I'm wondering if someone w/ some wider experience than my own can give me a sanity check on my iBike-calculated CdA value. Calm days are a serious rarity where I live, and I haven't been able to do some new CD's to check the value. My last CDs were last fall.
I am 5'9" and weigh 138lbs. I have a narrow build, including shoulders, and I have short aerobars on my bike (they extend about 1/2" beyond the brake hoods - I don't have arm rests on my bars - I just rest my forearms on the inner part of the bar) and I used these for my CDs last fall. (I presume, that given their short length, they're more efficient than the drops, but less efficient than a full-on aerobar setup - this really is my 'go fast' position though, and aside from hill climbing, I'm generally in this position when riding or pulling hard, which are the situations in which I'm most interested in my output.) My CDs were done in regular biking shorts and jersey. The CdA value that came out was .3248.
My climbing numbers seem sensible, as do my moderately fast section numbers, provided there's no headwind. Its when I get in a headwind and and/or am really hammering that my numbers seem to increase at suspicious rates, which leads me to question the CdA value that is being used.
I've done some poking around on bikecalculator.com for sections of rides, trying to get a sanity check on power output values. For particularly fast and/or headwind sections of ride, my power numbers (given speed, weight, position, slope, headwind, temp and altitude) are almost invariably higher than those that this page shows for being in the drops, and is of course much significantly higher than this page shows for being on (presumably real-man's) aerobars.
Does .3248 seem high for a skinny dude?
Thanks for any help. I'm waiting for a calm day to do a new set of CDs, but around here, that could be a very long time coming...
Jim
Sanity check on CdA value?
Re: Sanity check on CdA value?
I also occasionally see suspiciously high power at high speeds on my iSport. I believe this could be due to either a CdA that is too high or a wind scaling that it too high. On high speed descents in no wind conditions, at 30 or 35mph or more, I've compared displayed bike speed to displayed wind speed and more often than not, I see wind speed that is slightly higher than bike speed by maybe 5 or maybe 10% but no more. Of course, it is very hard to know that there is actually no wind, so I've repeated this test many times in hopes seeing if wind speed is on average higher than bike speed. I think it is, but I am not positive about this. I own an iSport and so there is no way I can manually change wind scaling to a smaller value to see if power becomes more sensible more of the time. You may want to try this test and try tweaking wind scaling. Of course you can also tweak CdA and see if the situation improves too.
Power scales linearly with CdA but cubically with wind. So a incorrect wind scaling should be most noticable at very high speeds and much less so at moderate and lower speeds. An improper CdA on the other hand should give a constant percentage error across all moderate and high speeds. Of course, errors in either one will make little or no difference on climbs, because their contributions are negligable at low speed compared to the power required to climb the hill itself.
Edit: I think what I said above is wrong. The iBike's Aero coefficient apparently equals wind scaling times CdA. So a proportional error in wind scaling should have the same effect on power as a similar proportional error in CdA. The iSport does let me tweak CdA, so I should be able to experiment. I'm guessing, but if this is true then power should be more accurate but displayed wind speed would remain incorrect.
Power scales linearly with CdA but cubically with wind. So a incorrect wind scaling should be most noticable at very high speeds and much less so at moderate and lower speeds. An improper CdA on the other hand should give a constant percentage error across all moderate and high speeds. Of course, errors in either one will make little or no difference on climbs, because their contributions are negligable at low speed compared to the power required to climb the hill itself.
Edit: I think what I said above is wrong. The iBike's Aero coefficient apparently equals wind scaling times CdA. So a proportional error in wind scaling should have the same effect on power as a similar proportional error in CdA. The iSport does let me tweak CdA, so I should be able to experiment. I'm guessing, but if this is true then power should be more accurate but displayed wind speed would remain incorrect.
Last edited by mds on Mon Apr 06, 2009 10:47 pm, edited 1 time in total.
Re: Sanity check on CdA value?
Do you happen to have the calibration file with the coastdowns (the ibcd4m file)?
One thing that might be something to think about is if the wind port of the iBike was blocked it would effect the wind scaling factor. . .and the cda. The cda of .325 does seem a bit low for a small guy in an aero position. In my normal riding position in the hoods my cda is .318 and that has shown very good consistency over time compared with other power meters.
