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Headwind vs Tailwind Perceived Effort WTF

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Headwind vs Tailwind Perceived Effort WTF

Old 05-19-11, 06:35 PM
  #1  
RecceDG
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Headwind vs Tailwind Perceived Effort WTF

Maybe someone can explain this to me, because I am baffled.

OK, the primary source of drag/resistance on a good road bike is air resistance. Bearing friction is minimal and linear with speed. Tire rolling resistance, also minimal and linear with speed. Air resistance starts low but is cubic with speed so builds very quickly.

So on flat ground, what really matters is the relative air speed. The higher the air speed, the more power required to overcome the drag.

So if we assume power to be more or less constant at a perceived effort level, one would expect that, within reason, (ie ignoring hurricanes) the net effect of a headwind or tailwind will be to change the ground speed.

In other words, if a rider can put out x watts at a given level of perceived exertion, and the air speed at which the power consumed by the drag equals x is n km/h, then a headwind will subtract ground speed and a tailwind will add it (assuming direct head and tail winds)

But you should be able to blindfold the rider, and, lacking visual cues as to groundspeed, he should be unable to differentiate between a headwind and a tailwind - because in either case, the limiting factor is airspeed not groundspeed.

But I'm not finding this to be the case.

Today's ride was a 50 km out-and-back over flat roads. "Out" was a headwind, "Back" a tailwind. Bearing was almost perfectly on the nose/tail, to the point where the windmills at the end of the road were pointed directly at me. I don't know wind speed, but somewhere between Beaufort 3 and 5 based on observations.

On the way out I was fighting to make 30 km/h, and was usually somewhere between 26 and 28 km/h. Legs felt like ****; heavy and leaden. No jump, no endurance, any attempt to build speed immediately fizzled out. I just wanted the damn ride to end. Heart rate 145-152

On the way back, speed jumped to 41-50 km/h. Actual measured average 41 km/h, but there are two stop signs that kill the average; moving average is 46 km/h. Heart rate 158-168. Tons of strength in the legs, plenty of jump, and could have cruised at 45 km/h all day (getting up to 50 started noticeably increasing fatigue)

Whiskey. Tango. Foxtrot.

What is crazy about this is that, using heart rate as a measure of exertion, I was making a much larger effort with the tailwind than with the headwind (and that jibes with my subjective observation as well) With the tailwind, I felt strong, with the headwind, I felt like ****.

But the resistance I was working against should have been equal in both cases - groundspeed shouldn't matter.

I wish I had a power meter so I could see if the actual developed power was the same... but based on heart rate, my guess is that I was putting out less power in the headwind than the tailwind.

Anybody have any insight into this phenomenon?

DG
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Old 05-19-11, 06:41 PM
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Morale?
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Old 05-19-11, 06:41 PM
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maybe because headwind works doubly against you by slowing you down and thus increasing the amount of time you need to sustain a given exertion level
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Old 05-19-11, 06:51 PM
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Originally Posted by RecceDG View Post
I wish I had a power meter so I could see if the actual developed power was the same... but based on heart rate, my guess is that I was putting out less power in the headwind than the tailwind.

Anybody have any insight into this phenomenon?

DG
Moving object tends to keep moving... If you got a tail, you are more likely to go faster and faster because it's easier and the perception of speed makes you wanna go even faster.

Headwind is the opposite. It's kind of demotivating and you are basically fighting against yourself because you feel like you are pushing hard and then you look at a computer. Not a good feeling.

I say forget about stats and other stuff that doesn't matter. Pedal with your whole being and dig down deep. It is what it is. Wind can be your friend or your enemy OR it can be just a wind. Your choice.

Last edited by spock; 05-19-11 at 07:04 PM.
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Old 05-19-11, 07:00 PM
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It's gratifying to know that headwinds suck as much in the Great White North as they do down here.

My reaction to headwinds is the opposite of yours - my HR goes up quickly and easily in headwinds, and I can't elevate it for beans in a tailwind. Not good when you're doing intervals on an out-and-back route!

