CdA Analysis

[mullerrj]

Wondering if somebody could help me out. I threw a Power Tap hub on the back of my bike with the iBike Aero on the front. The goal is to get continuous CdA while I'm riding so I can tweak my TT position. I setup everything by the book and performed a Calibration ride (2 miles out, 2 miles back). After the calibration ride I decided to check three positions on my bike: 1) on the tops 2) on the hoods and 3) in the drops. When reviewing the post ride file, I noticed that my position on the hoods was the lowest CdA of all three. How can that be? I would think that the tops would be the worst aero position, followed by the hoods and the drops would be the lowest CdA and best aero position. Can somebody check my file to see if I'm interpreting the data wrong? I don't think I am.

Mile mark 1 to mile mark 3= tops
Mile mark 3 to mile mark 5= hoods
Mile mark 5 to mile mark 7= drops

Thanks in advance. Rob

[rruff]

Where is your iBike or wind port located? You can get pretty extreme differences in wind scaling by changing position.

[mullerrj]

I have the remote wind sensor but I didn't put it on the iAero yet. I had the iAero mounted to my stem with a clear uninterrupted wind stream. Thanks Rob

[mullerrj]

Also, the wind was consistent when I tested the three positions. There was a 3-4 mph tailwind during the mile out, and a 3-4 mph headwind during the mile back. So, the wind was consistent on all three positions. Thanks Rob

[rruff]

I have the remote wind sensor but I didn't put it on the iAero yet. I had the iAero mounted to my stem with a clear uninterrupted wind stream. Thanks Rob

You might think it is "clear uninterrupted", but I'm pretty sure it is far from it. Even if/when you get the RWS mounted in a good location, it will probably be necessary to do a calibration for wind scaling in each riding position if you want accurate data. And by that I mean an extensive calibration... not just one trip out and back.

People go to a lot of trouble doing meticulous testing to get CdA accuracies of 2% or so via field testing. The iAero can help with this but it is still not easy. You need to know what you are doing and follow a good protocol.

[mullerrj]

When you say calibration for wind scaling are you talking about performing an AUTO WIND before the calibration rides? I'll tell you what I'm doing in about 15 minutes and it doesn't get any better than this. I'm going out on a flat level road void of any traffic at 0500 with absolutely no wind. In fact, it is so calm out right now I could fart and feel the breeze. I'm going to do all the requisite things I need to do: acclimating the ibike to the outside first, tilt, auto wind, etc. BEFORE the calibration ride. I'm going to do a cal ride (1 mile out and back) in each position AGAIN. If I can't get different distinct readings for each of the three positions I want CdA readings from..then I've got to conclude the iAero doesn't work as advertised for me..as far as CdA analysis. And, that's the reason why I bought the iAero. When you say "extensive calibration" I don't know what more I can do..within limitations. I'm not going to turn this into a PhD thesis project. If everything is setup properly, as I believe it is, why should I do EXTENSIVE calibration on wind scaling. Sounds to me, from what you are saying, the iAero isn't accurate enough. And, that to get accurate data I have to get a thousand data points and average them. Afterall, NOTHING I am doing is changing. The weather is NOT changing, my Cal ride position is NOT changing during the ride, the iAero mount position isn't changing, etc. The only thing that is changing from Cal ride to cal ride is my body position..which I want the CdA for. I have a Masters Degree in Aeronautical Engineering and if I can't do it..do you think any other layman that is buying the iAero will be able to? Should it say the iAero is for PhD aeronautical engineers ONLY? I'd like to think the iAero as a "poor mans wind tunnel" not a piece of junk that sometimes reads accurate CdA readings and sometimes not. Thanks for your advice.

[mullerrj]

1st ride (hoods) 2nd ride (drops) 3rd ride (hoods) 4th ride (drops)
Aero 0.41 0.388 0.346 0.367
Wind Scaling 0.919 1.027 0.956 0.991
Fric 6.508 6.508 7.375 6.508
Crr 0.0065 0.0065 0.0065 0.0065
CdA 0.446 0.378 0.362 0.37
Ride File 061810a 061810b 061810c 061810d
Tilt -0.10% -0.10% -0.10% -0.10%