One thing to check. When you go into the setup menu, then to the coast screen, when you are coasting at about 20mph does it show positive or negative watts in excess of 10 watts?
One thing that might be something to think about is if the wind port of the iBike was blocked it would effect the wind scaling factor. . .and the cda. The cda of .325 does seem a bit low for a small guy in an aero position. In my normal riding position in the hoods my cda is .318 and that has shown very good consistency over time compared with other power meters.
One thing to check. When you go into the setup menu, then to the coast screen, when you are coasting at about 20mph does it show positive or negative watts in excess of 10 watts?
Boyd Johnson
http://www.boydcycling.com - high performance carbon wheels and accessories
http://www.boydcycling.com - high performance carbon wheels and accessories
Re: Sanity check on CdA value?
Boyd - unfortunately, I lost my calibration ride data files a few months back when my computer crashed. I can export my current profile, but I don't know if that'll provide any value to you or not.
On the .325m^2 value - you say that's a little low - do you mean a little high? (ie, I should expect my frontal area to be smaller than that given my relatively small size and semi-aero position?)
On past CDs, I've gotten CdA values ranging from .307 to .334, and Crr values ranging from .0034 to .0064. (My current profile, from the most recent set of CDs, has CdA of .325 and Crr of .0044).
As always, I appreciate your assistance!
On the .325m^2 value - you say that's a little low - do you mean a little high? (ie, I should expect my frontal area to be smaller than that given my relatively small size and semi-aero position?)
On past CDs, I've gotten CdA values ranging from .307 to .334, and Crr values ranging from .0034 to .0064. (My current profile, from the most recent set of CDs, has CdA of .325 and Crr of .0044).
As always, I appreciate your assistance!
Re: Sanity check on CdA value?
Boyd - I went out at lunch today and did two sets each of 5 CDs and 1 4MR - one on the aerobars, and one in the drops, just so there'd be something to compare. It was a little windy, but for around here, it was relatively calm, so w/o waiting weeks for an abnormally calm day, this is about as good as it gets.
I was wearing a snug-fitting long sleeve shirt for these, whereas my prior set (the set that had yielded .325m^2 CdA), I was wearing a short sleeve shirt. I had to brake 3-4 times on my 4MR on the aerobars due to traffic on the path I was using and I somehow seemed to hit an unusual number of bumps on the way out on that one. I had to slow 2-3 times on the 4MR in the drops, and it wasn't as bumpy. There is a noticeable tilt decline after the correction on the aerobar 4MR (~20 ft over the 4 miles), while the drops 4MR seems pretty consistently flat after the correction.
The CdA for the aerobars CDs came out to .331 (even higher than the already suspiciously-high .325), while the CdA for the drops CDs came out to .360. Crr was .0039 and .0036, respectively, for the aerobars and drops. (My prior profile had it at .0044.) Wind Scaling was respectively 1.383 and 1.657. (Wind Scaling on my current profile is 1.948.)
I've attached the ibcd4m files here. Any insights you can provide would be appreciated. I wish that I was producing the watts numbers at higher speeds that my current profile says I am, but I don't quite buy it...
Thanks!!
Jim
I was wearing a snug-fitting long sleeve shirt for these, whereas my prior set (the set that had yielded .325m^2 CdA), I was wearing a short sleeve shirt. I had to brake 3-4 times on my 4MR on the aerobars due to traffic on the path I was using and I somehow seemed to hit an unusual number of bumps on the way out on that one. I had to slow 2-3 times on the 4MR in the drops, and it wasn't as bumpy. There is a noticeable tilt decline after the correction on the aerobar 4MR (~20 ft over the 4 miles), while the drops 4MR seems pretty consistently flat after the correction.
The CdA for the aerobars CDs came out to .331 (even higher than the already suspiciously-high .325), while the CdA for the drops CDs came out to .360. Crr was .0039 and .0036, respectively, for the aerobars and drops. (My prior profile had it at .0044.) Wind Scaling was respectively 1.383 and 1.657. (Wind Scaling on my current profile is 1.948.)
I've attached the ibcd4m files here. Any insights you can provide would be appreciated. I wish that I was producing the watts numbers at higher speeds that my current profile says I am, but I don't quite buy it...
Thanks!!