My guess is that you're closer to your limits when fighting a headwind, and the combination of your speed and the wind's are higher than you realize. If you're going 30 kph in a 30 kph headwind, your effort is almost going to feel like you're going 60 kph in still air. That might explain why you felt you had no jump.
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Old 05-19-11, 07:06 PM
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If you were doing a group ride, it makes sense.

Headwinds increase the delta in effort between the riders pulling and those sitting in. The front rider may be doing 500w, the ones sitting in 150w.

On a tailwind you have to work more consistently. The rider pulling may do 500w again, but the ones following may have to do, say, 350w to follow.

Also the speed variance is smaller in a headwind, maybe not in percentage but definitely in mph. So you may go +/- 2 mph in the headwind (18-22 mph) but +/- 8 mph in the tailwind (26-34 mph).

When it's really windy in a flat race I usually get dropped in a tailwind bit. In fact my second last race I got shelled as we turned onto a tailwind section. The headwind was easier - I could sit in. Tailwind... not so much. This was with a couple really strong riders putting down the hammer, and they were probably going relatively steady.

If you were riding solo, then I chalk up your higher tailwind HR to glee and joy.
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Old 05-19-11, 07:41 PM
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Originally Posted by RecceDG View Post
Air resistance starts low but is cubic with speed so builds very quickly.
That's not right.

Originally Posted by RecceDG View Post
So if we assume power to be more or less constant at a perceived effort level,...
That's a big assumption and certainly not supported by the experiences I'm aware of. Different inertial loads (as with head vs. tail winds) lead to different neuromuscular demands at equal effort leading to different RPE. It's easily seen riding at the same power up and down a hill.
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Old 05-19-11, 07:43 PM
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I was solo - I have to do a little more conditioning before I'll be ready for group rides.

Lots of suggestions that it's a morale issue... I don't think so.

I'm a data-driven guy (see former life as a racing engineer) and I don't take data points personally - in other words, when I look down and see a "26" on the speed, I don't get all dejected; I know that the headwind limits groundspeed so it is what it is.

The part that is baffling me is that the subjective effort was so clearly different. When I say that my legs felt like lead in the headwind... that's not hyperbole. I quite honestly had NO power in them. Any attempt to put in an effort that would raise heart rate higher than it was died almost immediately.

But with the wind behind me, suddenly the legs are OK? Ca ne fait pas de sense le.

The response that the wind speed might be higher than expected, thus killing the effort... there might be something to that. My observations of Beaufort 3 to 5 is a range of 12-38 km/h. Assuming 28 km/h groundspeed, that puts airspeed somewhere between 40 and 66 km/h, which is actually quite a bit higher than I'd expected.

That makes the tailwind airspeed somewhere between 34 to -20 km/h... it definitely wasn't negative; I felt wind on my face at speed. But yeah, that plus the higher rolling drag at speed might be enough to do it.

Maybe I'll source a hand-held anemometer and see what it says.

Power required to overcome wind drag:



Linear with area increase, drag coefficient increase, and cubed with speed increase.

DG

Last edited by RecceDG; 05-19-11 at 07:53 PM.
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Old 05-19-11, 07:47 PM
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Was your cadence the same into the wind verses with the tailwind?
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Old 05-19-11, 08:01 PM
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are you factoring in for the fact that the wind is not 100% straight and is actually on an angle, which means you have to calculate for resistance from both EW and NS?
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Old 05-19-11, 08:09 PM
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Originally Posted by RecceDG View Post
The part that is baffling me is that the subjective effort was so clearly different. When I say that my legs felt like lead in the headwind... that's not hyperbole. I quite honestly had NO power in them. Any attempt to put in an effort that would raise heart rate higher than it was died almost immediately.
I have noticed sometimes, depending on a fit, when you have a head wind compared to tail wind, my riding position would change for some reason and that would effect my comfort and my ability to give the effort. It is baffling I must say, but I think there might be something that has to do with gravity and speed. You have something pushing you down and from the front when there is a head wind. That's a double whammy. I don't know how to explain this, but i think there is something to it. Headwind gives you this / slope and tailwind gives you this \ slope.

