Coast Down Testing - 2000 Honda Insight
 

There have been those posting on EcoModder, discussing how people were able to determine not only the Cd of their vehicle, but also the effects of large and small modifications.

It was with this curiosity that this author recorded numerous coast down tests via a hand held GPS, during the time period of January 04th, 2010 through April 01st, 2010.

A section of road on the way to work was examined using Google Earth for flatness, and found to be suitable for coast down testing. The elevation of the road in feet above sea level is shown below.

The two vertical red lines indicate the primary section of road used for the coast downs, and within an elevation change of about two feet.

The graph below shows not only the digital elevation in feet, but a polynomial curve fit of the road surface as well.

http://ecomodder.com/forum/attachmen...1&d=1324501613

Once it was determined that coast down testing would be tried, a method for recording the coast down data was needed.

Reading the speedometer and recording time via stopwatch was considered, but not the most desirable approach.

A hand held GPS was already used for hiking and such, and thus considered ideal for exact recording of speed and time. The unit is a Garmin Legend eTrex and is set to record speed data every two seconds.

The GPS is started about two minutes before taking coast down data to allow the GPS unit to sync with the satellites and create stable readings.

Below is graph showing the raw output from the hand-held GPS in the coast down zone on the road. Note how much deviation the data has. As noted this is with the sample rate of two seconds.

This amount of speed variation between speed samples has been consistent over all the coast downs, and this graph is typical of that variation.

http://ecomodder.com/forum/attachmen...1&d=1324501613

Since the speed variation is so high, it is desirable to “smooth” the data via some mathematical method. The graph below shows what a smoothing method called “Linear Regression” looks like, as signified by the thicker red line.

http://ecomodder.com/forum/attachmen...1&d=1324501613

And here is what the resultant deceleration curve looks like, in response to the Linear Regression smoothing.

http://ecomodder.com/forum/attachmen...1&d=1324501613

The rolling resistance portion of the overall deceleration is shown below, again, based on a Linear Regression coast down smoothing.

Note that the Crr portion of the energy loss should be something closer to a straight line, not the downward sloping curve as shown below.

http://ecomodder.com/forum/attachmen...1&d=1324501613

With the above three curves in mind, another approach to smooth the raw GPS data is shown in the form of a second order polynomial that has three coefficients.

Note that the curve fit to the raw data seems much closer than the linear regression line above.

http://ecomodder.com/forum/attachmen...1&d=1324501613

And from this data smoothing, we can calculate what the resultant deceleration curve looks like. It appears that the deceleration curve shape is almost a straight line with a steep upward slope.

This curve is closer to what the deceleration curve should look like, but not quite.

http://ecomodder.com/forum/attachmen...1&d=1324501613

And from the above deceleration curve, we can then determine what the rolling resistance portion of the total energy loss looks like. It is certainly closer to a straight curve.

http://ecomodder.com/forum/attachmen...1&d=1324501613

From the above, it does not appear that applying either Linear Regression or a low order polynomial yields the desired smoothing, as the deceleration curves do not approach the ideal shape.

In order to smooth the raw GPS data without altering the true change in coast down speed, maybe another smoothing approach can give us an answer.

The screen shot below is from a custom Visual Basic program who’s sole purpose is to smooth rough GPS data as shown. The smoothing method in this case is a single pass, 5 point data averaging method, and one can clearly see that the raw GPS data, represented by red circles is indeed smoothed by presence of the blue line segments.

However, this data is still too rough in nature to allow proper deceleration of the vehicle to be calculated properly.

http://ecomodder.com/forum/attachmen...1&d=1324501613

So we take the process above and loop the freshly calculated output data a total of 2000 times to produce the very smooth curve shown below.

http://ecomodder.com/forum/attachmen...1&d=1324501613

This smooth curve is now placed along side the raw GPS data below.

http://ecomodder.com/forum/attachmen...1&d=1324501613

And the resultant coast down deceleration is shown below.

Well, now it seems we have a deceleration curve that is starting to have the characteristic curve of combining the rolling resistance and velocity squared aero component.

http://ecomodder.com/forum/attachmen...1&d=1324501613

If we then calculate the effective rolling resistance portion of the total energy loss, we end up with a curve that is shown below. This seems to indicate that the rolling resistance portion of the deceleration load is constant with speed.

