How much does a 10mph head wind slow you down?

Matno · Sep 7, 2005

It depends on whether the wind is blowing the same direction on the way back. I rode into a noticeable headwind yesterday. At 16 miles, my average speed was 17.3. Turned around and came back, and my average speed at 32 miles was 18.5. Some days I turn around and the wind has shifted and is a head wind again. So far, I've never experienced a tail wind in both directions! I did once get a steady tail wind on a one-direction ride though. Averaged just over 25mph for over 2 hours. I'm sure that's a best for me. It was SWEET until I realized we had taken a wrong turn and had to go back 10 miles with a strong headwind... :^(

BenWA · Sep 12, 2005

mmmm, depends...

Kerry Irons said:
My point exactly. That 10 mph we get from the weather people is taken 30 feet above the ground and away from any obstructions. We know that the actual wind speed is zero at ground level. My guesstimate is that what we experience on the bike is something like 1/3 of the reported wind speed. So if the weather says 10 mph, we cyclists see 3 mph, and that kind of head wind knocks 19 mph down to about 16. Per terry b, a true 10 mph head wind (which might be 30 mph reported) would cut 19 mph down to about 10 mph. But that kind of wind would be VERY difficult to ride in, to say the least!

This is generally correct, that is WHY many weather stations record wind velocity 30' above the surface: 10 m agl is generally a safe estimate for the top of the turbulent boundary layer that is affected by surface roughness and thermal turbulence in most places.

Although Kerry I think your guesstimate might be a bit exaggerated for more open parcels with large fetches, few obstructions, and mostly laminar air flow. In such conditions, the decay in velocity due to surface friciton begins VERY close to the ground; a rider sitting 1 m above the surface may experience more than 90% of the wind velocity that is measured 10 m above the surface in this scenario.

Fordy: yes sir, another pilot here!

tornado · Sep 14, 2005

Beaufort Scale for Wind Speed Estimation

For some visiual clues as to what the wind actually is:

Beaufort Scale for Wind Speed Estimation
The Beaufort Wind Scale is named after Sir Francis Beaufort, an admiral in the British Navy. He developed the scale in 1805 in order to estimate wind speed by noting how sails move in the wind. It turned out to be a great help and was later adapted for use on land.

Force 0
Strength: Calm
Speed: Less than 1 mile per hour (mph), less than 2 kilometers per hour (kph)
Observations: Tree leaves don't move, smoke rises vertically, sea is calm

Force 1
Strength: Light Air
Speed: 1-3 mph, 2-6 kph
Observations: Tree leaves don't move, smoke drifts slowly, sea is lightly rippled

Force 2
Strength: Slight Breeze
Speed: 4-7 mph, 7-11 kph
Observations: Tree leaves rustle, flags wave slightly, small wavelets or scale waves

Force 3
Strength: Gentle Breeze
Speed: 8-12 mph, 12-19 kph
Observations: Leaves and twigs in constant motion, small flags extended, long unbreaking waves

Force 4
Strength: Moderate Breeze
Speed: 13-18 mph, 20-29 kph
Observations: Small branches move, flags flap, waves with some whitecaps

Force 5
Strength: Fresh Breeze
Speed: 19-24 mph, 30-39 kph
Observations: Small trees sway, flags flap and ripple, moderate waves with many whitecapes

Force 6
Strength: Strong Breeze
Speed: 25-31 mph, 40-50 kph
Observations: Large branches sway, flags beat and pop, larger waves with regular whitecaps

Force 7
Strength: Moderate Gale
Speed: 32-38 mph, 51-61 kph
Observations: Whole trees sway, large waves ("heaping sea")

Force 8
Strength: Fresh Gale
Speed: 39-46 mph, 62-74
Observations: Twigs break off trees, moderately high sea with blowing foam

Force 9
Strength: Strong Gale
Speed: 47-54 mph, 75-87 kph
Observations: Branches break off trees, shingles blown from roofs, hight crested waves

Force 10
Strength: Whole Gale
Speed: 55-63 mph, 88-101 kph
Observations: Some trees blown down, damage to buildings, high churning white sea

Force 11
Strength: Storm
Speed: 64-74 mph, 101 kph-119 kph
Observations: Widespread damage to trees and buildings, mountainous waves

Force 12
Strength: Hurricane
Speed: 75 mph or greater, 120 kph or greater
Observations: Severe and extensive damage

fast14riot · Sep 14, 2005

I think most people seriously overestimate the wind they encounter...Answers - 10, 12, 15, 20. Reality - 4 mph gusting to 8.

