Checking your calibration

[rruff]

Seems that several people have been having troubles getting good calibrations lately, so I thought I'd reiterate some simple ways that you can check your values without going through the entire procedure again.

To check to make sure your Aero value makes sense:
a) Ride for at least 5 minutes in your normal position so the iBike can self adjust for tilt.
b) Make sure your wind offset is zeroed.
c) Put your iBike in the Setup-Coast screen.
d) Coast down a mild decent (~30mph should be good) and see what the reading is. Of course you need to be in the proper position.

The Setup-Coast screen will display both positive and negative watts so it is easier to tell what is going on. It will switch back to the normal screen after a few minutes, but you can always switch it back. The reason for the fairly high speed is to make the Aero factor large compared to Crr.

The reading will fluctuate quite a lot depending on your speed and the gustiness of the wind, but you are looking for the reading to fluctuate around zero rather than always be postive or negative. If it is always above zero then this indicates that Aero is too large, and vice versa. You can do this check quite easily several times during a ride and you don't even need to interrupt what you are doing.

So what if you determine that Aero is off? Best to do the coast-downs again. You could also try just tweaking the Aero factor but it is difficult to tell how much would be correct. The power to overcome wind resistance goes up with V^3... so it is highly dependant on the speed you are going. As a for instance though, the power to overcome wind resistance at 30mph will be around 500W for a typical rider in a typical position. So if you are seeing around +50W at 30 mph then this would indicate that the Aero factor is about 10% too high.

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Even if you get the Aero constant correct, your power can still be off if the wind scaling is wrong. This is determined during your cal ride, by zeroing the net wind. You can check this at any time by doing something similar on your own. The best venue would be a quiet residential area with little traffic. Even better would be a spot where you can ride in the same part of the road for both directions... this will ensure that obstructions are not causing the wind to look different on one side of the street vs the other. Light winds will give the best results... at any rate you will want to avoid spending a significant amount of time *ever* going slower than a tailwind. If you have a tailwind, slow down quickly at the turnaround to minimize this.

This actually works the best if the out-back course is fairly short and you do several laps... this will help account for any shift in the wind that may occur.

a) Make sure your wind offset is zeroed.
b) Do a Trip Reset.
c) Ride back and forth over the course at a normal riding speed... the more laps the better.
d) End at your starting point and do another Trip Reset.

You can create as many of these files as you like to check repeatability.

When you get home, go into iBike3 and check "Analyze Wind" for this file. Ideally you want to see zero, but a few 1/10ths of a mph plus or minus isn't bad either. The adjustment to wind scaling that would be applicable is (Vb/Vw)^2. For instance if your avg bike speed is 19.0 and your avg wind speed is 20.0, the correction to your wind scaling would be (19/20)^2= .90. Multiply this by your old wind scaling to get the new value.

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The last one is your Fric value and Crr. Unfortunately, I don't know of an easy way to check for this, other than to say that most riders with good tires and tubes and roads should get values in the .0040-.0060 range. Rough or too-tight wheel bearings, and rough roads will tend to increase your Crr. Fortunately, Crr is a pretty small part of the overall resistance at typical riding speeds so if you are off a bit on this, it won't have a big effect.

Like the Aero check above, you can do coastdowns but at a slower speed where Crr is the dominant factor. This would work great except that a small tilt error will have the same effect as a Crr error. For instance a tilt error of .1% is exactly the same as a .001 error in Crr (~20%), so it is difficult to ascertain errors that are smaller than that.

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What if the results are highly variable? Maybe your wind scaling is 1.5 one day and 1.1 the next? Maybe you were getting good results with an Aero factor of .40 and now it seems to be closer to .30? First check your procedure and make sure it is correct. Also repeat the checks and/or the calibration procedure a couple more times to see if you can eventually get some consistancy. If not, then it is time to email Velocomp and see if they can shed some light on the problem.

I hope this is helpful, and any comments, suggestions, or questions would be appreciated.