Jim
- Attachments
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- 090409_cal_data_Plantation_CD_Huck_4MR_drops.ibcd4m
- (166.43 KiB) Downloaded 589 times
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- 090409_cal_data_Plantation_CD_Huck_4MR_aerobars.ibcd4m
- (166.45 KiB) Downloaded 559 times
Re: Sanity check on CdA value?
Large variations in wind scaling across multiple calibration runs is not good.
Re: Sanity check on CdA value?
I just took a look at your calibration stuff. Your coastdowns look VERY good. Now, the only thing that I would recommend is to do your coastdowns from the drops and aerobars on separate runs.
Another thing to think of, if you do your coastdowns on a very good road surface, but you typically ride on "normal" roads, then you will just want to increase your crr a little bit (but keep the cda the same). The very smooth road will roll a little bit faster than the "normal" roads.
If you could, try two separate runs. One where you only do coastdowns in the drops then your four mile ride.
Then another where you only do coastdowns in the aero bars with another 4 mile ride. Make sure to download and clear the iBike before starting the second one.
Another thing to think of, if you do your coastdowns on a very good road surface, but you typically ride on "normal" roads, then you will just want to increase your crr a little bit (but keep the cda the same). The very smooth road will roll a little bit faster than the "normal" roads.
If you could, try two separate runs. One where you only do coastdowns in the drops then your four mile ride.
Then another where you only do coastdowns in the aero bars with another 4 mile ride. Make sure to download and clear the iBike before starting the second one.
Boyd Johnson
http://www.boydcycling.com - high performance carbon wheels and accessories
http://www.boydcycling.com - high performance carbon wheels and accessories
Re: Sanity check on CdA value?
Also, do you have a picture of your iBike mounted with the aerobars? If your wind port is blocked at all that could lead to the different wind scaling values and also the different aero values, as the iBike would think you have more of a tailwind than normal.
Boyd Johnson
http://www.boydcycling.com - high performance carbon wheels and accessories
http://www.boydcycling.com - high performance carbon wheels and accessories
Re: Sanity check on CdA value?
I've attached (I think... 2 pictures of how the iBike is mounted on my bike. I have a diamond-section stem, and the Topeak extender wouldn't clamp to it (I tried), so I went the ghetto approach with an old mtb bar-end and an adaptor that I machined from alu. The air port of the iBike sits approx 1.5" from the bottom edge of the bars (which slope upward at the end, where I put my hands, so its actually about 2.5" below the bottom edge at the ends of the bars, and subtract about 1/2" from that by the time I wrap my fingers around the bars. The port sits about 3.5" back from the end of the bars, which translates to about 4.5-5" behind the furthest point forward on my hands when they're on the bars. I don't think the port is at all obstructed, but its possible that it could be getting some wake turbulence from the air coming off of my hands, perhaps?
I've done numerous tests while riding along where I maintain the aero posture, but take my hands on and off the aerobars, just to make sure that there is no discernible effect on the power readings (ie, checking for this potential wake interference), and I have not been able to notice any change in power reading with hands on or off the bars.
So if my CDs seem reasonable, can I assume that the CdA values I'm getting are 'real', and if so, why is a small guy like me, in a semi-aero position, pushing so much doggone air?
Thanks for the help!!
Jim
2 pictures of how the iBike is mounted on my bike. I have a diamond-section stem, and the Topeak extender wouldn't clamp to it (I tried), so I went the ghetto approach with an old mtb bar-end and an adaptor that I machined from alu. The air port of the iBike sits approx 1.5" from the bottom edge of the bars (which slope upward at the end, where I put my hands, so its actually about 2.5" below the bottom edge at the ends of the bars, and subtract about 1/2" from that by the time I wrap my fingers around the bars. The port sits about 3.5" back from the end of the bars, which translates to about 4.5-5" behind the furthest point forward on my hands when they're on the bars. I don't think the port is at all obstructed, but its possible that it could be getting some wake turbulence from the air coming off of my hands, perhaps?I've done numerous tests while riding along where I maintain the aero posture, but take my hands on and off the aerobars, just to make sure that there is no discernible effect on the power readings (ie, checking for this potential wake interference), and I have not been able to notice any change in power reading with hands on or off the bars.
So if my CDs seem reasonable, can I assume that the CdA values I'm getting are 'real', and if so, why is a small guy like me, in a semi-aero position, pushing so much doggone air?
