The Why's and How's of Wind Offset

The Why's and How's of Wind Offset

Postby travispape » Sat May 17, 2008 1:29 pm

The two most important things to do for getting the best power measurements from the iBike are doing consistently good tilt calibrations (as I just discussed in another post) and wind offset calibrations. Just like the tilt cal procedure, it is very important to get the wind offset calibration right before doing your calibration rides and then on a daily basis before each of your rides.

Wind Measurement during Calibration Ride

A bad wind offset on the day that you do your calibration rides will cause a bad WindScaling factor in your profile and will negatively affect all of your subsequent rides based on the profile. The WindScaling factor takes into account all of the geometry factors that influence how much stagnation pressure is formed at the nose of the iBike for your setup. It depends strongly on the relative position of the iBike to the bars and stem and it also depends on somewhat on your size and position. Think of yourself as a bulldozer blade pushing air out in front of you. (Actually, the less like a bulldozer blade you are, the faster you will be, but that’s another topic: CdA.) WindScaling also depends on how nose-up or nose-down your unit is mounted. The pressure at the nose of the iBike unit depends on all these geometry factors and where it is located within your advance airstream. It also depends on the characteristics of the wind sensor in your unit.

The algorithm that processes your calibration ride measures the scaling factor that takes into account all of the above factors to measure the relationship between wind speed and the stagnation pressure of your iBike unit where it is mounted; however, this is only as good as your wind offset calibration before the 4 mile ride.

How to Do a Good Wind Offset Calibration

There are only two simple ingredients for a good wind offset calibration: 1) get completely out of the wind and 2) do it when the internal temperature of the iBike unit is in the same temperature range as it will be during the ride. That’s all there is to it.

I think the importance of #1 is obvious—if you do your wind offset cal when there is a 3 mph puff of wind hitting your unit at the time you push the center button, you will have a 3 mph error in your wind readings which will cause huge power errors. The other thing to be aware of is that you need to keep your fingers clear of both the front air port and the little hole in the bottom of the unit when you do the wind offset. The wind sensor is a differential detector and the back-side pressure comes in through that little hole on the bottom.

#2 is important because the offset of the wind sensor in the iBike is temperature-dependant, more so in colder temperatures. The easiest way to mess up the wind offset calibration is to store your iBike at room temperature in the winter and then do your wind offset before the unit has cooled all the way down to the freezing temperatures that you will be riding in. I know this can be a pain, but the best way to get the iBike to riding temperature is to ride with the iBike for 3-5 miles. The moving air is the fastest way to get the internals to riding temperature, plus it will take into account whatever temperature offset is caused by the sun warming your unit. After you ride for 3-5 miles, get the iBike out of the wind so that dangerous 3 mph puff of wind doesn’t hit it and do your wind offset.

The exact temperature isn't critical, all that matters is that you get your iBike to within the range of the riding temperatures before you do the wind offset. If you are going to be riding in 30 – 40 degF winter morning weather, doing the wind offset when the unit has only cooled down to 50 degF is going to cause the iBike to under-report wind and power. If you wait until it gets all the way down to 38 degF, then you will be fine. (If you know your morning ride is going to be in the 30-40 degF range, you might stick the iBike in the fridge overnight—I have done that in the past.)

The wind sensor is less picky in the summer, but it can still make a difference to get the temperature of the unit in the range of your riding temperature before doing the wind offset.

So back to the profile: don’t cut corners on the day you do your calibration rides. If you want a good profile, ride with your iBike for a few miles before you do your wind offset cal. The quality of all your future power data depends on it.

On a daily basis, be as meticulous as you want. If you are doing intervals or racing, be sure to get a good wind offset cal. If you are only commuting to work and you don’t care as much about getting the last Watt of accuracy on the road, just do the wind offset correction later in the iBike2 software.

How to Post-Process Wind Data

You can fix your wind offset calibration later in iBike2 using the “Analyze Wind…” button in the detailed tab. Here’s what I wrote a few weeks back about how to do it:

If the ride is a solo (i.e., non-drafting) out-and-back ride, set the average wind speed to zero. (By the way, the average ground wind speed in the wind analysis window is a distance-based average. This kind of average takes care of the fact that you spend more time on average riding slower against the wind than the faster wind-aided portion of the ride.)

