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2010-09-27 2:14 PM
in reply to: #3119910

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Subject: RE: Aero position in a tailwind
Rogillio - 2010-09-27 2:48 PM
jsiegs - 2010-09-27 1:26 PM
Rogillio - 2010-09-26 7:29 PM
gsmacleod - 2010-09-26 5:56 PM
Rogillio - 2010-09-26 12:49 PM

Looking back at my OP, I believe your proved my assertion to be correct and that the sail effect is not a myth at all but sound physics.  Like all things, there are limits and if you allow the wind to push you to the point where you are no longer going faster than the wind, the benefit is gone.


The problem with you assertion is that there is no point where, assuming you are still able to put power to the pedals, that you aren't facing a headwind.  If you aren't going to pedal when you have a tailwind, then by all means sit up to make yourself a sail.  However, if you are planning to put even a small amount of power to the pedals, then staying aero is still the better option.

In the attached file you can see that when air speed and ground speed are equal (and in the same direction) your position doesn't matter as you have zero air resistance due to the effective windspeed being equal to zero which only leaves the power to overcome rolling resistance (in this case, ~35W).

However, if the rider is still putting a small amount of power to the pedals, it will be easy to travel faster than the speed of the tailwind.  The second column shows what happens in the same situation if the rider applies enough power to travel only 10km/h faster than the tailwind (so 40km/h ground speed); the aero rider requires 61W while the rider sitting up as a sail, requires 67W.  So, in this case the aero position saves ~10% of the wattage required to travel 40km/h.

Shane


If the air is moving 10 mph faster than you are biking, what does getting 'aero' do for you?  The air out in front of you is moving away from you faster than you are moving.  

My assertion is 100% correct.  You are changing the problem to fit your argument.  Read my OP again and tell me if it is factually correct or not.  What I wrote, is "if the wind is traveling faster than you, it behooves you to sit up....".   You are changing the problem by saying if you continue to pedal, you will NOT be faster than the wind.  OK, if that is the case, read the rest of my OP, "if you are traveling faster than the wind, stay aero..".

~Mike


Well, obiviously it all depends if the air flow is laminar, and if you're dimpled.  Anyone know your Reynolds number?

Obviously, if the air is pushing you from behind, you will be best served by catching as much of it as possible, that's just common sense and sound physics.  The point gsmacleod and many other are trying to make is that that scenerio will virtually never play out in real world traning/racing situations.  For all intents and purposes (to us triathletes), the "sail effect" is a myth.  For what I would call impractical purposes, i.e. 200+mph tailwind, not pedaling, or pedaling @ < 35 W, it is very true.  So maybe if this were mythbusters, I might call it "plausible". 



In the real world most of us are not going to be biking 30+ mph...even with a 20 mph tailwind.   You also need to consider how push you still have left on say 53/23 gears at 30 mph.  So on paper you might be able to continue to pedal and gain speed indefinately but in practice, on flat ground there is a very real limit and you would be spinning like a hampster put not delivering much (if any) power to the drive train.

So you, et al can call it a myth all you want and I'll just sail merrily on.

~Mike 


Biking 30 mph with a 20 mph tailwind is only very slightly (something like 35 W someone mentioned ~ almost negligable) harder than biking 10 mph with no tailwind.  I'd say everyone on this forum could bike at least 10 mph and then add > 35 W to that. 

On my compact, at 100 RPM (very low cadence to max out, but it is something almost everyone can do), I hit 35.5 mph (50/11 combo, you'll go slightly slower in you std 53/12 - 34.5mph). At 53/23 you will max out at 21.6 and 120 RPM, but I assume that was a typo, since I've never seen a bike without a harder gear than that.  My more realistic max out is 120 RPM where I'll be going 42.6 mph (which i regularly hit on downhills).  I'd say anyone on this forum can get above 30 mph with no problem and without maxing out, unless another reason is slowing them down, e.g. they are not confident enough (again, I don't think that applies to many people here, at least not for long).

So sit up with your 20 mph tailwind, apply almost no power and "sail" along.  Myself and the average triathlete will pass you with very little effort though.

Again, I'm not arguing that a tailwind going faster than you are will not push you along.  i'm arguing that it would almost never behoove you to do that in a race.


