Tool for visualizing aero vs rolling resistance

[Matt Timion]

Originally Posted by MetroMPG

http://metrompg.com/tool-aero-rr.htm

i left out some of the fields that are constants. still need to add some form field / table column descriptions, credits & links.

feedback ?

Looks good to me. What was it written in? Java? Any chance you want to give me the source code so I can post it up here? Credit will be given, of course.

[MetroMPG]

it's coldfusion. i was going to do a javascript version, but i'm faster with CF (though javascript would have had a portability advantage). i can zip the file and PM you a link to download it. you can translate it to php or whatever suits you.

but let's just wait for a bit of feedback first. no point in both of us revising the same code at the same time.

also, i should compare the formulas used against another set i've seen.

[krousdb]

Originally Posted by MetroMPG

http://metrompg.com/tool-aero-rr.htm

i left out some of the fields that are constants. still need to add some form field / table column descriptions, credits & links.

feedback ?

If you are so inclined, see http://en.wikipedia.org/wiki/Density_of_air

FE increases as the density of air decreases which happens with increasing temperature, increasing humidity, increasing elevation and decreasing barometric pressure. I could have figured this out 15 years ago but my brain hurts when I look at it now. At least there is a table of density vs temp. You could try to fit a curve to it if you can't figure out the formula.

[MetroMPG]

If you are so inclined...

well, i was *partially* inclined.

i updated the tool with the table data from wikipedia, so you can select from a range of pre-calculated rho (air density) values based on various temperatures. the table doesn't specify the other values used in the rho calculations, but i suspect they used "standard" values for pressure & water vapour content (humidity).

http://metrompg.com/tool-aero-rr.htm

while roaming around the web reading about this, i came across a very interesting page which actually does the rho calculation (javascript) from input values of air temp, absolute pressure, and humidity.

most interestingly, it then takes that info and calculates *relative engine HP*.

since our cars' engine hp figures are calculated by SAE rules at standardized atmospheric conditions, this tool shows the effects on HP of different atmospheric conditions (as they affect density of the intake charge).

For the SAE J1349 relative horsepower calculations, the standard reference conditions are: Air temp 77 deg F (25 deg C), 29.235 Inches- Hg (990 mb) actual pressure and 0% relative humidity.

http://wahiduddin.net/calc/calc_hp_abs.htm

[krousdb]

Nice, I suspect standard pressure and humidity values also. The temp selection works just fine the way it is. I think you might want to mention that this calculator is primarily for calculating aero drag and rolling resistance. The MPG calc is subject to lots of speculation about how one calculates ICE efficiency. MPG figures should be taken with a grain of salt. I backed into a 26% efficiency based on the highway testing I did at 55 MPH. At 26% that is very high compared to the 13% figure that I have seen for modern ICEs. But maybe the 13% is for normal driving through a whole tank, not at a constant speed.

[MetroMPG]

agreed - it needs to be fleshed out with disclaimers, variable desrcriptions, links to relevant info & credits.

added one more feature: a quick select for summer / winter fuel energy densities.

note that the temperature calculation doesn't have an effect on rolling resistance, where in real life it would (tire rr decreases as temp increases). the temp calc on my page only affects aero drag.

i have a couple of other simple additions in mind. i'll do them tomorrow.

[MetroMPG]

one other thing i noticed in the spreadsheet formula: tire drag is unchanging at all speeds (vehicle mass * CRR * gravity constant), but doesn't rolling resistance increase with speed?

(EDIT: my mistake; the spreadsheed doesn't show this; see below.)

i was looking at other sites, and saw this formula for HP to overcome tire/mechanical drag. it's more precise, and varies with speed:

HP= V/375 times[0.01184 W + 0.000353(V-66)W].

Velocity V is in miles per hour,Weight W is in pounds.If your speed
is below 66-mph,then the second term within the parenthesis falls
out.Again,to get flywheel power, take your result and divide by
0.95. This yields Bhp the engine or electric motor must provide to the
powertrain, at 95% mechanical efficiency for the powertrain.

it would be nice to have a way to play with head/tail winds on the page as well. it shouldn't be too hard to revise the formula. i'll think about that a bit tomorrow as well - unless someone wants to jump in before then

[krousdb]

Yes, rolling resistance should increase with speed. Column U on the spreadsheet clearly shows that it does. The formula used was posted here by someone else and I cannot conform the tire resistance part. By all means, if you have found a better formula, you should use it.

Calculating the effect of a DIRECT headwind or tailwind would be easy, just add/subtract the windspeed to the aero part only. Calculating winds at other directions than 0 degrees and 180 degrees involves the side area of the vehicle and is much more complicated to do, if at all.

[Matt Timion]

Originally Posted by krousdb

Calculating the effect of a DIRECT headwind or tailwind would be easy, just add/subtract the windspeed to the aero part only. Calculating winds at other directions than 0 degrees and 180 degrees involves the side area of the vehicle and is much more complicated to do, if at all.

ACtually this is very easy to do as well. All that we would be doing is solving for an unknown side of a triangle, which is pretty simple trigonometry.

[MetroMPG]

you're right krousdb. if you remove aero drag from the picture (by setting column M values to zero), you can see HP Tires continues to increase with speed. my bad.

when i compared the two different formulas for rolling drag, you get these figures:

MPH - HP TIRES (orig. formula) - HP TIRES (diff. formula)

05 - 0.261422561 - 0.398304667
10 - 0.522845122 - 0.796609333
15 - 0.784267683 - 1.194914
20 - 1.045690244 - 1.593218667
25 - 1.307112805 - 1.991523333
30 - 1.568535366 - 2.389828
35 - 1.829957927 - 2.788132667
40 - 2.091380489 - 3.186437333
45 - 2.352803050 - 3.584742
50 - 2.614225611 - 3.983046667
55 - 2.875648172 - 4.381351333
60 - 3.137070733 - 4.779656
65 - 3.398493294 - 5.177960667

the difference is consistent: 52% more drag in the 2nd formula at all speeds. maybe it reflects the difference between LRR tires and non-LRR tires.

if you want, you can enter a column with the new formula in Row 2: =A2/375*(0.01184*B2 + 0.000353*B2)

(it applies only up to 65 mph. for some reason, rolling resistance as calculated in the second formula is set to increase above 66 mph. i'll see if i can find out more about it.)