[mds]

On the iSport if Aero is off I have found that rather than do another coast down I get better results by changing Aero via the manual CdA screen. It does not matter if the error is due to a bad wind scaling or a bad CdA or both. Since Aero is the product of both, its value can be tweaked by changing CdA no matter the cause of the error. So I make a note of the value of Aero and then change the CdA body parameters until I see a new Aero value appropriately larger or smaller. It can take a number of trials until you get the value you are looking for. I suggest making a 5% change, redo the high speed coast test, and repeat if necessary.

[wellmt]

rruff - what an excellent explanation and testing protocol. Thanks.

[rruff]

It does not matter if the error is due to a bad wind scaling or a bad CdA or both. Since Aero is the product of both, its value can be tweaked by changing CdA no matter the cause of the error.

Aero is the product of CdA and wind scaling, but only the CdA relates to power. If you change the CdA when the wind scaling is really the one that is wrong, then your power reading will be off.

For instance, a CdA of .30 with a windscaling of 1.33, and a CdA of .40 with a windscaling of 1.00 yield the same Aero factor... but in the first case your aerodynamic power will be only 3/4 of the second.

[mds]

Aero is the product of CdA and wind scaling, but only the CdA relates to power. If you change the CdA when the wind scaling is really the one that is wrong, then your power reading will be off.

For instance, a CdA of .30 with a windscaling of 1.33, and a CdA of .40 with a windscaling of 1.00 yield the same Aero factor... but in the first case your aerodynamic power will be only 3/4 of the second.

If what you say were true, it would be pointless to introduce a constant like Aero that was defined to be the product of wind scaling and CdA. Here is what I believe is going on. The aero component of power equals k * CdA * (v^3) where k is some constant and v is air velocity. v equals s * w where s is some constant and w is the output of the air speed sensor. Since s is a constant, it can be pulled out of the cubed term. Hence the aero component of power equals k * CdA * (s^3) * (w^3). I believe the product CdA * (s^3) is Aero. In other words, wind scaling equals s^3. It is actually the cube of some underlying air speed sensor slope constant.

All of this above of course is a wild guess, but I think a reasonable one.

On my iSport, I have no way of tweaking wind scaling. I wish there were. All I can do is tweak CdA. But doing it does have a downside. Although I believe it corrects displayed power it obviously does not correct displayed wind speed. Also I would like to have a direct way of entering CdA, rather than doing so indirectly via tweaking body parameters.

[rruff]

The aero component of power (force * velocity) is CdA*.5*Da*Va^2*Vb... where Da is air density, Va is air speed, and Vb is bike speed.

The wind scaling factor is merely a calibration constant applied to the dynamic pressure reading to give correct values for air speed. Variations in the hardware as well as the effect of different mounting locations make this necessary. Dynamic pressure is .5*Da*Va^2, and is measured in Pascals (N/m^2). When you display the wind offset, these are the units that are shown.

When you do CDs the iBike doesn't care what your wind scaling or riding tilt is. It doesn't need to know. But... the Aero and Fric factors that it determines from the CDs must have a calibration applied to give you CdA and Crr... and this is the purpose of the Cal ride. In that the wind scaling is determined, as well as the riding tilt. Then we can get CdA and Crr which are necessary for calculating power.

[mds]

If as you say wind scaling is a calibration constant applied to dynamic pressure (call it Ws) then aero power is CdA*Ws*0.5*Da*Va^2*Vb. Is this what you mean? If so, then what I claim in my first post is correct: Because CdA and Ws are multiplied in this expression, a proportional change to CdA can accurately adjust for a proportional error in Ws.

[rruff]

You could substitute CdA = Aero/Ws if you like... so Pa= Aero/Ws*.5*Da*Va^2*Vb.

[mds]

Are you saying that CdA*Ws*0.5*Da*Va^2*Vb is not correct? If not, Ws has to appear somewhere. Where?

[rruff]

It's the equation I gave above... Pa= Aero/Ws*.5*Da*Va^2*Vb

[mds]

I am still claiming my original post. Let dynamic pressure reading from the hardware be Dp.