ETA: I realized I misused that 35 W number, so that number isn't valid, but the idea that it is very low and almost negligable is.

Edited by jsiegs 2010-09-27 2:17 PM


2010-09-27 2:31 PM
in reply to: #3119981

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Subject: RE: Aero position in a tailwind
jsiegs - 2010-09-27 2:14 PM
Rogillio - 2010-09-27 2:48 PM
jsiegs - 2010-09-27 1:26 PM
Rogillio - 2010-09-26 7:29 PM
gsmacleod - 2010-09-26 5:56 PM
Rogillio - 2010-09-26 12:49 PM

Looking back at my OP, I believe your proved my assertion to be correct and that the sail effect is not a myth at all but sound physics.  Like all things, there are limits and if you allow the wind to push you to the point where you are no longer going faster than the wind, the benefit is gone.


The problem with you assertion is that there is no point where, assuming you are still able to put power to the pedals, that you aren't facing a headwind.  If you aren't going to pedal when you have a tailwind, then by all means sit up to make yourself a sail.  However, if you are planning to put even a small amount of power to the pedals, then staying aero is still the better option.

In the attached file you can see that when air speed and ground speed are equal (and in the same direction) your position doesn't matter as you have zero air resistance due to the effective windspeed being equal to zero which only leaves the power to overcome rolling resistance (in this case, ~35W).

However, if the rider is still putting a small amount of power to the pedals, it will be easy to travel faster than the speed of the tailwind.  The second column shows what happens in the same situation if the rider applies enough power to travel only 10km/h faster than the tailwind (so 40km/h ground speed); the aero rider requires 61W while the rider sitting up as a sail, requires 67W.  So, in this case the aero position saves ~10% of the wattage required to travel 40km/h.

Shane


If the air is moving 10 mph faster than you are biking, what does getting 'aero' do for you?  The air out in front of you is moving away from you faster than you are moving.  

My assertion is 100% correct.  You are changing the problem to fit your argument.  Read my OP again and tell me if it is factually correct or not.  What I wrote, is "if the wind is traveling faster than you, it behooves you to sit up....".   You are changing the problem by saying if you continue to pedal, you will NOT be faster than the wind.  OK, if that is the case, read the rest of my OP, "if you are traveling faster than the wind, stay aero..".

~Mike


Well, obiviously it all depends if the air flow is laminar, and if you're dimpled.  Anyone know your Reynolds number?

Obviously, if the air is pushing you from behind, you will be best served by catching as much of it as possible, that's just common sense and sound physics.  The point gsmacleod and many other are trying to make is that that scenerio will virtually never play out in real world traning/racing situations.  For all intents and purposes (to us triathletes), the "sail effect" is a myth.  For what I would call impractical purposes, i.e. 200+mph tailwind, not pedaling, or pedaling @ < 35 W, it is very true.  So maybe if this were mythbusters, I might call it "plausible". 



In the real world most of us are not going to be biking 30+ mph...even with a 20 mph tailwind.   You also need to consider how push you still have left on say 53/23 gears at 30 mph.  So on paper you might be able to continue to pedal and gain speed indefinately but in practice, on flat ground there is a very real limit and you would be spinning like a hampster put not delivering much (if any) power to the drive train.

So you, et al can call it a myth all you want and I'll just sail merrily on.

~Mike 


Biking 30 mph with a 20 mph tailwind is only very slightly (something like 35 W someone mentioned ~ almost negligable) harder than biking 10 mph with no tailwind.  I'd say everyone on this forum could bike at least 10 mph and then add > 35 W to that. 

On my compact, at 100 RPM (very low cadence to max out, but it is something almost everyone can do), I hit 35.5 mph (50/11 combo, you'll go slightly slower in you std 53/12 - 34.5mph). At 53/23 you will max out at 21.6 and 120 RPM, but I assume that was a typo, since I've never seen a bike without a harder gear than that.  My more realistic max out is 120 RPM where I'll be going 42.6 mph (which i regularly hit on downhills).  I'd say anyone on this forum can get above 30 mph with no problem and without maxing out, unless another reason is slowing them down, e.g. they are not confident enough (again, I don't think that applies to many people here, at least not for long).

