Out and Back Wind Effect...
03-29-16, 01:47 PM
#26
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Originally Posted by Campag4life
when the wind is at a rider's back the force of the wind is acting on a smaller drag coef....a convex surface versus a direct headwind where windspeed + bike speed is acting on a concave surface resulting in a higher drag coef. Putting it in more layman's term. The body from behind is less of a sail then a wind into the body. The body natively catches more air when wind is into it.
Originally Posted by FBinNY
So if one starts out in a headwind, and turns around when he reaches the halfway point in time, he'll ride home faster and get home sooner than he expects.
03-29-16, 02:05 PM
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Originally Posted by Campag4life
No it isn't for the reason I explained with due respect to FB. The reason is..when the wind is at a rider's back the force of the wind is acting on a smaller drag coef....a convex surface versus a direct headwind where windspeed + bike speed is acting on a concave surface resulting in a higher drag coef. Putting it in more layman's term. The body from behind is less of a sail then a wind into the body. The body natively catches more air when wind is into it.
The wind in front always produces more drag than the same wind reduces in back. Because drag proportional to apparent wind speed squared.
03-29-16, 02:06 PM
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It seems on windy days (+/- 18) my average about 1 mph less over the same E-W 26 mile course, wind E or W of course. I can't explain it, it just seems to be that way often
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03-29-16, 02:07 PM
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Originally Posted by PepeM
...
If he turns around at the halfway point in time, then he will reach home exactly when expected. If he turns around at the halfway point in distance, then he will reach home earlier (if he thought the out was going to take the same amount of time than the back.)
If he turns around at the halfway point in time, then he will reach home exactly when expected. If he turns around at the halfway point in distance, then he will reach home earlier (if he thought the out was going to take the same amount of time than the back.)
Ben
03-29-16, 02:13 PM
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One thing only one or two posters mentioned is that rarely as riders do we present the same aerodynamic drag area and coefficient when riding upwind vs riding downwind. We almost universally sit up, sometimes a lot when headed downwind. I rode about 30 miles upwind Sunday with the flags straight out. I was in the drops, arms bent or forearms horizontal nearly the entire time. Coming home, I did long stretches riding no-hands. (I was riding fix gear so that put something of a limiter in my downwind speed but I did flip the wheel and rode home one tooth higher than going out.)
BEn
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03-29-16, 02:20 PM
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Originally Posted by Campag4life
Thought I would posit a question to those that have thought about it. This question is part comedy but nonetheless rooted in reality aka aerodynamics...
When wind is say 10-20 mph and a route is out and back...my route is generally 10 miles + North and then 10+ miles South return. When the wind is dead downwind one direction and a pure headwind back, how much does average mph vary for the round trip compared to the same route on a dead calm day?
When wind is say 10-20 mph and a route is out and back...my route is generally 10 miles + North and then 10+ miles South return. When the wind is dead downwind one direction and a pure headwind back, how much does average mph vary for the round trip compared to the same route on a dead calm day?
Riding 20 miles out then 20 back in no wind takes one hour each way, two total, and yields a 20 MPH average.
Riding the same with a 10 MPH wind along the course allows a 10 MPH ground speed into the wind taking two hours, 30 MPH ground speed with the wind which takes 40 minutes, and 40 miles in 2:40 is a 15 MPH average.
Rolling resistance makes the arithmetic more complex and slightly reduces the speed difference changing directions.
03-29-16, 02:20 PM
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Originally Posted by caloso
I've done long intervals (such as 2x20's) on an out and back course in the wind. It's much easier to keep the target wattage going into the wind than with a tailwind. I think mostly it's psychological.
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03-29-16, 02:47 PM
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Also realize if this "in and out" ride is only 20 miles, it takes some people (me included) about 10+ miles to totally warm up. The first few miles I tend to spin, then gradually increase power. I don't often push harder until mile 10+, so it's difficult to compare power in this scenario. If I warmed up for 10 miles, then did the "in and out" ride it would get a better comparison. I'm too lazy at the moment to look at the power numbers for this type of ride, which I do often.
Regarding the windy vs. calm days, I agree that the aggregate speed during calm days is often higher, especially when riding solo. Oddly enough, it seems when doing group rides the aggregate speed during windy rides if often faster, since some of the stronger riders can push through the wind to help the full group, while most can haul ass in the downwind.
Regarding the windy vs. calm days, I agree that the aggregate speed during calm days is often higher, especially when riding solo. Oddly enough, it seems when doing group rides the aggregate speed during windy rides if often faster, since some of the stronger riders can push through the wind to help the full group, while most can haul ass in the downwind.
03-29-16, 02:54 PM
#34
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Alright, simple math time.
Lets assume a perfectly flat out and back ride, 10km each way. Our rider has a CdA of 0.32 m^2, density of air that day is 1 kg/m^3 and our rider has some really nice tires which have zero rolling resistance.
