Posts Tagged ‘Coefficient of Aerodynamic Drag



A poor head-on shot of my TT position.

The world of Time Trials is really, really complex. In many ways, it’s “The Race of Truth”, but in many ways, it’s also a race of technology. The pursuit of an aerodynamic edge, be it wheels, frames, helmets, or skinsuits, really can mean the difference between hitting the podium, and missing it.

This weekend, I raced an awesome looped course, complete with rolling terrain, different types of pavement, and an increasing wind. It was incredibly fun and challenging, but when the results were posted, I had missed the money by ONE second, and I had missed the win in my category by just about a minute. Let’s play around with some numbers here and there, look at wattage values, and see if we can figure out a way for me to increase my speed and reduce my time, just by using technology.

Over the years, I’ve tried to use technology to influence my positioning on the Time Trial bike, and also to influence my purchasing decisions, so everything tends to be tilted towards that which will produce the lowest amount of drag, while still allowing me to generate adequate power to the pedals. For this event: here’s my equipment list:

  • Aluminum Cervelo P3.
  • Aerobars
  • Oval TT fork
  • Nimble Trispoke up front.
  • Bontrager Aeolus Disc Wheel in rear (2007 model?)
  • TIGHT skinsuit
  • Louis Garneau aero helmet.
  • Pearl Izumi Booties

I opted to not bring a water bottle, though honestly, I should have considered wearing my camelback with a couple of ice cubes inside, but I don’t think my performance suffered from dehydration. I drank adequately prior to the effort, knowing I would lose fluids through perspiration and respiration from the stress and the environmental conditions.

Using Golden Cheetah’s experimental “AeroLab” and also a tried-and-true program from “”, I took a section of road that was smooth and steady in its’ slope, where I knew I was in my aero position as consistently as I could remember, and I tried to determine my Coefficient of Aerodynamics, or CdA.

You try to get the lines to mesh as much as possible.

For the second image, take Frontal Area and multiply it by the Coefficient of Wind Drag. Both numbers come up close to a .265m^2, which is better known as the “HOLE” you cut through the air when you’re in a certain position. Remember, I lost 3rd place by 1 second, and I lost the victory by one minute exactly. The difference then, between 4th, which I got, and first, which we want, is about 1.8%. Thus the title of this post – a 2% improvement in my time would have earned me the win. I may have been able to pedal faster, but honestly, from what I know about aerodynamics, my .265m^2 is probably a little high. I’d like to see if I can lower that CdA down to a .25 or a .24 without losing power, and see what that would achieve.

Here are the results of some Analytic Cycling calculations. Notice the DROP in watts required to travel at the same speed. We’ll go back to my original power average on the last image….

Wattage required at .25m^2

Wattage Required at .24m^2

The savings on watts at the same speed, 10.2 meters per second, goes from 252w to 243w to 237w, or a savings of 3.5% and then 2.4%, or a total of 6% decrease in the amount of power required to hold that speed. So, you’re doing less work, using less energy, to get down the road at the same speed. Now, let’s show the final chart, and reveal just what speed I would have held on that section, had I been able to hold a .24m^2 aero position, and still generate 250w…

250w at a CdA of .24m^2

My speed goes up from 10.2m/sec to 10.44 m/s, or….


Now, this is never a perfect science, but let’s just say that I was able to mostly hold that position, stay at a perfect .24m^2 CdA, and generate 250w, which is about what I pushed on Sunday.

I averaged 25.3 miles per hour, or 11.3 m/s.

a 2% improvement would have yielded an average speed of 25.8mph, or 11.53m/s.

Covering the distance of the TT route, a 2% improvement would have yielded a 55:07, which would have put me 2 seconds out of 2nd place. Raising my power to 252w would have put me down in the 54 minute range, which would have led to a a win in my category.

So what’s the moral of the story? Well, as much as I love power, let’s face it – time trials are almost always won by mere seconds. Never let up, push as hard as you can as long as you can, but remember the little stuff that can, and does, make a difference.

Now – if I can just find a way to shed that drag, ever so slightly! Stay tuned!!!


An interesting experiment in Time Trial ‘what ifs?’

A Cluttered look at Frontal Surface Area.

A Cluttered look at Frontal Surface Area.