Last edited by spock; 05-19-11 at 08:15 PM.
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Old 05-19-11, 08:13 PM
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Originally Posted by spock View Post
I have noticed sometimes, depending on a fit, when you have a head wind compared to tail wind, my riding position would change for some reason and that would effect my comfort and my ability to give the effort. It is baffling I must say, but I think there might be something that has to do with gravity and speed. You have something pushing you down and from the front when there is a head wind. That's a double whammy. I don't know how to explain this, but i think there is something to it.
gravity only comes into play when you are on a slope or off the ground. When you are on the ground with no slope, then Force normal will cancel out force gravity.
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Old 05-19-11, 08:14 PM
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Originally Posted by RecceDG View Post
But you should be able to blindfold the rider, and, lacking visual cues as to groundspeed, he should be unable to differentiate between a headwind and a tailwind - because in either case, the limiting factor is airspeed not groundspeed.

What is crazy about this is that, using heart rate as a measure of exertion, I was making a much larger effort with the tailwind than with the headwind (and that jibes with my subjective observation as well) With the tailwind, I felt strong, with the headwind, I felt like ****.

Anybody have any insight into this phenomenon?
Sure. Inertia is purely a function of ground speed and you have more of it to carry you through the dead spots in your pedaling circle when you have a tail wind at a given power output since you're going faster.

Riding inside on a trainer with a puny little flywheel and negligible inertia feels harder (at least initially; I only racked up 14 hours in a couple weeks when I'd had surgery and was forbidden to ride outside) at the same measured power than road riding and lots of people can't manage the same power on a trainer as they can on the road.

The same situation exists with a lesser degree comparing climbing to riding on flat ground or down hill.

It's also worth noting that 'flat' ground isn't. At 18 MPH with no wind it can take 50% more power to go up a .3% false flat than down it even though it doesn't look like there's a hill.

Last edited by Drew Eckhardt; 05-19-11 at 10:48 PM.
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Old 05-19-11, 08:31 PM
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Originally Posted by AEO View Post
gravity only comes into play when you are on a slope or off the ground. When you are on the ground with no slope, then Force normal will cancel out force gravity.
Force normal cancels out force gravity, but force head wind, will have an effect on force gravity and force normal on the object of force. Force tail wind has just the opposite.
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Old 05-19-11, 08:34 PM
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Air resistance starts low but is cubic with speed so builds very quickly.
Resistance increases with the square of velocity. Power required is cubic. I know that's what you meant, but from your posts I figure you'd appreciate having the trivial error eliminated.

But you should be able to blindfold the rider, and, lacking visual cues as to groundspeed, he should be unable to differentiate between a headwind and a tailwind - because in either case, the limiting factor is airspeed not groundspeed.
He would know by cadence. In fact, that's what I'd investigate first regarding your subjective effort question. I'll take it for granted that the gears chosen are optimal - given the speeds you're able to handle routinely. But is it possible that your cadence is subtly changed against the expectation of greater effort, perhaps reducing your efficiency?

I'd also want to check the actual wind speed and compare your relative velocities in both directions. I think you questioned this yourself, that you may have simply pushed a higher windspeed traveling into the wind.

The last little thing I can think of is going downhill into a headwind. I know you said it was mostly flat but I have little doubt that you recognize even small inclines more or less automatically. With a headwind, going downhill, you might be subconsciously expecting to attain a higher speed (given the deceptive nature of higher windspeeds cubed) and giving extra effort there.
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Old 05-19-11, 08:53 PM
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Originally Posted by spock View Post
Force normal cancels out force gravity, but force head wind, will have an effect on force gravity and force normal on the object of force. Force tail wind has just the opposite.
no it won't, because force wind is perpendicular to force gravity, which means they are unrelated.
that's the reason that two objects will hit the ground at the same time when one is dropped and another is launched horizontally.

