Can wind be faster than no wind? rruff and others

Jun 12, 14 9:03

Post #1 of 58 Views: 7426

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

Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]

Jun 12, 14 9:17

Post #2 of 58 Views: 7400

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

Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]

Jun 12, 14 9:19

Post #3 of 58 Views: 7398

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.

Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]

Jun 12, 14 9:25

Post #4 of 58 Views: 7367

Ok state TT course, which was pretty close to pancake flat

0mph wind: 56:18
270 deg wind at 8mph: 55:44

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

Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]

Jun 12, 14 9:26

Post #5 of 58 Views: 7361

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

Re: Can wind be faster than no wind? rruff and others [eb] [ In reply to ]

Jun 12, 14 9:26

Post #6 of 58 Views: 7361

out & back

Re: Can wind be faster than no wind? rruff and others [rruff] [ In reply to ]

Jun 12, 14 9:39

Post #7 of 58 Views: 7311

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

Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]

Jun 12, 14 9:59

Post #8 of 58 Views: 7260

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.

Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]

Jun 12, 14 10:22

Post #9 of 58 Views: 7214

Both Cd and A vary with the yaw vector, so these CdA figures are perplexing.

____________________________________
Pain is inevitable. Suffering is up to you.

Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]

Jun 12, 14 10:28

Post #10 of 58 Views: 7183

jackmott wrote:

Discus!

Take a short break from ST and read my blog:
http://tri-banter.blogspot.com/

Re: Can wind be faster than no wind? rruff and others [Tri-Banter] [ In reply to ]

Jun 12, 14 10:35

Post #11 of 58 Views: 7167

nice

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

Re: Can wind be faster than no wind? rruff and others [Runorama] [ In reply to ]

Jun 12, 14 10:36

Post #12 of 58 Views: 7160

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

Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]

Jun 12, 14 10:45

Post #13 of 58 Views: 7135

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)

Re: Can wind be faster than no wind? rruff and others [rruff] [ In reply to ]

Jun 12, 14 11:11

Post #14 of 58 Views: 7093

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

Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]

Jun 12, 14 11:36

Post #15 of 58 Views: 7051

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
Amazonian

Re: Can wind be faster than no wind? rruff and others [Mrcooper] [ In reply to ]

Jun 12, 14 11:42

Post #16 of 58 Views: 7031

Added complication:
Could there be a substantive increase in CRR at higher yaw?

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

Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]

Jun 12, 14 11:45

Post #17 of 58 Views: 7026

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

Re: Can wind be faster than no wind? rruff and others [rruff] [ In reply to ]

Jun 12, 14 11:55

Post #18 of 58 Views: 7007

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

Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]

Jun 12, 14 12:05

Post #19 of 58 Views: 6992

Fine. Now I'm going to have two TT bikes, one for low yaw, the other for high yaw. Thanks dude.

Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]

Jun 12, 14 12:08

Post #20 of 58 Views: 6980

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

Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]

Jun 12, 14 12:09

Post #21 of 58 Views: 6979

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.

Re: Can wind be faster than no wind? rruff and others [NordicSkier] [ In reply to ]

Jun 12, 14 12:18

Post #22 of 58 Views: 6962

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
Amazonian

Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]

Jun 12, 14 12:27

Post #23 of 58 Views: 6917

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)

Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]

Jun 12, 14 12:30

Post #24 of 58 Views: 6908

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

Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]

Jun 12, 14 12:44

Post #25 of 58 Views: 6876

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.

Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]

Jun 12, 14 12:48

Post #26 of 58 Views: 3613

Can wind be faster than no wind?

In my (anecdotal) experience, the answer is yes ... though not necessarily in a flat, 40k TT sense.

I have a regular 25-mile out and back that accrues about 2000ft of vertical (including two 800ft, south-facing ascents on the outbound leg). I typically return with an NP of 242-245W for the ride, in about 1h13-1h16.

Our winds have been quite stiff this season, and I have at least a dozen rides from which I've concluded that rides against N+W wind directions are comparable to no wind, and rides against S+E wind directions are typically 2-3 minutes faster than the aforementioned. So, yes; depending on the topography, wind can be faster than no wind.

"The right to party is a battle we have fought, but we'll surrender and go Amish... NOT!" -Wayne Campbell

Re: Can wind be faster than no wind? rruff and others [Jason N] [ In reply to ]

Jun 12, 14 12:51

Post #27 of 58 Views: 3607

Humid conditions actually lower air density and can be helpful. Also, rainy conditions generally coincide with low barometric pressure which lowers air density as well. You can correct for these factors pretty easily though.

