Headwind/Incline Equivalence

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Mick F
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Re: Headwind/Incline Equivalence

Post by Mick F »

I think that may be the difference here.
You know your speeds and how it varies with slope.

Personally, I don't.
I plod along in my own sweet way and absorb hills as part of the ride. Wind affects my average speeds of course and so do hills, but a 1% slope in still air is un-noticeable. I doubt it slows me down at all.

I'm a "tractor" not a racer. 1% is nothing to me. 50ft in a mile is almost flat as far as I'm concerned.
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Brucey
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Re: Headwind/Incline Equivalence

Post by Brucey »

Mick F wrote:...You know your speeds and how it varies with slope.....Personally, I don't.
.... but a 1% slope in still air is un-noticeable. I doubt it slows me down at all....


that you (apparently) don't notice or care is not the same thing as it not happening... some folk 'wouldn't notice' a 5kph headwind either....

cheers
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Mick F
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Re: Headwind/Incline Equivalence

Post by Mick F »

Yep.
I must agree there.

Is this not the same in all things?
Just coz you don't notice something or are aware of it, doesn't mean it's not there.

:oops: This is sounding like a philosophical argument!
Mick F. Cornwall
roubaixtuesday
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Re: Headwind/Incline Equivalence

Post by roubaixtuesday »

Brucey wrote:
that you (apparently) don't notice or care is not the same thing as it not happening...



Ahhh, Schrodinger's headwind.
iviehoff
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Re: Headwind/Incline Equivalence

Post by iviehoff »

gaz wrote:
Brucey wrote:using this calculator
http://bikecalculator.com/veloMetricNum.html
on its default settings, it suggests that every 1% gradient is roughly equivalent to 10kph of headwind. It feels worse than that to me..... :wink:

I do not pretend to understand the calculator, however I've ridden up a 25% gradient but I don't fancy my chances into a 250kph headwind :shock: .

As was pointed out above, the resistance of headwinds rises with the square of windspeed, but gravity is linear. Naively that suggests to meif you relate 1% to 10kph, then a 25-fold increase in gradient should relate to a 5-fold increase in windspeed (square root of 25). I have read the rubrics to the calculator, which suggests that it is using a squared relation for windspeed, but we do not see actually see a simple square relationship like that, especially not at lower gradients and windspeeds. Indeed at lower gradients and windspeeds, we initially see a roughly linear relationship, at least in the area of 1% to 3% and 10kph to 30kph. The windspeed becomes more significant rather above that, it tells us a 10% slope is equivalent to about 63 kph wind, and a 20% slope a 95kph wind. At those higher slopes and speeds, the square relationship is more nearly approximately satisfied, though not quite. I suspect that in part this is because there is a substantial aerodynamic resistance even at a 0kph wind, and aerodynamics is more complicated than just this one relationship.

Now I have encountered winds I couldn't cycle in, and hills I couldn't cycle up. But trying to use that to get a relationship is tricky. I have got up Hardknott with its 33% sign, but only because the 33% sections are very short. There are 20% hills I have failed to get up, at least in one go, because the 20% section went on for a long time. Winds, on the other hand, tend to be more persistent than steep hills. They also tend to be more variable, and it is probably the gusts that had me off the bike, not the averages. It is pretty clear to me that I can't cycle in a 95kph wind, and I should probably acknowledge that I can't cycle up a 20% hill for more than a pretty short distance.

I remember days in Patagonia and Iceland where I grovelled along at about 8kph in a strong wind. If I said that the wind was probably of the order of 60kph, and it was a bit like going up a 10% hill all day, that wouldn't be far wrong. In fact a 10% hill all day would probably have been a bit worse. I am making myself comfortable with the relationships of this calculator, remembering the shortcomings of approximating real gusty winds to constant windspeeds the calculator uses.

I have often wondered whether there was materially lower air resistance at altitude and this calculator answers the question. Being at 4000m is equivalent to a tailwind of 8 or 9kph. Though there are other issues in relation to being at 4000m which make riding there much harder. Also, it is often very windy at such altitudes.
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Re: Headwind/Incline Equivalence

Post by roubaixtuesday »

iviehoff wrote:I have often wondered whether there was materially lower air resistance at altitude


Formally, air resistance is proportional to density multiplied by velocity squared in fully turbulent flow.

Density is proportional to air pressure, and inversely to (absolute) temperature; air pressure falls with altitude.

So yes, there is lower air resistance at altitude, and also in hot weather. Even at ground level, air pressure can vary by +/- 5%

Which is why Bradley Wiggins had the velodrome heated for his hour record attempt, and cursed the high air pressure on the day.

For mortals like myself, however, a bit more effort and a few less cakes is what really matters...
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Re: Headwind/Incline Equivalence

Post by bovlomov »

BrianFox wrote:...there is lower air resistance at altitude, and also in hot weather. Even at ground level, air pressure can vary by +/- 5%

Is that so? I often feel that the air is somehow 'thicker' on hot days, and imagined that it was something to do with air currents rising from the hot asphalt. Is that wrong?

