CdA test Part I

With the first half of the season finished I throught about some goals for the second half. One of the goals is to ride a TT.  Not just a 40km TT (or shorter), but a 'monster TT'. Basically it is a TT of 123km. The TT is end of September. I have selected this goal not in terms of trying to win (because there is nooo way that will happen), but merely for the experience.

I have never done a TT and I don't have a TT bike. I do have some gear: a Castelli San Remo speedsuit, some shoecovers and Mavic Cosmic SR wheels.  I don't want to invest too much money in this 'small experiment / experience. After reading some articles on TT gear I conclude I can get most bang per euro by buying some aero clip-ons. And that's what I did: the 3t Aero clip-om pro.

Why do I bother about aero? Because with generating speed it's about the fight of overcoming air resistance which makes 90% of the effort of moving your bike forward. You might have a very big engine in terms of being able to produce a lot of watts, but you are not very aero, you might loose it from someone who produces way less watts, but is very aerodynamic.

How to quantify aero? With a measure called CdA, which is a combination of frontal area x the drag coefficient. It is measured in metres squared. The lower this number the more aero you (and your setup) are.You can imagine that with riding on top of the handlebar you will be less aero than when you ride in the drops. For a TT it is important to have a good watt / CdA factor. It's a factor like for climbing but there you have the watt / kg which is a good indicator who will ride fastest up the mountain. For a more extensive review on CdA see: 

http://www.cyclingpowerlab.com/cyclingaerodynamics.aspx and

http://www.cyclingpowerlab.com/WattsOverCdA.aspx

 

In a post coming later when I have finally got my equipment installed properly (I am facing some problems with the clip-on width) and conditions to test are more ideal (i.e. no wind), I will explain the method I used to derive CdA and I will then try to give a better estimate of my CdA.  I will also try to make some scenario’s of how fast I would be able to ride given some parameters (a.o. CdA).

 

For this first test - do aero clip-ons give a CdA advantage for me?-  I went to a small industrial area in the evening (to decrease the chance of traffic) and rode the same laps over and over again for different scenario’s:

1)      3 laps on the top

2)      3 laps on the hoods

3)      3 laps on the drops

4)      3 laps on the aero-clip ons

 

The conditions were not ideal (there was wind although consistent head and tailwind on the same parts of the course).  However, I am only interested to see if aero-clip ons do have an advantage as opposed to the real (estimate) of CdA. Below you see the results (index) of the estimated CdAs from drops, hoods and aero-clip ons compared to the CdA estimate for drops. 

 

Now it is quite clear that the aero-clip ons show an improvement in CdA estimate, also in relation to drops and hoods. So, the conclusion is: yes, I do get more aero from this.You might think drops to be better, but hoods turn out slightly better now. Possible explanation: when I sit on the hoods I tend to pull my elbows (arms) more towards my body (I get narrower), which seems more aero.  If it is hoods or drops which are more aero, I can test that later under better conditions; I am not interesten in that now.

Next thing which is nice to see is the difference in speed and wattage average for the 3 laps on drops and clip-ons. This is quite nice. I go from FTP range on drops for a lower speed (38.7 km/h) to SST at higher speed (39.3 km/h).

 

 

As said I am still fumbling a bit with my setup. This has firstly to do with the fact that my handlebar goes from 31.8mm in the middle to around 26.5mm. These 3t clip-ons are for 31.8mm so I have used rubber (from a tire) and all of that stuff to fill up the gap. However, the clip-ons are very unstable then and after a while they start to slide. I have to find something for that otherwise I am going to buy a vert cheap handlebar on which I can fit the clip-ons. Getting this fixed is priority number one.

 

Second is that I am also trying to find a good position, in terms of that it feels comfortable and it is as aero as possible. These are adjustments w.r.t the position of the stem and the saddle. I still have 3 rings between my frame and my stem and I want to get rid of those so my handlebar will go down. This also implies I can sit more deep and I have to make sure that there is enough room for my legs not to bump into my chest. Maybe I have to put the saddle just a bit upward. Those rings should be gone somewhere next week.

 

After I have done these modifications I will be able -of conditions are well- to calculate my CdA. I will then probably try to vary a bit in position of the clip-ons. I all I must say this testing and 'fumbling' takes more time than I thought and distracts me from putting time into proper cycling (or training). I hope I will be ready by the end of September.....stay tuned for some more test results in the coming weeks.