The thing that makes evaluations tough is that winds change in direction and intensity. If the "pure" crosswind shifts a little during your ride, it can hurt you or help you.

Re: Can wind be faster than no wind? rruff and others [Jason N] [ In reply to ]

Jun 12, 14 13:06

Post #28 of 58 Views: 3597

Jason N wrote:

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.

Are you saying it might be the...

Lance effect?

Re: Can wind be faster than no wind? rruff and others [rruff] [ In reply to ]

Jun 12, 14 13:16

Post #29 of 58 Views: 3586

As ruff says higher humidity actually helps you cut through the air (rain causes other issues obviously). So from a physics standpoint you can go faster on the same watts, from a physiological standpoint you may not be able to hold the same watts however (similar to altitude where it's easy to cut through but harder on the rider).

-----------------------------------------------------------------
Founder: BestBikeSplit
Amazonian

Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]

Jun 12, 14 13:21

Post #30 of 58 Views: 3577

jackmott wrote:

Added complication:
Could there be a substantive increase in CRR at higher yaw?

I had the same thought but didn't have enough conviction to say anything.

Re: Can wind be faster than no wind? rruff and others [rruff] [ In reply to ]

Jun 12, 14 13:26

Post #31 of 58 Views: 3567

rruff wrote:

Humid conditions actually lower air density and can be helpful. Also, rainy conditions generally coincide with low barometric pressure which lowers air density as well. You can correct for these factors pretty easily though.

The thing that makes evaluations tough is that winds change in direction and intensity. If the "pure" crosswind shifts a little during your ride, it can hurt you or help you.

So basically I have no more excuses for going slow. Can't blame wind, rain, or humidity. I did have a little bit of powerade in my BTA bottle to take one sip out of just to get that sugar taste in my mouth...so I suppose if I cramped up I could still blame that.

Re: Can wind be faster than no wind? rruff and others [GreenPlease] [ In reply to ]

Jun 12, 14 13:30

Post #32 of 58 Views: 3561

GreenPlease wrote:

jackmott wrote:

Added complication:
Could there be a substantive increase in CRR at higher yaw?

I had the same thought but didn't have enough conviction to say anything.

There is a *significant* increase in scrub and something else. The Crr model is not rich enough to describe the tire trying to roll off the rim, so we can't really call this "Crr", per se.

AndyF
bike geek

Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]

Jun 12, 14 13:37

Post #33 of 58 Views: 3557

I'd turn this around and get a tailwind.

Re: Can wind be faster than no wind? rruff and others [GreenPlease] [ In reply to ]

Jun 12, 14 13:37

Post #34 of 58 Views: 3556

My first order guess is that it wouldn't make a substantial difference. It's not like you're doing work laterally (unless you're getting moved around a bunch), nor inducing much of a slip angle.

I do reserve the right to be wrong, but I'm guessing any effect would be below our present levels of detection.

The question of who is right and who is wrong has seemed to me always too small to be worth a moment's thought, while the question of what is right and what is wrong has seemed all-important.

-Albert J. Nock

Re: Can wind be faster than no wind? rruff and others [rruff] [ In reply to ]

Jun 12, 14 13:43

Post #35 of 58 Views: 3555

I always wondered that with very strong crosswinds, the point where you drag started to increase again as surfaces stalled.... if you were better off riding a little harder to reduce your yaw angle and ultimately make yourself more aero.

Hmmm.... could you have a Garmin that 4g cell capable for internet access, but with no communication functionality to stay within the rules, that could upload real time weather data and calculate based on wind direction the optimum power output at that moment. Actually it would be cool if it could feed you live results from the athlete tracker as well. But for both you'd need to update USAT rules. Right now GPS is allowed, only because it's passive, and isn't really functionally different than a magnetic compass.

TrainingBible Coaching
http://www.trainingbible.com

Re: Can wind be faster than no wind? rruff and others [AndyF] [ In reply to ]

Jun 12, 14 13:45

Post #36 of 58 Views: 3552

From a side wind? I'd expect it on a track or from hard turning, but not so much from the normal-to-forward-travel vector. Have any data to show this? (I'm totally comfortable being wrong with underestimating this effect)

The question of who is right and who is wrong has seemed to me always too small to be worth a moment's thought, while the question of what is right and what is wrong has seemed all-important.