If the wind conditions changed significantly during the duration of a solo out-and-back ride, make the appropriate adjustment. For example, if the wind picked up for the return portion of the ride and it was a tail wind, I might enter a -0.2 or so for the average ground wind.

If the ride was a solo loop, set the average wind speed to zero; however, you might need to adjust if part of the loop had high wind exposure while the other part of the loop was shielded.

If wind conditions are calm for a solo ride, set the average wind speed to zero even if it was a one-way ride.

If the ride was one-way and wind was blowing, use the airport data, the direction of the route, and the amount of wind exposure on the road to reduce the result. The roads here where I ride are carved through a sea of trees, so I greatly reduce the airport wind measurement for my estimate of the road speed. For example, if I do a one-way ride towards the East and the airport reports the wind was out of the NW at 10 mph, later when I post-process the ride I would assign a wind of -1.0 mph. (Negative because it is a tail wind; much smaller than the airport data because it was not in the same direction as my ride and because most of the ride was very shielded by the trees.)

Even if you ride in an area with lots of wind exposure, remember that airport wind readings are 10 m off the ground and are much higher than you would get on the road.

Drafting is another complication. If you did some of the ride in a paceline or peloton, then the best thing you can do is highlight a portion of the ride where you were in the clear wind and do your windoffset analysis for the whole ride just based on the highlighted portion of the ride. If you are riding a crit, but ride solo to warm up, base your wind speed correction based only on the solo part of the ride. You can typically expect a large virtual tailwind for rides where you benefit from a good draft. Drafting is more difficult to repair later, so if you know you are going to draft, you have extra incentive to go out and do a good windoffset before the ride. If you do go to the trouble, you will get a really good measurement of effectiveness of the draft based on the measured wind speed data.

90% of the accuracy battle is getting good tilt calibrations and wind offset calibrations before both your calibration rides and then on a daily basis before each ride. Get these right, and you will almost always have very accurate power data both on the road and after post-processing.

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Re: The Why's and How's of Wind Offset

Postby Norm » Sun May 25, 2008 7:51 pm

Great description Travis! I noticed you didn't mention cupping your hand in front of the iBike to block any wind. Is that procedure no longer recommended?
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Re: The Why's and How's of Wind Offset

Postby travispape » Mon May 26, 2008 11:57 am

Norm wrote:Great description Travis! I noticed you didn't mention cupping your hand in front of the iBike to block any wind. Is that procedure no longer recommended?


No, the concern is that cupping your hand over the nose might cause the wind to swirl around and still get into either the front side of the sensor or into the tiny hole in the bottom of the unit, which is the back side of the differential detector. It is better to get out of the wind. I did some coastdowns yesterday and took my bike into the bathroom at a ballpark. I've also used tall grass in a ditch on the side of the road in the past.
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Practical questions

Postby carstenj » Wed May 28, 2008 4:44 pm

I have a couple of newbie questions regarding wind offset calibration. I understand from this thread that it is VERY important to get the wind offset calibration right so I am going to ask even through the answers might be obvious :-)

When I go into the wind offset menu in the setup screen and observe that the top number should drift towards 0 everything is fine and no further calibration is needed.

Often I see that the number do drift to 0 and stay there for a few seconds but then it drifts away from zero. How long should the number stay at 0 before the wind offset can be trusted?

When the number does not drift to 0 and I have to click the center button the force the number to 0 then same question as above. How long should the number stay at 0 before it can be trusted?

What is the proper way to leave the wind offset calibration? Should this be done by pressing the up-arrow?
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Re: The Why's and How's of Wind Offset

Postby Velocomp » Wed May 28, 2008 10:01 pm

The number will never settle to zero precisely. Think of a candle flame; even when the air appears to be exactly still the flame still moves around a bit. So, you'll see the number drift around a few tenths. That is fine.