Thanks for the help!!
Jim
- IMG_2411_low.jpg (226.92 KiB) Viewed 13742 times
- IMG_2412_low.jpg (226.82 KiB) Viewed 13738 times
Re: Sanity check on CdA value?
Hmm, you know what. It *could* be that when you are in those aero bars that you are actually more aero in the drops. I noticed they are shorter bars so you won't be stretch out on them. I don't see any elbow rests, so if your elbows sit farther apart, then you actually could be more aero in the drops.
When you did your coastdowns did you do them on the same section of road? I noticed that every one of your coastdowns lasted 10 seconds, so that tells me that there really wasn't a big difference in aerodynamics between the two positions.
When you did your coastdowns did you do them on the same section of road? I noticed that every one of your coastdowns lasted 10 seconds, so that tells me that there really wasn't a big difference in aerodynamics between the two positions.
Boyd Johnson
http://www.boydcycling.com - high performance carbon wheels and accessories
http://www.boydcycling.com - high performance carbon wheels and accessories
Re: Sanity check on CdA value?
Its very likely/possible that I'm not realizing much aero advantage on the aerobars. They are intentionally short - I ride a regular old road bike, and use the aerobars mainly because I can ride on them for 10, 20, 30 minutes pretty comfortably, whereas I cannot ride in the drops comfortably that long (bad back, pathetically thin upper body), but my bike isn't setup for big-ole aerobars hanging off the front. When I'm concentrating on it (such as when there's a killer headwind), I keep my elbows pretty close in by the stem, but when I'm cranking up a roller, or accelerating hard, or being lazy, I do tend to put my elbows out further (not way further - still just a matter of inches from the stem, but they can range as far as halfway from the stem to the end of the bar, which is a 40cm bar), as it gives me more stability. All that having been said, I think I do gain some aero or mechanical advantage on them, as its not uncommon for me to take my pull on a 'lets beat each other up' ride w/ gung-ho friends, go down to the aerobars, and 2 minutes later, look back and there's nobody there - whether that's aero or just plain comfort/ergonomics/mechanics, I'm not sure.
Whether I'm more aero in the drops, I don't know. Its possible, and it wouldn't surprise me altogether, but the respective CdA values coming out of the aero-vs-drops CDs today were .331 and .360 respectively, so it seems the aerobars are yielding a little more aero advantage. (Though my Crr's were inversely related - .0039 on the aerobars and .0036 in the drops - I presume due to more weight forward w/ the elbows resting on the handlebars when I'm on the aerobars - does .03 on CdA or .0003 on Crr buy/hurt me more?)
The thing is, I'll be riding on a mild downhill, going along at a reputed 300-some-odd watts (which I'm a little suspicious of to start with, based on perceived effort and comparative output to a slight uphill section) then it'll suddenly report that I'm doing 400-450w, and I may be putting in a little more effort, or may have encountered a little more headwind, but I don't think to the effect of 1/3 again more effort. So, the reported power output seems a bit inflated on slight downhill sections as compared to slight uphill sections, and when there's a headwind on the downhill section, it seems even more pronounced. From what I'm learning here, maybe this is in fact a wind scaling thing, since it seems to inflate more with increased speed? (or maybe I'm just a lazy bum, and I don't mind working harder when its downhill...
I did all 10 CDs today (5 on aerobars, 5 on drops) on the same stretch of road. The uphill on the section where I was coasting out was a little steeper than I'd expected, but still nothing major (I just presume that more rollout time is better than less, as far as the iBike doing its magic in collecting and analyzing data.) Your point about road surface is well taken - overall the section I did my CDs on is a least a little smoother than most of the roads that I ride on. If I were to tweak the Crr a little bit to account for this, how much would you recommend I do so?
Thanks Again - I really appreciate everyone's insight and input on this!
Whether I'm more aero in the drops, I don't know. Its possible, and it wouldn't surprise me altogether, but the respective CdA values coming out of the aero-vs-drops CDs today were .331 and .360 respectively, so it seems the aerobars are yielding a little more aero advantage. (Though my Crr's were inversely related - .0039 on the aerobars and .0036 in the drops - I presume due to more weight forward w/ the elbows resting on the handlebars when I'm on the aerobars - does .03 on CdA or .0003 on Crr buy/hurt me more?)
