IM Barcelona CdA analysis.

Due to the variety of nationalities and number of triathletes that raced this past weekend in Ironman Barcelona at Calella (Spain), I have decided to write my first post in this blog in English.

The aim of this post is to give my opinion, as usual with numbers, about what happened there: one of the most important frauds in the history of our sport, at least in Spain.

 

Some time ago I wrote an explanation of the physics and the equations that described how power transferred to the bike is employed and shared between: aerodynamic resistance power, rolling resistance power, climbing power and friction resistance power ( Aerodinámica en la bicicleta). It is written in Spanish but as math’s language is international and there are lots of blogs and websites describing this, I will not talk more about this.

However, I will use the equations explained in that post to show if the vast majority of bike splits (and as consequence, also the running splits) are reliable.

I will use some data from the longest Strava segment of the race I have found: Strava segment 5727036 , and I will assume the next hypothesis:

  •  0% climbing: 29 meters in 71 km can be assumed as completely flat, so no climbing power is need,
  • no friction resistance: it can be usually neglected, and moreover, as in this case I will compare different performances.
  • no acceleration or inertial power: There are no curves, no braking, only big roundabouts, and it is a non-drafting race, so no big accelerations would be expected.
  • no wind. In this case, we can assume this as the road is a straight line, 180 degrees roundabout, and again the same straight line. So in the case there was any wind, it would be compensated during the whole lap.

So, in this case the Power applied for each cyclist is considered employed only in aerodynamic resistance and rolling resistance, that is:

P= Paero+ Pfriction = 0.5*r*v3*CdA + M*g*crr*v

These simplifications are penalizing CdA, as the neglected power (friction, acceleration and climbing) are being accumulated in Paero, and as consequence, in CdA, so values obtained will be higher than those that can be found in some other articles. For people not familiar with CdA values, here you have some examples: http://www.cyclingpowerlab.com/cyclingaerodynamics.aspx

Therefore, if we consider:

r= 1.2

M= 75 ( triathlete+ bike) (conservatively low, as a bigger mass would derive in a even lower CdA)

Crr=0.004,

We can obtain the next CdA for several combinations of applied power and speed obtained.

Anyway, from my experience I think I can say a 0.27 is a quite good CdA, and values below that are difficult to achieve and maintain during 180 km, and moreover, difficult to run consistenly 42 km after more than 4 hours in that position.

We have two examples from this race that we now were completely legal:

Joe Skipper (PRO) (data from Strava): 40.4 km/h, 304w. I assume 86 kg, so CdA would be 0.314.

Michael Alonso-McKernan:(ironman barcelona drafting.html): 178 km ,85 (75+10) kg, 38.7 km/h, 251 w. CdA= 0.289.

Both Joe and Michael have with good time trial position, good material and they are not big men. So we can have an idea how difficult to achieve a CdA= 0.26 is… Now, just take a look to the Stratospheric speeds at the  Strava segment , then look to the power column, and the go to this post table and check in how many cases the CdA obtained is reliable. I have marked in yellow the suspicious ones, and in red the one that are more than suspicious.

It is a shame that we would never be able to check the power files of all these splits that then, could run “fresh”. But well, at least, in addition to the videos and photos, we some physic base to decide if we believe or not some amazing performances. Some of them can seay that they were in legal groups, and that save some watts… Well, it is already measured in wind tunnel that the improvement can be between 5 and 12 w, that can be translated to 0.009-0.020 in CdA. Not more.

Sadly, I think this is a lost battle as everybody in Calella think that it was a great success: Ironman company earned money and “athletes” achieved their (nosense) target time record (404 sub10h) and probably next year we will have exactly the same scenario. And let’s see what happens tomorrow in Kona. Bad month for the Long Distance Triathlon.

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