Author Topic: The effect of Wind  (Read 1845 times)

The effect of Wind
« on: March 11, 2019, 08:08:09 am »
Anybody here any good at sums?  Recently I have ridden quite a lot in high winds, and I would love to know how this is affecting my performance, bearing in mind I usually end up exhausted, but the more I delve into it the more I submerge myself in Fluid Dynamics, which is alien to me.  Please does anyone have a formula to calculate (perhaps approximately) if I am facing a headwind of 15mph (say), and I want to ride effectively at 15mph(say), what is the speed I need to actually ride?  It cannot simply be a subtraction, because if I am riding at 10mph into a 15mph headwind I do not go backwards.  Alternatively if Garmin says I averaged 15mph into a 15mph headwind, what did I really achieve?

Re: The effect of Wind
« Reply #1 on: March 11, 2019, 08:09:29 am »
<i>Marmite slave</i>

PaulF

  • "World's Scariest Barman"
  • It's only impossible if you stop to think about it
Re: The effect of Wind
« Reply #2 on: March 11, 2019, 08:20:22 am »
The trivial answer is 15mph but you'll have an apparent air speed of 30mph.

Are you looking for an equivalent power output rather than speed? Or put another way an equivalent "still air speed"?


Re: The effect of Wind
« Reply #3 on: March 11, 2019, 08:33:22 am »
I find that if the wind speed exceeds 30mph, my ground speed reduces to around 0mph.

Usually because I haven't bothered going out.

ElyDave

  • Royal and Ancient Polar Bear Society member 263583
Re: The effect of Wind
« Reply #4 on: March 11, 2019, 08:59:51 am »
your garmin gives speed across the ground, and actually only in a horizontal direction.  So it lies when you are on anything other than the flat.  The degree of lying depends on the slope.

In fluid dynamics terms this is a pretty complex set of euqations (I might have had a stab at it 20 years ago), mostly because you as a human bean is not uniform, and then attach you to a BSO which is more non-uniform and is varying in its non uniformity.  You could make several approximations to uniformity, but effectively the question you are asking is how much more drag is being created by me cycling into a wind, assumed headwind, than cycling into still air.

So effectively normal drag = 15mph (say) into still air, effective windspeed 15mph
New drag = 15mph "speed" plus 15mph wind = 30mph "drag"
I believe form drag scales with square of speed, so x 2 speed = x 4 energy output required to overcome that aspect of drag, then you have friction losses (drivetrain plus skinfriction), then you have your biomechanical efficiency, then your biochemical efficiency

I'd say go find a wind tunnel, or a CFD expert.
“Procrastination is the thief of time, collar him.” –Charles Dickens

T42

  • Tea tank
Re: The effect of Wind
« Reply #5 on: March 11, 2019, 09:22:30 am »
there is a website

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

Enjoy

He doesn't take the road surface into account.

The force exerted by aerodynamic drag varies as the square of the combined rider and wind speeds, which means that the power required to overcome it varies as the cube.  When I've incorporated this in programs I've taken the resultant of the two velocities, with the wind speed multiplied by the cos of the angle of incidence, but I'm by no means sure this is valid for drag.  I've always had the impression that it's easier to ride straight into the wind than at an angle of < 45° to it, probably because when you're riding straight into it you have a smaller frontage and can get into a better aerodynamic shape.
I've dusted all those old bottles and set them up straight.

Re: The effect of Wind
« Reply #6 on: March 11, 2019, 09:25:16 am »
Because physics is a git, the arc in which you're impeded (headwind) is greater than the arc in which you're assisted (tailwind). ISTR it's something like 200' headwind, 160' tailwind; so it has to be quite noticeably behind you before you get "help".

The effect of the wind on Saturday was that I was even more of a miserable bugger on our bike than usual.

Re: The effect of Wind
« Reply #7 on: March 11, 2019, 12:14:08 pm »
You are of course expressing an average speed and since it is not a linear relationship between speed and effort, you cannot just take that and multiple by some number to come up with an average "extra" effort.  However, taking the output of one of the online calculators the estimate would be that for every hour you are cycling at 15mph into a 15mph headwind you will burn an extra 170 Calories over what you would burn if the wind was still.

Re: The effect of Wind
« Reply #8 on: March 11, 2019, 12:44:20 pm »
The effect of the wind on Saturday was that I was even more of a miserable bugger on our bike than usual.

I think my most impressive strop was when the bike blew over for the second time while I was trying to mend my second puncture on a grass verge.

FifeingEejit

  • Not Small just Far Away at the back
Re: The effect of Wind
« Reply #9 on: March 11, 2019, 12:55:54 pm »
your garmin gives speed across the ground, and actually only in a horizontal direction.  So it lies when you are on anything other than the flat.  The degree of lying depends on the slope.

