Drag Data Digest
Wouldn’t it be nice if wind tunnel drag charts always created a clear, distinct ranking? That isn’t the case in many tests (this one included), so we calculated wind conditions to estimate “real life” in a triathlon to decipher which wheelset could be fastest based on our test results.
Wind speed impacts the wind angle experienced by the rider, which is a product of both rider speed and wind velocity, so we looked at weather data to estimate typical wind speeds experienced in races across the country. We found that 8.1 mph was the average wind speed based on data collected in 49 major cities from Seattle to Miami. Ideally, an athlete would measure real wind speed on each individual race course and run a calculation to find the drag he expects to face during a race, then pick accordingly from his stockpile of race wheels. This is, of course, not practical for most athletes, so we used this approximation.
Yaw angle also changes with rider speed (faster rider, shallower yaw; faster wind, wider yaw), so we calculated the fraction of time a rider would spend in various yaw angle ranges when riding at both 20 and 25 mph to estimate total drag difference among the five wheelsets that a cyclist averaging 22.5 mph would experience in a typical ride:
Grams of drag predicted
Mad Fiber Clincher: 796 grams
Enve SES 6.7: 766 grams
Hed Jet 6/9: 766 grams
Bontrager Aeolus D3 7: 757 grams
Zipp 404/808: 757 grams
What do these numbers add up to on the race course? Faster’s wind tunnel engineer Jay White uses a mathematical calculation to equate 100 grams of drag to 1 second per kilometer, regardless of rider speed. That equation creates the following time differences over the 112-mile Ironman bike leg:
Estimated time saved
Mad Fiber Clincher: 0
Enve SES 6.7: 54 seconds
Hed Jet 6/9: 54 seconds
Bontrager Aeolus D3 7: 70 seconds
Zipp 404/808: 70 seconds
Based on the aero data, faster riders might lean toward the Zipp wheels because they out-performed the others at shallow yaw angles, while slower cyclists could look at the Bontrager wheels for a little extra speed. Those were the top performers at broad yaw angles typically experienced by riders averaging lower speeds. Visit Insidetriathlon.com/dragdata to see all the test data and calculations used for this estimation.