So sit up with your 20 mph tailwind, apply almost no power and "sail" along.  Myself and the average triathlete will pass you with very little effort though.

Again, I'm not arguing that a tailwind going faster than you are will not push you along.  i'm arguing that it would almost never behoove you to do that in a race.


ETA: I realized I misused that 35 W number, so that number isn't valid, but the idea that it is very low and almost negligable is.


Well that happens even when I'm peddling my heart out aero or not. 

Let me put it this way, if I can FEEL the wind on my back, it makes no sense to get tucked into aero position.....despite all the math and coefficients of drag and power equations.  Now if I continue gain speed I will eventually out-pace the wind again, then I will go back aero.



~Mike


Edited by Rogillio 2010-09-27 2:34 PM
2010-09-27 4:54 PM
in reply to: #3117983

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Subject: RE: Aero position in a tailwind
I was just thinking about two things: 1.) what is the best way for a cyclist to determine which way the apparent wind is blowing and 2.) is it better to hammer up wind and rest downwind, vice-versa, or to be steady-eddy and have same power all the time?

1.) Determinig apparent wind direction (i.e. yaw angle). 

About 15 years ago I used to fly sailplanes (those beautiful big-winged fibreglass planes with no engine). A very important piece of info a pilot needs to know is the wind speed and direction. To determine the direction of the wind, pilots tape a 4" piece of yarn to their cockpit glass. Voila - a simple way of determining which way the apparent wind is blowing. (see photo below). Usually the idea is to adjust the plane's yaw to take the wind head-on.



You could do someting like this with a bike. Attach a piece of yarn to the sides of your bull-horns. Whichever way the yarn is pointing - that's where the apparent wind is blowing. If its pointing forward - sit up, if backward get aero, if sideways - HOLD ON FOR DEAR LIFE!

2. Back to the scenario. OK - its a 40km bike course (out and back). The wind is howling at 20mph (32.2km/hr). Its blowing directly in the direction of the course so one leg is upwind and one leg is downwind. Are you better off hammering on the downwind and resting on the upwind, vice-versa, or just riding at constant power?  Quick, take a guess... what do you do on windy days? 

Well, here is what the numbers say:


You are best off HAMMERING it on the downwind and saving some energy on the upwind... you will be both faster and burn less energy!

--------------------------------------
P.S. Mike, Thanks for the positive feedback!

Rogillio - 2010-09-26 11:49 AM  FYI, I've hired a lot of engineers over the years and you have two qualities that often do not go hand-in-hand, good anlytical skill and good communications skills.  ~Mike




2010-09-27 6:04 PM
in reply to: #3120393

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Subject: RE: Aero position in a tailwind
mgalanter - 2010-09-27 4:54 PM I was just thinking about two things: 1.) what is the best way for a cyclist to determine which way the apparent wind is blowing and 2.) is it better to hammer up wind and rest downwind, vice-versa, or to be steady-eddy and have same power all the time?

1.) Determinig apparent wind direction (i.e. yaw angle). 

About 15 years ago I used to fly sailplanes (those beautiful big-winged fibreglass planes with no engine). A very important piece of info a pilot needs to know is the wind speed and direction. To determine the direction of the wind, pilots tape a 4" piece of yarn to their cockpit glass. Voila - a simple way of determining which way the apparent wind is blowing. (see photo below). Usually the idea is to adjust the plane's yaw to take the wind head-on.



You could do someting like this with a bike. Attach a piece of yarn to the sides of your bull-horns. Whichever way the yarn is pointing - that's where the apparent wind is blowing. If its pointing forward - sit up, if backward get aero, if sideways - HOLD ON FOR DEAR LIFE!

2. Back to the scenario. OK - its a 40km bike course (out and back). The wind is howling at 20mph (32.2km/hr). Its blowing directly in the direction of the course so one leg is upwind and one leg is downwind. Are you better off hammering on the downwind and resting on the upwind, vice-versa, or just riding at constant power?  Quick, take a guess... what do you do on windy days? 

Well, here is what the numbers say:


You are best off HAMMERING it on the downwind and saving some energy on the upwind... you will be both faster and burn less energy!

--------------------------------------
P.S. Mike, Thanks for the positive feedback!