I try to click the center button as briefly as possible. Pressing on the button too long pressurizes the interior of the iBike and can cause errors.
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Re: The Why's and How's of Wind Offset

Postby rider of rohan » Thu May 29, 2008 4:04 pm

First of all, thanks for the great post. I feel like I am working on a PHD in Ibikeology. This is one fickle and high maintenance instrument. I am a new user and have done multiple coast downs and a 4 mile ride, made the profile, the whole nine yards. My rides are reporting less wind than wheel speed. No tail wind to speak of on the ride. When I clicked on analyze wind to correct it, I noticed the wind offset at the bottom read -5.7 which seemed really high. Is that an indication of a bad profile? My battery may have been fizzing and it was a little windy when I did it. Crappy Radio Shack battery lasted like 2 hours in warm Las Vegas weather! Thanks for the tip on temperature. Dude, the thought of hunkerding down in a ditch or a park bathroom to calibrate a $500.00 instument sounds about par for the course for the ibike. So many variables...Please tell me this unforgiving process will someday end up in reliable information. How many days in a row can you ride without changing a battery, a calibration, or have something go wrong to ruin power readings. I have yet to have one ride I feel good about. Can you train in specific power zones without crazy readings? I think I need to check the battery, leave my ibike outside, calibrate the wind offset and try again. Cycling has never been so complicated.
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Re: The Why's and How's of Wind Offset

Postby coachboyd » Thu May 29, 2008 4:18 pm

I have actually found it to be pretty easy. Let the ibike sit outside for 15 minutes before I ride, make sure the tilt is reading -.3 on the tape I put on my garage floor, and then zero the wind offset. I am usually out the door in less than 30 seconds from when I step outside.

And yes, I have been getting VERY reliable readings. . .enough so that my other power meters are currently on ebay.
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Re: The Why's and How's of Wind Offset

Postby rider of rohan » Thu May 29, 2008 8:02 pm

Thanks Boyd. I'll get it dialed in.
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Re: The Why's and How's of Wind Offset

Postby rider of rohan » Fri May 30, 2008 12:27 pm

I have a bunch of newbie questions: Do you do a tilt calibration before every ride as well as a wind offset? Or do you just check the tilt? Do you check tilt in the setup mode, or just in eviro? Can you redo the tilt calibration without performing the rest of the initial setup like coastdowns and 4 mile ride? In other words, can you redo the tilt calibration without messing up your profile?
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Re: The Why's and How's of Wind Offset

Postby coachboyd » Fri May 30, 2008 2:36 pm

Yep, I check the tilt and the wind offset before every ride. . .also the starting elevation, but that only takes 5 seconds.

For the tilt and wind, I do both in the setup screen. Like I said before, I have a spot marked on my garage that I know is -.3% If the bike is sitting there and not at that number, I redo the tilt calibration. You can perform as many tilt calibrations as you would like, it will not affect your stored profile numbers.

Wind offset is also best to check is the setup screen. You could have it as a negative number and then it wouldn't show in the wind/slope screen outside of the setup.
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Re: The Why's and How's of Wind Offset

Postby alienator » Sat May 31, 2008 4:43 am

coachboyd wrote:For the tilt and wind, I do both in the setup screen. Like I said before, I have a spot marked on my garage that I know is -.3% If the bike is sitting there and not at that number, I redo the tilt calibration. You can perform as many tilt calibrations as you would like, it will not affect your stored profile numbers.


I do this. I have a spot marked on the garage floor that, is ±0.1°, depending on which way I have the bike facing.

coachboyd wrote:Wind offset is also best to check is the setup screen. You could have it as a negative number and then it wouldn't show in the wind/slope screen outside of the setup.


This doesn't make sense. While it's obvious, from a calibration point of view, that wind offset is the most critical procedure in the setup of the iBike, any wind scaling factor, be it positive or negative, should have an influence on the overall measurement. The only way it wouldn't would be if the scaling factor was part of a piecewise function. That can't be because such a discontinuity would violate a few laws of physics and would likely cause the ghosts of Newton, Reynolds, Navier, and Stokes to rise up and strike dead the good people at iBike.
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Re: The Why's and How's of Wind Offset

Postby rruff » Tue Jun 17, 2008 11:47 am

travispape wrote:There are only two simple ingredients for a good wind offset calibration: 1) get completely out of the wind and 2) do it when the internal temperature of the iBike unit is in the same temperature range as it will be during the ride.

#2 is important because the offset of the wind sensor in the iBike is temperature-dependant, more so in colder temperatures. The easiest way to mess up the wind offset calibration is to store your iBike at room temperature in the winter and then do your wind offset before the unit has cooled all the way down to the freezing temperatures that you will be riding in. I know this can be a pain, but the best way to get the iBike to riding temperature is to ride with the iBike for 3-5 miles. The moving air is the fastest way to get the internals to riding temperature, plus it will take into account whatever temperature offset is caused by the sun warming your unit. After you ride for 3-5 miles, get the iBike out of the wind so that dangerous 3 mph puff of wind doesn’t hit it and do your wind offset.


