Drag HP vs mph

Although I would never expect that anyone would do this, (just thinking aerodynamics here), there is a way to make a Prime have lower drag then a Prius ECO. Replace the Primes: hood, front fascia and fenders with those from a 4th generation Prius (non-Prime).

From what I think I read the shape of the double bubble glass and perhaps the greater length in back of the Prime is to make up for the weaker aerodynamic Prime front end. I suppose the Prime got the more agressive styled front end for marketing reasons, and perhaps because Primes were expected to on average be driven at lower speeds, shorter distances, than the non-Prime Prius because the Prime really shines while driven in EV mode.

 

This EPA data most interests me in finding out whether my guess that an EV can be simplified down to mass, tyre rolling resistance, frontal area and Cd is reasonable.

 

Here is the CSV data without quotation marks. It should import cleanly into Google Docs

 

That's how I simulate them.

 

Did you compare to the roll-down data ?

 

I compared using the roll-down data (which includes an odd, linear term) to zeroing the linear term and fitting the same curve using only the constant and quadratic terms. Virtually the same.

 

I'd like to compare the roll-down data to the simple drag equation of tyres and air mentioned above.

 

Tires is the constant term, air is the quadratic term. In other words, remove the linear term.

The resulting terms matched the roll-down data well, but they don't match what I'd expect for rolling friction or CdA.

 

I may not be following, but I'm asking if
Coef B = tyre RR * mass
Coef C = Cd*frontal area

The Coef A term strikes me as a fudge factor to make the algebra work but I don't know that with any confidence

 

I can assure you, I am not capable of over thinking ;-)
Under thinking is no problem

So,
is the drag equation
RR*mass + v*v*Cd*frontal_area

a good substitute instead of using the roll-down data for EVs ?

 

drag force = RR_coef*mass + 1/2*air_density*v^2*Cd*frontal_area

 

Thanks for the correction

 

I thought the coefficients are submitted to EPA by the manufacturers, If this is the case, why ignoring part of them?

 

If I can find a physical reason for the linear term, I'll include it. But it doesn't seem to be necessary since I can form basically the same curve without it.

 

Which indicates that the linear coef. contribution is very low - may be transmission load in EV drive?

 

I successfully reduced the drag on my Prime by setting the following distance on the dynamic cruise to 1 bar, and the speed to 80mph while following 2 megabus's doing 77mph. 85 miles later, at an average cruise speed of 77mph, I had 59mpg in hybrid mode. I was getting about 57mpg on my own at 67mph prior to their arrival. So the simple solution is to buy a megabus and hire a driver to drive it ahead of your Prime.

 

My less "thinky" method :

use 60-30 coast down to consider the squared variable (aero drag)
30-0 coast down to consider the linear variable (rolling friction)

hardly precise, but gives some rough idea

Cheers, Folks

 

Wow I love the tech math talk even though I seriously don't understand it!

Lowering the intellectual kennel somewhat, I have ordered the "Protection Pack" which includes what we call "mud flaps". Does that translate into U.S.? If so, will these be likely to affect the aerodynamics much?

It is v muddy round here with our weather and farmers tractors on the road so I reckon the mud flaps are worth having to keep the Car looking cleaner. The (black) Gen2 after I wash it only has to look at a road to be dirty again behind the front wheels and along the sills (fenders?) below the front doors.

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