Rogillio - 2010-09-26 11:49 AM  FYI, I've hired a lot of engineers over the years and you have two qualities that often do not go hand-in-hand, good anlytical skill and good communications skills.  ~Mike




  What assumptions are you making for loosing power pedaling?  Say you are in 53/12 and spinning 90 rpm while biking with 20 mph tailwind.  How fast can you go with a 20 mph tailwind?  You are chasing the law of diminishing returns, no?  At some point you asymptoticlly approach zero for power output and your efficiency approcaches zero?  On your webpage I think you have some out of range or unachievable speeds. 

I'm not communicating this very well.  Let me try this, how long can one really continue to output 200 watts?  If I'm being blown in a 30 mph tailwind, I find it difficult to believe that I can deliver more than a few watts to the drive train.  So when you put the limits the gear ratios in, at some point you are going to stop gaining speed as the rolling friction is greater than the minimal power you are putting out.  Does this make any sense?

~Mike




Edited by Rogillio 2010-09-27 6:08 PM
2010-09-27 6:47 PM
in reply to: #3120496

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Subject: RE: Aero position in a tailwind
Sunday as it's looking like horrible racing weather!
  • NE at 28 mph
  • Gusts: 47 mph

Wind




I know one thing, when your pushing hard for the first half of a race on the bike fighting a stiff headwind then you get a favorable change in direction and your cranking out serious speed but not getting any significant cooling effect because lack of a head wind on the body you can start to get (over) heated rather quickly, like riding a trainer with no cooling fan!

Edited by Donto 2010-09-27 6:48 PM
2010-09-27 7:03 PM
in reply to: #3112182

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Subject: RE: Aero position in a tailwind
At first this thread was lame as we have already vetted this subject, but now it's all sorts of interesting.


2010-09-27 7:32 PM
in reply to: #3120496

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Subject: RE: Aero position in a tailwind
Rogillio - 2010-09-27 8:04 PM

What assumptions are you making for loosing power pedaling?  Say you are in 53/12 and spinning 90 rpm while biking with 20 mph tailwind.  How fast can you go with a 20 mph tailwind?  You are chasing the law of diminishing returns, no?  At some point you asymptoticlly approach zero for power output and your efficiency approcaches zero?  On your webpage I think you have some out of range or unachievable speeds.


According to my numbers, a 32km/h tailwind with 200W gives a speed of 58km/h; that is about 100rpm with a 53/11 so very possible to continue putting 200W to the road in that situation.  Eventually you will run out of gears, however, spinning out a 53/11 should happen after 120rpm which is just over 70km/h.

I'm not communicating this very well.  Let me try this, how long can one really continue to output 200 watts?  If I'm being blown in a 30 mph tailwind, I find it difficult to believe that I can deliver more than a few watts to the drive train.  So when you put the limits the gear ratios in, at some point you are going to stop gaining speed as the rolling friction is greater than the minimal power you are putting out.  Does this make any sense?


Now, if you are looking at riding with a 30mph tailwind, things get a little crazy (however, very few people would actually ride with sustained 30mph winds) as with the 30mph tailwind, 200W would result in 71km/h (which would be 120rpm of a 53/11).  However, if that isn't possible due to cyclist skill, putting 100W to the pedals would result in 61km/h which should be possible with either a 53/11 or 53/12.

While the power to overcome rolling resistance does increase with speed, it is still only ~100W at 100km/h and since very few cyclists will ever see this speed and every trained cyclist can put out at least 100W, it is not of great concern to this situation.  You will be limited by your gearing well before the rolling resistance becomes a limiting factor.

Shane
2010-09-27 7:57 PM
in reply to: #3120606

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Subject: RE: Aero position in a tailwind
gsmacleod - 2010-09-27 8:32 PM

Now, if you are looking at riding with a 30mph tailwind, things get a little crazy (however, very few people would actually ride with sustained 30mph winds) as with the 30mph tailwind, 200W would result in 71km/h (which would be 120rpm of a 53/11).  However, if that isn't possible due to cyclist skill, putting 100W to the pedals would result in 61km/h which should be possible with either a 53/11 or 53/12.


That's why I run 55/9.  Just to be sure I can put the power down in a 30mph tailwind.
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