Prowler 

It could have been a lovely day. Set out on my usual 32 miles ride on the SRT here in Philly and it went just great (on the '76 Raleigh Pro Mk IV). "Oh, look how fast I am! Must really be in shape". After a few miles I knew I was in trouble as there's only one reason (other than drugs) for a step change in performance: strong tail wind. The SRT follows the Schuylkill River (surprise!!) so if there is wind, often from the west or northwest, you get it all, unmolested or abated by trees and buildings. So I had a fast trip out 16 miles then turned and, ugh, hit the headwind. 16 miles back to the car.

After about 45 minutes in the drops, grinding away at that and getting bored of staring at the shinny fork legs on the Raleigh (lovely British chrome though) I started wondering. Wondering things I would normally pay no attention to. So:

What say you physics majors? Is the wind like ying and yang in that the advantage of a tail wind is balanced by the disadvantage of the same wind on your face? Or is the advantage in speed and reduced effort of a tail wind wiped out by the increased work effort and drop in speed of the headwind on the way back? I am convinced that the 'free ride' of a down hill section does not make up for the added effort to get up the next equally tall hill. Net increase in energy. A hilly loop is much harder, albeit better exercise. A windy hilly circuit is, sometimes, enough to make you cry.

So how about headwinds and tailwinds? I rode a metric century out of St Michaels, MD - all flat but a wicked headwind off the bay for half of it. Even with long rest stops I was pretty wobbly when I got back to the car.

Chrome Molly

I think you are correct, in the sense there is an unequal gain/loss.

From what I understand, wind causes more effort when riding unless it's about 3/4 of the way behind you. That is, if you're heading north, the wind is of no real benefit unless it's coming from the SSE or SSW. Conversely, when you turn around and head south again, you're going to really feel that SSE or SSW wind working against you. A pure cross wind adds effort.

My fastest days are humid with close to zero wind. I'm no rocket surgeon though, so someone will come along and tell you why.

RobbieTunes

There is never a tail wind. It's just a good day.

A head wind is just the way it is, a metaphor for my life, or a theme for Bob Seger...

I think I read the same article C.M. did, complete with diagrams.
A 3/4 tail wind is the cat's pajamas.

In my opinion, the headwind/tailwind thing is not a wash, but it's darn close.
Never seems like it while you're riding, though, but Mr. Garmin seems to confirm.
I just don't think the tailwind makes up for the cursing and frustration, if you allow yourself that.

While C.M. likes calm humid air, I've done triathlons in the dead air around NC coastal plain lakes.
While it's not wind, it sucks, and the entire field was much slower than I know them to be.
It seems thick.

I do like near-zero wind, though. No headwind, and a chance at consistency.
My best 40K triathlon leg was on a near-zero wind day. It was one less thing to worry about.

By the way, "calm" wind is less than 5 knots. Hardly calm if you're on a bike, though.
Also, a "standard" 5'x3' flag, straight out from the flagpole, takes about 12 knots.

I'm not a physicist, and my "sail area" is substantial, but I do watch the Big Bang Theory once in a while.

iab

I find I will do a pr when there is little to no wind.

More wind, slower times. Any advantage a tailwind my provide does not overcome the penalty of a headwind.

Sitting in though makes me even faster.

Velognome 

Head winds are always stronger that tails winds.....it's just life.

RR3

Ten mile out and back course into a 20 mph head wind and then downwind after the turn around.

10 mph into the 20 mph head wind and 30 mph downwind provides about the same effective windspeed. 20 mph average, right? (I am old and slow....plus, these are easy numbers to make the point)

Of course the average would 15 mph....it took 80 minutes to go 20 miles but wind is never right at you or right behind, so this illustrative example is extreme. With no wind, the time would be closer to 1 hour.....20 mph out and 20 mph back.....30 minutes for each leg.

No real need for a Physics degree

jetboy 

I actually am a physicist (or was) and as far as the world knows its a wash- the real questions is probably for a biologist or kenesthesiogist. is your body able to equate the advantage of the tail wind with the disadvantage of the head wind? I suspect not.

if you were a robot it would be easier.

Sir_Name 

Good point. As I see it, the headwind creates two penalties: its slows you down as it pushes into you, and you burn more energy (expend calories) as you push into it. It slows you down twice, one short-term, one long-term. I'd guess your gain from a tailwind is more one-dimensional. -- You burn more energy pushing into the wind than you get from it pushing at your back. Data trumps hypotheses, so take that for what it's worth. Also:
Aerodynamics play into the balance of the wind affecting the rider from varying directions (wind pushing you more effectively from one direction over another, i.e. from which direction the rider is most/least aerodynamically efficient - assuming average road rider and gear with hands on the drops tuck, airflow from/over the back may be more laminar than coming into the pocket between the rider's head/arms/torso where airflow may be a bit more unruly/turbulent). Plus rider/wind relative speed having an effect. Plus...

jimmuller 

I have a marvelous proof of this concept but there is not enough space here to write it.