Another cluttered look at aerodynamic profiling

Another cluttered look at aerodynamic profiling

Well, I need to start by summarizing what the last two weeks have been like. I returned to Dallas on a Monday, and when I got off the plane in Dallas, the heat and humidity hit me like a moist brick. Welcome to summer! I was supposed to race down in Fredericksburg, TX the last weekend of June, but honestly, I was pretty exhausted, and I ended up basically just using my commuter bike to ride around town, and even then, not much. Sleep came and went, a checkup with my doctor came and went, it continued to get hotter, and the ozone levels started to peak in the orange and red areas.

But this weekend, the Tour de France began, and my fast-bike juices started roiling again. The Stage 1 TT was held in Monaco, and we sat in front of the TV for about 3+ hours as we watched Lance set the early fastest time, eclipsed by Levi and a Columbia rider, and then finally by some of the other GC favorites, including two other Astana riders. I’d been asked about participating in the Texas State Time Trial Championships in August, and I still hadn’t quite gotten my TT saddle adequately broken in or adjusted right, so I decided right then and there that I would drive over to the Texas Motor Speedway on Sunday morning and go do a 40K TT on my P3.

Now, all of this was supposed to happen at around 5am, but when the buzzer went off, my hesitancy was rewarded with the sound of RAIN falling outside! YES! So I rolled over and went back to sleep until well after 8am, when the Tour stage came on. That provided another excuse to delay, and I didn’t get out until after 1 o’clock.

The good news is that the rain had kept the temperatures around 82 degrees fahrenheit. The bad news was that there were scattered showers all over the place, and I got rained on lightly as I drove out there.

Now, laugh all you want, but when I do these workouts, I do them with all my kit on. That means aero helmet, skinsuit, booties, the works. I do this because A) I don’t get to mess around with my expensive stuff much, and B) I like to truly simulate what it’s going to take in terms of heat and suffering in a lid with tiny vents, and a zipper pulled up to the neck, to finish the damned 40K in the most aero position possible. Every second counts, right?

So I programmed the Garmin to advance laps every 8K, then put labels on my aero wings to try and come up with a good time-based pacing strategy. I was going to let wattage take care of itself. The Quarq was zeroed and calibrated perfectly, the Garmin was accurate to within 2 meters, and I started from a track stand.

Lap 1 was supposed to be a 12:20 – a slow way to work yourself up from negative splits/positive watts, but I ended up blowing through the damned thing at an 11:38. That proved to be the fastest lap of the ride, with the others coming in, albeit consistently, at 11:55, 11:51, 12:02, and 11:54. Negative splits? Hah! I was lucky to have broken the hour! My power was low, averaging 232 PNorm, as was my cadence (80 rpm), and my freakin’ HR was at 180-190 bpm the whole time! NUTS! What happened?

Tail between my legs, and about to pass out in the humidity (the rain fell just enough to make humidity soar beyond 80%, not enough to actually accomplish anything in terms of cooling me down), I sat in the car with the A/C on, and finally drove home, only to hear that Andy Roddick, still playing the best game in the history of Tennis, had made one small mistake and lost the Wimbledon Finals to Roger Federer-er. Fortunately, I hadn’t been racing. Otherwise, I would’ve been last place in the 3’s probably. Ugh. I finally made it back home to look at the numbers and start figuring out what happened, and then also try to figure out ‘what if’?

The good news is the consistency. The average lap was an 11:52, with a +- of -13, +10, and the wattage numbers were really interesting, going in this order:

1) 246 – 11:39

2) 232 – 11:56

3) 230 – 11:51

4) 224 – 12:02

5) 228 – 11:54

So, depending on how you look at it, and how consistent I was in terms of standing or staying in the aero bars, 4 watts between laps 4 and 5 bought me 8 seconds. Yet I must’ve stood longer on the 5th kilometer of lap 2, because while I was 2 watts stronger on Lap 2, I was 5 seconds slower than on lap 3. Lap 4, I know exactly what happened – I mentally quit for about 10 seconds, in an effort to try and force my heart rate down. I know, I know, I’m not supposed to use HR, but it IS effective, especially in situations just like this, when the heat and humidity combine to provide stressors that you’re not accustomed to. Want to adapt to heat and humidity? Ride in it!