The thing is, I'll be riding on a mild downhill, going along at a reputed 300-some-odd watts (which I'm a little suspicious of to start with, based on perceived effort and comparative output to a slight uphill section) then it'll suddenly report that I'm doing 400-450w, and I may be putting in a little more effort, or may have encountered a little more headwind, but I don't think to the effect of 1/3 again more effort. So, the reported power output seems a bit inflated on slight downhill sections as compared to slight uphill sections, and when there's a headwind on the downhill section, it seems even more pronounced. From what I'm learning here, maybe this is in fact a wind scaling thing, since it seems to inflate more with increased speed? (or maybe I'm just a lazy bum, and I don't mind working harder when its downhill...
I did all 10 CDs today (5 on aerobars, 5 on drops) on the same stretch of road. The uphill on the section where I was coasting out was a little steeper than I'd expected, but still nothing major (I just presume that more rollout time is better than less, as far as the iBike doing its magic in collecting and analyzing data.) Your point about road surface is well taken - overall the section I did my CDs on is a least a little smoother than most of the roads that I ride on. If I were to tweak the Crr a little bit to account for this, how much would you recommend I do so?
Thanks Again - I really appreciate everyone's insight and input on this!
Re: Sanity check on CdA value?
It is likely either the CdA or wind scaling that is too high. One thing about your setup is that I'd expect a pretty huge difference in wind scaling between the aero position and any other. The position of your arms and hands on the aerobars are quite close to the unit. Did you do all of your CDs and cal ride in the same position? IMO it would be best to wait for a calm day and do the full cal in each position.jim_s wrote:My climbing numbers seem sensible, as do my moderately fast section numbers, provided there's no headwind. Its when I get in a headwind and and/or am really hammering that my numbers seem to increase at suspicious rates, which leads me to question the CdA value that is being used.
The wind offset can be checked at any time by simply doing the cal ride... or do a series of out-back "laps" on your own, and when you get home check to see what the net wind was in IB3 (should be close to zero).
Those CdA numbers seem a little high for someone your size, but then I don't know your complete setup and your position on the bike. At calibration speeds (20-8mph) the ratio of roling drag to aero drag will be ~30% at 20mph and ~200% at 8mph, assuming a flat road and no wind. I wish there was a way to peg the Crr and have the program calculate the appropriate CdA from the CDs... but in lew of that maybe try changing the CdA in the opposite direction half as much as you change your Crr. For instance, if you increase Crr from .0039 to .0055 (a 41% increase), try decreasing CdA 20%... or from .331 to .275. Of course a better option would be to redo all the calibration procedures if you have a decent day.Whether I'm more aero in the drops, I don't know. Its possible, and it wouldn't surprise me altogether, but the respective CdA values coming out of the aero-vs-drops CDs today were .331 and .360 respectively, so it seems the aerobars are yielding a little more aero advantage. (Though my Crr's were inversely related - .0039 on the aerobars and .0036 in the drops - I presume due to more weight forward w/ the elbows resting on the handlebars when I'm on the aerobars - does .03 on CdA or .0003 on Crr buy/hurt me more?)
Your Crr is on the low side... people who have done meticulous field testing with racing tires and latex tubes generally get values in the .0040-.0045 range. With more typical tires and tubes, values of .005-.007 are common. It is difficult for the iBike to extracted accurate CdA and Crr values unless the conditions are good. It is much better at total drag... ie the sum of the two. Another possibilty for your low Crr is a poor tilt cal. .1% error in tilt is .001 (~20%) error in Crr.
Make sure your wind scaling is correct and see if the readings are still erroneous. The best check on CdA is to put the iBike on the "coast" screen and then coast downhill. The "coast" screen will display positive and negative watts. Ideally you'd want this to fluctuate around zero (the amount will vary a lot depending on how fast you are going and the gustiness of the wind). If it is always positive then it is likely that your CdA is too high.I've done some poking around on bikecalculator.com for sections of rides, trying to get a sanity check on power output values. For particularly fast and/or headwind sections of ride, my power numbers (given speed, weight, position, slope, headwind, temp and altitude) are almost invariably higher than those that this page shows for being in the drops, and is of course much significantly higher than this page shows for being on (presumably real-man's) aerobars.
