Aerodynamics- What Is It Worth To You?
by Shayne Kondor
A recent ad for a hydration system caught my eye; the ad claimed that this hydration system was developed in a wind tunnel to be the most aerodynamic option available. As an experimental aerodynamicist, this piqued my interest; could the claim be true? Could I shave some time from my bike split with a new water bottle? When I saw that the price of this bottle was about the same as a new set of cycling shoes, I became far more skeptical; not about their aerodynamic claims, but skeptical of where my bike budget would be best spent on equipment for next season.
The inspiration for this idea came from wind tunnel drag measurements that I made on two cyclists of identical weight, 188 lbs. Both were tested on time trial/triathlon bikes with state-of-the art aerobars and deep section racing wheels. Cyclist #1 was fitted on a 53 cm frame while Cyclist #2 was fitted on a 58 cm frame. Now the difference in the two test subjects may start to become obvious, if they are the same weight then they must have different builds to be riding on such different size frames. There was big difference in Body Mass Index (BMI) between the two test subjects. Cyclist #1 had a stocky build with a BMI of about 27, while Cyclist #2 was taller and proportionally leaner with a BMI of about 24. Wind tunnel drag measurements showed that Cyclist #2, with the lower BMI, generated about 10% less wind resistance at the same speed, despite being on a bigger bike.
Since their bikes were nearly identical, except for frame size, could BMI be a significant aerodynamic variable? Wind tunnel tests by other aerodynamicists have found that the cyclist's body, alone, is responsible for about 2/3 of their total wind resistance. Logically, the item that generates the most wind resistance would be the first place to start improving aerodynamics. To get an idea of how much time can be shaved by a lower BMI, let's put Cyclist #1 and #2 in a hypothetical 40K time trial on a flat course with no wind. Assume that both cyclists can generate the same power at the rear wheel. Next, to make comparison easy, let the stockier cyclist (#1) finish the race in exactly 60 minutes, averaging 24.9 m.p.h. Some quick math (which I'll spare the reader) shows that Cyclist #2 could average 25.7 m.p.h., for the same effort! Lower aerodynamic drag lets the leaner cyclist finish 2 minutes ahead at the end of 40K.
Dropping a few unnecessary pounds is where aerodynamic improvement should start. A leaner body presents a lower projected area to the wind in any position on the bike; thus, generates less wind resistance all around. Now lets say you've slimmed down to a 20 BMI, how can equipment further cut wind resistance? Back in the 1980's, Chester Kyle conducted wind tunnel tests on time trial cyclists to determine how nearly every conceivable aerodynamic tweak affected performance. His data showed that most exotic equipment such as skin suits, aero profile bakes, aero water bottles, etc. only saved a few seconds over 40K. The biggest savings came from aero profile helmets and aero wheel sets, shaving 30-60 seconds from a 40K time, but these were very expensive seconds. Conversely, just addressing body composition will shave minutes from a 40K time, without having to buy a thing. While it could be argued that aero wheels have come a long way in 20 years, shaving some inches off your waist can have equal impact on your bike splits (and surely cost a lot less!).
Shayne Kondor is an experimental aerodynamicist at Georgia Tech Research Institute in