Does this also apply if you use a wheel rotation sensor?

Kim

  • Timelord
Re: The effect of Wind
« Reply #10 on: March 11, 2019, 01:00:45 pm »
The effect of the wind on Saturday was that I was even more of a miserable bugger on our bike than usual.

I think my most impressive strop was when the bike blew over for the second time while I was trying to mend my second puncture on a grass verge.

The effect of incessant headwind on a loaded Streetmachine is surprisingly hard to distinguish from that of a rear-wheel puncture.  I kept stopping to reassure myself that the tyres really were properly inflated.  Unfortunately, they were.
To ride the Windcheetah, first, you must embrace the cantilever...

Re: The effect of Wind
« Reply #11 on: March 11, 2019, 01:08:27 pm »
your garmin gives speed across the ground, and actually only in a horizontal direction.  So it lies when you are on anything other than the flat.  The degree of lying depends on the slope.

Does this also apply if you use a wheel rotation sensor?

I don't see why it would
Old enough to know better, but young enough to do it anyway

ElyDave

  • Royal and Ancient Polar Bear Society member 263583
Re: The effect of Wind
« Reply #12 on: March 11, 2019, 01:33:56 pm »
Not if the rear wheel sensor is the sole judge of speed, that would then be with reference to the road surface rather than point to point as by GPS.
“Procrastination is the thief of time, collar him.” –Charles Dickens

ElyDave

  • Royal and Ancient Polar Bear Society member 263583
Re: The effect of Wind
« Reply #13 on: March 11, 2019, 01:34:44 pm »
You are of course expressing an average speed and since it is not a linear relationship between speed and effort, you cannot just take that and multiple by some number to come up with an average "extra" effort.  However, taking the output of one of the online calculators the estimate would be that for every hour you are cycling at 15mph into a 15mph headwind you will burn an extra 170 Calories over what you would burn if the wind was still.

Cool, you're suggesting an integral approach. I wish I could remember how to do that. ???
“Procrastination is the thief of time, collar him.” –Charles Dickens

Re: The effect of Wind
« Reply #14 on: March 11, 2019, 10:35:10 pm »
your garmin gives speed across the ground, and actually only in a horizontal direction.  So it lies when you are on anything other than the flat.  The degree of lying depends on the slope.

The horizontal plane accuracy of GPS is generally 3 times better than the vertical accuracy, and so the smoothing algorithms that most GPS devices use will first assume that any vertical changes can be ignored, but continue up that slope for more than a few seconds and the smoothing algorithms will begin to incorporate the vertical rate of change too and the speed reported will reflect that.

Public GPS signals simply aren't accurate enough to do any reasonably accurate/contemporaneous determination of speed anyway, it's all a bit of a fudge. (Consider a 3m accuracy per reading and moving at 20mph =~ 9m/s. Two positions one second apart whilst moving at 9m/s could be reported as anything between 3m and 15m apart. The more points you smooth in the lower the error will be, but the slower the algorithm will be to responding to a change in velocity.)

If you have a wheel sensor available the GPS will use that augment the smoothing algorithms. It can't just trust the wheel size the user has input as this is (a) often wrong and (b) changes with tyre pressure. Indeed my Forerunner 935 doesn't even ask for a wheel size, it calculates it (and adjusts it) based on the GPS distance traveled and the number of wheel revolutions reported. The wheel sensor helps fill in the gaps where the GPS signal isn't strong enough (tree cover, tunnels, etc) or gives misleading values (due to reflections of buildings) compared to the expectation of moving steadily in roughly a straight line.
"Yes please" said Squirrel "biscuits are our favourite things."

Kim

  • Timelord
Re: The effect of Wind
« Reply #15 on: March 11, 2019, 11:01:57 pm »
Public GPS signals simply aren't accurate enough to do any reasonably accurate/contemporaneous determination of speed anyway, it's all a bit of a fudge. (Consider a 3m accuracy per reading and moving at 20mph =~ 9m/s. Two positions one second apart whilst moving at 9m/s could be reported as anything between 3m and 15m apart. The more points you smooth in the lower the error will be, but the slower the algorithm will be to responding to a change in velocity.)

AIUI this isn't how GPS speed works:  It measures the doppler shift of the signal, rather than deriving from changes in position, and is therefore surprisingly accurate (as long as the signal's decent and you're not moving too erratically).

The wheel sensor is still advantageous for the reasons you mention, and because it works properly at low speeds (particularly 0) where it's hard to distinguish genuine movement from GPS error.
To ride the Windcheetah, first, you must embrace the cantilever...

Re: The effect of Wind
« Reply #16 on: March 11, 2019, 11:31:51 pm »
Yes, should have been clearer. It was more about how GPS positional data alone can't be relied up for accurate velocity determination.