Re: Sanity check on CdA value?
rruf - thanks for the detailed and insightful info. I agree, I need to wait for one of those rare calm days and do another set of CDs and 4MR and see where things fall out again.
From what I've read on this and other threads, this does sound like a potential wind scaling issue. I did a ride today, and afterward I picked a section where I did an out/back on a road - I looked at the 3 mi before the turn-around point, and 3 mi back the other way. When I go into the 'Analyze Wind' dialog, the Avg Ground Wind is showing at 1.7mph. This is based on my current profile, which has CdA of .325 and Crr of .0044. (In response to your question about the Crr - I'm no expert on this stuff, but I do use pretty quality tires (Vredestein Forteza Tricomp) and run them at a relatively high pressure (140psi). Being lightweight, in combination w/ the high pressure, it wouldn't surprise me if my rolling resistance was abnormally low.)
I made a copy of my profile and played w/ the wind scaling factor (1.948 in my profile), and found that by setting the wind scaling value down to 1.66 and applying the profile to this extracted section of the ride, the Avg Wind value in the Analyze Wind dialog went to zero. The side-effect of this however was my already-suspiciously-high CdA being jacked up to .381, which I'm guessing must introduce its own issues. Is there a way to manually tweak the wind scaling factor w/o mucking up the CdA value?
I got to thinking about Boyd's hypothesis as regards the short aerobars and the relative aero values compared to teh drops - I find on long downhills that are steep enough for me to not bother pedaling anymore, that I coast faster when I get into the drops and hunched way down than when I'm on the aerobars. (coupled with my general fear of being on those things when going 35+ mph anyway... I notice the 'really scrunched down' position (chin nearly resting on the stem) is significantly faster than a more upright 'drops' position (not shocking), as well. Given the seeming wide variation in aero drag in various positions in the drops though, I'm wondering how people reliably choose their drops position for CDs and general riding, when they base their profile on a position in the drops. Anyway, I digress, but I think that Boyd is definitely onto something there - ie, my 'aero' position isn't likely especially aero. (Though again, I use that position primarily for the extended comfort factor rather than for any expected aero advantage.)
On the potential for the wind scaling to be affected by my hands/arms being close to the unit when I'm on the aerobars - do you suspect its getting rough air from my hands/arms? There is definitely nothing directly obstructing or in the airflow in front of the port, but it is quite possible that it could be getting a wake off of my hands. (I've tried to visualize the airflow around my hands, but lacking a wind tunnel, visualization is about the best I can do... I suppose that I could go find a hill and put the iBike into CD mode, then try to establish a baseline 'hovering around zero' value/range w/ my hands on the bars, then try to very subtly move them up on top of the bars and see if that makes any difference - you think that could be a useful exercise, or you think it would be way too subjective?
Anyway, I'm on the lookout for a calm day that's warm enough for me to wear shorts and short sleeves, and plan to go out and do 5CDs and a 4MR for 2-3 riding positions (downloading in between position setups) and see what comes of it all. As it currently stands right now, I must be one of the widest, least-aerodynamic skinny dudes on the planet...
From what I've read on this and other threads, this does sound like a potential wind scaling issue. I did a ride today, and afterward I picked a section where I did an out/back on a road - I looked at the 3 mi before the turn-around point, and 3 mi back the other way. When I go into the 'Analyze Wind' dialog, the Avg Ground Wind is showing at 1.7mph. This is based on my current profile, which has CdA of .325 and Crr of .0044. (In response to your question about the Crr - I'm no expert on this stuff, but I do use pretty quality tires (Vredestein Forteza Tricomp) and run them at a relatively high pressure (140psi). Being lightweight, in combination w/ the high pressure, it wouldn't surprise me if my rolling resistance was abnormally low.)
I made a copy of my profile and played w/ the wind scaling factor (1.948 in my profile), and found that by setting the wind scaling value down to 1.66 and applying the profile to this extracted section of the ride, the Avg Wind value in the Analyze Wind dialog went to zero. The side-effect of this however was my already-suspiciously-high CdA being jacked up to .381, which I'm guessing must introduce its own issues. Is there a way to manually tweak the wind scaling factor w/o mucking up the CdA value?
