Effects of CdA on speed

[cruzKurt]

I signed up for Windsock Premium today and am going to start monitoring my CdA. I don't have deep dish wheels, race case nor do I wear skin tight riding gear.

Today, here are my numbers. I did a search of the forums and read some here and there about your CdA's, unfortunately can't search for CdA because it only has 3 letters. If anyone can point me to some discussions, that would be great.

I am wondering if I lower my CdA from the current .34, what will the effect be on speed? Also, how long of a ride do I need to do as I make changes to see the effects of CdA?

 

[Rob Lloyd]

Do you have a power meter? To me, that 0.34 looks like the 'standard' CdA MyWindsock starts with. If you have a power meter, it will automatically calculate the CdA based on your power measurements and the weather/slope data for your ride.

I typically see variations of ~0.05 in CdA on the same bike, but different rides/days/weather. I have seen 0.14 and as high as 0.21 on the same bike. To truly use it to determine improvements, you need to use the same course, with similar weather conditions. Even then, you'll likely see some variation in calculated CdA.

 

[cruzKurt]

Do you have a powermeter? ....

I do not, I am planning on getting some speedplay pedals once I get acclimated to the clipless experience.

 

[Rob Lloyd]

Yeah, without a PM, the CdA number in MWS is just the default (and therefore meaningless).

 

[Henri]

Be aware that that programm does not "measure" wind resistance, it calculates it based on speed (including acceleration) in comparison to power and elevation change. From what I caught reading here, it should also take into consideration weather data. Maybe it has some data entry field to tell it your tire width and preassure as well? But there remain many unknown and unprecise factors like road surface and how exposed you are to the wind (open fields versus forest).
So really the question in regards to that program would be "How does speed effect estimated CdA?"

But I can give you a tool to do that kind of calculations with your experienced or made up values and have it calculate your CdA an see how speed changes when you than change CdA.

Bicycle Speed (Velocity) And Power Calculator

Bicycle Speed (Velocity) And Power Calculator, comparing different kinds of bicycles

www.kreuzotter.de

(Still there will be lots of inprecision when trying to recreate the situation you experienced.) But of course be aware that this is steady state speed at maintained conditions. When there is some de-/accaleration in it, the results will be flawed! (When keeping speed steady with varying power on varying slopes, averages might actually work ot, I'm not sure. Won't work with windspeeds.)

 

[Frito Bandito]

I think Strava will guestimate your power based on a typical rider on a DF bike, and combine it with the wind and slope as Rob wrote. Since you are not on a DF bike it is going to assume you are putting out significantly more power based on your speed. That is why it is important to get a power meter at some point. While your CdA measurements may be off, you can still use them to see if the adjustments you make are in the right direction towards a lower CdA. Just don't take that data as gospel since you aren't on a DF bike and you don't have a power meter. Once you get the power meter then you will have a better idea about what your CdA is. Many of us are using PMs and MyWindSock to tweak our positions and setups to get faster on less power, and while that system has its flaws and limitations, it is real world data, not from a wind tunnel, which also has its flaws. Since most of us don't have access to a wind tunnel we are left with not many options, and this is about the best we can do.

Here is a pic of the titles of several of my rides that includes that ride's CdA, speed and power for a bit of reference on what a lower CdA does for your speed according to your power output. The CdAs are different according to my hydration setup, bars used, wheels used and maybe other minor stuff such as which lense used and clothings. However, they were always an up and down a flattish river path to even out the headwinds and tailwinds. The last on is titled "T1" because that is my road bike with clipon aero bars. It was a fast ride for me on that bike, but the amount of power needed for that speed, and the riding position left me largely immobile.


Before I had a MyWindSock to reduce my CdA, I had several long rides at +200w that could have been MUCH faster had I reduced my CdA to anywhere around 0.200. Then my speed would have been at least 39kph.

 

[Henri]

My Wind Sock gets the Power data from Strava? Yeah, when you have no power meter, Strava calculates some power based on a predefined average CdA for roadbikes. My Wind Sock then calculates a CdA based on that power. So Sock's CdA is mainly based on Strava's stock CdA assumption. That should not really do anything. Even if you actually change your real CdA massively, that recursive calculation should still give about the same results. Changes in outcome would then only be result of changing road and whether conditions and such and differing assumptions about them. (When you have headwind and Strava doesn't know, it estimates lower power based on lower speed. Sock knows your headwind and sees you have the same relative speed with seemingly much lower power, so it thinks your CdA must have been much lower.)

If you have no power meter, get one quick or see if you can cancel your Sock subscription. (Maybe ask support to pause/postpone it, so you can use the left over time, once you got the power meter.)

 

[vosadrian]

Yep.... MyWindSock and any other similar tool for measuring CdA needs accurate power data. The result is meaningless without power data. A result of 0.3-0.35 is about right for a typical road bike. A stock V20 should be closer to 0.2. A well modified V20 should get <0.2.

If you need to work out how much fast you would go with a lower CdA for a given power output, use an online calculator like this:

Power to Speed Calculator | Endurance Data

Choose temperature, altitude, tires, CdA, drivetrain efficiency and calculate the speed for a given power output.

www.endurance-data.com