unless you have a function that proves force gravity has an effect on wind, which would only happen if force wind is blowing downwards on an angle.
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Old 05-19-11, 09:06 PM
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no it won't, because force wind is perpendicular to force gravity, which means they are unrelated.
Maybe he's talking about the lift force on his torso from the headwind. It would change his balance, hence possibly he alters his position. If so that would seem to indicate that he tends to push harder -faster wind-speed - into a headwind.
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Old 05-19-11, 09:40 PM
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Originally Posted by AEO View Post
no it won't, because force wind is perpendicular to force gravity, which means they are unrelated.
that's the reason that two objects will hit the ground at the same time when one is dropped and another is launched horizontally.

unless you have a function that proves force gravity has an effect on wind, which would only happen if force wind is blowing downwards on an angle.
They are all just forces, but if there is an object that uses its own power to go with those forces, they kind of combine and effect that object in certain ways. Muscles that do the pedaling also use that force to carry or propel themselves, so the external forces are that much more powerful in effecting the way those muscles act or the way we perceive our predicament.

It's all kind of visual to me, so I'm having some trouble putting it into words. Go inside my head and come out, and tell me I'm wrong.
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Old 05-20-11, 12:06 AM
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Unless you have a ground speed greater than the wind speed, you will always have a "relative" headwind. If you are in stagnant air your perceived headwind is equal and opposite your ground speed. A 5kph headwind relative to the ground vs a -5kph tailwind relative to the ground is a 10kph spread. Since effort to overcome air resistance is exponentially increased with speed, there is a tremendous increase required to overcome that 10kph difference. Tailwinds dont push you, they reduce the headwind.
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Old 05-20-11, 12:38 AM
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I have the same problem. Very difficult to get my HR up in a headwind. I think a lot of it is the distraction of the buffeting. Increasing cadence seems to help.
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Old 05-20-11, 02:06 AM
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Originally Posted by znomit View Post
Increasing cadence seems to help.
For a high HR, do this.
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Old 05-20-11, 04:40 AM
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You keep using the word "perceived" which is subjective. My question is about making other factors equal. Did you remain in the same gear and at the same cadence throughout the entirety of the trip? I would think that would have an effect on your "perception."
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Old 05-20-11, 07:47 AM
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Originally Posted by RecceDG View Post
But you should be able to blindfold the rider, and, lacking visual cues as to groundspeed, he should be unable to differentiate between a headwind and a tailwind - because in either case, the limiting factor is airspeed not groundspeed.
Incorrect.
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Old 05-20-11, 08:33 AM
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I bet with a power meter you would have seen you were putting down more power into the headwind. I seem to do this too because I bike with a computer and don't like seeing the speed drop below a certain threshold--when really I should just ignore it and go with power output.

I like playing with bikecalculator.com so that I can understand the expected speed difference at constant power by putting in grades/headwinds/tailwinds.
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Old 05-20-11, 12:35 PM
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Originally Posted by RecceDG View Post
In other words, if a rider can put out x watts at a given level of perceived exertion, and the air speed at which the power consumed by the drag equals x is n km/h, then a headwind will subtract ground speed and a tailwind will add it (assuming direct head and tail winds)
This is the root of your dilemma. Excluding rolling resistance and other non aero effects, power is equal to force x velocity. In this case, the force is due to aero drag and is proportional to the square of the net wind speed. The velocity, however, is your actual ground speed.

For example, if you have a 10kmh headwind your ground speed will be lower than riding with a 10kmh tailwind so in order for the power to be equivalent the drag force will need to be higher when riding into a headwind. The only way to have a higher drag force is to have a higher net wind speed. So when you ride into a headwind the net wind speed is considerably higher than riding in a tailwind.

Some actual numbers (from kreuzotter.de):
Headwind:
P = 300W
Wind = 20kmh
Speed = 27.6
Net Windspeed = 47.6

Tailwind
P=300W
Wind = -20kmh
Speed = 52.3
Net windspeed = 32.3

But you should be able to blindfold the rider, and, lacking visual cues as to groundspeed, he should be unable to differentiate between a headwind and a tailwind - because in either case, the limiting factor is airspeed not groundspeed.
See above. Note that the relative windspeed 47.6 vs 32.3 generates about twice as much drag ((47.6/32.3)^2 = 2.1) but the ground speed is roughly half as much.
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