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blowing in the wind(20 posts)

blowing in the windajgibbons
May 22, 2002 1:15 PM
Rough winds do shake the darling buds of May.
--William Shakespeare, Sonnet 18: Shall I Compare Thee to a Summer's Day?

Nice, but these winds happen to be busting my chops this spring, more than most years.

So I have a wind question; one that will probably be best answered by the racers out there.

I know the simple physics: if you go in a loop on a windy day, and you're equally exposed to it the whole time, and the wind doesn't change speed or direction, then it will impede you half way and help the other half – a wash.

However, my longer training times while doing windy loops don't indicate "a wash".

Are there some more subtle physical variables I'm not thinking about? Or is more human/muscle related? Or am I coincidently having bad training days on windy days? Or.?
I'm no scientist, don't play one on TV either...Wannabe
May 22, 2002 1:31 PM
I have as far from a scientific mind as is possible but I'll give this a go. As I am a light rider, wind is not my friend so I have given this some brief thought. I have no evidence to back this up either!

Anyway, I noticed the same thing you have. I am guessing it has to do with the fact that speed and wind resistance are not directly related. Rather, they have an exponential (right term?) relationship. IE, as your speed doubles, wind resistance quadruples. I think this has got to be the reason why it is not a "wash" on our rides.

Maybe some of the smart people here will be able to provide the math behind my supposition...

Andy (has a BA but can be full of BS)
also, not a scientist...mwood
May 22, 2002 1:40 PM
but, a friend who does TT at a very competitive level and who does take a very scientific approach to the whole deal told me last week that any wind which is more than 18 degrees off from dead behind you does you no good. Something to do with your forward velocity creating its own aero dynamic, which is impacted negatively by the wind...
Dyamic pressure is proportional to ro v squared ...Humma Hah
May 22, 2002 2:43 PM
I do this for a living, was just doing the calculations today. In that equation, ro is air density, v is velocity, and dynamic pressure (Q) is proportional to air density times velocity squared. Using ro and v in proper SI units, the pressure produced is in Pascals. This is the pressure measured by a pitot tube on an airspeed indicator.

Actual drag is more compicated, the formula above simply giving pressure acting on a pipe end stuck into the breeze, but usually follows the same trend.

Power required to overcome this force, force times distance per unit time, goes up as the CUBE of v because you have not just the v squared in the drag term, but another v in that distance per unit time.

So wind causes a triple-whammy when it comes to how your legs feel.
...and i was just about to say EXACTLY that...(nm)JS Haiku Shop
May 23, 2002 12:00 PM
re: blowing in the windyfoiler
May 22, 2002 1:38 PM
You guys will have to do the math for yourself but here's the deal. You're in the downwind part of the course a lot less time than you are in the upwind part of the course.
That's all the hint I'm giving you...

There will be a quiz on Friday.

The extreme example with aircraft ...Humma Hah
May 22, 2002 2:37 PM
Say you have an airplane that can do 100 kts, and you need to fly to a point 100 nautical miles downwind, then turn around and come back.

Now assume the wind is 100 kts. Going downwind, you have a tailwind and you go 200 kts, taking 1/2 hour to get there.

Coming back upwind, however, you simply hover in the headwind, with no groundspeed. It takes infinite time to get back. Bad deal, particularly with 4 hours of fuel onboard.
I know! I know! (at least I know what I read...)cory
May 22, 2002 1:45 PM
Bicycling, back when it was a real magazine, did a story on this. Had graphs and physics and everything, and it determined that at typical wind and bike speeds, anything within a 270-degree arc of dead ahead was perceived as a "head wind." For the wind to help you, it had to be coming from within 45 degrees each side of dead astern.
Can't prove it scientifically, but anecdotal evidence certainly supports it.
Agree with 45 degrees astern as beneficial. -nmTig
May 22, 2002 2:02 PM
It is never a wash ...Humma Hah
May 22, 2002 2:33 PM
... there's only one way wind helps you (I've had it happen once), and that's to ride around the same way as a circulating system. If you ride CCW around a low pressure system, you always have a tailwind!