To be quite honest, I am not sure if on a theoretical level, this is actually the case or not.

http://ecomodder.com/forum/attachmen...1&d=1324501613

The data below shows the entire mathematical process of taking the raw GPS data, smoothing it, calculating the Total Deceleration, Aero Force in Newtons, the Crr Force in Newtons, and finally the calculated deceleration (model).

In this case, the actual Total Deceleration and the Model Deceleration had the best match with the following settings:

Cd: 0.234; Crr: 0.0134; Wind: -1.16 m/s

http://ecomodder.com/forum/attachmen...1&d=1324501613

During the coast down testing over the winter of 2010, 39 coast down runs were recorded and the deceleration of each run is shown below.

Looking at the graph, one can easily see that there is quite a bit of variance in the data. The heavy red curve represents the “average” of all 39 coast downs, and this curve is surprisingly close to the “ideal” coast down deceleration curve.

http://ecomodder.com/forum/attachmen...1&d=1324501613

So what does the “average” curve tell us? The car apparently has a Cd of 0.228, a Crr of 0.0137 in cold Wisconsin winter conditions. Remember that the Cd probably is very close to actuality since the stock Insight has a Cd of 0.25 and this car has, at this point in time, fiberglass under-body smoothing panels, with about 70% coverage. Several of the panels next to the rear wheels are not created at this point in time.

It is also obvious from the data that the moving velocity in which the aero effect equates to the same rolling drag is close to 20 m/s (44.7 mph).

http://ecomodder.com/forum/attachmen...1&d=1324501613

What the data in this study also tells us is that, even though the coast down testing is performed on the same section of road during the drive to work and the car fully warmed up for 20 miles before reaching this location, the data between each run is highly variable.

It is this author’s opinion that the conditions and/or measurement methods during these coast down tests are still too variable to be of much use regarding aero improvements to our cars.

One possible improvement would be to arrange for the collection of much more data, maybe in the hundreds of data points, and utilize a method that is not so prone to speed variance, such as the GPS unit is now.

_______________________

I can see why Aerohead gave up on this sort of testing long ago, as he has already made mention of in the past. It will take some dedicated equipment to record enough data, and with enough consistency, to really add value to our aero endeavors at EcoModder.

Jim

...can't "explain" ALL the variances, but these 'variables' came to mind:

• differences in wind velocity
• differences in wind direction
• differences in driver control (we're seldom 100% repeatable)
• differences in air temperature
• differences in air density (cold/dry winter vs. hot/humid summer)
• differences in GPS satellite lock-on's (jumping between different satellites?)

...individually, these might be small, but their "interactions" might be significant?!?

...what kind of regression correlation coefficent (R^2) do you get when you tell the Trendline function to "set intercept = 0", which forces the "average" line to go through zero at Y = X = 0.

Wow. This is very interesting. I've been wondering myself about the reliability of my testing. Does anyone know of the reliability of this compared to measuring fuel economy while holding speed to test improvements? I suspect coast down testing would be a little more reliable, but as many of us know, even the tiniest variations in a driving commute can contribute significant variations in fuel economy. Ideally, it would be nice to average 39 runs in A-B-A style for testing, but that is very time (and gas) consuming.

...this posting needs to be "STICKIED"!

I could barely follow this but I'm geeking out over your details. lol

Your most accurate fit to the data is from a least squares fit of a quadratic equation to the raw data. Filtering the raw data makes look prettier while reducing accuracy. The lost accuracy comes from the phase characteristics of your filter.

Your road has enough slope to fuzz the results. I regularly drive a route of about 60 miles. One end is 800 feet higher than the other end, for an average slope of 13 feet per mile. Average mileage is about 5 MPG higher going downhill.

Your top speed was only 40 MPH, which is not fast enough for accurate results. A 5 MPH headwind increases aero drag by 27% at 40 MPH, and more at lower speeds. A 5 MPH head or tailwind will change my trip mileage by about 2 MPG.

Averaging several runs is good. Starting at a much higher speed is even better. The best starting speed is where the aero drag is at least ten times higher than the rolling resistance. Coasting down until almost stopped will also improve accuracy.

MTA: OK, I see that you already spotted some of this. You calculated that air drag is equal to RR at a speed higher than your highest test speed. That means that it is difficult to accurately estimate Cd from your data. Your Crr should be pretty good because the test data is almost all rolling resistance, with very little contribution from air drag.

seems to me there are 3 major uses of fuel.

One is overcoming aerodynamic drag.
One is overcoming rolling resistance.
One is accelerating the vehicle.

Of the three, I suspect the middle one is the least significant.

