School me on aero

Triathlon Forum
1

Bear with me, I am pretty ignorant on all things aero.

The S5 and Cervelo’s claims are impressive. Now that I have a powermeter I can understand what “9-32 watts” feels like (Claimed savings vs typical road bike)

I ride a ‘typical road bike’ - a Look 585

When wind tunnel tests are done and claims are made like the one above, how can I translate this to the road?

The test claims yaw of -15 to +15. I think this means a headwind from approx 11 o’clock and 1 o’clock.

Lets assume when i am riding the yaw angle is anywhere between -180 and +180

Is this assumption correct? Basically when I ride the wind can come from any direction and there is no ‘prevailing’ direction / yaw angle

However since the wind speed is usually below 30kph (my avg riding speed) I will be riding into an effective yaw angle of -90 to +90 most of the time.

So if we call the drag savings between -15 and +15 20 watts, what is happening between -90 and -15? And from +15 to +90

I guess the savings are less since the design is more optimized for head-on winds. Lets say the savings there are 10 watts.

So:
30 degrees (-15 to +15) = 20watts
150 degrees (-90 to -16 and +15 to +90) = 10watts

Weighted avg = 11.7watts savings

Is this a better representation of how many watts I can save by changing frames?

Also, how much is the aero benefit reduced when you start taking yaw angles at 45degrees and beyond? Are the watts savings halved? Cut even more drastically?

Any help understanding is appreciated

Sincerely,
Aero mafia wannabe

2

I would like these answers also…Jackie boy where are you???

3

Yaw is apparent wind angle, the factors are the wind speed, the angle to your bike, And your speed. Your speed is an important factor. Figuring the ave yaw is between 0-15 is pretty safe.

Styrrell

4

Hed has a good yaw windspeed calculator or something to that effect. But basically yaw =/= where the wind is coming from. Even wind coming from 90 degrees will be much much less yaw angle that the rider is actually experiencing.

5

While we wait for an aerospace engineer to chime in, I’m going throw this out there:

You will never win or lose a road race based on how aero your frame is.

6

Hed has a good yaw windspeed calculator or something to that effect. But basically yaw =/= where the wind is coming from. Even wind coming from 90 degrees will be much much less yaw angle that the rider is actually experiencing.

This is highly dependent on where you live. In the northeast, you’d be correct. There are days here on the front range where the wind will be 70mph. That’ll feel like a pretty severe yaw no matter how big your legs are.

7

Not to be picky but yaw is exactly what the rider is experiencing. For instance when the wind is at your 2 oclock it feels like a head wind. when you turn around and its then at your 8 oclock, it still feels like a headwind, just less so. The wind didn’t change direction, but the yaw (apparent wind) is a headwind in both cases.

Styrrell

8

Figuring the ave yaw is between 0-15 is pretty safe.

Styrrell

What do you mean by pretty safe? No matter your speed, won’t -15 to +15 be the wind conditions only 17% of the time all else equal? Those 30 of 360 degrees multiplied by 2 in case your speed is greater than wind speed.

So I am cool with taking the claimed watt savings and saying you will get those savings 17% of the time.

What about the other 83% I suspect the 30watt savings fall dramatically at say 45degrees yaw.

I can understand that bike manufacturers measure at -15 to +15 because thats what they base their designs on since it appears the differences between designs is most apparent at these angles.

But unless I am totally wrong on the idea that the wind comes at you from all 360 degrees, the savings that are being claimed are true only for a small subset of conditions you will face during your daily riding.

9

While we wait for an aerospace engineer to chime in, I’m going throw this out there:

You will never win or lose a road race based on how aero your frame is.

I always thought this too,
but did you see how Gripel pipped Cancellera for the bronze at the world champs?
Not saying it had anything to do with the frame (Fabian should learn to sprint in the drops), but still.

10

But unless I am totally wrong on the idea that the wind comes at you from all 360 degrees

That is correct.

11

You have to factor your speed into yaw. When you do this you find that yaws greater than 15 degrees will be very rare. Start with this, absolutely no wind. The wind speed is 20 mph if you are riding 20 mph. Go to this link and click on the Yaw calculator. http://s321476941.online.de/hedgermany/aerodynamics_technology/yaw_calculator.asp

Keep in mind that its very rare for the wind to be directly at your side for an entire ride. Also the typical wind speed that is given by the weather channels is measure above the ground and in open conditions. Most or at least many of us ride where buildings, trees, terrain, etc slow down the average wind(sorry if you live in Iowa).

Styrrell

12

I’m just going to throw this one out: never trust statements by uniformed individuals.

13

You’re right, there is a tiny chance that at the pro level, because it is often so close and the speeds a so high in the final sprints, the frame may end up making the difference.

But you totally see where I’m coming from with the drops comment…there are always a million other things that make way more difference: do you attack from 5km out or 30km out? Does the peleton want to chase that day? Are you in a break of 3 or 9? Are you even taking any pulls? Maybe the decisive moment is on the hills (often is), where aero is irrelevant. Etc etc etc.

14

You will never win or lose a road race based on how aero your frame is.

never?

15

and no matter what happens, a more aero frame is making you faster.

cat 5 races are just as close as cat 1 races

You’re right, there is a tiny chance that at the pro level, because it is often so close and the speeds a so high in the final sprints, the frame may end up making the difference.

But you totally see where I’m coming from with the drops comment…there are always a million other things that make way more difference: do you attack from 5km out or 30km out? Does the peleton want to chase that day? Are you in a break of 3 or 9? Are you even taking any pulls? Maybe the decisive moment is on the hills (often is), where aero is irrelevant. Etc etc etc.

16

saunaking - you are vastly underestimating your own speeds affect on apparent wind speed.

yaw over 15 degrees is very rare

17

and yaw of 90 degrees never happens unless you’re standing still.

18

For the vast majority of races, at any level, fitness, luck, and guile, will be the major factors. Every once in a while small differences in equipment will make THE difference. It happens, but its seldom enough that I wouldn’t go overboard on anything equipment related.

Styrrell

19

could you explain how almost any wind at low speed will feel like a head wind? I’m reminded of the old saying that the wind is always against you but never realized it’s scientifically true.

20

Thanks for that link.
Playing around with it for a bit, plugged in a rider speed of 30kph and windspeed of 15kph and took a reading at each 10degree increment

33% of the cases the yaw was <15 (avg was 7)
67% yaw was >15 with avg of 25

So taking my made up numbers where <15yaw is a 30 watt drag reduction and at 25 degrees that reduction is only 10 gives me a weighted avg of 16.6watts drag reduction

So in my little ‘real world’ the drag savings are there but about half of the numbers in the whitepaper.

**I realize the whitepaper is very clear on the yaw, but that is often clouded in the ‘new bike hype’

So I am working through the s5 whitepaper and my takeaways so far are:

-Aero savings drop considerably when yaw is >15
-In real life you have an effective yaw of <15 about one third of the time
-To translate claimed savings to real life, knock it in half to give you a ballpark - this could be calculated more accurately if the tests gave numbers for yaw angles of 15-30 degrees, which in my example a cyclist is facing around 67% of the time!!

Thanks for the responses, I am learning…