We always hear a lot about watts per kilogram when it comes to cycling, but what if I told you that isn’t the most important thing sometimes, dare I say even most of the time. You see, watts per kilogram is crucial for the ultra mountainous events since the lighter and stronger a rider is, the faster they can scurry up a climb, but for the majority of cycling events, how much power you can produce in relation to how aerodynamic you can get is what really matters, aka Watts/CdA. The weight of the rider is important here too of course as more muscle usually = more power, but more fat never = more speed! Before we delve into the sciencey stuff though, how can we define Watts/CdA?
Watts = Power being produced by a rider. Cd = The coefficient of drag (wind). A = How much frontal area a rider projects. Watts/CdA = Power being produced by a rider divided by the coefficient of drag multiplied by the frontal area of said rider. In essence, how good you are at turning your hard work into forward speed! The lower the CdA the better.
You have really 2 options of measuring CdA: 1) Purchase time at a wind tunnel 2) Use software like Golden Cheetah, or an online calculator like this one from Cycling Power Lab’s website. You will need a power meter for the most accurate CdA value. You can also check out this great article from our friend, Dean Phillips of FitWerx, on field testing and figuring out CdA using the “Chung Method”.
Decreasing and Optimizing CdA
So now that you understand how to find out what your CdA is, how can you lower it? Well, the greatest return on your investment is achieving a more aerodynamic position on your bike while being able to maintain your wattage output. The latter half is crucial because you can get as aerodynamic and reduce your frontal area all you like, but if you can’t produce the power necessary to have it be advantageous, why do it? This is why frequent field testing is important. Figure out what worked well, what didn’t do much, and learn to optimize your position accordingly. Then once you have that figured out you can play around with other aspects of your cycling garb like your helmet, shoes, kit, etc. Just don’t be THAT guy at your next event that shows up on a $10,000 bike and can’t stay in his aero tuck for more than 2 minutes 😉
To give you a little better idea of where to spend your hard earned dough:
Typical CdA Values
- Riding on tops = >.4 CdA
- Riding on hoods = .32 CdA
- Riding on drops = .30 CdA
- Riding on aero bars (clip on) = .29 CdA
- Riding on aero bars (optimized) = .26 CdA
- Road Bike, Road Helmet, Drops = .30 CdA
- TT Bike, Road helmet, Aerobars = .24 CdA
- Road Bike, TT Helmet, Aerobars = .25 CdA
- TT Bike, TT Helmet, Aerobars = .23 CdA
Ridiculous Values for the Hour Record:
- Merckx = .26 CdA @380w
- Moser = .25 CdA @400w
- Obree = .17 CdA @359w
- Indurain = .24 CdA @510w (holy cow!)
- Rominger = .19 CdA @456w
- Boardman = .18 CdA @462w
Imagine if Big Mig could get just a little more aero? He would probably still have the record with that monster wattage!
So, what are watts/CdA? A measure of how effective you are at transferring the power you create into forward motion. You can decrease this value by optimizing your position and purchasing more aero kit, but be sure to perform frequent field testing to ensure you aren’t sacrificing overall speed for drag reduction.
Lindsey Underwood’s thesis regarding Aerodynamics of Track Cycling
Scientific approach to the 1-h cycling world record: a case study
(1) High Performance Cycling (Jeukendrup, 2002)
(2) Scientific approach to the 1-h cycling world record: a case study (Padilla et al, 2000)
(3) “How Aero Is Aero” (2008)