I agree aero drag would be MUCH easier to calculate at higher speeds.

Also, I suspect some of your curve is actually from the wheels energy being non linear with respect to speed - the wheels are rotating, so they give energy to the system as a function of the square of the mph.

accelerating the vehicle is a function of rotational inertia and weight of the vehicle.

I'll second that !
Please make this a 'stickie'

Holy moly Batman! That is some serious data crunching.

How did you determine the Crr? In my coastdown tests using the Instructables spreadsheet, this is the hardest part to get -- it is basically an educated guess?

@drmiller100 -- I suspect you are correct. The aerodynamic drag swamps everything else at a constant speed. And accelerating (from a stop and/or uphill) is huge but if you can limit the time it lasts in proportion to the whole drive time, then it becomes less critical.

Thanks for the interesting and very thorough report. I have been affraid all along that some of the techniques we hold so dear don't render accurate results without considerable control and considerable data. I don't think any one could have exercised much more dedication than you have and I compliment you on your effort.

I have felt for a long time, since 2008 when I first joined, that the easiest measurement would be MPG improvement over long two way courses. With such controls as low wind, constant driving techniques, constant average speed, the small variations of conditions and driving technique average out if the course is long enough. I have achieved good repeatability with this technique. True that it doesn't serve up the number we would all like to get - the improved Cd - but it does point to which mods work and relatively how well they work.

Rolling resistance is not the least significant, at least not at lower speeds, one really needs to do the calculations, but below a certain speed the road load force due to the vehicle mass and rolling resistance plays a larger role than the aerodynamic drag. After this point then the contribution remains fairly consistant where as the aerodynamic drag increases as a squared function of the velocity of the vehicle.

You have to be going pretty slow to make aero insignificant though. In my Paseo, at about 35 mph its a ~50/50 division between aero and RR and it has decent aero and a very small frontal area. Even at 25 mph its still ~30% of the energy used.

I did a bunch of calcs, and rolling resistance is ALWAYS the least significant of the three items I listed. At low speeds we assume city traffic with start/stop, and in that case the energy to accelerate the car is by far the biggest contributor.

At high speed (over 40 mph) aero is the biggest issue.

If you have a heavy car at low speed, then acceleration of the car is the biggest issue. If you have a light car at low speed, then acceleration of the car is still the biggest issue.

Your thoughts?
What con

Jim,

I've been studying your data and I have one suggestion as to a possible error source.

We all know how very sensitive the Insight fuel economy measurement is to tire pressure. Looking over the part of the year you were testing, I'm wondering if you may have had significant tire pressure variations. Tire temperature changes tire pressure. On sunny warmer days, the road would have been warmer, and on cold overcast days the road surface would have been colder. Cold stiff tires have higher rolling resistance.

I have a slight hill in front of my house which runs for .4 miles. When I give my Insight a little push, I gain speed to 8 MPH at the first intersection in the summer. In the winter, I can barely make 5 MPH at that point. You say that the car is thoroughly warmed up, but perhaps the tire temperatures are still varying significantly on different days.

From my racing I also know this. We use a bit more pressure on cold days to put the traction into the sweet spot.

Hi Tele,

Yes, I understand what you are saying about "influences" that could affect the coast down numbers.

I absolutely agree that wind and temperature have affected the repeatability of these number presented, and I have made no attempt to correlate them with individual coast down decelerations towards the end of this study.

Why?

The GPS readings are simply too few, and littered with too much scatter.

I have probably spent well over 200 hours or so "trying" to get more stable or properly "filtered" data from the raw GPS output. All in vain in my opinion however. The GPS data is simply too scattered to be of much use for this precision application.

As for the statistical R^2 value, in this case I think that "goodness of fit" number is of not much use with this much data variation.

As for the Instuctables.com posting, you can now see that much more data needs to be collected to get really accurate results.

Jim

Hi Aveo,

As for the reliability of straight-line A-B-A testing, I would rely on that type of testing more than coast down testing until I can develop some type of automated setup that takes the VSS speedometer readings from the Insight and logs it onto an SD card for later retrieval.

MetroMPG and others have seemed to provide reliable data by this method.

Jim.

Actually my top speed was closer to 60 mph (over 25 m/s), so that should be pretty useful for getting the split between Cd and Crr.

The rest of your comments are based on the assumption that my maximum speed is only 40 mph, and because of this, these are not correct because my start speed was actually higher than that.

Jim.