Seems like this could be fixed via firmware. Since the iBike has a temperature sensor, then why not allow it to do it's own temperature compensation? Wouldn't the sensitivity of the units to temperature be fairly consistent and roughly the same for each one? Or is it random... what causes it?
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Re: The Why's and How's of Wind Offset

Postby racerfern » Tue Jun 17, 2008 1:22 pm

Since the iBike has a temperature sensor, then why not allow it to do it's own temperature compensation? Wouldn't the sensitivity of the units to temperature be fairly consistent and roughly the same for each one? Or is it random... what causes it?


Actually, I think the real factor is barometric pressure. Most people have no way of telling barometric pressure so we look at the temperature. When that has stabilized then the barometric pressure sensor has stabilized. This affects both the altitude displayed independently of the temperature and the wind offset because it is so sensitive to the slightest pressure changes.

I live at sea level and the temperature is relatively constant. However barometric changes from day to day show changes in altitude from -100ft to +100ft. The wind offset changes accordingly. Let your unit settle in to ambient temp and it will have settled in for barometric conditions also.
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Re: The Why's and How's of Wind Offset

Postby rruff » Tue Jun 17, 2008 4:22 pm

I'm not following you. Barometric pressure is independant of temperature... but the thing that iBike really needs to know is the air density... which will vary depending on the pressure and temperature. iBike has direct sensors for both of these, so there should be no problem getting a good air density. Humidity is another variable, but it doesn't have a large enough effect to make a big difference. Actually the temperature effect is pretty small too... effects density proportional to the absolute temperature, so a 20 degree change only effects the air density ~4%. The reason why your altitude changes is because iBike uses barometric pressure to guess at altitude, and this varies... but that shouldn't effect it's determination of air density or it's accuracy.

The only issue I can see here is that the dynamic pressure sensor is sensitive to temperature. The static pressure sensor doesn't seem to be, though. But for whatever the reason that the dynamic pressure sensor is sensitive to temperature, there should be some way to compensate for it in firmware... unless it is random... which would be strange.
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Re: The Why's and How's of Wind Offset

Postby racerfern » Tue Jun 17, 2008 6:07 pm

We're pretty much on the same page. The temperature indicated is simply the display we should use to indicate that all the sensors have equalized to the ambient air, nothing more.
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Re: The Why's and How's of Wind Offset

Postby cwwees » Tue Jun 17, 2008 8:48 pm

I just had to pipe in...

Barometric pressure is not affected by temperature. But the sensor that measures barometric pressure IS affected by temperature. If altitude and temperature are known, the barometric sensor can be calibrated. I know it's a high-order polynomial (5th order?) between temperature and voltage reading from the barometer, but whatever... You have to know two of those three terms to get the other. That's why every barometric altimeter has a thermometer in it.

My question on this discussion has to do with temperature variation during a ride. I often see 30 to 40 deg F change from start to end of a ride. If I zero the wind offset at 60 deg F and finish my ride at 100 deg F (quite easy to do in Arizona), am I losing accuracy because of that? How does the PC software handle temperature changes through the ride?

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Re: The Why's and How's of Wind Offset

Postby rruff » Tue Jun 17, 2008 10:09 pm

cwwees wrote:If altitude and temperature are known, the barometric sensor can be calibrated.


I'm confused. The barometric pressure at a particular location will vary even if the temperature doesn't (check the weather stats)... and you can't calibrate the sensor on an unknown variable. It only varies ~.5% normally, so maybe you are saying that error of that amount is close enough?

I know it's a high-order polynomial (5th order?) between temperature and voltage reading from the barometer, but whatever... You have to know two of those three terms to get the other. That's why every barometric altimeter has a thermometer in it.


So, you are saying is that the electronic pressure sensor is effected by temperature? I assumed that was true... which means that firmware should be able to compensate. What is your source of info on these sensors? I'd like to learn more about them...

My question on this discussion has to do with temperature variation during a ride. I often see 30 to 40 deg F change from start to end of a ride. If I zero the wind offset at 60 deg F and finish my ride at 100 deg F (quite easy to do in Arizona), am I losing accuracy because of that? How does the PC software handle temperature changes through the ride?


