Cda Questions

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JulianBoulter
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Cda Questions

Post by JulianBoulter »

Hi Guys,

I tried snapshot Cda today and noticed the Cda reported is quite different from what I have in my profile. The profile displays Cda as 0.353 whereas the snapshot Cda was varying between 0.39 and 0.45. In the initial setup I chose the Hoods Elbows Bent option which seems to us 0.351 for Cda. I noticed in Issac that if I tweak the Cda up to where I was seeing it in the snapshot Cda, it changes power significantly, so my questions are:

1. Should I now modify my profile to use the Cda I see when I perform the snapshot Cda or try a coastdown procedure to refine it further?
2. Is the Cda actually calculated during the initial calibration ride or does it just use the user option of Hoods Elbows Bent? Am thinking it is calculated by using the Hoods Elbows Bent option as a baseline as the numbers are slightly different e.g. 0.353 in the profile and 0.351 when the Hoods Elbows Bent option is selected?
3. And related to aerodynamics, how does iBike handle out of the saddle efforts where the usual ride position changes significantly?

Cheers

Julian
KenS
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Re: Cda Questions

Post by KenS »

The CdA in Newton is calculated based on your height, weight and position.
Snapshot CdA can be affected by various factors: head/tail wind, road surface, your position on the bike, braking. Basically, Newton works out how fast you should be coasting based on the calculated CdA then reports the CdA that would give you your actual coasting speed.
If your snapshot CdA is consistently a long way off the calculated CdA it could be worth tweaking. Start by checking that your Crr is correct because the snapshot assumes it is. But if, say, your Crr in the Newton is too low then the snapshot will compensate by reporting a higher CdA.
If Crr is right then you can tweak both CdA and Crr with coastdowns. I would not adjust CdA based on the snapshot.

To answer your last question, Newton does not make any adjustments for out of saddle efforts. If you are out of saddle for a climb then your speed means aerodynamics is not that significant. For training, the short times you are out of saddle are also insignificant to your overall training effort. If you are sprinting out of saddle I imagine you should be concentrating on other things than your power reading :)
-- Ken
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Russ
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Re: Cda Questions

Post by Russ »

Hi JulianBoulter (and Kens!),

I would only add to what Ken said if you did coastdowns vs the iBike height and weight setup approach.
If you did coastdowns, then iBike attempts to measure both Cda and Crr with complex math that looks for the Cda part at higher speed and the Crr part at the lower speed portion of the coastdown.

That said and even if you did not do coastdowns but thought things were right, say last summer or fall (if you are N. hemisphere) when temperatures were higher, now in winter with lower temps, the Crr probably is higher and certainly is form most, if not all tires. So the Cda displayed would be expected to be higher when cold weather prevails. Also if you are wearing more clothing that raises your Cda. I think the assumed Crr's for the height and weight method are for warmer temps.

Russ
Velocomp
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Re: Cda Questions

Post by Velocomp »

Excellent responses, Ken and Russ!
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JulianBoulter
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Re: Cda Questions

Post by JulianBoulter »

KenS wrote:The CdA in Newton is calculated based on your height, weight and position.
Snapshot CdA can be affected by various factors: head/tail wind, road surface, your position on the bike, braking.
But in theory the snapshot Cda should be close to the Cda you have set in the profile (calculated as you say from height, weight and position) if you are riding in the position you have set in the profile and with the same bike setup and clothing (because the other settings should take wind, road surface, ride position into account) should it not? I only did the calibration ride a couple of weeks ago so was expecting something similar

So for my understanding of the profile details, do I have this correct :

1) The calibration ride only works out the wind scaling factor and the riding tilt
2) Aero is calculated from wind scaling and Cda
3) Friction is calculated from riding tilt and Crr
4) Riding tilt is an offset based on the position of Newton on your bike and is used used in gradient calculations
5) Wind scaling is an offset based on the position of Newton on your bike and is used to measure the wind

One thing I'm still not too sure about is if your Cda is used to calculate wind scaling or is it as I say above, purely an offset to take into account position of the unit on the bike?

