### Matching Aero Wheel Design to Wind Conditions

posted Feb 24, 2010, 2:39 AM by Donald Vescio   [ updated Sep 12, 2011, 8:10 AM ]
 Matching Aero Wheel Design to Wind Conditions  It used to be that a rider would simply purchase a set of wheels with a relatively low spoke count for time trials and triathlons. Today, wheel technology has advanced significantly, and it's possible to match an aero wheel design to the wind conditions on the course to achieve optimal performance.  There are two terms that you need to know: wind direction; and apparent wind direction, which also is known as yaw.  Wind direction is pretty simple: it's the direction from which the wind originates, which also is the same direction that the wind is felt by a rider when standing still.  When a rider is in motion, however, the direction from which the wind appears to originate will vary, depending on his or her speed.  In simplest terms, faster riders experience smaller yaw angles than slower riders.  You don't have to be a meteorologist of expert in mathematics to determine the likely apparent wind angle for race day, which can factor into your wheel selection.  Steve Hed, one of the pioneers of bicycle aerodynamics, has an easy to use online calculator that you can use to make sure that you've taken into account environmental variables as you prepare for your important race.  Just visit: http://www.hedcycling.com/aerodynamics_technology/yaw_calculator.asp  Examples:  Let's take an average, middle of the pack rider who anticipates that he will ride at an average speed of 15 miles per hour.  Looking carefully at the weather records of the race venue, the rider anticipates that the likely wind during race day will be from the west at 15 miles per hour--it's a very windy course.  Plugging these values into Hed's apparent wind calculator yields the following results:   The wind angle on the rider in the example above is 45 degrees headwind.  Now, take a second rider who tends to win her age group  on a regular basis; she anticipates that her average speed will be 30 miles per hour.  Entering her values yields the following results:   For this rider, the apparent wind angle while she is powering north will be approximately 27 degrees, which will feel much more like a head wind for her than the strong crosswind experience by the rider in the first example. What's important is that the anticipated apparent wind angle can be used to help each rider select the best wheels for the event.  For the rider in example one, a wheel that performs in high apparent wind angles would be a good choice; for the second rider, a wheel that performs better in low yaw angles might be a better choice.  What wheel, then, should each rider use?  Steve Hed also makes this determination easy through the use of his wheel drag calculator:  http://www.hedcycling.com/aerodynamics_technology/ The wheel drag calculator enables a direct comparison of wheels in varying yaw angles.  In the example below, three wheels are compared: a traditional spoked wheel; a Hed trispoke wheel; and a Zipp disk wheel.   The traditional spoked wheel is represented by the top line.  As compared to a modern aerodynamic wheel, it isn't very effective at any wind speed and it represents at least twice the potential aerodynamic drag as the other two wheels in this example.  The green line represents a Zipp disk; at an apparent wind angle of 5 degrees, there is no difference between it and the Hed trispoke in terms of aerodynamic drag; the same holds true for a yaw angle of approximately 17 degreed.  In between 5 and 17 degrees, a Zipp disk will out perform a Hed wheel.  But in yaw angles greater than 17 degrees, the performance of the Hed trispoke continues to improves, and even surpassed that of the Zipp disk.  Similarly, at very low yaw angles (0 to 5 degrees--that experienced by extremely fast riders) a Hed Trispoke might proved to be the best choice.  What does all of this mean?  Slower riders most often experience high apparent wind angles, which means that they should select a wheel that performs well in high yaw angles, such as a Hed trispoke; very fast riders tend to experience very low yaw angles, which means that pretty much any modern aerodynamic wheel will yield excellent performance.  Using Steve Hed's two simple tools enables riders of any ability to select wheels that will be the best match for them and the wind conditions that they face.