Relation between aero drag and power?

[TonyM]

Hi,

I always hear in the marketing claims of the manufacturers that their product X will save Y watts in terms of "drag".
But how do I "transform" this save in terms of drag into a savings in terms of power made from the cyclist itself?

So for example if a wheelset "A" will save 5 watts of drag compared to a wheelset "B", how much watts savings in terms of power do I get ? 5 watts also ?? So If I need let's say 200 watts to achieve 40km/h with my wheelset "B" I would then need 195 watts with this wheelset "A" to achieve 40 km/h??

Is it to be understood like this? Or is it to be understood like for example 5 watt aero drag savings means x seconds time savings for a 40 km ride at 30 km/h?

[mrlobber]

https://www.cyclingpowerlab.com/Yaw.aspx

[sawyer]

Hi,

I always hear in the marketing claims of the manufacturers that their product X will save Y watts in terms of "drag".
But how do I "transform" this save in terms of drag into a savings in terms of power made from the cyclist itself?

So for example if a wheelset "A" will save 5 watts of drag compared to a wheelset "B", how much watts savings in terms of power do I get ? 5 watts also ?? So If I need let's say 200 watts to achieve 40km/h with my wheelset "B" I would then need 195 watts with this wheelset "A" to achieve 40 km/h??

Is it to be understood like this? Or is it to be understood like for example 5 watt aero drag savings means x seconds time savings for a 40 km ride at 30 km/h?

Just two different ways of expressing aero benefit for marketing purposes:

- "lower power output required to achieve X speed with this product"
- "Y seconds faster over 40kms with this product for a given power output"

[TonyM]

@ mrlobber:

Thanks! [emoji106][emoji106]

[RyanH]

Watt and time savings are specific to a certain speed, temp, altitude/barometric pressure and humidity that the test was done. So, it can make it fairly hard to use and nearly impossible to compare results from different mfg unless they provided the Rho number. You really need to convert that into a CdA figure so that you can then calculate your total CdA using the difference for a given speed and environmental factors.

I found it extremely difficult to compare data from different manufacturers.

[cyclenutnz]

I've written a little bit about interpreting marketing claims before

The key point is to scale the claimed savings for the speed you care about


Usually the marketing will be based on 30mph or 50kph - because that magnifies the differences. So if you see a 5w claimed saving it is going to mean 2.5w saving at 40kph. Then reduce that some more if you're group riding - so 1-1.5w.

[Ahillock]

Hi,

I always hear in the marketing claims of the manufacturers that their product X will save Y watts in terms of "drag".
But how do I "transform" this save in terms of drag into a savings in terms of power made from the cyclist itself?

So for example if a wheelset "A" will save 5 watts of drag compared to a wheelset "B", how much watts savings in terms of power do I get ? 5 watts also ?? So If I need let's say 200 watts to achieve 40km/h with my wheelset "B" I would then need 195 watts with this wheelset "A" to achieve 40 km/h??

Is it to be understood like this? Or is it to be understood like for example 5 watt aero drag savings means x seconds time savings for a 40 km ride at 30 km/h?

This is the most useful formula/conversion that I have come across:

0.1 lbs (50 g) of drag (at 30 mph) = 0.5 s/km = 5 W = 0.005 m^2 CdA = 0.0005 Crr

[53x12]

Watt and time savings are specific to a certain speed, temp, altitude/barometric pressure and humidity that the test was done. So, it can make it fairly hard to use and nearly impossible to compare results from different mfg unless they provided the Rho number. You really need to convert that into a CdA figure so that you can then calculate your total CdA using the difference for a given speed and environmental factors.

I found it extremely difficult to compare data from different manufacturers.

I agree with Ryan. Which is why many prefer the data to be expressed as CdA as it is much easier to compare data across wind tunnels and products than it is as "watts" or "watts saved" or "time saved" which all have a bunch of other issues with them.

The change in CdA is all that matters.

[TonyM]

How could we understand for example this chart in terms of "x watts of power savings" between the 25c and the 28c with the DT Swiss wheels - at the speed that analysis was made (45 km/h) and at let's say 35 km/h?



https://www.dtswiss.com/en/technology/w ... echnology/

[basilic]

and while we are at it, what's with the 28c, 25c? it's 28 mm, 25 mm tires. Here I thought DT Swiss came from a metric country.
c traditionally refers to the rim central channel, e.g. 17c is 17 mm central width
C belongs in the French sizing system for wheel diameter, so 700C aims for 700 mm outside diameter, with tire, and 622 mm bead seat diameter
cc is cubic centimeter
xxx is not safe for work

carry on

[TonyM]

Looking at some results in terms of drag savings I am quite surprised how much (or little) it is for an average cyclist (the one that the Marketing departments are targeting).....

Here the analysis in terms of saving in drag (grams):
https://cyclingtips.com/2010/04/biggest ... quipment/#
(calculation based on a rider at 48mins over 40Kms) ...that means 50 km/h !


Now turned into savings in terms of watts for an average cyclist biking at let's say 35 km/h:
(using the following website for the calculation: https://www.cyclingpowerlab.com/Yaw.aspx)

- Aero tubing for the frame: -1.7 Watt !

- Wheels (at 0° and 0° yaw angle and at 10° and 0°): -5.3W to -7.2W

and the best value for the money!
- Shoe covers: -3.04W (double than the frame with aero tubing! )

Now of course this is different if the cyclist is riding @45 km/h, in average....

[kgt]

What does "standard tubing TT frame" mean? Something like that?:

[BeeSeeBee]

That blurb from VeloNews was never explained well. I'd say the chart gives a pretty good hierarchy of aero importance, but the numbers are a bit weird since drag differences between aero frames are often greater than that. The rear wheel recommendation and shoe cover (though depending on the cover) seem a bit wonky too.

Its part of why understanding the protocol of the test (e.g. wind speed, yaw angles, positional differences, equipment differences, tunnel design, etc.) is important before you try to understand the results.

[Rudi]

What does "standard tubing TT frame" mean?

I'd assumed it to mean a TT bike made with round alu/steel/whatever tubes vs. shaped, carbon tubes like most if not all modern TT bikes have.

Not the angles at which the tubes are assembled.

Think if you got someone to build you a brand new frame out of round tubes that had the exact geometry of a Cervelo P5 vs. an actual Cervelo P5.

Edit for clarity

[cyclespeed]

I did this vid last year and concluded that shoe covers were the best aero bang for your buck, simply because they're so cheap.

https://www.youtube.com/watch?v=ASFunOcHe48