Then Ws*Dp = 0.5*Da*Va^2. Substituting in your original Pa = CdA*0.5*Da*Va^2*Vb I get Pa = CdA*Ws*Dp*Vb. It follows that an error in Ws can be compensated by a tweak in CdA.

On a different subject. In your original Pa equation, Va^2*Vb appears. I am not sure this is correct. Take two cases. Case 1. Bike speed 20 in calm winds. Va=20 and Vb=20. Case 2. Bike speed 10 in head winds of 10. Here Va=20 (still since relative wind remains 20) and Vb=10. In your equation Pa in the two cases differs. I believe actually that Pa in both cases should be the same since in both cases the bike sees a relative wind of 20. If this is true, then the Vb in your equation should really be Va. Va can be found by solving Ws*Dp = 0.5*Da*Va^2. Need Da to so however. Taking this further Pa = CdA*Ws^(3/2)*... What this means is that a Ws error can still be compensated by tweaking CdA, only the proportion to do so differs. As long as CdA and Ws to some power appear in product form, one can be tweaked to account for the error in the other.

[wellmt]

Well those equations are beyond me, but once you've got your figures agreed, if you could let me know whether the tests posted originally still stand that'd be great..

[mds]

The original post is great. This debate should not take anything away from that excellent post. I probably should have started a new thread.

[rruff]

Then Ws*Dp = 0.5*Da*Va^2. Substituting in your original Pa = CdA*0.5*Da*Va^2*Vb I get Pa = CdA*Ws*Dp*Vb. It follows that an error in Ws can be compensated by a tweak in CdA.

When you tweak CdA on the iSport does the wind scaling stay the same? Or does Aero stay the same... in which case tweaking CdA alone would not help you. In the software at least, Aero and Fric factors are dominant... if you change CdA then the wind scaling will automatically change in the other direction.

I believe actually that Pa in both cases should be the same since in both cases the bike sees a relative wind of 20. If this is true, then the Vb in your equation should really be Va.

Power is force x velocity. CdA times dynamic pressure gives you the force... and then you need to multiply by the bike speed.

Consider riding at 10mph into a 30mph headwind. This is much easier than riding at 40mph in calm winds.

[Norm]

rruff -- great initial posting! Thanks for the details!

Mod -- please consider making this thread Sticky.

[mds]

When I tweak CdA on the iSport, Aero changes. I believe wind scaling stays the same. There is actually no way on the iSport to display the wind scaling value nor to confirm that it has either changed value or stayed the same. I believe the only way to change wind scaling on the iSport is to do another calibration ride.

I disagree. The aero component of power is the same when riding at 10mph in a 30mph headwind and riding at 40mph in calm winds. Relative wind is 40mph that you are pushing against in both cases. What does change in these two examples is the amount of rolling resistance, which of course is larger in the second case.

[rruff]

When I tweak CdA on the iSport, Aero changes.

In that case you should be ok.

I disagree. The aero component of power is the same when riding at 10mph in a 30mph headwind and riding at 40mph in calm winds. Relative wind is 40mph that you are pushing against in both cases. What does change in these two examples is the amount of rolling resistance, which of course is larger in the second case.

I suggest you investigate this further. With the following parameters I get 320W required to ride into a 30mph headwind at 10mph, and 1250W to ride at 40mph in a calm wind.

Rider weight (lbs) 170
Equipment weight (lbs) 22
Temperature (F) 70
Altitude (ft) 0
Bar. Pres. (in Hg @ 0 ft) 30
Dry bulb temp- Dew point (F) 50
Transmission Loss (W) 10
CdA (m^2, nominal) 0.34
Crr (rolling res. coef.) 0.005

[mds]

OK, I agree with you now about your 10/30 and 40 example. Your Va^2*Vb make sense now. Relative wind may be the same, but work is proportional to displacement which, over a fixed period of time, clearly differs, so power must differ too, by a lot in this case. Thanks.

[wellmt]

Hi rruff, I tried your tests over the weekend and have a couple of questions.

d) Coast down a mild decent (~30mph should be good) and see what the reading is. Of course you need to be in the proper position.

