Effects of Wind on riding effort

liquor box

How much effect should the wind have to the amount of effort you put into a ride?

This morning I rode 20km away from home and it took 40minutes, the return 20km took 56minutes into the wind. I had a look at the weather information and it said that there were winds of 20kmh at the time of my ride.

So the question I have is if there is a head wind (assume exactly head on) and it is at 20kmh and I ride at 15kmh, is this the same as usually riding at 35kmh if there was no wind?

I hate riding into the wind but think it is a good way to get some type of resistance training and an easy way to force me to try to increase my cadence.

sungod

liquor box wrote:

<...>
So the question I have is if there is a head wind (assume exactly head on) and it is at 20kmh and I ride at 15kmh, is this the same as usually riding at 35kmh if there was no wind?
<...>

pretty much

the power needed to overcome aerodynamic drag increases as the square of speed + any headwind, i.e. to double speed takes 4x the power

at low speeds, a fair chunk of the power goes into overcoming rolling resistance, drivetrain friction, etc., all this is fairly constant, the rest is overcoming drag, at higher speeds it's dominated by drag

assuming you need 200w to do 30kmh (with no headwind), if 5% is lost in the drivetrain, and 30w to rolling resistance, that's 160w drag

but double the speed to 60, or add a 30kmh headwind, now drag is taking 640w, then add 30w and allow for the drivetrain, and you need about 705w

so going 30 kmh is easy, but going 60 kmh, that burns

it's the drag being proportional to the square of speed that makes riding into a strong headwind tough, getting onto the drops and holding a good position can save you a lot of energy here

aerodynamics is complex, so in practice it's not as simple as above (and as a simple bloke, this pretty much exhausts my knowledge of the subject!)

for training, you can make your own headwind, just go faster!

Secteur

Anecdotally, I reckon that a tail wind helps you maybe 5%, but a stiff headwind slows you 20%.

How can that be?!!

StorckSpeed

Secteur wrote:

Anecdotally, I reckon that a tail wind helps you maybe 5%, but a stiff headwind slows you 20%.

How can that be?!!

This.....If there's no wind I can do an out and back at 18.5mph. With wind this drops to between 16-17mph. In my early years I thought that it would average out but thats not the case at all.

RonB

Haven't touched this type of query for 30 yrs (and now I get the chance to prove it!). I think it's because you are moving as well as the air around you. With the headwind, say it's you at 25 and the wind at 30 kph, that's a relative velocity of 55. On the return leg then you only get 5kph as an "advantage". Faster you go the worse it gets!

bompington

StorckSpeed wrote:

Secteur wrote:

Anecdotally, I reckon that a tail wind helps you maybe 5%, but a stiff headwind slows you 20%.

How can that be?!!

This.....If there's no wind I can do an out and back at 18.5mph. With wind this drops to between 16-17mph. In my early years I thought that it would average out but thats not the case at all.

It's simple maths. Look at out this way: suppose you can average 18mph in still air. You cycle a 9 mile leg out into a strong headwind that slows you down so badly it takes you 59 minutes.
How fast do you have to go to keep up your 18mph average on the way back? That's right, you have to do 9 miles in one minute, i.e. 540mph.

keef66

I thought this was a question about flatulence!

Same considerations apply. The momentary loss of propulsion as you stand up to let one rip; is this to some extent offset by the propulsive force of said anal blast? To what extent is this ameliorated by the chamois pad?

Show your calculations in full.

rothers_bikeo

http://www.sheldonbrown.com/brandt/wind.html

bompington

rothers_bikeo wrote:

http://www.sheldonbrown.com/brandt/wind.html

Aha! Graphs, just what we need!
... and they do indeed show that a 90° crosswind is in fact a headwind:

- definitely shows a 90° wind takes more power than no wind.