A perfect headwind is better than a headwind slightly off to either side. First, you'd have to run staggered file to paceline. Second, it rakes down the side of the bike causing drag on the whole thing. Straight on, the leading edge of the front wheel makes a wake that keeps wind off the spokes, rear half of the wheel, rear wheel, etc. With any crosswind, you lose this advantage.

With a tailwind, you can definitely pick up speed, but there are other factors that then come into play that may keep you from taking full advantage of the tailwind.
It's quite simple: 2 pointsKerry
May 22, 2002 4:09 PM
1) as has already been noted, you spend more time at the slower speed. A 20 mile out and back course, if you can average 20 with no wind, takes an hour. If the wind is such that you go 10 mph on the way out, and 30 on the way back, it's going to take you an hour and 20 minutes.

2) the power to over come wind drag is speed times drag, and drag is proportional to the square of the velocity. So power is proportional to the cube of the velocity. A pure 11 mph head wind (11 mph at the bike, which might be 40 mph measured in the open air, 30 feet above the ground) would slow a 20 mph rider to 10. A pure 11.2 mph tail wind would speed that 20 mph rider to 30.
Not so simplefiltersweep
May 22, 2002 4:42 PM
Point 1 is very valid, however, for me at least, I always average higher speeds during wind- simply because of psychology. I can't bear to ride below a certain speed (somewhat variable by mood) and I ultimately push myself harder against the wind- and also reap the benefits of going with the wind (like really pushing it to stay at 20 in a stiff head wind, then practically coasting back at 25). The still days drive me nuts... I feel I am ALWAYS going against the wind, and the average usually drops, since I don't ever have a proper tailwind.

I have no idea what you are referring to in your point #2-
"a pure 11 mph head wind" ? But anyway....

BTW- I can't imagine a wind that would slow me to 10 mph.... are we talking about a gale?
Imagine winds like these.amflyer
May 22, 2002 5:03 PM
Here in SE Nebraska:

35-55 mph.
Higher gusts.

Big accident on the interstate today becasue of low visibility due to blowing dust (a la dust bowl days)

I've ridden on days like this and I was lucky to keep at 10 mph. God I wish I lived in Colorado...

Force 9Kerry
May 22, 2002 5:14 PM
Sorry about my abstract reference to a "pure" wind. The wind speed we all know is the one quoted on the local news or listed on the weather chanel. However, that has only a limited relationship to the wind we feel when riding the bike. Posted wind speeds are taken about 30 feet above the ground, and well away from any buildings or trees. At the ground, we know that the wind speed is zero. So close to the ground, the wind speed is substantially less than the posted speed, and that is the true speed we encounter when riding a bike. My comment about the 11 mph "true speed" being more like 40 mph is tied to this. So if you were riding across open country (no houses, trees, bushes, etc.) directly into a 40 mph headwind, you might indeed be slowed to 10 mph.
KS WindJamieB
May 22, 2002 5:35 PM
I don't know about all the physics either, but today we did an interval session in horrible wind (like that seen in NE.....35 mph with 50 mph gusts). I never felt like I had a tailwind, despite the fact we did the workout on a loop. It seems like it was just as hard coming back as it was going out. Oh well.....I, too, wish I lived in Colorado or the Southwest.
That tears it then.amflyer
May 22, 2002 5:51 PM
I'm sueing the state of Nebraska for having crappy winds. Maybe we can include KS and OK and make it a class-action suit.
1x50% + 1x200% = 2.5 hourselviento
May 22, 2002 5:59 PM
a 2 hour loop turns into 2.5 hour loop. Probably not most accurate but you get the point.
1x50% + 1x200% = 2.5 hoursamflyer
May 22, 2002 6:01 PM
Sort of like 10 mph up the hill and 30 mph down doesn't equal a 20 mph average, right?
that's the idea. nmelviento
May 22, 2002 6:10 PM
Time more than distancehaydensimons
May 23, 2002 9:20 AM
The hill example is a good one. Say you have a 10 mile hill that you cruise 10 mph up. It will take 6 minutes to get up it. If you travel 20 mph down it will take 3 minutes. That puts your average below 15. If you ride 1 mile at 10 mph, you have to ride 2 miles at 20 mph to get the average up to 15.