Sir_Name 

I for one am interested. Any link?

belacqua

Fermat's Last Theorem

I think this is a running joke among mathematicians.

JohnDThompson 

The biggest problem I see is that starting out with a tailwind deceives you into biting off more than you can chew. Payback comes when you turn around after all those delightful tailwind miles.

Velognome 

Gentlemen, avoid headwinds by tacking

old's'cool

Part of the problem is that aerodynamic drag increases with the square of the resolved airspeed. The other part is the inequality of frictional effects in positive versus negative direction.
It is easy to figure out if you take it to an extreme.
Lets assume there is a wind speed high enough to produce so much drag, that nearly all your surplus effort is expended in overcoming the static friction of the machine (drivetrain friction and rolling resistance), with virtually nothing left to make progress against the wind. So your trip time against the wind tends toward infinity. To even have a chance of making it up on a round trip (which you don't, since the upwind leg trip time is already approaching infinity), your trip time with the wind would need to tend toward zero, which it does not (rather, your velocity would max out at (resolved wind speed) + Xmph, X being the velocity you can add via pedalling, taking into consideration practical gear ratios, cadence, mechanical resistance, and sustainable watts). So your downwind velocity would be finite, suggesting that the inverse, i.e. the trip time, would not tend toward zero.
Compare this with a zero wind situation, where you can average a respectable Xmph in both directions, and thus a finite trip time in both directions. Much better than a trip time tending toward infinity on the upwind leg, not even considering additional time of the downwind leg.

xiaoman1 

Not a physicist either, so this equation may not be the correct/best one to use.....but as mentioned you should factor in your loss of performance as muscles fatigue....as jetboy says we're not robots and non controlled environmental conditions will exist eg. wind direction, wind consistency and so on.
F[SUB]d[/SUB] = c[SUB]d[/SUB] 1/2 ρ v[SUP]2[/SUP] A
c[SUB]d = drag coefficient = 1.1
[/SUB]A = characteristic frontal area of the body = 5.5
Disclaimer use above at your own risk....when in doubt wet finger hold up in the air, cool side direction of the wind....head down ....tail wind open jacket, head wind zip-up, pedal harder.
Regards, Ben

thinktubes 

Wind Map (dataviz)

FBinNY 

Consider a typical example with easy numbers. You're riding 20mph into and down wind.

First, keep in mind that wind drag is proprotional to the square of wind speed, so if you're riding 20mph in still air, the drag would be proportional to 400.

Now, do the same into a 10mph wind and you have 30mph wind speed, and drag proportional to 900. When you turn around, the wind speed drops to 10mph with drag proportional to 100. So into the wind, you add 500 drag units, and down wind you get back 300 drag units. Not exactly an even trade.

repechage

Not always possible on a bike. however, as one rides through a wind coming from just forward of abeam, you develop apparant wind that moves forward and makes you feel as it you are heading into one more Than the true wind if you were stationary.
When sailing a boat at best is sailing 45 degrees into the wind, but the apparant wind angle will show about 33 degrees.
A true headwind is an emotional killer.
I have had it double a commute time, gets depressing going down a descent in a small ring and having to work for the km.

Jeff Wills

Another inequality is that wind will shorten the amount of time you ride with the wind, but lengthen the amount of time you ride against it on a simple out-and-back course. Around here, it's sometimes possible to use the terrain to shield yourself from the wind coming out of the Columbia Gorge, then catch a one-way 20mph tailwind. Just like surfing, baby.

Ed.

Take up sailing and it will all be apparent.

repechage

Want to really get befuddled? Watch a foiling Moth.

Jeff Wills

Careful there- this is a family forum, buddy.

davester

The elephant in the room is that there is never a tailwind. If you hit a strong headwind on the way out expecting that you'll get a tailwind on the way home then the wind will shift by 180 degrees while you're eating lunch. Tailwinds only happen in your fantasies. Deal with it!

Prowler 

Did you really get all this from Mad magazine? I seem to remember that issue.

In my stupor, yesterday, I did ignore aerodynamics. I see where the wind force helping you on the downwind leg (over your smooth back and around your smooth profile) would be much less than the wind force against you going upwind (duh!!). Now I wonder if, in a stiff headwind, do we harm our cause by riding in the drops? All curled over we create a pocket that may create more drag than the more open profile when sitting up. Nah, can't be. I do know that after an hour in the drops my back hurt and my neck ached. AOK today though.

I'm not a physicist but I do wear glasses and a pocket protector and I stood next to one in a lunch line one time. I could, maybe, even find my old K&N slide rule if I tried hard enough. That orange case should catch the flash light beam.

RR3

Do you really think it is a wash? Not even close. Here is calculator to play around with....


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