Login required to started new threads

Login required to post replies

Prev Next
Can wind be faster than no wind? rruff and others
Quote | Reply
Wanted to move this into its own thread, it is pretty interesting and this may go on a while.

Anyway the basic question is, can an out and back, or loop TT be faster with wind in some cases than with no wind at all. As we know from various wind tunnel results, drag can be a good deal less at medium and high yaw than at zero yaw.

But any wind will also mean more apparent wind speed when riding, at least some of the time.

So can you be net faster? I mentioned that we seemed to notice faster times at one of our local TTs that used to occur monthly in Austin when there was moderate wind at certain directions. So here is are the results from best bike split I ran on that course:

Out and Back
8.09 miles
250 watts normalized power limiter

cda - yaw
0 - .21
5 - .2082
10 - .1972
15 - .1948
20 - .1875

0mph wind
time 18:56

5mph wind 180 degrees 18:52

8mph wind 180 degrees: 18:46




Of course bestbikesplit's physics could be wrong, the CdA vs yaw numbers could be unrealistic. But I believe analytic cycling's dynamic wind page will get you similar results if you set it up using their wheel drag vs yaw inputs. I haven't done it in a while because it is cumbersome to set it up.

Discus!



Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
Aeroweenie.com -Compendium of Aero Data and Knowledge
Freelance sports & outdoors writer Kathryn Hunter
Quote Reply
Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]
Quote | Reply
Just to check if this was merely due to getting cross-tailwinds on the uphill sections, I checked some other wind angles:

45 degrees is a little faster too, which is direct crosswind on the uphill bit.

The hills do appear to play a role though, I'm going to try with a flat course.



Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
Aeroweenie.com -Compendium of Aero Data and Knowledge
Freelance sports & outdoors writer Kathryn Hunter
Quote Reply
Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]
Quote | Reply
So can you be net faster?

But of course. Ask any sailor if you can be faster with wind.

We've known for at least 30 years that you can sail a bike.
Quote Reply
Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]
Quote | Reply
Quote Reply
Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]
Quote | Reply
As we know from various wind tunnel results, drag can be a good deal less at medium and high yaw than at zero yaw.

Define how those CdA vs yaw numbers were derived... and also define how they are being used to determine drag force in modeling programs.

For instance, when they yaw the table 10deg they don't increase the tunnel speed to simulate a crosswind... they leave that the same. The apparent rider speed actually *drops* in that case. When you are riding on the road and you have a crosswind, the wind hitting your body is greater than when there is no wind.

So is the CdA value based on the tunnel speed and measured in-line drag force, or do they calculate the apparent rider speed and base it on that? I'm guessing they don't do any calculation, which is why the dropoff in CdA with yaw is so large.

I do believe that many people can experience a slight benefit from a pure crosswind, but it's a lot less than those tunnel numbers would indicate.
Last edited by: rruff: Jun 12, 14 9:31
Quote Reply
Re: Can wind be faster than no wind? rruff and others [eb] [ In reply to ]
Quote | Reply
out & back
Quote Reply
Re: Can wind be faster than no wind? rruff and others [rruff] [ In reply to ]
Quote | Reply
Good question, I was just using the default CdA vs yaw values that BBS picks for you once you enter a 0 degree CdA value.

So just now I referred to the thread where Damon Rinard indicated how to correct for drag force at yaw as measured in the tunnel, and used data from the Zabriskie-dummy on a P4

If I am doing this right (damon can maybe check me?) then the cda reduction at yaw in my test cases was less pronounced than zabriskie on a p4. It seems like the adjustment is so small you can mostly ignore it, even.

edit: (the cda conversion there is just approximated using the 50g drag = .005 CdA rule of thumb)




rruff wrote:
As we know from various wind tunnel results, drag can be a good deal less at medium and high yaw than at zero yaw.

Define how those CdA vs yaw numbers were derived... and also define how they are being used to determine drag force in modeling programs.

For instance, when they yaw the table 10deg they don't increase the tunnel speed to simulate a crosswind... they leave that the same. The apparent rider speed actually *drops* in that case. When you are riding on the road and you have a crosswind, the wind hitting your body is greater than when there is no wind.