[mullerrj]

oops..somehow the previous reply posted before I had a chance to explain. Anyway, that was the results of this a.m cal rides. Instead of doing tops, hoods and drops I did hoods and drops only because that would be the biggest change. The first two cal rides are what I expected, the CdA in the drops being less than on the hoods. i.e. 0.45 CdA hoods vs. 0.38 CdA drops. However, I would have expected the 3rd cal ride (hoods again) to be inline with the first cal ride, around 0.45 CdA. Instead, it was lower (0.36 CdA) than any of the other two CdA values I got in the drops..which can't be correct. What I did notice was that the FRIC number was higher on the 3rd cal ride (than the rest) and the AERO number was lower than the rest. That doesn't make sense. Nothing had changed on the ride. Anybody know what that's all about?
Anyway, I don't like the inconsistencies I'm seeing and hope I don't need to do a sample size of 8 cal rides instead of 4 to get some "reasonable assurance" that my numbers are correct. I guess if it were up to the statisticians of the world they'd have me doing a sample size of 20 to get some statistical significance...bleh.
Thanks again for any/all input. Cheers Rob

[rruff]

Your Fric # should be the same for all if your tilt and Crr are the same.

Also, I thought you wanted to tweak your TT position? Doesn't that mean you will ride in the aerobars? That is where getting the wind scaling correct is toughest, because changing positions will often effect the airflow that the unit (or RWS) sees.

By an extensive calibration of the wind scaling I mean that you do a lot of out-back laps. I don't use the iBike cal function for this, but rather just ride back and forth many times and determine what is required to make the wind cancel out. In addition you can look at each out-back segment to see how much scatter there is.

[rruff]

Sounds to me, from what you are saying, the iAero isn't accurate enough. And, that to get accurate data I have to get a thousand data points and average them. Afterall, NOTHING I am doing is changing. The weather is NOT changing, my Cal ride position is NOT changing during the ride, the iAero mount position isn't changing, etc. The only thing that is changing from Cal ride to cal ride is my body position..which I want the CdA for. I have a Masters Degree in Aeronautical Engineering and if I can't do it..do you think any other layman that is buying the iAero will be able to? Should it say the iAero is for PhD aeronautical engineers ONLY? I'd like to think the iAero as a "poor mans wind tunnel" not a piece of junk that sometimes reads accurate CdA readings and sometimes not. Thanks for your advice.

You should have enough knowledge to appreciate how difficult this task is. People pay $1k/hr in the wind tunnel to achieve accuracies of 1-2%.

The iAero isn't junk, but if you want accurate CdA readings, you'll have to work at it. The wind scaling is the big one that can change when you change positions, so you really need to nail it for each one. Many of the other errors can be minimized by taking a lot of data points.

[mullerrj]

rruff- thanks for your help. I've been in contact with John and we're going to work it out. I just want to know the best method for capturing the most accurate data. I have no problem putting the time, patience in..just need to know the best way to do it. I will document everything and post it for future iAero owners.

Yes, I'm going to eventually use the Wind Sensor on my TT bike but wanted to workout the methodology/protocl on my road bike. I figure the road bike was easier to establish a baseline. Yes, you are right. I appreciate how difficult the task is. I just can't believe no one has tried to do what I have yet. If they have, they haven't offered any help thus far. I realize the iAero isn't junk but sometimes I wonder why the discrepancies on what I believe are ideal conditions for gathering CdA numbers. When I tested this morning, the wind was dead calm. The mount did NOT move nor did my riding position. Or maybe the mount did move when I ran over a manhole cover or bump in the road. I'm not smart enough to know how sensitive the iAero unit is. Or, perhaps there is a wind even though I can't feel it. Regardless, I want to learn how to do it right..and when I do..I will document it and pass the information on to other iAero owners.

Thanks so much for your help. I realize now that some of my posts sounded like I was whining. It's just been a loooooooong week at work and I'm already feeling better now that it's the weekend and I have a drink in my hands. Cheers Rob

[mullerrj]

Ok, got an update with more CdA data points. Seems like I'm getting smarter each time I do the cal ride. I've got a good idea that my CdA on the brake hoods is 0.345. That's based on the cal rides (enclosed) and observation while riding long flat stretches of road w/ little to no wind. Enclosed is a .jpg file with the data so far. The reason I believe my CdA in the drops is not lower, which I expected, is because I have the iAero mounted on my handlebar stem and I'm thinking that when I'm getting lower I'm getting "dirty air" from my head/helmet which is no more than 6-8 inches (above) from the iAero port. Yes/No?