Good question... and also, why can't it compensate for temperature while you are riding?
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Re: The Why's and How's of Wind Offset

Postby Zecutive » Tue Jun 24, 2008 4:46 pm

My question is the same as one of Charlies. What happens when the temperature varies substantially during a ride. I live in Florida. In the summer we are usually rolling at dawn so that we can so as much of a ride in as possible before the hottest part of the day. It is not unusual for the temp. at the beginning of a ride to be 75 and at the end of a 4 hour ride for it to be 100 or more in the sun. I see the altitude change significantly over that time even though it is very flat here. How much is the power reading being effected?
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Re: The Why's and How's of Wind Offset

Postby rruff » Wed Jun 25, 2008 3:44 pm

Pretty sure the altitude reading is based on barometric pressure only... which does vary, but not enough to make a big difference (~1W) unless maybe a major weather system is coming through. How big of an altitude change are you seeing between the beginning and end of a ride?

AFAIK the effect of temperature on the wind offset is the major temperature issue.
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Re: The Why's and How's of Wind Offset

Postby Russ » Thu Jun 26, 2008 8:56 pm

A new trick I use to calibrate my wind offset mid ride....
I have a few left over small baby wash rags from when the kids....
I was carrying one to wipe sweat from my eyes. I tried folding it several layers thick
and gently holding it against the front of the iBike while zeroing the wind offset, seems
to work fairly well. I try to cover both ports without any real pressure on the unit.
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Re: The Why's and How's of Wind Offset

Postby travispape » Sun Jun 29, 2008 4:13 am

Just a little interesting aside to this conversation, there is a diurnal pressure cycle. I didn't even realize that the phenomenon existed until I noticed the my iBike almost alway over-reported altitude for my early-evening ride home based on the morning calibration of starting elevation by about 30 ft or so. If you look at the daily pressure cycle where I live, the pressure always drops as the day wears on and the effect is usually larger than changes due to any weather systems for the day. Cold fronts usually cause a bigger pressure change on the day they come through.
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Re: The Why's and How's of Wind Offset

Postby travispape » Sun Jun 29, 2008 4:39 am

Zecutive wrote:My question is the same as one of Charlies. What happens when the temperature varies substantially during a ride. I live in Florida. In the summer we are usually rolling at dawn so that we can so as much of a ride in as possible before the hottest part of the day. It is not unusual for the temp. at the beginning of a ride to be 75 and at the end of a 4 hour ride for it to be 100 or more in the sun. I see the altitude change significantly over that time even though it is very flat here. How much is the power reading being effected?


The best way to get the answer is to look at your own ride data. In the wind analysis window, highlight the beginning portion of the ride and compare it to the wind data later in the ride after the temperature has gone up. Compare the wind data to the weather station wind data for the day (www.weatherunderground.com is good for historical weather data) and try to estimate how much the wind offset changed during the course of the ride. Be sure to take into account the wind direction for each portion of the ride.

What you are looking for is whether there is a different amount of wind speed error at the end of the ride compared to the beginning. I remember one of my own rides in April ended up hot. Looking at the weather station data, the temperature only went up from 66 degF to 81 degF; however, since the ride started at 9 am and ended at close to 1 pm, the sun heating the road caused a bigger heat-up on the road than 15 degF. Anway, looking at my data, I can't see that there was any signfinicant error in the wind data due to the heat-up after compensating for the direction of the wind that day. As far as I can deduce, the wind speed measurment is good.

In general, the iBike is less sensitive to temperature changes at hot temperatures compared to winter time temperatures.

Anyway, if you do find that the wind speed data is off a noticeable amount at the end of the ride, you can play around with the wind offset correction in the analyze wind page to get an idea of how much error that causes in the power measurement. I just checked one of my rides and a 1 mph change in wind speed causes a 12 W change in power. You might see that much error in the winter time, but I don't think you'll see that much error very often in the summer.

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Re: The Why's and How's of Wind Offset

Postby aytchkay » Tue Oct 14, 2008 3:08 pm

Regarding Wind Calibration, I want to be absolutely clear regarding the manual and proper procedure.
While out of the wind (and in wind offset mode) watch that the number settles down close to zero. If it does than you say that calibration is valid. Do I NOT press the center button? Do I immediately go OUT of the Wind Offset screen?
If the number does not settle, I lightly press the center button and watch for it to settle close to zero. Then what? same questions above! Do I carefully touch arrow button to go out of Wind Offset screen or what?
You don't really instruct what to do once Wind Offset is zeroed....

