Conclusions:
  • Carrying a drink bottle is better than not.
  • If you carry two drink bottles, keep them on the frame.
  • Two drink bottles, one on down tube and one one seat tube of frame.
    One drink bottle on seat tube of frame.
    One drink bottle on down tube of frame.
    Two drink bottles behind seat.
    Plot of difference between two drink bottles on frame and two drink bottles behind seat. The case of the two drink bottles on the frames is better than two drink bottles behind the seat under windy conditions.

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    If a rider needs to wear a drink supply strapped to his back, should it be worn over or under the jersey? Wear it under the jersey or better yet mount drink bottles on the frame.

    Plot of Aeropak worn over jersey vs riding on the hoods.
    Plot of Aeropak worn under jersey vs riding on the hoods.
    Plot of the difference between riding with an Aeropak over or under the jersey. These is an advantage to wearing it under the jersey.
    Plot of the difference between riding with an Aeropak under the jersey or two drink bottles on the frame. The advantage goes to two drink bottles on the frame. There is more difference in a strong headwind and much less difference in strong side winds.

    Copyright © 2000 Tom Compton All rights reserved.