Thanks

Julian
KenS
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Re: Cda Questions

Post by KenS »

JulianBoulter wrote:
KenS wrote:The CdA in Newton is calculated based on your height, weight and position.
Snapshot CdA can be affected by various factors: head/tail wind, road surface, your position on the bike, braking.
But in theory the snapshot Cda should be close to the Cda you have set in the profile (calculated as you say from height, weight and position) if you are riding in the position you have set in the profile and with the same bike setup and clothing (because the other settings should take wind, road surface, ride position into account) should it not? I only did the calibration ride a couple of weeks ago so was expecting something similar

So for my understanding of the profile details, do I have this correct :

1) The calibration ride only works out the wind scaling factor and the riding tilt
2) Aero is calculated from wind scaling and Cda
3) Friction is calculated from riding tilt and Crr
4) Riding tilt is an offset based on the position of Newton on your bike and is used used in gradient calculations
5) Wind scaling is an offset based on the position of Newton on your bike and is used to measure the wind

One thing I'm still not too sure about is if your Cda is used to calculate wind scaling or is it as I say above, purely an offset to take into account position of the unit on the bike?

Thanks

Julian
Your points are all correct and CdA is not used to calculate wind scaling.
Snapshot CdA is calculated by Newton looking at your speed (or more correctly deceleration) and from that determines what force is needed to cause that. Since you are coasting the only force is from Aero + Rolling resistance + Gravity. Gravity we know (assuming your total weight and tilt are accurate). Snapshot CdA assumes Crr in the Newton is accurate and calculates rolling resistance accordingly. Snapshot CdA then says that whatever is left "must" be due to CdA. But on a rough road Crr is not accurate so snapshot CdA will be overstated.

Also, the calculated CdA is "just" a very educated estimate of your CdA that works for most people most of the time. But for some people it wont be quite right. For instance, my road bike CdA is 0.34 and touring bike CdA is 0.46 (both determined by coastdowns) versus a calculated 0.32. But in both cases I ride "on the hoods elbows bent".

So if you are consistently seeing a different snapshot CdA even when everything else is right, including road surface, you would probably benefit from doing a coastdown
-- Ken
JulianBoulter
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Re: Cda Questions

Post by JulianBoulter »

This is great information, thanks guys, is there a way to work backwards then so that I can take the Cda figure I saw in the snapshot Cda of 0.45 and calculate what the Crr should be to give that Cda?

Also I noticed that in Issac Tools\Cda analysis, which I know is intended for DFPM users (of which I am not one), that peaks of varying Cda are shown and they appear to roughly coincide with the points where cadence is zero, is this therefore the visual representation of snapshot Cda or something else?

Thanks

Julian
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Russ
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Re: Cda Questions

Post by Russ »

Hi JulianBoulter,

I will try to tackle your first question:
"is there a way to work backwards then so that I can take the Cda figure I saw in the snapshot Cda of 0.45 and calculate what the Crr should be to give that Cda? "

My analysis of your question brings up a subtle point that you may be missing which I will try to explain and I will also try to suggest a usable method for what you want.

The problem I see is that the snapshot CdA that you see among the other things mentioned (kit, position, etc) is dependent on Crr and wind in order to be actually correct or not. So if the Crr is exactly correct for the surface you are riding and there is NO wind, then the displayed CdA should be fairly accurate. So problems for absolute accuracy are potentially varying surface, non matched Crr to surface, wind variations including yaw angle etc.

These issues do not render the feature useless, they just complicate it's usage. A steady wind on a straight hill slope can be fairly useable too, best not a tail wind as this reduces the CdA contribution to the power total. Although if you simply want Crr, a tail wind matching your speed suppresses any CdA contribution! Hard to get such a steady state wind and its angle, hill, etc. Also I can only see this case working with 'soft peddling', see below about that.

Now exact accuracy is not necessary for the intended purpose of trying kit and position changes as a relative improvement is sufficient. Of course lacking total still air (including traffic), you probably will have to make multiple runs and average, which is a good idea anyhow. This extra work (climbing the hill and coasting down again) can be part of a training session (hill repeats) but also is one reason the DPMF combination is superior for tweeking your kit and position.

Especially if you have done the Coast Downs with your cal ride then the combination of CdA and Crr for your kit and position, at least with a good quality set of CD's and cal ride results, means that your power displayed is going to be quite close regardless of the absolute values of CdA and Crr used, except in high speed situations where the incorrect values increase the error contribution with speed since CdA trumps Crr at high speeds. (cube instead of square contributions respectively).