If the wind was gusting against you down the hill would this cause the reading to read +n watts?

When you get home, go into iBike3 and check "Analyze Wind" for this file. Ideally you want to see zero, but a few 1/10ths of a mph plus or minus isn't bad either. T

On the Anaylze wind screen, which is the figure i should be checking? Is it the wind offset box, ground offset box, or should I just looking at the bar graph reading? IIRC My ground offset box is prefilled with -4 (kph).

Thanks

[coachboyd]

Was this a solo ride that you just did? Did it start and end at the same place? If so, I can take a look at the file and see if your wind scaling can be tweaked at all. If it was a really windy day (gusty conditions especially) it will be harder to do.

[wellmt]

Hi CoachBoyd.

I did a new calibration as I got some new wheels/tyres. The results from my actual 45km ride look OK.

I then wanted to double check my calibrations with rruffs suggestions, just to be sure so did a quick back and forth along the same bit of road a couple of times to check the wind scaling. It was a reasonably gusty day.

At this point I don't want to take up your time, since I think the results look OK. It was just a question of what I should be looking at on the Analyse wind screen.

If I get a ride on a calmer day and I decide I'm not happy then I'll take you up on your offer, if I may

[coachboyd]

With the new 3.09 firmware it's easier than ever to check your wind scaling to see if that's correct.

On a relatively calm day, or a day with consistent winds (not gusty), stop and do a wind offset.

Then ride down a road for a minute looking at absolute wind speed, turn around and ride back and the absolute wind speed should be the inverse of the way out. Very easy.

[wellmt]

OK my bad, the two tests had -0.4 in the ground offset box in one test and -0.7 in the other (not 4kph!). Actually I remember watching the wind offset in either direction and it looked pretty good. The absolute wind feature is great BTW. Thanks.

[rruff]

With the new 3.09 firmware it's easier than ever to check your wind scaling to see if that's correct.

That's a good feature! How is it accessed?

I assume it is in realtime and it will fluctuate... so it would still be good to look at your net ground wind in iBike3.

[rruff]

OK my bad, the two tests had -0.4 in the ground offset box in one test and -0.7 in the other (not 4kph!).

That is pretty decent. Check a few more times and see if it's consistent, and if it is, you could adjust your wind scaling (and maybe Aero) upward.

Gusty winds are not ideal, but if you do several laps it should even out. Another thing about wind... if you experience cross-winds when doing the test, the *ideal* net ground wind would not be zero, but rather some positive number. This is because cross-winds increase drag compared to no wind.

[coachboyd]

With the new 3.09 firmware it's easier than ever to check your wind scaling to see if that's correct.

That's a good feature! How is it accessed?

I assume it is in realtime and it will fluctuate... so it would still be good to look at your net ground wind in iBike3.

In the setup there is a screen that says ABS Wind - Just set it to yes. Then if you look at the averages it will tell you average absolute wind speed. . .very cool stuff.

[daidnik]

Rruff has commented at some length on the algorithmic aspects of the power calcs. I would like to ask a few questions along that line.

In base units, the units of Force are [(M*L)/(T^2)]. Similarly, Work, or a Force applied over a distance has base units of [(M*L^2)/(T^2)] and by further extension, Power or the rate at which the Work is done has base units of [(M*L^2)/(T^3)]

It was stated that the "Dynamic pressure=0.5*Da*Va^2" where: Da=Air Density ; Va=Air Velocity. This pressure has the units of Force/Area; Pa or N/(m^2), which equates to [M/(L*T^2)] in base units of M=Mass, L=Length, T=Time. This is indeed the units of a Force/Area.

It was also stated that "Aero component of power (Force*Velocity)=CdA*0.5*Da*Va^2*Vb". In base units, Force*Velocity=[(M*L^2)/T^3]. Using this and the above implies that the units of CdA are [L^3/T] or (Volume/Time).

Is this correct? If so, could you convey any insight into the physical meaning of the constant CdA?