So is the CdA value based on the tunnel speed and measured in-line drag force, or do they calculate the apparent rider speed and base it on that? I'm guessing they don't do any calculation, which is why the dropoff in CdA with yaw is so large.

I do believe that many people can experience a slight benefit from a pure crosswind, but it's a lot less than those tunnel numbers would indicate.



Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
Aeroweenie.com -Compendium of Aero Data and Knowledge
Freelance sports & outdoors writer Kathryn Hunter
Last edited by: jackmott: Jun 12, 14 9:40
Quote Reply
Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]
Quote | Reply
On an out and back, a 90 degree cross-wind is clearly faster than no wind, at least theoretically. In my experience it's pretty hard to replicate this on an outdoor course, though, because outdoor courses tend to have turns and also it's rare that the wind is aligned correctly, so you usually end up with a headwind/tailwind situation which counteracts the effect. I did experience it once for sure, though. My CdA measured out at around 0.015 less than usual on a (fairly) straight out and back with a strong crosswind.
Quote Reply
Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]
Quote | Reply
Both Cd and A vary with the yaw vector, so these CdA figures are perplexing.

____________________________________
Pain is inevitable. Suffering is up to you.
Quote Reply
Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]
Quote | Reply
jackmott wrote:
Discus!








Take a short break from ST and read my blog:
http://tri-banter.blogspot.com/
Quote Reply
Re: Can wind be faster than no wind? rruff and others [Tri-Banter] [ In reply to ]
Quote | Reply
Quote Reply
Re: Can wind be faster than no wind? rruff and others [Runorama] [ In reply to ]
Quote | Reply
Runorama wrote:
Both Cd and A vary with the yaw vector, so these CdA figures are perplexing.

Why are they perplexing because of that?



Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
Aeroweenie.com -Compendium of Aero Data and Knowledge
Freelance sports & outdoors writer Kathryn Hunter
Quote Reply
Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]
Quote | Reply
Let me see...

Tunnel speed = Vw
Yaw angle = a
Rider eq. speed = Vr

Vr = [Vw^2-(Vw*sin(a))^2]^.5

But since the drag is proportional to V^2, the correction factor for CdA if the tunnel speed is used would be: Vw^2 /[Vw^2-(Vw*sin(a))^2] or ...1/(1-(sin(a))^2)

Yaw, CdA correction

0,1
5, 1.0077 (.77%)
10, 1.0311 (3.11%)
15, 1.0718 (7.18%)
20, 1.1325 (13.25%)


So going back to your listed CdA vs yaw numbers:

cda - yaw (corrected CdA)
0 - .21
5 - .2082 (.2098)
10 - .1972 (.2033)
15 - .1948 (.2088)
20 - .1875 (.2123)

Quote Reply
Re: Can wind be faster than no wind? rruff and others [rruff] [ In reply to ]
Quote | Reply
Using that correction for the Zabriskie/P4 drag numbers we get:

0.225
0.211607396
0.206218241
0.200961894
0.192520636

decrease of .0325

vs the values I used in BBS decrease of .0225

So sounds like the yaw/cda inputs for BBS need to be corrected values, if using wind tunnel data as an input, or you will get optimistic results. Since if it were doing the correction for us, my simulations wouldn't have been faster with wind.



Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
Aeroweenie.com -Compendium of Aero Data and Knowledge
Freelance sports & outdoors writer Kathryn Hunter
Quote Reply
Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]
Quote | Reply
Ah this makes sense. We used wind tunnel data recently with overly optimistic results (though only about a 20 sec difference over a 70.3), but that was primarily at low yaw angles.

-----------------------------------------------------------------
Founder: BestBikeSplit
CS: TrainingPeaks
BestBikeSplit | BBS Twitter | BBS Facebook | The Cycling Coop
Quote Reply
Re: Can wind be faster than no wind? rruff and others [Mrcooper] [ In reply to ]
Quote | Reply
Quote Reply
Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]
Quote | Reply
You got the Zabriskie data from this chart... but where did you get the other numbers you listed above?


Quote Reply
Re: Can wind be faster than no wind? rruff and others [rruff] [ In reply to ]
Quote | Reply
The initial yaw/cda numbers were the default estimates from bestbikesplit. You enter in a 0 yaw CdA and it estimates the at yaw numbers.


rruff wrote:
You got the Zabriskie data from this chart... but where did you get the other numbers you listed above?




Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
Aeroweenie.com -Compendium of Aero Data and Knowledge
Freelance sports & outdoors writer Kathryn Hunter
Quote Reply
Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]
Quote | Reply
Fine. Now I'm going to have two TT bikes, one for low yaw, the other for high yaw. Thanks dude.
Quote Reply
Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]
Quote | Reply
Absolutely.

Let's say you did a straight North-South TT. And the wind was directly from the East or West. So you saw the exact same apparent wind in both directions, meaning, assuming a roughly flat course - so that your speed is the same - then the yaw angles should be the same.

In that case, you'd obviously be faster with wind than without.

How much that changes as it becomes less of straightforward case of course, "depends."

"Non est ad astra mollis e terris via." - Seneca | rappstar.com | FB - Rappstar Racing | IG - @jordanrapp | Game Designer @ Zwift
Quote Reply
Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]
Quote | Reply
After thinking about this some more, it would be more accurate to say that the problem with the tunnel data (if my speculation is correct) is that they base CdA on the tunnel speed and the drag force in the direction the rider is traveling (or facing)... rather than the drag force in the direction of the wind vector. Obviously you are going to get a smaller CdA when you do that. If you take it all the way to 90 degrees, you'd expect CdA to drop to ~0.

The correction factor is the same one that I gave above.

When I do calculations with this stuff, I determine the force on the rider in line with the apparent wind vector, *then* calculate the component of this force in the direction the rider is traveling. IME there is a small drag reduction with yaw when defined this way, but it small enough (for me at least) that it doesn't compensate for the higher apparent wind I experience with crosswinds.
Quote Reply
Re: Can wind be faster than no wind? rruff and others [NordicSkier] [ In reply to ]
Quote | Reply
Felt for high yaw angle ... trek for low... ? I just gave up and ride with a kid trailer everywhere. Need to calculate drag there, but I have a feeling my Crr is a bit on the high side...

-----------------------------------------------------------------
Founder: BestBikeSplit
CS: TrainingPeaks
BestBikeSplit | BBS Twitter | BBS Facebook | The Cycling Coop
Quote Reply
Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]
Quote | Reply
direct personal experience ... yes. Two years ago I set a course record on a local 20km TT that I have not been close to since, given the same power and calmer conditions. On that day there was a pretty stiff cross wind that was close to 90deg relative to the direction of travel (out & back)
Quote Reply
Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]
Quote | Reply
I don't have any fancy calculations, but after this past weekend, I think it's very possible. I rode 48 seconds faster on just 7 more watts over 40k despite there being more crosswinds, a little rain, and poorer road conditions this year compared to last. When I got to the start line, I was just hoping to ride 1 minute slower than last year if even if I maintained a slightly higher wattage. Who knew I could be just as fast, if not faster despite the other conditions (rain and rougher road) playing against me.

ETA: My fastest HIM bike split at Honu was in 2011 when the winds were ridiculous. It was also the year LA smashed the bike course record and several other pros rode under 2:10 when normally the fastest bike split of the day is over 2:10.
Last edited by: Jason N: Jun 12, 14 12:34
Quote Reply
Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]
Quote | Reply
The initial yaw/cda numbers were the default estimates from bestbikesplit. You enter in a 0 yaw CdA and it estimates the at yaw numbers.

Any idea where they got their data?

Note that a drop in CdA doesn't necessarily mean you will go faster with a crosswind. You will still have a higher V^2 component in the direction of travel.

For instance, say you are going 26mph with a 8mph 90deg crosswind. That is 17.1deg yaw, and a wind speed of 27.20mph. The force is proportional to V^2, so it is (27.2/26)^2 = 1.095 higher than with zero wind, and the component in the direction of travel is 1.095 * cos(17.1) or 1.046... 4.6% higher. So for the crosswind to have no effect on your speed you would need to experience a drop in CdA of that magnitude to compensate.

Based on the numbers you listed it looks like BBS is showing a 9-10% lower CdA at 17 deg... which might be right for someone with modern equipment and a good sail effect.

Quote Reply

Prev Next