Now for my practical use for the feature regarding Crr and Cda tweaks:
Since I am not a racer, I rarely do the hill repeat thing for reducing my CdA. Also with the improvements in the Newton, I have less concern about tweaking the accuracy as it is usually quite good and easy to achieve the good results with or even without the coast downs!

That said, here is how I have used this in the past. Now I spread my usage over weeks or longer of observations because I rarely find the low wind conditions among a variety of reasons. I have, and probably will again, done the hill repeat thing for calibrations. First, to keep the power data from being suppressed if a coasting hill descent, you must 'soft peddle' above 30rpm. You could also use a flat with steady power applied instead.
Now you would go, in Isaac, to the 'Analyze' menu on the tool bar and choose 'Tweak CdA, Crr, Cm....' and there you enter the CdA value. Do this after identifying the part of you ride (hill descent or flat) and highlighting it.
Note the power value in the left hand window before applying a change and note the original CdA value with is from your profile. After the value is applied, the power will change. Now if you go back and adjust your Crr until you get a power match with the original power value for the highlighted section. For this to be really meaningful for a Crr tweak, again, your position, kit and road surface and low or steady wind are a must be for your test case.

The result of this probably won't be all that accurate of a Crr. You should sanity check this against similar tire Crr values, etc. You also may need to iterate through this whole process a number of times. I also think that a speed of about 15mph more or less should suffice for a Crr check as you do not want a small CdA error to contribute heavily. This is part of why I observe my displayed CdA on hills that I use a lot as I can sanity check my profile and even gauge the changing effect of road surface and sometimes wind over many months, even years of riding the same favorite route.

Hope this makes sense :-)
Russ
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Russ
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Re: Cda Questions

Post by Russ »

I used some of the techniques that I mentioned in above post today after my ride. Because I had the issue in mind, I soft peddled in several places during my ride and tinkered with my CdA and Crr values after the fact.
N_Loop_140301_z31.jpg
N_Loop_140301_z31.jpg (92.84 KiB) Viewed 15310 times
It turned out that this steep downhill followed by uphill (8% ish) worked best to check my profile.
You can see, for the selected section, Aero at 691 watts and Rolling resistance at about 84 watts.
So this is a good test of the CdA value as the residual average Power = 0.2 watts. The road is somewhat rough chipseal here and the .006 Crr should be approximately correct. If it isn't quite right on, it's error contribution is quite minimal. I was not intentionally 'soft peddling' here but rather gearing up and peddling until my gear inches and cadence were sufficient to produce power, so the interim peddling amounted to soft peddling!

Earlier in the ride, I soft peddled on a section of worn nearly smooth chip seal. The 'residual' power was about twenty watts. I had an 8kh tail wind that seemed stable but the Crr to balance it out was .003 and I do not buy that. I am using Conti 28mm 'All Season' tires at about 70psi front and 80 rear, the temperature was around 53F.
My bike is an oversize cross bike so I could get disk brakes and I can easily take larger tires (my favorite bike now). So I think the .006 for the whole ride may be a bit high but there is some even rougher road on some of my ride and I had chosen it as near a 'median' value.

Perhaps if I had not had the tail wind, I might have gotten a better shot at a good Crr for the smoother portion of the road.

Russ
JulianBoulter
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Re: Cda Questions

Post by JulianBoulter »

Hi Russ,

Many thanks for taking the time to explain this to me - so I have been watching my snapshot Cda over a couple of weeks now and I would say it consistently averages out to about 0.43 which obviously does not match the 0.353 in my profile. Being as I race TTs and Crits I want the best accuracy I can get so I plan to perform coast downs when I get the time. The discrepancy I suspect (with your guidance) is related to the Crr value which I have set in the profile for 'Good Asphalt' and 'Utility' tires whereas the country roads I ride on here in rural UK vary greatly from OK to very rough, I also ride in the winter on Schwalbe Durano Plus which is a heavy duty road tire. Anyway until I get the chance to play with this further hopefully you won't mind if I ask you the following questions:

1) So I take the point about the snapshot Cda being not so much one of absolute Cda accuracy but more of a way to try out different positions and watch the relative differences but do you think that until I perform the coast downs, I could simply use the 0.43 Cda figure in the 'Tweak Cda, Crr, Cm' tool as a rough guide to the actual watts I'm pumping out? i.e. do you think that this would give me a similar wattage result as discovering and entering my true Crr or do the other factors such as wind play too much of a role in the snapshot Cda to render this a worthless exercise?