Perhaps it's the tyres sticking to the road.
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Re: Headwind/Incline Equivalence

Post by roubaixtuesday »

Is that so? I often feel that the air is somehow 'thicker' on hot days, and imagined that it was something to do with air currents rising from the hot asphalt. Is that wrong?


I fear it is.

The air is actually not only less dense with rising temperature, but also with *rising* humidity, as the molecular weight of water is less than that of air.

So, entirely counterintuitively, for low air resistance, you want a hot, humid day. Think really muggy summer day - with low air pressure, so likely thunderstorms.

On the other hand, a cold crisp day in winter will actually have considerably higher air resistance, featuring low temperature, high pressure and low humidity. A quick back of the fag packet sum suggests by as much as 15%!

I guess it feels "heavy" because of the difficulty in losing heat.
karlt
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Re: Headwind/Incline Equivalence

Post by karlt »

BrianFox wrote:
On the other hand, a cold crisp day in winter will actually have considerably higher air resistance, featuring low temperature, high pressure and low humidity. A quick back of the fag packet sum suggests by as much as 15%!



Bloody hell! That explains a good proportion of the observation that it's slower in winter, even with the same bike as used in the Summer.
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bovlomov
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Re: Headwind/Incline Equivalence

Post by bovlomov »

BrianFox wrote:
Is that so? I often feel that the air is somehow 'thicker' on hot days, and imagined that it was something to do with air currents rising from the hot asphalt. Is that wrong?

I fear it is...

Well, either there are other confounding factors, or... ...I'll have to learn to appreciate this 'thin' air.
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Mick F
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Re: Headwind/Incline Equivalence

Post by Mick F »

:shock: :shock:
I put it down to wearing all that clobber!
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roubaixtuesday
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Re: Headwind/Incline Equivalence

Post by roubaixtuesday »

Looks like Sir Bradley agrees with my back of the fag packet, so it must be right ;-)

In keeping with the recent British cycling tradition of marginal gains... ...temperature in the velodrome set to 28 degrees, warm enough to give a performance advantage – an increase in temperature of three degrees results in a 1% increase in speed, due to the increased movement among the air molecules that makes it easier for the cyclist to shove himself and his bike through them...

...The one element that was out of Wiggins’s control was the British weather, and that failed to co-operate; an anti-cyclone with pressure at 1036 cost him about 700 metres, he believed.


http://www.theguardian.com/sport/2015/j ... our-record
iviehoff
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Re: Headwind/Incline Equivalence

Post by iviehoff »

BrianFox wrote:
iviehoff wrote:I have often wondered whether there was materially lower air resistance at altitude

Formally, air resistance is proportional to density multiplied by velocity squared in fully turbulent flow.
Density is proportional to air pressure, and inversely to (absolute) temperature; air pressure falls with altitude.
So yes, there is lower air resistance at altitude, and also in hot weather. Even at ground level, air pressure can vary by +/- 5%
Which is why Bradley Wiggins had the velodrome heated for his hour record attempt, and cursed the high air pressure on the day.
For mortals like myself, however, a bit more effort and a few less cakes is what really matters...

Good points Brian.

I think we can reasonably assume that wind flow around a cyclist without a fairing is turbulent.

I always suspected that air resistance had a relationship to pressure, I suppose it was easy to look it up but I never did. It still surprises me that nevertheless in the very low pressure of Mars there is enough force from winds to lift up huge dust storms.

It was clearly rather unlucky for Wiggo that the air pressure was 1036 on the day, as that is pretty unusually high. Although I do fully take the point that warm (because low pressure) humid air has less resistance, there are also physiological disadvantages about riding in warm humid air, I wonder where the balance lies.
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Re: Headwind/Incline Equivalence

Post by Samuel D »

iviehoff wrote:Although I do fully take the point that warm (because low pressure) humid air has less resistance, there are also physiological disadvantages about riding in warm humid air, I wonder where the balance lies.

The infamous but very knowledgeable Dr Ferrari reckons the Wiggins temperature was excessive.

Cars are a more straightforward case and deliver higher mileage in the summer principally for the reason BrianFox explains.
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Re: Headwind/Incline Equivalence

Post by Bigdummysteve »

robing wrote:
karlt wrote:I 'ate it when you climb a hill into a wind, but it's not too bad because you're sheltered by the hill itself, so you comfort yourself thinking of the whizz down the other side, only to be met with a wall of wind trying to blow you back up it taking all the fun out.

This! Nothing worse than having to pedal downhill!


Me too, I once took my 6 and 11 year old kids for a ride on the back of my bigdummy through north Devon. Up hill mostly 10 miles, turned around looking forward to an easy ride downhill home and flogged myself silly pedalling downhill into a 20mph headwinds.
The hil was perhaps 6% and I found it easier going up than fighting the wind downhill!
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