Calculating Drag Coefficient from top speed?

[JQmile]

This look right to you guys?

Power (watts) = 1/2 (1.2(air density))(velocity cubed(m/s))(frontal area(m/sq))(drag coefficient).

I know the car goes 80mph (35.2 m/s) and it has made 36hp at the wheels (1hp=745.7 watts) so ........

36(745.7)=26845

26845=(.5)(1.2)(35.2(cubed))(1.9)(X=Cd)

26845=49720X

26845/49720=.53!

So by this math, that means my Cd is .53!! Looks like my fuel economy is all light weight and small engine, and no aero.

Thoughts?

I have an G-Tech, so I might try a coastdown measurement later provided I can do the math LOL

[pawilkes]

i don't have time to give you a full answer but you aren't considering your vehicle weight as far as i can see which will add some draw added horse power.

Friction Drag = Cf x cos theta x weight of vehicle
where Cf is the friction coefficent of your tires, i would use somewhere around 0.012 if you don't have the exact number. theta is the angle away from horizontal, so 0 for level ground.

also, be VERY careful with your units. I taught a Mechanical engineering lab where we calculated drag for vehicles and many students messed up the units. i am not sure where the 1.2 comes into your equation.

hope that helps

[JQmile]

1.2kg/m cubed is the density of air at sea level at about 70 deg.

[pawilkes]

one other thought, is it making 36 HP at the wheels when it is going 80 mph or is that just its peak? just because it can make 36 HP doesn't mean its making it all the time. look on Instructables.com to find instructions on how to determine your Cd. i haven't done it but the instructions looked reasonably easy

[MetroMPG]

Measure the drag coefficient of your car

I believe he provides a spreadsheet as well, so you don't have to do any math if you're not so inclined.

[JQmile]

I found this on the web, and we all know the internet wouldn't lie to us

Looks like I was a little off, probably because I didn't factor in any rolling resistance.

"Among commercially available 1984 model street cars, the most aerodynamically efficient was the Audi 5000S, with a Cd of 0.33. The Chevrolet Corvette was close behind with a Cd of 0.34. Most other street models have approximate values of 0.40 (my wife's Chevrolet Chevette, I have discovered, is at the high end of the spectrum with 0.45)."

Mine's an '82, but you get the gist of its aero properties. They're bad. Nosecone maybe??

[aerohead]

Originally Posted by JQmile

This look right to you guys?

Power (watts) = 1/2 (1.2(air density))(velocity cubed(m/s))(frontal area(m/sq))(drag coefficient).

I know the car goes 80mph (35.2 m/s) and it has made 36hp at the wheels (1hp=745.7 watts) so ........

36(745.7)=26845

26845=(.5)(1.2)(35.2(cubed))(1.9)(X=Cd)

26845=49720X

26845/49720=.53!

So by this math, that means my Cd is .53!! Looks like my fuel economy is all light weight and small engine, and no aero.

Thoughts?

I have an G-Tech, so I might try a coastdown measurement later provided I can do the math LOL

I think the formula needs a value which brings force per unit time into the equation.I'll have to compare to my US standards stuff at home.Sorry,I'm not metric!-------------------------- Also,you need the entire road load of the car at top speed which will include power absorbed by the tires.-------------------------- Finally,does the 36-hp value include powertrain efficiency loss?--------------------------------- Hucho,in his book,provides a rule of thumb for drag reduction vs top speed: 30% drag reduction = 10% increase in top speed.-------------- If you have the original top speed,then modify the shape of the car(ONLY),and you realize a 10% improvement in speed.then you've cut your drag coefficient by 30%.

[cfg83]

Hello -

Sounds like a job for the Bonneville Salt Flats :

The Open Road: Racing the Bonneville Salt Flats

People have descended on the Bonneville Salt Flats to drive everything from rocket-ships-on-wheels to rental cars. We're in the pits where three long lines of canopies make a temporary village where drivers, spectators and crews hang-out to talk cars, occasionally firing up these babies to warm the oil and fine tune 'em. This is an event called The World of Speed ...one of three race trials that takes place at the Bonneville Speedway between August and October each year. People come from all over to see the flats, break world speed records, or top their personal best, like Bob Powers from Tucson, Arizona.

At one point during the year, the Salt Flats are open to anyone. I think it's Speed Week, but I'm not sure :

2007 Speed Week Results

CarloSW2

[aerohead]

Originally Posted by cfg83

Hello -

Sounds like a job for the Bonneville Salt Flats :

The Open Road: Racing the Bonneville Salt Flats


At one point during the year, the Salt Flats are open to anyone. I think it's Speed Week, but I'm not sure :

2007 Speed Week Results

CarloSW2

I didn't know about that one,but did run in 1991.I joined the Utah Salt Flats Racing Assoc.,paid I believe $35.00 US and a donation to "Save The Salt" and once past safety inspection,could run as many times as the line of cars would allow.------------- The good thing about sanctioned events is that there are safety people,paramedics,and air ambulance,as Wendover doesn't have any medical facilities,and it's a looooooooooooooong way to Salt Lake City hospitals,should something go wrong.----------------- A race-neighbor,Charles Farmer,in Krum,TX, went last year with his Ford SVO-powered streamliner.He said lines were very long now,and if you got in line "today" you might get to run "tomorrow".Too bad! I had planned to return with the T-100,but my enthusiasm is dulled after Charles report.---------- Do it if you can,it's a great experience,and the data is as accurate as anything we as amateurs could hope for.

[gone-ot]

...if you know the engine's HP-rating at the engine speed you're driving, you can backsolve for the value Cd·A in the below equation that's commonly used to estimate drag-power (HP) versus vehicle velocity (V):

V = 53.26 × [ HP/(Cd·A) ]^(1/3)

HP = (V^3)×(Cd·A)/151130

(Cd·A) = (151130×HP)/(V^3)

...because this equation assumes HP(aero) = HP(engine) and doesn't include HP(tires), you probably need to assume only 90% of HP(engine) is usable.