-Albert J. Nock

Re: Can wind be faster than no wind? rruff and others [motoguy128] [ In reply to ]

Jun 12, 14 13:47

Post #37 of 58 Views: 3550

Agreed. Not that far off. If only there was a model that ran fast enough .. speaking of which we have an api if anyone is interested in creating that.

-----------------------------------------------------------------
Founder: BestBikeSplit
Amazonian

Re: Can wind be faster than no wind? rruff and others [Mrcooper] [ In reply to ]

Jun 12, 14 13:55

Post #38 of 58 Views: 3536

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.

Doh! Just noticed who you were...

How did you get your CdA vs yaw numbers? When making calculations do you assume that CdA represents drag in the wind vector direction or drag in the rider's direction of travel?

Re: Can wind be faster than no wind? rruff and others [rruff] [ In reply to ]

Jun 12, 14 14:10

Post #39 of 58 Views: 3525

I'll double check the algorithm but I believe it is in line with riders direction with apparent wind experienced (broken into components) as you mentioned earlier with some special adjustments for specific equipment effects. Anything we can do to enhance the accuracy is welcomed. Working on a field test drag model now. So trying to estimate yaws for various positions based on previous races or test performances.

-----------------------------------------------------------------
Founder: BestBikeSplit
Amazonian

Re: Can wind be faster than no wind? rruff and others [Derf] [ In reply to ]

Jun 12, 14 14:22

Post #40 of 58 Views: 3515

Derf wrote:

My first order guess is that it wouldn't make a substantial difference. It's not like you're doing work laterally (unless you're getting moved around a bunch), nor inducing much of a slip angle.

I do reserve the right to be wrong, but I'm guessing any effect would be below our present levels of detection.

You are doing work laterally. That work is rolling the rubber off the rim and creating scrub, which generates heat. You are working against a dissipative force through a distance. Similar to the deformation of the tire with the Crr*m*g term.

AndyF
bike geek

Re: Can wind be faster than no wind? rruff and others [AndyF] [ In reply to ]

Jun 12, 14 14:37

Post #41 of 58 Views: 3500

So there is a significant slip angle (scrub) to the tire. I figured the relatively small side force would be generally benign. Any idea the magnitude of this effect?

The question of who is right and who is wrong has seemed to me always too small to be worth a moment's thought, while the question of what is right and what is wrong has seemed all-important.

-Albert J. Nock

Re: Can wind be faster than no wind? rruff and others [Mrcooper] [ In reply to ]

Jun 12, 14 14:37

Post #42 of 58 Views: 3498

Hey that is awesome that you are doing a field test model! I was going to ask if you were interested in that because it would be a natural for your site. Aerolab would be great if there was a way to input wind.

I've been using a spreadsheet I made: http://forum.slowtwitch.com/..._latest_reply;so=ASC It's probably too tedious though... don't think anyone uses it besides me.

I've updated it since then... if you are interested I'll send you the latest.

I don't make any assumptions for CdA vs yaw, but have noticed that CdA tends to be lower if yaw is high.

Do you model tire Crr vs temperature? That can have a pretty big effect also. And I've noticed that road temperature is almost as big of an influence as the air.

Re: Can wind be faster than no wind? rruff and others [Derf] [ In reply to ]

Jun 12, 14 14:40

Post #43 of 58 Views: 3494

Derf wrote:

So there is a significant slip angle (scrub) to the tire. I figured the relatively small side force would be generally benign. Any idea the magnitude of this effect?

It is, of course, highly dependent on the conditions. But let's just say that wherever you're going to make significant use of the improved yaw CxA, you must also account for an increased apparent Crr. And the result is different for clinchers and tubs. Tubs are much more lossy.

AndyF
bike geek

Re: Can wind be faster than no wind? rruff and others [AndyF] [ In reply to ]

Jun 12, 14 14:43

Post #44 of 58 Views: 3491

Good stuff, Andy!

I was pondering that while riding on my Vittoria Corsas that just replaced GP4000s. They seem to behave differently to side forces, most noticeably when out of the saddle, with the Vittorias seeming to scrub less.

Do you have any data you can share?

Oh... tubulars are worse? I keep wondering if there is some "unknown" but real performance benefit to those that would justify why all the pros use them...