2) Related to the above, I took this a step further (similar to the process you describe), here is what I did:
a) Using the 'Tweak Cda, Crr, Cm' tool I noted the increase in power I got by using the 0.43 Cda figure (approx 20 Watts)
b) Then using the 'Tweak Cda, Crr, Cm' tool again (after canceling out after step a), I increased the Crr value until I got approx the power I noted in step a)

This gave me a crr of 0.0089 which is close to what I get if I choose the 'Asphalt Blend' and 'Touring' options in the profile, do you think this is valid?

2) Just to clarify, when you say 'soft peddle' the reason for this is that it will allow me to see the snapshot Cda but also to record some wattage that can be used in the 'Tweak Cda, Crr, Cm' tool? (unlike when I stop peddling altogether) is that correct?

3) Can you clarify that the Cda analysis tool in Issac (which I know is intended for DFPM users of which I am not one), is providing me a visual record of snapshot Cda? I have been studying this for a while now and although it is not mentioned in the documentation, it really does appear that the peaks of varying Cda coincide with the points where cadence is zero i.e where I coasted to view my snapshot Cda

Cheers
Julian
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Russ
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Re: Cda Questions

Post by Russ »

Hey JulianBoulter,

Another cause of higher CdA is yaw angle. If you have ideal no wind conditions and do the coast downs, this snapshot will still read higher if you have a headwind from off axis due to the larger 'picture' or area that is actually facing the wind vector directly. This is because your smallest most aerodynamic position is heading directly into the wind. So my opinion on that is that the perfect CdA value should probably be slightly raised to better meet average headwind conditions. And yes the Crr not matching also will be reflected in the calculation as an offset. This is because the total drag is composed of both and they are static values. Still the iBike manages to be remarkably accurate when the profile is close.

1. Depends on the speed at which you are measuring the CdA. This is because of the dominance of CdA at higher speeds. Typically at mid speeds, the total is split close enough for good power unless your profile is off to much.
[edit to add afterthought] I would likely tweek the Crr upward for the tire and road conditions. I suspect if you search Crr in the forum you will several in depth discussions. Basically typical mixed road takes a +.001 or so additon to Crr for smooth surface, rough is likely more. Cobbles, no idea :-) [end edit]

2. The soft peddle is only for the tweek method that I outlined above. The CdA Analysis (see next answer) requires coasting.

3. Under Tools... CdA Analysis...
Isaac will display CdA for a selected portion of a preselected portion. That is you select a portion where you were coasting and holding your position, then open the CdA Analysis tool and be sure to set the smoothing values in both the main window and this pop up tool to minimum (I think both are necessary) then select a portion in the tool pop up window and the CdA value per analysis will display in the same CdA sub pane or sub window. You cannot be soft pedaling for this as it will only give you your profile value in that case (as best I remember it). And no (I do not have a DPFM either) you cannot do the other analysis that you can do if you have a DPFM, such as time advantage or any other except exactly this. This of course requires the Newton+ or and older model with Aero feature turned on.

Regards,
Russ
Pete
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Re: Cda Questions

Post by Pete »

Hi All (1st post)
I have a query hopefully related enough to use this thread...
(About to purchase a Newton but looking to gain some confidence first via questions)

If I update a profile to change my weight,which may have changed over time,will my previous rides with the same profile be re-calculated with the new weight or stay as per original? Or can it be either - i.e. a manual step to initiate a re-calc?

I'd like to know that I can track watts/kilo over time.
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Russ
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Re: Cda Questions

Post by Russ »

Hi Pete,

Welcome to the forum! The previous rides will not be recalculated automatically. Actually each ride file has the profile information for that ride in it. You can extract the profile from a ride, you can also recalculate a ride with a different profile which is sometimes useful if something was wrong during the actual ride, such as a wrong profile selected, etc.

