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Wind on Rider |
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question often heard is, "What is the effect of wind on performance?" The wind blows on a rider on a road course. The wind can come from any direction and be of any speed.
Two Input Forms are provided. One has menus of choices for courses and wheels. The other lets you enter raw data. Response times depend on the complexity of the fundamental integration. Complex courses with lots of turns (Sydney) or long courses with lots of turns or lots of apparent wind changes (Ironman Hawaii) require long compute times. The Est Min Ave Speed parameter controls how long the computation runs (try 8 m/s for the default data on the Ironman Hawaii Course). This parameter should be set as close as possible but not greater than the speed you expect from the slowest rider. Responses for the default data take about 5 to 10 seconds. A one-lap evaluation of the Sydney 2000 TT course takes about 30 to 45 seconds. An evaluation of the Ironman Hawaii Course takes about a minute and a half or more. The topic will timeout before reaching the end of a full three laps of the Sydney course (the full TT distance) and is likely to time-out before reaching the end of the Ironman Hawaii Course if conditions are exceptionally complex. Don't be surprised and do be patient. Contact us privately if you have a special interest in longer calculations. Since the input form is detailed and takes a lot of screen space, this topic is best viewed on a high resolution monitor. The Model uses the Estimated Speed parameter to make a guess in seconds (distance divided by estimated speed) as to how long to run. If you are computing a slow, uphill effort, you may get an error, something like "Interpolation function is out of range". If this is the case, set the Estimated Speed to something slower. If you expect your model rider to ride fast, set the Estimated Speed to something faster. For example, an evaluation of the Sydney 2000 TT course takes about 30 seconds with default estimated speed and takes about 20 seconds with an 8 m/s Estimated Speed. Top Questions This Model Can Answer  Using the Advanced Input Form, reset it and enter a value for wind variability, say 2, for each course segment in the Course and Wind input table, check the Plot Wind Data box, and evaluate several times. One series of evaluations produced the following list of time differences (seconds): {7.37, 9.82, 9.11, 9.81} Notice from the plot of wind speed how it changes over the course and with each evaluation.Top Course and Wind You can select a course from the menu or use the Advanced Input Form to enter your own data. The course for the model is described by a series of course segments. Each segment has a distance, direction, slope, coefficient of rolling resistance, prevailing wind direction, wind speed, and wind variability. The direction of the segment is the compass direction in which the rider is heading. The wind direction is the compass direction from which the wind is blowing. The wind variability is an amount by which the wind speed increases or decreases randomly over time. If the "Course Closed" box is checked, the course is expected to close both in distance, direction and elevation. In the case of a course selected from the menu, the wind speed, direction, and variability is the same for all course segments and is specified on the Input Form. The model incorporates wind into the differential equations using the approach explained by Jobst Brandt in his paper, "Headwinds, Crosswinds, and Tailwinds, a Practical Analysis of Aerodynamic Drag", Jobst Brandt, Bike Tech, pp4-6, Aug 1988. Course Notes  data was taken from a map available at Official Site of the Sydney 2000 Olympic Games. Please recognize that the configuration of the course with its many turns makes an exact model difficult to do from an image on a web site. In the image at the right, the black arrows are represent the course directions and distances and the red arrows represent the prevailing wind. A wind speed of 10 m/s and direction of 315 degrees are a best-guess of typical conditions.Top Drag on Wheels Wheel drag is often quoted as force in pounds or force in "grams" at some specified speed, often 30 mph. Greenwell, et al, quote drag coefficients. You can enter drag in various units in the Advanced Input Form and the conversions will be done automatically. Or you Can use the Menu Input From. Menu Input Form—The default values in the Menu Input Form are drag coefficients taken from Greenwell's paper. The Standard rider uses coefficient-of-drag values for conventional 36-spoke wheels as default values. The Test Rider uses coefficient-of-drag values for a Specialized (Now Hed 3) in front and a disk in the rear as default values. The following tables list the wheel drag values on which data in the Menu Input Form is based. Conversions to consistent units are automatic.
Comments on Wheel Drag Data—If you compare results from Greenwell's data on a Specialized wheel with Cobb's data on a Hed 3 (both wheels are identical, the name is the only change), you will find a difference in performance. Cobb's data covers 15 degrees of yaw angle; Greenwell's, 65 degrees. Both tested wheels in fixtures in wind tunnels. Both are credible sources. The shapes of the drag functions are consistent and the magnitudes are consistent. This gives you some sense of the precision and reproducibility of the data. Advanced Input Form—You may enter values for drag on wheels in the Advanced Input Form as values of force due to drag or drag coefficients. The model reads the values for the drag for the yaw angles and converts drag in units of force to a coefficient of drag if necessary. It then fits a continuous function through these points. The yaw angle values must range from 0 to 90 and must be strictly increasing. There must be at least four data points (four yaw angles). A whole line of data can be left blank, but if a line has any data, no blank data elements are allowed. You can get drag coefficients or drag forces from the web sites of various wheel manufacturers. However, most that do list data are not complete and seldom offer any basis for the data. An explanation of wheel parameters is given in Wheels Concepts. Wheel Weight & Inertia—Although discussed a lot in the popular forums, the wheel weight and rotational inertia do not make much difference in the outcome for riders on courses such as these. However, if you want to see the effect, select the Generic Point-To-Point course and run some cases with both riders using the same wheels only with different weights and rotational inertias. Check the "Plot Rider Speed Data" box. The last plot, "Speed Difference Between Riders vs Time" shows the difference in speeds of the riders. The large peak during the acceleration at the start is all the difference there is.  Top Model The model uses the standard equations for forces on a rider. The model sums the forces, writes the differential equations for the riders and solves them for distance, velocity, and acceleration. This concept is explained in the Glossary. Top How do you measure these things? One way to measure distance is by riding a course with a bicycle equipped with an ordinary bicycle computer and noting distances at points where direction changes. Measure the slope using a machinist's bubble protractor on the top tube of the bike. Note the zero point when the bike is on a known level surface. Turn the bike in several directions to verify the levelness of the surface. Take these measurement while stopped. Taking them at several places on the same hill will give some sense of the precision of the measurement. Measure the direction of the course by using a compass. Measure the direction of the wind by using a compass. Measure the wind speed by using a hand-held anemometer or use data from the local weather report. Make a guess at the variation in wind speed. Note that valley's and such commonly have wind blowing from different directions than the prevailing wind. The data for the Sydney 2000 TT course was taken from a map provided by the Organizing Committee. It looks reasonable, but it did require some "tweaking" go get the course to close. As courses go, it has to be one of the more complicated ones to describe. |
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Ironman Hawaii Course