dmitrivich

I'm usually an evening rider, and where I live, the geography virtually guarantees a 8-15 mph headwind on the way out on my ride (south), and thus the equivalent tailwind on the way back (north). I actually enjoy this, as it's the only thing resembling a "hill" in my area, and it's nice to have the hard effort early on. Still, it is a bit weird always heading out at 15-16 mph and feeling sluggish, and coming back at 24-26 mph. I work hard on the way out, but I think I may take it a bit too easy on the way back, because I already feel as though I'm moving fast.

My question, though, is whether the headwind and tailwind cancel out exactly (let's assume an out and back course; I think I can understand why they wouldn't in a loop exactly - extra work when the wind blows from the side). The ground is completely flat. Does anyone know the actual aerodynamics/physics behind this (rather than just a guess)?

All other things equal, would I expect my average speed on a day with wind (most days) to be the same as that on a still day?

Poppaspoke

You spend more time going slow than you do going fast (to cover
the same distance)...so the average speed is pushed down.

The same holds true for riding hilly terrain: it takes longer to go up the
hill than back down it. You spend more time riding slow.

salsabike

Okay--here's the word from my engineer spouse on exactly how headwinds affect you.

The force that acts against you riding into the wind is proportional to the square of the relative wind velocity. If you are going into a 10 mph wind at 10 mph, the force is K*(10+10)^2 = 400 * K. If you are going 20 mph into a 10 mph wind, the force is K * (10+20) ^ 2 = 900 * K

The multiplier K depends on your frontal area and how aerodynamic you and your bike are, and the density of the fluid you are moving through. It's harder to bike through water.

gcl8a

No, they don't cancel. All else being equal (eg, you're not changing your position on the bike), your average speed will be less on windy days, just as your average speed over hills is less than on flats.

The 'common sense' argument is as follows: For a given distance out and back, you spend more time going slower and less time going faster. Let's say your normal average speed is 20mph. If you went 20 miles out and 20 miles back, it would take exactly 2 hours. Now let's say the wind makes you go 15mph out and 25mph back, your total time is 1.33+0.8=2.13 hours.

That assumes of course that the headwind hinders you as much as the tailwind helps you (+/- 5mph). In reality, it's even worse than that (I think, but I'll have to do some math, first).

gcl8a

To elaborate:

The power needed to overcome wind resistance is related to the bike speed times the square of the bike speed plus headwind:

power = Cd * v * (v + wind)^2

The last term is, as salsa notes, related to the force needed to move through the wind, and the first 'v' converts it to the rate you must do 'work' to overcome the resisting forces (remember force times distance is work, or energy, and force times speed is power).

Cd is the magic coefficient of drag which takes into account bike position, air density, etc., but the problem is that Cd itself if a function of speed and wind speed. _Assuming_ Cd is more or less constant (although I seem to remember that at low speeds it goes like 1/speed, so you get a v 'back'), then we can play with some numbers:

Let's say you do 20mph at 400W (you've dialed it up, of course), then Cd=0.05. Working through some math, with a 5mph headwind, your speed at 400W is 16.813mph, while with a 5mph tailwind, I calculate 23.465mph. Interestingly, the headwind slows you by 3.2, but the tailwind helps by 3.5. (This seems counter-intuitive to me; anyone catch any mistakes?)

Your 20 miles out and back (see my other post) now takes 20/16.813 + 20/23.465 = 2.042 hours.

DrPete 

Just be careful of the train that left a station 50 miles away 1 1/2 hours ago traveling due west at 50mph.

DrPete

rruff

No... wind always slows you down. I made a spreadsheet awhile back to calculate speed vs power, so I just plugged in some numbers to give you an idea. If you can average 20 mph on a windless day, then a 8 mph headwind/tailwind will change your speed to ~15.55mph going out, and ~25.06mph coming back, for an avg of 19.19mph. Note that an 8mph wind at bike level relates to about a 15mph wind as reported at 30ft. Bushes, trees, houses, etc can reduce this even more.

Using the average of the out and back runs, ie (15.55+25.06)/2 = 20.30mph ...which is closer to what you'd do on a windless day.

Sidewinds slow you down, too... an 8mph sidewind would slow you down to 19.54mph.

spunky

Just ride harder into the wind to bring your speed back up.
It's great resistance training.

merlinextraligh

There are a couple of reasons that wind slows your average down, even on an out and back. The first has to do with the squared function of wind resistence as set forth above. But its get worse because the above analysis assumes a pure headwind and a pure tailwind. However in the real world almost all winds are to a degree cross winds. This adds an element of Vector physics, with the bike's speed and direction being one vector, and the wind being another vector. Assume straight ahead is 12 O'Clock on clock dial For most riding and wind speeds, it ends up that any wind from 8am to 4pm hinders you to some degree. And only winds from 4pm to 8am help you. That's why it feels like most winds are headwinds, because they are. Bicycling had an article on this with graphs that explained it much better a few years ago..

Jinker

The other factor at play is that the wind ALWAYS seems to swing around to a headwind if I'm riding in any direction for more than 5 minutes or so.

Wind sucks when trying to maintain a draft too... A headwind will slow the group down a lot. A tailwind, while picking up your speed, will reduce the benefit of the draft (you'll have to work harder to keep with the leader), and crosswinds blow the draft sideways across the road, usually over a ditch or into a lane of traffic, where it's less useful.

Mind you, if you stop staring at your speedo and focus on the ride itself, you can either enjoy yourself just as much, or if you're in 'training' mode, work equally hard in any wind.

dmitrivich

Thanks for the info, everyone.

caloso

I have nothing to add to this august scientific discussion but just wanted to say, I feel ya man. There are lots of days that I'll ride 5 miles extra past my house just so I can have a tailwind home.

va_cyclist

Just another variable to consider, but you get more cooling effect from a headwind on a hot day.

bmclaughlin807

I love the wind.... on the way home it's always blowing down the river I'm riding on, (I'm going slightly uphill, into the wind) ...

No problem, it'll be a slight cross wind on the real upwind portion of my ride, right?

Nope... I turn 90 degrees and start the climb, as soon as I get up away from the river, the wind is right in my face again.

Gotta love the way wind behaves around obstacles and terrain.