One of the nice things about "accelerating" a vehicle, is getting all that momentum back in the form of "coasting".

This where serious MPG's can be made. Much more than straight line driving with the motor running continuously.

Jim.

Hi Neil,

Well, I did spend a ridiculous amount of time "trying" to filter data that was very noisy. I like working with math, and this study was a serious study in noise filtering, and definitely shows the need for more and more stable data.

The Crr was calculated using the same technique as posted in the Instructables site. And please note that ambient air temperature make a "huge" difference is exactly what Crr number is generated.

Colder temperatures in the winter, compared to summer, changed the Crr value by about 20% or so. I don't know how accurate this calculation is because of the current GPS accuracy. But the the calculated number was generated by a "best fit" calculation was iterated many times until the error reached a minimum from the actual data compared to the "model" data.

The assumption that air drag swamps everything else at a constant speed, is only correct if the speed happens to be 150 mph or above. If however the car is a Honda Insight, and the speed is 45 mph, then the Cd and Crr are equal to each other. Obviously at slower speeds, the Crr predominates.

Jim.

Hi Jim,

At this point in time I would have agree with you.

A constant speed gas mileage test such as what MetroMPG or others have done, seems to be relatively simple and yet very effective to producing relative gains in aero changes and other effects.

In fact I was quite enamored by Metro's testing of his cardboard tail and later, the mirror, radiator block and other details, using the ABA technique.

I have not given up on coast down testing, but for now I will focus on other aspects of energy savings until a better approach to speed recording can be implemented.

Jim.

Hi Mr Miller,

Can you list your calculations?

I would like to know how Crr is always less than Cd?

At the speeds I typically drive, Crr is actually larger than Cd (38 mph).

Jim.

ok, at low speeds acceleration of the car from a stop is the biggest issue. then comes rolling, and then aero.

I mis"spoke".
But still, rolling resistance is not nearly as important as the energy to start and stop the vehicle in typical city traffic.

Interesting that you put most of your miles at under 38 mph. Do you feel safe at those speeds on the freeway?

Hi Jim,

You could very well be correct about this.

I simply do not have enough data or experience with coast down testing to say that tire pressure is an issue or not, but I would think that this is indeed a major factor in Crr.

There are several bicycle sites that I have investigated, and rolling resistance seems primarily a "road/tire" function. Tire carcass deflection is a large factor.

I typically keep tire pressure from 55 to 60 psi, but still ambient temperature, road conditions, dust and so on *could* be a big factor and I may not even be aware of the many factors.

All this testing and data collecting is just part of our aero "research and development" process here at EcoModder.

Jim.

what kind of mileage do you typically get?

Does this correlate directly to your Crr???? For instance, for a 2500 pound car, with a Crr of about .008, at 35mph, my math comes back with about 2 horsepower rolling resistance.

which by my math says you should get about 90 mpg if your drag is identical.

Do you really get 90 mpg?

This statement needs to be qualified.

In a stop/go situation, then yes, acceleration can be a large factor. However coasting in the same situation can nullify this effect and then some.

Anyone who gets let's say, 150 mpg during hypermiling contests, is using this same technique to great effect.

I can get higher mileage in town under the proper conditions, than out on a deserted country road.

It is for that reason that I do NOT drive on the highway, but instead go out of my way to take backroads to/from work. This gives me the freedom to drive at much slower speeds where there is relatively lower traffic.

In this situation, the effects acceleration is nil and Cd next, with Crr being the largest drag factor.

Jim.

How???? What makes the city better then country?

Actually, yes I get quite a bit more than 90mpg during the warmer months.

Lot's of coasting is involved to get these high numbers, but does show what is possible. In these conditions, Crr predominates.

http://ecomodder.com/forum/attachmen...1&d=1324614169

Jim.

When driving in the country, let's say my average speed is 38 mph. My sustained effective gas consumption is equivalent to 120mpg while utilizing lean burn of the Honda 3-cylinder engine.

This is the maximum MPG's of this engine design on hot summer days (90F).

To get higher than this, coasting is involved.

The choice now becomes:
1) Coast from 35 mph to 20 mph on a country road.
2) Do the same thing in town in an appropriate speed zone.

(Note: I am not practicing pulse and glide, but simply utilizing the mass of the car to coast to the next stop or change in direction of the road).

The country road coasting is obviously more dangerous, since the posted speed limit there is 55 mph.

So doing the same thing in town is easier if the traffic conditions allow this to happen, since there is less speed differential than the country road approach.