In a more general algorithmic sense, I as we analyze each unit of time, the rider does Work against several oppositional Forces:

(Force of Air Drag)+(Force of Rolling Friction)+(Force of Gravity)

Is these, (with the applied force of the rider), all the terms in the "Free Body diagram" of this cyclist?

If so, the change in Potential Energy associated with the last term is easy, but could you elucidate a bit on the Crr factor associated with the second term of the Force of Rolling Friction?

I would think that power term associated with the Work done on the bearings, tires, road, etc. would scale linearly with bike velocity, i.e. Prr=k*Crr*Vb where k=constant, or simply Prr=Crr*Vb.

Is this correct?

If so, then Crr would have the units of Force.

Is this correct?

[mds]

This is my understanding on units, power terms and iBike coefficients:

iBike Aero coefficient (meter^2)
iBike Fric coefficient (newtons or kilogram meter / second^2)
Cda (meter^2)
Crr (unitless)
wind scaling (unitless)
riding tilt (degree) but can be converted to unitless radian as used below (multiply by pi / 180°)

Here are the power (watt or kilogram meter^2 / second^3) terms:

Gravitational component of power = weight (kilogram) * acceleration of gravity (meter/second^2) * vertical velocity (meter / second)
Aerodynamic component of power = 0.5 * air density (kilogram / meter^3) * Cda (meter^2) * wind velocity^2 (meter^2 / second^2) * bike velocity (meter / second)
Rolling resistance component of power = weight (kilogram) * acceleration of gravity (meter /second^2) * Crr (unitless) * bike velocity (meter / second)

The iBike's Aero coefficient equals wind scaling (unitless) * CdA (meter^2)

The iBike's Fric coefficient equals weight (kilogram) * acceleration of gravity (meter / second^2) * (Crr (unitless) - tan(riding tilt (unitless radian)))

[daidnik]

I don't see how the "Dynamic Pressure" with units of Force/Area can be equated to Power with units of Force*Velocity by multiplying by a constant with units of Area.

The expressions were cited before, and all looks good up until we get to bringing CdA into the equation.

The best intuitive physical meaning that I can come up with for CdA is that it relates a sensed (detected) pressure at the pressure transducer to a volumetric air mass moving by per unit of time. It sort of makes sense that we need this constant to calculate the work done by the rider in this unit of time as he pushes thru this volume in a unit of time.

I'm pretty sure that the units for CdA need to be as I stated.

Question: You make a distinction between "Friction Coeff" & Crr. I thought these were the same thing. What nomenclature or name is assigned to this Friction Coeff?

Your cited an algebraic expression for the Force in the term associated with the rolling resistance makes some sense up until the (Crr-Tan(Tilt)) in the product. I don't see which way the angle is relative to gravity and I don't see why it wouldn't just be a Sin or Cos rather than Tan.

It has the right units for a Force, but I don't get that last part.

[mds]

daidnik, the stuff above is my best guess. I could be wrong.

The iBike's pressure sensor reads something proportional to dynamic pressure (kilogram/(meter second^2)). Multiplying its reading by wind scaling gives actual dynamic pressure. Multiplying actual dynamic pressure by Cda (meter^2) and by bike velocity (meter/second) gives power. CdA is coefficient of drag (unitless) times area (meter^2).

The iBike Fric coef is what is labeled Fric on the iBike screen and in the software. It is not Crr.

The forumla for Fric I gave is my best guess. It could be wrong. In fact the tan could be a sin. I am not sure. I will try to motivate why Crr and riding tilt are combined as they are. If you look at grav power and roll power, one is proportional to Crr * bike velocity, the other to vertical velocity. The proportionality constants, weight and gravity, are the same. Vertical velocity is proportional to bike velocity, with a constant of proportionality related to road grade, measured by the iBike's accelerometer. So the sum of grav and roll power is proportional to (Crr + something) * bike velocity. This "something" is the a function of accelerometer output, but it must corrected for a zero offset bias, which is riding tilt. Ignoring the actual accelerometer output, we are left with (Crr + something related to riding tilt) * bike velocity. So it makes sense to add these two seemingly different things. Bottom line: Crr and riding tilt are equivalent in how they affect power.