Last edited by: rruff: Jun 12, 14 14:46

Re: Can wind be faster than no wind? rruff and others [AndyF] [ In reply to ]

Jun 12, 14 14:46

Post #45 of 58 Views: 3487

Interesting. Losses in the glue from the side load. I bet different tire constructions behave differently, too.

I might need to pm you soon as I just finished a data assimilation class and kind of want to play with a "real-world" data set. Might try and squeeze some more out of the Chung method by looking at yaw (and overconstraining on GPS). Even if nothing really useful comes out of it, I'll be better off for the problem.

The question of who is right and who is wrong has seemed to me always too small to be worth a moment's thought, while the question of what is right and what is wrong has seemed all-important.

-Albert J. Nock

Re: Can wind be faster than no wind? rruff and others [rruff] [ In reply to ]

Jun 12, 14 14:47

Post #46 of 58 Views: 3484

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.

Not true. I know for a fact that the SD LSWT applies a sine Beta correction to the force plate data.

http://bikeblather.blogspot.com/

Re: Can wind be faster than no wind? rruff and others [AndyF] [ In reply to ]

Jun 12, 14 14:54

Post #47 of 58 Views: 3480

AndyF wrote:

Derf wrote:

So there is a significant slip angle (scrub) to the tire. I figured the relatively small side force would be generally benign. Any idea the magnitude of this effect?

It is, of course, highly dependent on the conditions. But let's just say that wherever you're going to make significant use of the improved yaw CxA, you must also account for an increased apparent Crr. And the result is different for clinchers and tubs. Tubs are much more lossy.

Range of apparent Crr increases?

http://bikeblather.blogspot.com/

Re: Can wind be faster than no wind? rruff and others [Tom A.] [ In reply to ]

Jun 12, 14 15:00

Post #48 of 58 Views: 3473

Tom A. wrote:

AndyF wrote:

Derf wrote:

So there is a significant slip angle (scrub) to the tire. I figured the relatively small side force would be generally benign. Any idea the magnitude of this effect?

It is, of course, highly dependent on the conditions. But let's just say that wherever you're going to make significant use of the improved yaw CxA, you must also account for an increased apparent Crr. And the result is different for clinchers and tubs. Tubs are much more lossy.

Range of apparent Crr increases?

Apparent Crr increases, yes. But we really need a second coefficient, which accounts for the lateral loading from side wind forces.

AndyF
bike geek

Last edited by: AndyF: Jun 12, 14 15:01

Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]

Jun 12, 14 15:10

Post #49 of 58 Views: 3463

Wind is a complicated issue. I was at the Texas TT also (54:47 for men's 55+). If you were fortunate to have started early enough, then you had a growing head wind going out and a still increasing tail wind on the return. That seems to often be the case in morning TTs-- the wind velocity increases as the morning goes on. I did the SoCal TT champs a few weeks ago on a loop course. The course is set up so that about 6 miles of the course overlaps itself-- to take advantage of the prevailing winds. Wind certainly helps there too. Then you have the aberration of Moriarty, on really good days it is possible to catch desert cooling winds going out, then have a shift in wind to prevailing Southern wind for the return. Its no wonder so many national records are set there (It doesn't hurt that its at altitude, the pavement is okay, and that it is a certified course too.). So wind can help. Wind can hurt. Often its like fishing, you should have been there yesterday.

Re: Can wind be faster than no wind? rruff and others [TPerry] [ In reply to ]

Jun 12, 14 15:15

Post #50 of 58 Views: 3458

I loved the Galveston 70.3 where some waves had a cross tail out and a cross tail back :)

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Re: Can wind be faster than no wind? rruff and others [Tom A.] [ In reply to ]

Jun 12, 14 15:34

Post #51 of 58 Views: 1512

Not true. I know for a fact that the SD LSWT applies a sine Beta correction to the force plate data.

If the data shown in the plot is already corrected for CdA in the wind direction, then the speed gains with a crosswind strong enough to produce a +20deg yaw are massive. According to my calculations enough to account for a speed increase from 52.54 km/hr with no wind, to 56.17 km/hr with a crosswind. Nearly a 7% *speed* increase. Seems dubious.

Re: Can wind be faster than no wind? rruff and others [rruff] [ In reply to ]

Jun 12, 14 16:11

Post #52 of 58 Views: 1497

rruff wrote:

Not true. I know for a fact that the SD LSWT applies a sine Beta correction to the force plate data.