I would appreciate the next step - my offsets have not been good, as analysis on iBike2 indicates...

thanks,

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Re: The Why's and How's of Wind Offset

Postby racerfern » Tue Oct 14, 2008 3:53 pm

Once wind offset is very close to zero in calm conditions you can exit the setup screen. Of course if you sit there and stare at the wind offset number it will move since no air is PERFECTLY still. As long as you for all practical purposes at zero in calm air, you're good to go. Depending on your preparation at that point you may still have to do your tilt, set your altitude, whatever.

Hope this helps.
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Re: The Why's and How's of Wind Offset

Postby ChrisS » Thu Dec 18, 2008 11:13 am

It seems like wind measurment is affected by 2 parameters- Wind scaling and wind offset. I see that the wind offset zeros the wind sensor but how does the wind scaling parameter work? How do the two work together to measure wind speed?

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Re: The Why's and How's of Wind Offset

Postby coachboyd » Thu Dec 18, 2008 12:14 pm

The wind scaling is a measurement of how much wind comes into your wind sensor at speed (I think that makes sense). Basically, when you do your calibration ride, it's measuring the wind speed vs. the wheel speed in both directions. This measurement converts the aero number into a cda number. So if you have an aero of .400 and a wind scaling of 1.0, your cda would be .400.

If you have a wind scaling of .9 though, your cda would be .360. Wind scaling can be affected by position on the bars or stem, angle of the unit, and it can vary from unit to unit. That's why it's so important to do the calibration ride.
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Re: The Why's and How's of Wind Offset

Postby mds » Fri Jan 02, 2009 11:12 pm

I've been doing a wind offset on my iSport both before and after my rides. I've noticed that at the end of my ride the wind offset never reads zero even though it did read zero at the beginning of my ride after a reset. On my unit I usually get a reading after my ride of a couple mph, and sometimes 6 or even 8mph. Is this normal? What is a normal drift for wind offset after a several hour ride?
Thanks,
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Re: The Why's and How's of Wind Offset

Postby Russ » Sat Jan 03, 2009 1:26 pm

Humm.. to dovetail on mds question,

I rode with my new gen3 for 43 minutes with the temp droping from 54 to 44F. I had let the unit settle outside for at least ten minutes before starting and did the offset cal. I had ridden fairly hard and my brain get funky but at the end, my best memory of Wind Offset at end of ride -2.x to -3.x in Km/Hr. On another 51 minute ride with temp start/finish of 47F to 43F,
wind offset at the end was -3.5 Km/Hr. I had taken pains to write down the -3.5 one after that ride.

What is the impact of this on the data and how best to handle it in post-processing?

If I stop periodically and rezero, conditions permitting, how does this affect the recorded data stream? Is this the best policy?

Thanks,
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Re: The Why's and How's of Wind Offset

Postby Russ » Sun Jan 04, 2009 3:41 pm

Yesterday's longer ride was interesting. Over about 2.5 hours, from 1500-1730 the temperature was about 50 from 1500-1600 then dropped steadily to 43 at 1730.
I run the external battery mount (new for Christmas with my new gen3) with a 123 battery.
I stopped and zeroed or checked wind offset several times and back home at the end.
First stop offset about -0.7 after about 10 minutes of riding, forgot the voltage.
Second stop at about 1530 voltage 2.85 and offset +1.0, zeroed.
Third stop at 1605 voltage 2.84 and offset 0. Temp just starting to drop
Home at 1730 voltage 2.59 and offset +5.0 Temp now 43F
The last 11 minutes was a separate cool down ride after doing the trip reset, it was roughly a
loop with nearly calm conditions (most of the last hour was near calm). When I post processed
setting ground wind to zero, the wind offset in the wind analysis screen went to +5.0.

All this to continue my above post, same questions.

Thanks,
Russ
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Re: The Why's and How's of Wind Offset

Postby Russ » Sun Jan 04, 2009 3:42 pm

Ooops type on voltage back at home, 2.59 should been 2.79v back at home.
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