Russ
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Russ
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Re: Cda Questions

Post by Russ »

Correction....

In answer to JulianBoulter above I said:
2. The soft peddle is only for the tweek method that I outlined above. The CdA Analysis (see next answer) requires coasting.

In my ride today, Isaac displayed Analyses CdA for a soft peddling section and NOT for the coasting section.
I guess my memory gets cornfused sometimes (Mangled English Fun for confused :-). At my advanced age, I beg forgiveness and mercy and with no beatings :-)

Russ
JulianBoulter
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Re: Cda Questions

Post by JulianBoulter »

Today I tried snapshot Cda twice in the same spot and in the same riding position, the only difference is that once it was cycling straight into +10 mph headwind and once facing the opposite direction with the +10 mph wind now a tail wind.

The results were as follows:

1. Snapshot Cda with headwind = 0.48
2. Snapshot Cda with tailwind = 0.38

I understand why the wind makes such a difference here as the headwind reading must see that I'm coasting much slower and therefore puts it down to drag but given that the Newton knows the wind speed, why can't it take that into account?
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Russ
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Re: Cda Questions

Post by Russ »

JulianBoulter,

I will give you one theory, perhaps Velocomp or others can give more.
If your headwind was not EXACTLY head on, ie say 10 or 15 degrees or even more off to the side, then the wind actually sees a higher area. This condition is called yaw angle. Just look at your bike head on and from 45 degrees (to make it easier to see) and observe the increase in area that you see. Since the CdA is Cd (drag coefficient) times area, an increase in CdA should make sense. I have seen claims that average wind is 15 degrees off center for bike riding. If this is true, perhaps the CdA should be set slightly higher than measured in a perfect coast down with no wind.

Now think about the reverse case of a tailwind. Now you probably were going faster by a good margin than the 10 mph tailwind. However the resultant vector will be at an even larger angle, perhaps one that the iBike doesn't measure as well, even effectively something approaching a cross wind. Further, since the CdA related force is reduced, then it is less valuable in the relative measurement of CdA. Remember the purpose of this tool is really to be able to sense relative changes in CdA as observed of several runs for each position and not really for calibrating the profile, though over several rides, I have made small tweeks to my profile based on it ( probably to Velocomp's amusement and chagrin!).

My analysis description may not be exact but hopefully it makes my point.

Hope this helps,
Russ
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racerfern
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Re: Cda Questions

Post by racerfern »

@JulianBolter:
Please post ride files, it helps to make sense of it all.

That said, be careful under what conditions you take snapshot CdA. I'm used to seeing .37-.38 under relatively calm wind conditions. I don't see how you can get a snapshot CdA into a 10mph headwind unless there's a significant downward slope. Either that or you're VERY strong. Regardless, I'm not strong at all, in addition I'm just trying to get back into the groove. Just look at my avg speed on the plus wind leg, 13.5 at best. I couldn't get a good snapshot CdA if I tried.

My coasting results were useless because of the extreme winds. Although the overall results were spot on with other rides on this same course, the few times I was able to get a snapshot CdA the reading was way off. I'm not concerned because under these circumstances that rarely occur, the amount of time you're actually making power is insignificant.

Add to that, that Russ is spot on, to truly judge wind you have to consider yaw angle and its effects. As Robert Chung said on the wattage forum years ago, "Wind is thy frenemy"
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JulianBoulter
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Re: Cda Questions

Post by JulianBoulter »

Thanks for the response guys, yeah I guess it's more complicated than my simplified version of things, and like you say, its more about relative changes than actual values which can be used reliably for the calibration.

I was just having fun messing around to improve my understanding of this great tool!
texmurphy
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Re: Cda Questions

Post by texmurphy »

Snapshot Cda is displayed when cadence rpm=zero.
Is there any reason if Snapshot Cda is active and rpm=0 for long enough to show Cda, that rpm should be shown?
Now one must wait through HR, then rpm, then Cda on the display.
When looking at the Snapshot Cda, rpm will always be zero and need not be displayed.
Velocomp
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Re: Cda Questions

Post by Velocomp »

Someday we might have Intel Inside... :-)

Until then, we have to live with the reality that there are real tradeoffs to be made to accommodate both algorithm and display requirements.
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