If the data shown in the plot is already corrected for CdA in the wind direction, then the speed gains with a crosswind strong enough to produce a +20deg yaw are massive. According to my calculations enough to account for a speed increase from 52.54 km/hr with no wind, to 56.17 km/hr with a crosswind. Nearly a 7% *speed* increase. Seems dubious.

Here's a link to the spreadsheet they gave me from the 0 and 5 deg runs I got to do. As you can see, the CxA (drag area in the direction of bike travel) is corrected for Beta:

https://www.dropbox.com/...9%20Data%20Sheet.xls

edit: However, this data doesn't have fixture drag tared out either...

edit 2: Oh, and it's a cosine Beta correction...I had mis-remembered above :-)

http://bikeblather.blogspot.com/

Last edited by: Tom A.: Jun 12, 14 17:40

Re: Can wind be faster than no wind? rruff and others [Tom A.] [ In reply to ]

Jun 12, 14 17:21

Post #53 of 58 Views: 1476

Thanks... but getting an error with that link.

Re: Can wind be faster than no wind? rruff and others [rruff] [ In reply to ]

Jun 12, 14 17:41

Post #54 of 58 Views: 1469

rruff wrote:

Thanks... but getting an error with that link.

Sorry about that...user error when setting up the link on the post. Try the link from the original post, or this one below:

https://www.dropbox.com/...9%20Data%20Sheet.xls

http://bikeblather.blogspot.com/

Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]

Jun 13, 14 4:37

Post #55 of 58 Views: 1383

jackmott wrote:

Runorama wrote:

Both Cd and A vary with the yaw vector, so these CdA figures are perplexing.

Why are they perplexing because of that?

Because they are not absolute figures, just relative. They result from looking only at the axial force vector in the direction of the rider.
Total aerodynamic drag, and actual CdA of the rider-bike system increases with yaw. If we'd only rotate the rider-bike without moving the scale we'd see that.
It's just that we ignore the lateral force vector because we say the frame-wheels-tires pick it up, and it's easier to test that way. The frame-wheel-tires system is not going to be perfect at transfering this force to the ground (hysteresis losses in the system plus slippage impacting Crr). I don't think it should be totally ignored. I feel it would be better to start with total drag on the system, separate axial and lateral force vectors and work from there. It would be interesting to test Crr with a lateral force component corresponding to the load on the system from lateral force at various yaw angles and speed.

____________________________________
Pain is inevitable. Suffering is up to you.

Last edited by: Runorama: Jun 13, 14 5:07

Re: Can wind be faster than no wind? rruff and others [Runorama] [ In reply to ]

Jun 13, 14 5:20

Post #56 of 58 Views: 1367

Runorama wrote:

Because they are not absolute figures, just relative. They result from looking only at the axial force vector in the direction of the rider.

" it has been SOP to correct wind tunnel data for ground-based vehicles from tunnel-axis to body-axis for decades "-Coggan (also Tom mentioned it 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

Re: Can wind be faster than no wind? rruff and others [jackmott] [ In reply to ]

Jun 13, 14 7:47

Post #57 of 58 Views: 1340

jackmott wrote:

Runorama wrote:

Because they are not absolute figures, just relative. They result from looking only at the axial force vector in the direction of the rider.

" it has been SOP to correct wind tunnel data for ground-based vehicles from tunnel-axis to body-axis for decades "-Coggan (also Tom mentioned it above)

I get that. Does it mean we should ignore out of body-axis force vectors like it doesn't impact the system? That's why I feel it makes more sense to start with tunnel-axis drag, which is real aero induced drag, and then split the vectors. All body-axis figures will translate optimistically to rider power requirements because we ignore the lateral component impact on the whole system, and I feel it is not negligible for a cyclist.

____________________________________
Pain is inevitable. Suffering is up to you.

Re: Can wind be faster than no wind? rruff and others [Runorama] [ In reply to ]

Jun 13, 14 9:51

Post #58 of 58 Views: 1317

In the spreadsheet that Tom linked, CdA is translated to the tunnel axis. I don't know if they make lateral measurements or not... probably not... but assume that the drag force is in line with the tunnel flow.

And it goes down with yaw... which seems impossible. Or more likely the drag vector is *not* in line with the tunnel... the sailing effect skews it quite a lot.

Regarding the lateral component, that will effect tire scrub (Andy F above) along with some altering of the steering angle and tilting. Those seem like tough things to calculate... but you can get a handle on the total effect by field testing with crosswinds.

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