I got to thinking about Boyd's hypothesis as regards the short aerobars and the relative aero values compared to teh drops - I find on long downhills that are steep enough for me to not bother pedaling anymore, that I coast faster when I get into the drops and hunched way down than when I'm on the aerobars. (coupled with my general fear of being on those things when going 35+ mph anyway...
I notice the 'really scrunched down' position (chin nearly resting on the stem) is significantly faster than a more upright 'drops' position (not shocking), as well. Given the seeming wide variation in aero drag in various positions in the drops though, I'm wondering how people reliably choose their drops position for CDs and general riding, when they base their profile on a position in the drops. Anyway, I digress, but I think that Boyd is definitely onto something there - ie, my 'aero' position isn't likely especially aero. (Though again, I use that position primarily for the extended comfort factor rather than for any expected aero advantage.)On the potential for the wind scaling to be affected by my hands/arms being close to the unit when I'm on the aerobars - do you suspect its getting rough air from my hands/arms? There is definitely nothing directly obstructing or in the airflow in front of the port, but it is quite possible that it could be getting a wake off of my hands. (I've tried to visualize the airflow around my hands, but lacking a wind tunnel, visualization is about the best I can do...
I suppose that I could go find a hill and put the iBike into CD mode, then try to establish a baseline 'hovering around zero' value/range w/ my hands on the bars, then try to very subtly move them up on top of the bars and see if that makes any difference - you think that could be a useful exercise, or you think it would be way too subjective?Anyway, I'm on the lookout for a calm day that's warm enough for me to wear shorts and short sleeves, and plan to go out and do 5CDs and a 4MR for 2-3 riding positions (downloading in between position setups) and see what comes of it all. As it currently stands right now, I must be one of the widest, least-aerodynamic skinny dudes on the planet...
Re: Sanity check on CdA value?
Another item of note - I took the profiles that were generated as a result of my still-in-some-question CD/4MRs last week (one on aerobars, one in drops, though I did them back-to-back and didn't download between, which is apparently a no-no). Anyway, taking the profile that was generated in the drops and applying it to this 6 mi out/back ride today, guess what - Avg Wind is zero (it was 1.7 mph using my current profile). Applying my aerobar profile generated from the other day, Avg Wind for this same section came out to -1.8 mph. So, perhaps you're onto something w/ your thoughts on the position of my arms as relates to the unit. (Again, I'm sort of just guessing and flailing away here until I can do another, more proper set of CDs/4MR, but its interesting to ponder, and it certainly *seems* to point to a significant difference in wind readings between being in the drops vs on the aerobars)
Taking this drops profile and going back and applying it to some rides that I've been suspicious of numbers reported on fast sections, the numbers on those sections come down to more believable values. ex, one section on a ride w/ an ave speed of 25mph had an ave watts for 10 min reported at 366 w/ my original profile - which I don't buy, vs w/ the drops profile, that same section reports at 322w - I can just about believe that. Granted, I was on the aerobars for that section, not the drops, but assuming the two are pretty close aerodynamically, even w/ the drops profile CdA being higher than the aerobars profile CdA (.360 vs .325), the associated wind scaling factor (1.657 vs 1.948) seems to be providing more reasonable numbers at the higher speed. (Incidentally, comparing the watts on climbs between the two different profiles applied to the same climb sections, the effect was pretty minimal - on the order of 2-3% difference, vs the 10-12% difference on the higher-speed sections.)
If there is something going on as regards the proximity of the unit to my hands/arms on the aerobars, how much more separation of the two should I try to get?
Taking this drops profile and going back and applying it to some rides that I've been suspicious of numbers reported on fast sections, the numbers on those sections come down to more believable values. ex, one section on a ride w/ an ave speed of 25mph had an ave watts for 10 min reported at 366 w/ my original profile - which I don't buy, vs w/ the drops profile, that same section reports at 322w - I can just about believe that. Granted, I was on the aerobars for that section, not the drops, but assuming the two are pretty close aerodynamically, even w/ the drops profile CdA being higher than the aerobars profile CdA (.360 vs .325), the associated wind scaling factor (1.657 vs 1.948) seems to be providing more reasonable numbers at the higher speed. (Incidentally, comparing the watts on climbs between the two different profiles applied to the same climb sections, the effect was pretty minimal - on the order of 2-3% difference, vs the 10-12% difference on the higher-speed sections.)