Well I do agree with this, but being a bowman and navigator/watch captain on some of the boats I race on, I am surprised that nobody has measured their apparent wind speed while riding, i.e. wind felt only as a matter of moving through it. I would ask these same guys to subtract their ground speed from their guess and then see if they still think they are correct.

As a matter of fact the general public really has no way of quantifying perceived wind speed, unless you happen to be trained to pay attention to this; sailors, pilots, kite flyers, shooters/hunters, etc. I routinely check the weather stations for their wind forecast and am very surprised at the differences between them. Have you ever listened to the marine forecast for SF Bay in the summer? Winds 10 to 25 with gusts to 30 in the afternoon. I mean, jeez, this is a HUGE wind range, so I am not really surprised by great overestimation of wind speeds.

As far as how much you slow down in a head wind, don't forget to get down in the drops as this will help significantly. I don't know how many people I see riding into the wind on the flats and head up just mashing big gears to get through it when all you have to do is get down and aero to get throught it a bit quicker. I have noticed in IMO that TT wheels will help with riding into the wind as you dont have to keep fighting ultra light wheels to rountinely get them back up to speed, just spin 'em up and keep your head down.

Cheers-

P.S. the Bf scale really isn't used much in the US, at least by sailors, but is very popular in Europe.

CFBlue · Sep 14, 2005

Big Flat Ride and Winds

I was on a T-Shirt ride this past weekend in the farmlands around Sacramento. I had expected to be able to maintain 23+ on the pancake flat course and was surprised at the struggle I had as the course changed directions through the orchards. The day before the wind was really howling...note scientific term for wind speed... so I was pushing it so I would finish the course early and miss the breeze I expected in the afternoon. Bike speeds were lower than I expected and I attributed it to the expected winds.

I was really surprised when I got off to find that there was just the lightest breeze, barely moving the leaves. that was a lot of self generated headwind. And it left me fresh out of excuses for why I didn't do better than 18.5 on a course with only 950 feet of climbing...(freeway overpasses). Had to be obeying the stop signs

Kerry Irons · Sep 15, 2005

Speed variations

CFBlue said:
I had expected to be able to maintain 23+ I didn't do better than 18.5

No offense, but this cracks me up. You expected to deliver around 250 watts, but you actually rode at 150 watts. For me, 18.5 mph is a relaxed pace, and 23+ is time trialling. That's like saying you expected to do a 1:05 25 mile TT, but instead you did 1:21. Either you totally don't know yourself or you had a REALLY bad day

Triker · Sep 15, 2005

Here is the answer:

http://www.kreuzotter.de/english/espeed.htm

RodeRash · Sep 15, 2005

Besides bikes, I fly kites at the beach -- power kites for mountain boards and kite buggies. Because the power generated by the "wing" (the kite is an airfoil) needs to be under control, it's important to have an accurate estimate of wind speed.

A large kite can literally drag you down the beach on your face in a wind that's too strong. Sooooo, we have anonometers. But we also learn to look at the flags on buildings. It's possible to figure within a knot or two (knot = 1.1 MPH more or less) the wind speed from looking at the flags.

Beauford uses movement of things in nature, but it's a long list. Flags and trees are pretty good guages and generally available.

But too, about wind speed and where it's taken. Look at the tree tops next time you're in a gale. The wind is stronger up there than down on the ground.

How much it slows you down sort of depends how aerodynamic you are. Wear a loose, baggy, flapping nylon shell sometime in a breeze. That slows you down more than wearing a skin-suit. Helmets, wheels, even the shape of the cranks, and surface of shoes will affect wind drag.

How much does a 10mph head wind slow you down?

Matno

BenWA

tornado

fast14riot

CFBlue

Kerry Irons

Triker

RodeRash

Top Contributors this Month

Recommended Communities