I also played around with some values in, in an effort to figure out how aero I was with that kit on, and in those conditions. After tooling around with it quite a bit, I came up with this:

CdA estimate for July 5 2009 232w

If you take the 232w I averaged, the distance traveled, and the time required to travel that distance, then add in air density (taken from Alliance Airport’s weather station), and temperature, you get a pretty good idea of just what your Frontal Surface Area is. In this case, for me, it was 0.47m^2. When you multiply it by 0.5 (don’t ask me why), you come up with CdA, which in this case reads .235. From experience trolling the bike geek forums, I think this is a pretty low number. It may actually be TOO low, in that it may be affecting my overall ability to generate such power, though I’m hoping that this is not the case, and let me explain that next.

Let’s play ‘what if’. What if I had been able to generate 250w. I’ve done it out there before, and finished with a faster time, though never with a value as accurate as it was with the GPS. 232w yielded 11.22m/sec. 250w would have yielded 11.54m/sec, a gain of 2.77%.

18 watts yields a 2.77% increase in speed...

Then, as we edge up to 260w, you get an average speed of 11.7m/sec, a gain of 1.3675%…

260w yields a smaller gain over 250w...

Finally, if we move up to 267w, which is supposedly close to the FTP I would have had in mid-June at sea level, given the fact that I was at a theoretical FTP of 297/60 in a road racing position, and you’re theoretically supposed to drop your FTP by about 10% when you move in to the aero position (changes in hip angles, compressed lungs, etc.), you come up with 267. So, it’s a theoretical number based on two separate guesses, but they’re sort of standardized, so yeah, let’s see what 267 yields…

267w over 40k would yield...

11.82m/s, or 1.0152% in delta over 260w.

SO —-

An almost linear increase in wattage, would yield a diminishing curve of speed improvements. Granted – the improvements would still have led to a 40k time of 56:24.1 (a new PR by well over a minute), but this exercise is meant to show just how incredibly hard it is to get time gains just by pedaling harder.

Now, let’s throw one or two more wrenches in the gearing.

Once again, don’t laugh, but this morning I actually rode with my Dinotte lighting 200’s on, front and rear. One mounted on the left wing, one under the saddle. This was for safety at 5:30am. However, I didn’t remove them when I rode at 1pm. That, plus the fact that I moved back to an aluminum seatpost on the aluminum P3, made the thing weigh in at about 22lbs. Let’s say I rode the thing ‘clean’, sans lights, and batteries. Let’s remove 1 kilo from the 232w equation, and 1cm^2 from the surface area…

What happens when you drop 1 kilo and 1cm^2 from your bike and FSA.

11.31m/s… a gain of 0.8%, which would’ve yielded a 40K of 58:52.9 seconds.


30 seconds cut just by removing ‘that much’ drag. WOW.

Let’s cut the weight a bit more – I’ll switch back to a carbon seatpost… .5 kilos.

Uh oh. The benefit? ZERO. Nothing. Still 11.31m/s.

Let’s now get radical. Say I hit the lottery and can afford to buy a P3 Carbon… We’ll lose another 2 kilos…

What happens when you drop another 2 kilos

A measly .02 meters/second gain. BUT WAIT – there’s supposed to be some aerodynamic advantage to the P3!! Maybe we drop the Frontal Surface Area by another 2cm^2?

What happens when you switch to an even more aero frame.

KABOOM! A MONDO gain in speed! 11.45m/s! COOL! And all of this, at 232w!!!!

So what’s the moral here? I think the moral is that while I love wattage, love studying it, applying it, and finding ways to get more of it, aerodynamics is a VERY, CLOSE, COUSIN.

So don’t be afraid to invest in aerodynamic improvements. To achieve a sub-60 40k for me, at just 232w, is actually pretty freakin’ cool. Discs help, Aero front wheels help, narrow tires help, aero helmets help (a ton), and ALL of this combines to yield better speed for less energy. I’m still going to crush my nether regions in August and a lot next year in an effort to gain more power in the aero position, but as of right now, I’m saving up for a TT vunderbike to replace my P3. Who knows, it may even be ANOTHER P3, but as of Saturday, and maybe even when I first saw one at the Baker City race in Oregon, I’ve got total silly pants bike lust for a Ridley.

If my wife reads this, she’s absolutely going to KILL me.

Thanks for re

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