If there is something going on as regards the proximity of the unit to my hands/arms on the aerobars, how much more separation of the two should I try to get?
Re: Sanity check on CdA value?
Those are decent tires, but the Crr is multiplied by weight to get resistance... so being light doesn't matter. For now I think that is close enough though... it is not off by much if at all. At higher speeds Crr is a small factor.jim_s wrote:(In response to your question about the Crr - I'm no expert on this stuff, but I do use pretty quality tires (Vredestein Forteza Tricomp) and run them at a relatively high pressure (140psi). Being lightweight, in combination w/ the high pressure, it wouldn't surprise me if my rolling resistance was abnormally low.)
This just indicates that your Aero factor is way off. I'm going to take a wild guess that your CdA is really around .275 on the aerobars. I weigh 170 and my CdA is .34 on the road bike and .255 on the TT bike with all the good stuff. Until you get a low-wind day try setting your CdA to .275 and wind scaling to 1.66... which would be a .456 Aero factor.I made a copy of my profile and played w/ the wind scaling factor (1.948 in my profile), and found that by setting the wind scaling value down to 1.66 and applying the profile to this extracted section of the ride, the Avg Wind value in the Analyze Wind dialog went to zero. The side-effect of this however was my already-suspiciously-high CdA being jacked up to .381, which I'm guessing must introduce its own issues. Is there a way to manually tweak the wind scaling factor w/o mucking up the CdA value?
Comfort does influence power quite a lot. It is the reason we don't ride with a flat back all the time. I am very aero with my chin on the bars and everything pulled in... but I can't ride in that position. IMO setting the profile for your normal extended hard riding position is the way to go. In the rare times that you are more aero it will also cost you physiologically, and when you sit up or stand on a climb it will be easier physiologically... so though the iBike power reading will be off a little, it will actually give you a good reading of your physiological stress.Given the seeming wide variation in aero drag in various positions in the drops though, I'm wondering how people reliably choose their drops position for CDs and general riding, when they base their profile on a position in the drops. Anyway, I digress, but I think that Boyd is definitely onto something there - ie, my 'aero' position isn't likely especially aero. (Though again, I use that position primarily for the extended comfort factor rather than for any expected aero advantage.)
Your problem is that your hand position on the aerobars likely has a big effect on the unit... so there is no good way to have a setup that works for both that and hoods or drops. This is a different issue that might be hard to resolve.
Yep. Do the full calibration on the aerobars and then one on the hoods and you will know how much the effect is. I'd expect wind scaling and CdA to both be different.it is quite possible that it could be getting a wake off of my hands.
On my TT bike I have the iBike mounted a couple of inches above the front tire. The wind scaling is 1.7. The same unit on the handlebars has a wind scaling of 1.05.
Re: Sanity check on CdA value?
Wow, thanks again rruf - I wish I had about 1/2 as much of a grasp of this stuff as you seem to have!
I'll try those numbers for a few rides and see how they go - I'm also going to go back and try to apply those to a series of past rides and see how things look.
I'm in full agreement w/ setting the profile for the position I'm most frequently in when I'm riding hard, which in my case is on the stubby aerobars. When I'm just riding along, I'm generally on the hoods, but that typically means I'm just doing an easy ride (or section of a ride), or that I'm drafting. When I want to get down and boogie for any appreciable period of time, I go to the stubbies just about every time. (The other times I'm interested in my power output is on climbs, and in that case, aero doesn't really come into play, and I've found that applying different profiles generally only results in a very minor % of difference in the reported numbers.)
Thanks Again!
I'll try those numbers for a few rides and see how they go - I'm also going to go back and try to apply those to a series of past rides and see how things look.
I'm in full agreement w/ setting the profile for the position I'm most frequently in when I'm riding hard, which in my case is on the stubby aerobars. When I'm just riding along, I'm generally on the hoods, but that typically means I'm just doing an easy ride (or section of a ride), or that I'm drafting. When I want to get down and boogie for any appreciable period of time, I go to the stubbies just about every time. (The other times I'm interested in my power output is on climbs, and in that case, aero doesn't really come into play, and I've found that applying different profiles generally only results in a very minor % of difference in the reported numbers.)
Thanks Again!