Here are some test particulars for me and the bike:
Height: 6'0"
Weight: 177#
Bike Weight (as tested): 18# (Trek Madone w/ water bottle and Power Tap SL+ hub and Open Pro wheel, Conti 4 season front tire on Bontrager Al rim and Conti Gatorskin rear tire on Open Pro wheel)
Road conditions: flat, smooth asphalt, with 2-3 speed humps NOT bumps in a one mile section
Weather conditions: calm (no wind), early morning, 62F
Traffic: no cars
Average speed: 20 mph out, 18 mph back
Cal ride: 1 mile out, 1 mile back
Equipment: Giro helmet, std racing kit

Comments? Thanks Rob

BTW, I'm only leaving the iAero mounted on the stem of my road bike until I get control/baseline data. Then, I will mount the remote wind sensor to see how that changes my CdA values in the same two positions. Next will be doing the same on the TT bike.

[lorduintah]

I think you are doing some great prep work. But for accuracy on some parameters a 1-mile out/1-mile back is not enough to get some things nailed. 3by3 or better is the way to go.

Also, how many CDs do you do? 10 is more like the number to capture one outlier that could throw parameters off.

Tom

Like Rob has already stated - more data, more confidence in the mean value. 80% confidence requires >36 data points for the average; more than 100 to get the standard deviation...... and all of this is just one position!


Tom

[mullerrj]

ok..no problem..I'll do 3 out 3 bak instead of 1 and 1. And, I'll keep doing them each week until I start to get some statistical significance. Like I said..I'll do what I have to do.

[Velocomp]

This is good work but, as others have pointed out, you need a lot of data, gathered carefully, to get the degree of accuracy you're looking for.

Others may have a different opinion but if I were doing this series of experiments I would do the following:

1) I would use a remote wind sensor, with the intake port located as far forward as possible, for example, a few inches beyond the end of the aero bars. The reason for this location is to minimize as much as possible the changes in stagnation pressure caused by your body's shifting from hoods to drops.

2) Since you're trying to measure small differences, make sure to do your testing in calm wind conditions

3) I would do a 4 mile long cal ride. This distance is long enough to get more accurate ride tilt data, but not so long that wind conditions will change.

4) I would do my first Cal Ride in the hoods position. After that first Cal Ride I would do 5-10 coast downs in the hoods position, followed by 5-10 coast downs in the drops position. NO CAL RIDE IN BETWEEN.

5) Then, I would do a second Cal Ride in the drops position, followed by coast downs in the hoods and drops positions as above.

6) I would hold Crr constant as you are doing, but I would also analyze with Crr allowed to be measured. You have a lot of data you're gathering and your overall results might be better.

If your RWS is located properly you should see little difference in wind scaling and ride tilt factors between the two Cal Rides. And, you'll have a ton of data that compares CdA for both Cal Rides in both positions. Hopefully you'll see similar results for each set of runs.

Let us know what happens!

[rruff]

John's protocol looks pretty good. I'm not the best person to offer advice since I don't have a DFPM... but then it doesn't look like you are using it either... or am I missing something?

The way I've done it without a DFPM, is to map the elevation profile of the coast-down course precisely by riding slowly in both directions a few times and constructing a slope vs distance table. It's tedious work, but once you get it, you never need to do it again. The reason I do this is to remove one unnecessary variable, since there is some scatter in the iBike's determination of slope. Then I do a series of coast downs... not using the iBike's function, but merely recording the speed and wind data. For each position I also need to get the wind scaling, but since I'm doing many out-back laps for each position, I have the data to do this also. Then when I get home I use Excel to do a curvefit on the coast-down runs, solving for CdA (Crr is fixed). It seemed to work pretty well, but I haven't done it much... it takes a lot of time... several hours to compare two positions.

One popular method using a DFPM (but no iBike) is the "Chung Method"... basically solving for virtual elevation on a loop course and determining the CdA that gives a consistent elevation profile. IMO the big problem with this method is that wind seems to screw it up... especially if it varies a bit. I think a better idea would be to use the iBike to map the slope profile, so it is no longer a variable, and instead solve for virtual wind. Then you can solve for the CdA that gives the best overall fit, and you can also have a map of the wind independent of the iBike's wind sensor... or in other words you have two sources of virtual wind data, and CdA as the only unknown. Well, that isn't exactly true... Crr will not be known precisely, but if you don't change your tires and tubes, this shouldn't be varying during the runs.

[mullerrj]

Yes, I have a DFPM...stated that in first post. I was wondering why John recommended Coast Downs. I did them anyway. Here is my latest data after I installed the RWS=Remote Wind Sensor. I think I'm getting more accurate readings with the RWS in the drops. Regardless, here's the latest. Thanks Rob

[rruff]

I'd check your Crr in run 3 on the hoods, but it looks like you are getting pretty good consistency. Are each of the Aero numbers an average of many runs? Usually there are a bunch of coast-downs per cal ride. Also, what tires and tubes do you use and what is the road surface like? .0065 is pretty high.