I'm starting to like town driving more than country in certain conditions.

Jim.

Wow.

My apologies.

You are doing something completely different then I thought you were doing.

Your mileage is spectacular!

Thanks!

Actually, most of my driving on back country roads is very typical of most other motorists, except that I drive at a slower speed to keep losses to a minimum.

On the way to work there are at least 10 sections of road that go from the posted 55 mph limit to a lower speed such as 35 or even 25 mph.

When this transition happens, I shut off the injectors to the motor and stop the ICE, and then coast all the way to the lower speed limit sign.

Sometimes I restart the ICE to maintain a 25 mph speed. Other times I coast down to 10 mph or so towards a stop sign.

I am constantly tailoring my driving to accommodate current conditions, such as traffic or colder/hotter weather conditions and so on.

The ICE engine in the Honda is amazingly versatile, as I can routinely drive all the way down to 28 mph in fifth gear with a very light throttle, and using HV battery assist when needed, and even in the colder conditions. I've had the car three years now, and it's inspiring how the car has taught me how to ecodrive. And that is the main reason why the gas records have continued to climb. The aero mods are only part of the equation.

Driving the way I am right now, I would have to venture that the brake pads would last the life of the vehicle, since they are rarely used to slow down on the drive to work.

Obviously, city driving at rush hour is a whole other matter, of which I am spared due to the nature of my current drive route.

Jim.

We finally get to see you backlog of fill-ups - impressive :thumbup:

Equally impressive is the fact that your mid-2010 highs, are lower than your current errr ... winter-low of only 90mpg .


A real shame Honda isn't producing an updated version of this Insight.

I have noticed that really slick, silent asphalt produces higher FE than stoney, rough surfaces. But, this should not be a factor in your testinng since you always used the same road section.

Can you elaborate a bit of your comment about the "road/tire" function?

Can you draw any decent estimate of fuel economy difference in your tail on/tail off. data? I'm not sure how to draw conclusions from the fuel logs since we don't know when the tail was on and when it was off.

Jim,

About "road/tire" function, please see here for start....

The Recumbent Bicycle and Human Powered Vehicle Information Center

I have seen postings of how tire resistance is constant with speed, and then John Tetz shows that for his recumbent bicycles, that this may not be completely true.

As for the tail on the car, the car did not have a tail in the winter of 2010. It was not made yet.

Also, note that I discussed the underbody smoothing panels were only providing 70% coverage during this time.

Hope this helps, Jim.

Hi Euro,

I thought of you, when I was putting the finishing touches on the tail last fall.

Remember when I was opening up the gap between the tail and the car to match the rest of the body, and you mentioned, "Arg, now the drag will go up"?

Well the last several pictures in my posting on the tail show that this gap has been closed up again.

Thanks for the nudge!

Jim.

Thanks Jim
 

Thanks for the bike article. I read about half of it and intend to study it in more detail, but I'm a bit under the weather.

It was particularly interesting that he identified the cold weather effect similar to one I identified by rolling my Insight downhill from my house. I still haven't fully digested his discussion of the variability of Crr but I'll back to it.

I still think that in your case, ambient temperatures for your otherwise well controlled coast downs may have been a problem.

I think I kinda misspoke my question about the tail. I was simply asking if your fuel logs could be analyzed to give a fairly decent measure of fuel economy improvement. I know it isn't the way you wanted to go at the analysis, but it would be helpful to all of us if you did the analysis - which wouldn't be very difficult. Problem with someone else trying to do that analysis is that we don't actually know when the tail was on and when it was off. Of course, there may be other changes which will have distorted the data. Just asking:)

