The Why's and How's of Wind Offset
Posted: 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.
Travis
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.
Travis