The only force the rider is facing is drag Fd = 0.5 * 1 * 0.32 * v^2 = 0.16 kg/m * v^2
Now lets say our rider goes out on a windless day and keeps a constant 200W throughout.
P = W/t = F*d/t = F * v = 0.16 * v^3 = 200W
Our rider would keep a constant velocity of 10.77 m/s or 24mph on a windless day.
Now he goes out on a day where the wind is blowing at 22.4mph (10 m/s)
For the headwind leg:
P = F * v = 0.16 * (v + 10)^2 * v = 200W
v = 5.32 m/s for 31.3 minutes
Tailwind leg:
P = F * v = 0.16 * (v - 10)^2 * v = 200W
v = 18.27 m/s for 9 minutes
Average velocity:
vavg = d/t = 20km/40.3min = 8.27 m/s or 18.5mph on a day with 22mph winds keeping constant 200 watts.
Now, if he hammers at 250W the first leg and keeps it at 180W the second leg, following the same calculation he would get an average speed of 20.2mph.
If he goes crazy and keeps 300W upwind and then 200W downwind, he gets 21.9mph.
Did that quickly so feel free to check my numbers.
Lets assume a perfectly flat out and back ride, 10km each way. Our rider has a CdA of 0.32 m^2, density of air that day is 1 kg/m^3 and our rider has some really nice tires which have zero rolling resistance.
The only force the rider is facing is drag Fd = 0.5 * 1 * 0.32 * v^2 = 0.16 kg/m * v^2
Now lets say our rider goes out on a windless day and keeps a constant 200W throughout.
P = W/t = F*d/t = F * v = 0.16 * v^3 = 200W
Our rider would keep a constant velocity of 10.77 m/s or 24mph on a windless day.
Now he goes out on a day where the wind is blowing at 22.4mph (10 m/s)
For the headwind leg:
P = F * v = 0.16 * (v + 10)^2 * v = 200W
v = 5.32 m/s for 31.3 minutes
Tailwind leg:
P = F * v = 0.16 * (v - 10)^2 * v = 200W
v = 18.27 m/s for 9 minutes
Average velocity:
vavg = d/t = 20km/40.3min = 8.27 m/s or 18.5mph on a day with 22mph winds keeping constant 200 watts.
Now, if he hammers at 250W the first leg and keeps it at 180W the second leg, following the same calculation he would get an average speed of 20.2mph.
If he goes crazy and keeps 300W upwind and then 200W downwind, he gets 21.9mph.
Did that quickly so feel free to check my numbers.
03-29-16, 05:56 PM
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Originally Posted by tiiger
I somehow have a headwind on the ride to work AND on the way home.
it's a conspiracy.
it's a conspiracy.
And the smoke from a campfire follows me wherever I sit...
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03-29-16, 06:33 PM
#37
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Originally Posted by FBinNY
That's usually true, and not an illusion for most people in most places. A decent rider maintains speeds between 15 and 20mph or higher. 15mph is pretty decent wind on most days, so you're usually riding faster than the wind, meaning even with a tail wind, you're riding into a head wind.
Add to that the reality that you'll ride faster with a tailwind. As a very rough guestimate, you'll probably ride at least fast enough to produce a net headwind of 10mph (if you have the gearing, because that's what it takes for the effort to feel normal.
So headwind or tailwind, you'll alwaus be riding into the wind. The difference is the gear you use, and your ground speed.
Add to that the reality that you'll ride faster with a tailwind. As a very rough guestimate, you'll probably ride at least fast enough to produce a net headwind of 10mph (if you have the gearing, because that's what it takes for the effort to feel normal.
So headwind or tailwind, you'll alwaus be riding into the wind. The difference is the gear you use, and your ground speed.
03-29-16, 08:08 PM
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Originally Posted by caloso
I've done long intervals (such as 2x20's) on an out and back course in the wind. It's much easier to keep the target wattage going into the wind than with a tailwind. I think mostly it's psychological.
03-30-16, 10:51 AM
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My single speed is geared for riding with no wind on flat ground, and so when the wind changes it takes me out of my most efficient powerband (rotational speed). I think that is what affects me the most mentally.
Of course the dynamic pressure from the wind is what is causing the slower rotational speed in the first place.
Of course the dynamic pressure from the wind is what is causing the slower rotational speed in the first place.
03-30-16, 10:55 AM
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On this morning's ride I had a chance to check this out. 2x20' into and then with a 5mph wind. This is in the delta, so the roads are flat as a pancake other than climbing up and down a levee. https://www.strava.com/activities/530832682/analysis
First interval: 261w AP, 20.3mph.
Second: 262w AP, 22.7mph.
So, pretty significant speed difference for the same effort with not much wind.
First interval: 261w AP, 20.3mph.
Second: 262w AP, 22.7mph.
So, pretty significant speed difference for the same effort with not much wind.