I think I'm going to take to complaining that I always have to pedal in a crosswind: it sounds more believable to a non-cyclist, and it gives me the chance to dismiss all objections (and so make myself look faster, the ultimate goal) with a quick burst of "ah yes, you see W = SQRT((U+V×COS(α))2+(V×SIN(α))2) where β = ACOS((U+(V×COS(α)))/W), D = W2 and P = U×D×COS(β)"

rolf_f

RonB wrote:

Haven't touched this type of query for 30 yrs (and now I get the chance to prove it!). I think it's because you are moving as well as the air around you. With the headwind, say it's you at 25 and the wind at 30 kph, that's a relative velocity of 55. On the return leg then you only get 5kph as an "advantage". Faster you go the worse it gets!

It's not just that. To get a tailwind that actually feels like a tailwind (ie a push on your back) the wind has to be going faster than you are. Given that you have a tailwind, that will actually need to be a very strong wind. And by the time you are 45 degrees away from the wind, you'll be heading towards half the effective tailwind.

Of course, still air feels like a headwind equal to the rate you are moving as well. It all adds up to a basic unfairness in the laws of motion

Muffintop

This thread is making my head hurt.

I was out on friday, there was a headwind of 11mph with pretty much constant 20mph gusts, and I managed 11.1miles per hour does that mean that had I not had the head wind I would have averaged upwards of 25 mph?

bompington

Muffintop wrote:

This thread is making my head hurt.

I was out on friday, there was a headwind of 11mph with pretty much constant 20mph gusts, and I managed 11.1miles per hour does that mean that had I not had the head wind I would have averaged upwards of 25 mph?

No. Probably. Or maybe, perhaps.

paul2718

So the question I have is if there is a head wind (assume exactly head on) and it is at 20kmh and I ride at 15kmh, is this the same as usually riding at 35kmh if there was no wind?

No.

If we ignore all other than the wind then, in each example, the force you have to produce to overcome the wind is the same, but in one case you are pushing that force along at 35k, in the other at only 15k. Power is Force times Velocity, so it takes something more than twice the power to ride at 35k on a still day as it does to ride at 15k into a 20k headwind.

Paul

amaferanga

Muffintop wrote:

This thread is making my head hurt.

I was out on friday, there was a headwind of 11mph with pretty much constant 20mph gusts, and I managed 11.1miles per hour does that mean that had I not had the head wind I would have averaged upwards of 25 mph?

No. Definitely not.

But anyway, how do you know it was 11mph with 20mph gusts? If it's from the weather forecast then all bets are off. Or perhaps you had an anemometer with you?

nickel

I've got to say, on those rare occaisons you get a really powerful tailwind thats directly behind you it doesnt half feel brilliant, makes me think "this is what its like to be a pro" as I blast along the flat effortlessly at 25mph+

markos1963

Nickel wrote:

I've got to say, on those rare occaisons you get a really powerful tailwind thats directly behind you it doesnt half feel brilliant, makes me think "this is what its like to be a pro" as I blast along the flat effortlessly at 25mph+

Sad to say if you were a Pro, riding on the flat with a tailwind then you'd be doing 30mph+

Pigtail

To get the average to work in that way you need to spend the same time at the higher speed - not the same distance. So go 9 miles in one hour and 27 miles in the next hour to average 18 mph.

nickel

markos1963 wrote:

Nickel wrote:

I've got to say, on those rare occaisons you get a really powerful tailwind thats directly behind you it doesnt half feel brilliant, makes me think "this is what its like to be a pro" as I blast along the flat effortlessly at 25mph+

Sad to say if you were a Pro, riding on the flat with a tailwind then you'd be doing 30mph+

haha, well me+40mph tailwind= normal riding for a pro with no tailwind :P

Muffintop

amaferanga wrote:

Muffintop wrote:

This thread is making my head hurt.

I was out on friday, there was a headwind of 11mph with pretty much constant 20mph gusts, and I managed 11.1miles per hour does that mean that had I not had the head wind I would have averaged upwards of 25 mph?

No. Definitely not.

But anyway, how do you know it was 11mph with 20mph gusts? If it's from the weather forecast then all bets are off. Or perhaps you had an anemometer with you?

http://www.metoffice.gov.uk/education/t ... uk-archive

http://www.metoffice.gov.uk/education/t ... 0#Scotland

I suppose though it really doesn't matter what the wind was doing, all I know is I was riding in it and it was forking hard.