The BIG thank you
 

Jim,attempting to reduce the data from the last trip has forced me to revisit all the dynamics a vehicle can undergo which affect mpg,and it is a constant reminder of what you've had to fight over all this time.
So we all owe you the big thank you for good fight.
When CAR and DRIVER tested my CRX we had the luxury of a dead flat,tree-lined and windless straightaway at Chrysler's Proving Ground,where we could test un-molested by traffic.
C/D carries there own weather data acquisition hardware with them,so those variables were covered.
The Daytron Messtechnik optical fifth-wheel offers zero rolling resistance of its own,and the on-board computer was automatically recording all the speed/time data at 1/10th second intervals for later data reduction.
Speeds were started from above 70 mph,then coastdown data was captured between 70mph and 20mph.
The low velocity contour would help nail the power absorption coefficient for the tires to be compared later against the speed trace to 'see' the aero effects.
The car had been weighed on four, drive-on scales and was accurate to the pound.We knew the test mass with driver and recording equipment to the pound.
10,back-to-back runs were conducted as quick as the car could be
u-turned,with a weather check at the conclusion.
We ended up with Road Horsepower at 30-mph,50-mph,and 70-mph.
Knowing the R-R power absorption coefficient obtained,R-R power could be subtracted from the Road Load to isolate the aero contribution,and then by knowing the frontal area of the CRX,the Cd fell out of the data.
C/D used a custom program on a HP programmable calculator to do the data reduction.
--------------------------------------------------------------------------
By using the Proving Grounds,we didn't have to deal with traffic,elevation,curves,wind spectra,etc.. With our own weather station those criteria was accounted for.As testing goes,it was good as it was ever going to get.
In hindsight,the only hitch,was the attachment of the fifth-wheel to the passenger door.
It was only when I reduced the C/D data that I realized that the fifth-wheel itself was aggravating the Cd,with its own frontal area and composite Cd.
No regrets,and it was a very interesting experience watching my car go around the high speed oval at top speed.
Since GM has moved onto a military reservation to escape the poveratsi (sp?)
perhaps they'll sell ecomodder.com their old Mesa Proving Grounds for cheap.
Then we can do our own proper testing out of harms way.
I want to personally thank you for all the hell you went through for all of us.
And I'll keep thinking of ways we can do these exotic tests and maintain our sanity.
Your a hero! Thanks,Phil.:thumbup:

Good work!
 

I agree. It will also require a much higher starting speed, particularly with an aerodynamically clean car. If the car is clean (or heavy for its frontal area) then rolling resistance has larger effect, and for the differences in deceleration to show up a greater speed differential is helpful.

Repetitions on the same day going in both directions are helpful to filter out wind effects. Working on a still air day is almost essential, because cross winds increase drag when going either direction, both from tire distortion and disruption of flow due to undesirable apparent wind direction.

A fifth wheel is easy to set up, and bicycle speedometers (the magnetic counters) give consistent, repeatable speed indications.

Rolling resistance is fairly easy to measure directly, by careful towing, but requires removing axle shafts, etc. With a known rolling resistance, then aero drag can be calculated more confidentally.

Unless you have really awful tires, your Crr cannot be .0137 -- so this figure is taking into account final drive and transmission friction, which is not perfectly constant -- part of it is hydrodynamic. The usual coastdown test calculations assume one constant drag, and one exponential.

But at very least, consistent measurement speed is required.

Thanks for the impressive work and tireless efforts!

Hi Jim,

OK, finally got around to looking at pictures taken of the car with the tail on and off and have updated the fuel mileage logs to reflect this....

Tail was taken off car at 06-05-11 and found water logged

Tail was put back on car at 09-25-11 after painting with primer.

Happy number crunching Jim. :)

Jim.

Hi Phil,

Well thanks for the very kind words!

I really agree with you about the need for having a very refined location for doing these coast down tests.

I do recall reading in one of the national design publications about one of the NASCAR teams actually using an old railroad tunnel to test the Cd of their car for more repeatable conditions. Obviously there is going to be issues with compressed air in the tunnel, but at least they can measure refinements to the their vehicle shape on a relative basis.

Phil, I'm certainly not giving up on coast down testing just yet. I am attempting to learn microprocessor programming, which is going quite slowly right now. Eventually I would like to build a circuit board that takes the car speed pulses from the transmission (VSS) and process them to allow proper coast down testing, with hopefully the required accuracy.

If you run across any of those old data files mentioned above, please let me know.

Jim.

One other forum member calculated that my Crr should be closer to 0.008 or so, and this seems reasonable if it's warm outside.

The above Crr of 0.013 that you are referring to is probably fairly accurate because this was measured in the dead of winter, during some very cold temperatures. Obviously the Crr in the winter will be at least 20% greater than in the warmer months in the summer. This was pretty obvious from all the coast down tests, whether I published the results on EcoModder or not.

Surprising, the Cd did not change as much, since the equations listed on the Instructables consider barometric pressure and temperature (air density) and thus correct for any change in these values, giving a reasonably stable Cd number, no matter what season the coast down testing was recorded.

At this point in time I have no reason to either agree or disagree with my findings, but simply publish what I found, so for me a more thorough investigation will need to wait until a more refined process is developed.

Jim.