Here's a pretty good (but detailed) description of how it is done:

http://www.aprs.net/vm/gps_cs.htm

The most relevant section is:-

Quote
In early GPS receivers, four PRs from 4 satellites was converted into a 3-D (XYZ, Lat/Lon/Hgt or whatever) position plus the calibration of the timing bias of your receiver, and 4 PRRs were converted into a 3-D velocity plus a measurement of the frequency error of the oscillator. More modern receivers take all the PR+PRR data from all the N satellites in view for the past T seconds and feeds the 2*N*T PR+PRR samples it into a single mathematical "black box" (BB) (usually a Kalman filter) to produce an over- determined estimate of the same 8 parameters. So in modern receivers, this BB is using both the combination of past & present PRs and PRRs from many satellites to improve the Position, Velocity & Time (PVT) estimate. So Paul's statement about velocities being determined by changes in position is sorta, partially correct, but (when you look at the equations inside the BB), the measured "apparent Doppler" frequencies are even more important.

So velocity is going to be most heavily influenced by the doppler shift values (which aren't specific to the horizontal plane, i.e. speed will take into account the slope) but still takes recent positional updates into account albeit less significantly within the calculations.
"Yes please" said Squirrel "biscuits are our favourite things."

ElyDave

  • Royal and Ancient Polar Bear Society member 263583
Re: The effect of Wind
« Reply #17 on: March 12, 2019, 06:37:39 am »
That still doesn't clearly demonstrate to me that it is taking speed from distance along the plane of the road vs distance on a flat xy grid imposed on the ground
“Procrastination is the thief of time, collar him.” –Charles Dickens

T42

  • Tea tank
Re: The effect of Wind
« Reply #18 on: March 12, 2019, 08:00:15 am »
You are of course expressing an average speed and since it is not a linear relationship between speed and effort, you cannot just take that and multiple by some number to come up with an average "extra" effort.  However, taking the output of one of the online calculators the estimate would be that for every hour you are cycling at 15mph into a 15mph headwind you will burn an extra 170 Calories over what you would burn if the wind was still.

Cool, you're suggesting an integral approach. I wish I could remember how to do that. ???

You'd need a Pitot tube on the bike for starters. Something like this.

It'd need a bit of hacking, though...
I've dusted all those old bottles and set them up straight.

Re: The effect of Wind
« Reply #19 on: March 12, 2019, 08:35:52 am »
That still doesn't clearly demonstrate to me that it is taking speed from distance along the plane of the road vs distance on a flat xy grid imposed on the ground

From the page...

Quote
The Doppler shift includes the vector sum of the satellite's ~7 km/sec orbital velocity plus the 400 m/sec (at the equator) rotational velocity of the earth plus your receiver's motions (in a moving car, ~10-50 m/sec).

So, some of the inputs to the calculation are derived velocities (and positions) of known objects (the satellites) that massively dwarf the eventual computed velocity (10m/s) that it still manages to calculate accurately, and are in a whole load of random directions (each satellite will be moving in a completely different direction to the others at any one time).

For it to only compute the velocity in the horizontal plane (relative to the Earth's surface) it would have to compute the velocity of the device using this information (since it has no idea what the horizontal plane is at this point), then calculate where the device is on the Earth, what the horizontal plane is at that point (with reference to the geodetic datum), then calculate the vertical component of the velocity now it knows what 'horizontal' is, then recalculate the velocity of the device in the horizontal plane only and report that.

Or it could just use the original velocity it calculated.
"Yes please" said Squirrel "biscuits are our favourite things."

Re: The effect of Wind
« Reply #20 on: March 12, 2019, 08:45:48 am »
You'd need a Pitot tube on the bike for starters. Something like this.

It'd need a bit of hacking, though...

It already exists:
https://m.probikekit.co.uk/cycling-power-meters/powerpod-powermeter-v2/11514777.html


T42

  • Tea tank
Re: The effect of Wind
« Reply #21 on: March 12, 2019, 08:59:52 am »
About bloody time, too.
I've dusted all those old bottles and set them up straight.

Re: The effect of Wind
« Reply #22 on: March 12, 2019, 09:12:17 am »
It gets a reasonable review from DC Rainmaker too: https://www.dcrainmaker.com/2016/03/powerpod-depth-review.html
"Yes please" said Squirrel "biscuits are our favourite things."

T42

  • Tea tank
Re: The effect of Wind
« Reply #23 on: March 12, 2019, 04:05:27 pm »
At that price I might have forked out for it a few years back but now I couldn't really justify it.
I've dusted all those old bottles and set them up straight.

Zed43

  • prefers UK hills over Dutch mountains
Re: The effect of Wind
« Reply #24 on: March 12, 2019, 05:49:26 pm »
I never got the PowerPod to work. Expensive paper weight.