Tool for visualizing aero vs rolling resistance - Page 3 - Fuelly Forums

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Old 03-20-2006, 08:28 AM   #21
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Re: Yes, rolling resistance

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Originally Posted by Matt Timion
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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.
OK, and how do you find the CD and area for the side of your vehicle? That data is not published, nor is it measured that I know of.
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Old 03-20-2006, 08:30 AM   #22
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Quote:ACtually this is very

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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.
i think what krousdb was getting at is crosswinds have additional effects beyond just figuring out the new apparent windspeed and adding it on to the vehicle speed. (calculating "apparent wind" using true wind and vehicle speed is common for sailing, using vectors.)

e.g. rolling resistance changes in a crosswind (but not in a tail/head/no wind). in a crosswind the tires scrub (you have to counter steer against the wind), and the amount of scrub depends on the particular aerodynamics of the vehicle (and the cross wind strength/direction)
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Old 03-20-2006, 08:33 AM   #23
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Re: you're right krousdb. if

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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.)
Perhaps the RR increases over 65 MPH to reflect additional downward force due to aero drag which manifests itself as additional weight?
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Old 03-20-2006, 08:38 AM   #24
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Re: you're right krousdb. if

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Perhaps the RR increases over 65 MPH to reflect additional downward force due to aero drag which manifests itself as additional weight?
i thought of that too, but then i think i've heard that most non-racing cars produce lift, not downforce, at highway speeds. which would mean a *lessening* of rr from reduced apparent weight. i'll look into this and get back.
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Old 03-20-2006, 08:58 AM   #25
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Re: Yes, rolling resistance

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Originally Posted by krousdb
OK, and how do you find the CD and area for the side of your vehicle? That data is not published, nor is it measured that I know of.
IN that case, it's not easy.
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Old 03-20-2006, 09:07 AM   #26
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side wind

Ignore the side forces of the wind and just deal with the cosine of the angle times the wind velocity which gives you the frontal effects vector.
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Old 03-20-2006, 09:20 AM   #27
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Re: side wind

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Ignore the side forces of the wind and just deal with the cosine of the angle times the wind velocity which gives you the frontal effects vector.
That's actually exactly what I was thinking. An angled wind will just produce less of a frontal wind. I'm thinking that maybe all that can be done is ignore the side wind forces as it is impossible to really calculate.
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Old 03-20-2006, 09:57 AM   #28
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Re: side wind

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Originally Posted by Matt Timion
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Originally Posted by JanGeo
Ignore the side forces of the wind and just deal with the cosine of the angle times the wind velocity which gives you the frontal effects vector.
That's actually exactly what I was thinking. An angled wind will just produce less of a frontal wind. I'm thinking that maybe all that can be done is ignore the side wind forces as it is impossible to really calculate.
A physicist friend of mine has developed a calculator for Prius MPG. As it turns out, crosswinds cause more drag than headwinds:

http://privatenrg.com/Crosswinds.htm#Crosswinds

The 70/30 rule is also interesting.
http://privatenrg.com/index.htm#WindAssistScreen

See all of his Prius related info here:
http://privatenrg.com/
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Old 03-20-2006, 10:06 AM   #29
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CD

Actually I have heard that you can ignore the CD Drag Coeffient at lower speeds (under 100mph) as it is such a small portion of the rolling resistance and frontal area drag as to not matter much. Hard to say without testing further - maybe a few foam shapes held out the window of a moving car on a spring scale (a poor mans wind tunnel) will test the drag effects at various speeds.

Took the xB out for some highway miles yesterday after I gggggggassed it up OUCH! glad I pay after I pump because I didn't expect it to take $25 to fill it up yikes - really should have filled it last week for a few bucks less. Anyway I was on a long down hill and pushed the pedal down and the gps registered 86.3mph in just a few seconds from 60mph. On the tight on-ramp coming back I opened it up in third to about 5500 rpm and then hit 4th for a little more then dropped it into 5th as the cars behind me started getting smaller - man this box really moves out! Kills the mileage for the day however only about 43mpg. HA HA HA HA HA was getting used to the high 40's.
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Old 03-20-2006, 10:46 AM   #30
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Re: side wind

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Originally Posted by krousdb
A physicist friend of mine has developed a calculator for Prius MPG. As it turns out, crosswinds cause more drag than headwinds:
this makes sense when you think about it. vehicles are not at all aerodynamically optimized to be travelling "sideways" through the wind.
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