I got a little toy recently where i can input settings for my car to get other infomations outputted. I'm sure everything wouldnt be all that accurate, but I was wondering if anyone had a approx value for the S2000's drag coefficient while the top is up?

Thanks for the help

 

Somewhere around 0.39. Very close to a brick.

 

yup. about equal to a mack truck.

 

Doing some really rough math...

It has been reported on the board that the S2000 is mostly aerdynamically limited on speed somewhere near 165 mph (which happens to be not too far from our power peak in top gear).

Working from that assumption of aerodynamic limitation (if the assumption is wrong, the math is wrong from here on out).

The power to overcome air resistance is equal to: (At 165 mph, the rolling resistance and other power terms are acutally pretty small compared to the aerodynamic term, so I'll ignore them here.)

P = 0.5 * rho * A * Cd * V^3

Where P = Power (in Watts)
A is Characteristic area (in square meters)
Cd is the drag coefficient (unitless)
V is the velocity (meters / second)
rho is the density of air (kg / meter cubed (~1.3 is pretty close))

At top speed, the peak power of the car (220 Hp at the wheels, for our calculation here) is being used entirely to move the air, so we can back out the A*Cd term.

Quick math: 164053 Watts (220 Hp) = 0.5 * 1.3 * A * Cd * (73.76 m/s)^3

A*Cd = 0.62 square meters

The car is roughly 6 feet wide at least 3 feet tall on average, so call it 2 square meters frontal area. (Does anybody have a good measurement of this? Or any other characteristic area?)

Cd = 0.62 m^2 / 2 m^2 = 0.31. This probably represents the upper limit of what Cd really is, with the other power usages, and probably higher frontal area. I would estimate that the Cd is actually a bit lower than this, maybe 10% or so.

To get a really good measure of the Cd, I would need the following:
Speed vs. time curve for a car with known weight, and known dyno curve, pulling in top gear at relatively high speed on a road of known grade. (Do NOT try this on the roads, please find a track to try this at.)

I'll leave the top-down problem as an excercise to the reader. (Hint, I've heard the top speed with the top down is only ~135 mph...)

Hope this helps...
Eric

 

hint, gps electronics testing with the top down, 5th gear, wot, passing 130, feels like she might do 150 top down

 

220 HP at the wheels is widely optimistic for a stock car, depending on the dyno 200 is a better high average.

The S2000 will certainly exceed 130 top down, and there are many first hand reports that the top up top speed is about 150.

A measured CD was once reported on the board and even top up it was pretty mediocre, about .35 as I recall.

 

For what it is worth, when the 04s came out, I recall reading that the factory advertising features claimed the new front and rear bumpers improved the cd by about 5 percent.

 

i could see how the new rear would help with the gap honda engineered under the right taillight allowing an escape path for trapped air

 

Your numbers are pretty innaccurate. Our car is much more than 3 feet tall. Average height has nothing to do with a drag calculation. 3 feet or 36 inches is rediculously low. The Ford GT40 was 40 inches tall. That is rediculous. Ours is closer to 50 inches. 5.74 feet wide as well, not 6 feet. The density of air is also 1.2 kg/m^3 not 1.3.

I have seen several published values for drag coeffecient. They are all in the upper 0.3s. Check this thread I started. I used CFD to calculate a similar number.

https://www.s2ki.com/forums/index.ph...ic=327235&st=0

 

Slimjim,

I appreciate your points, my work was only meant as a back-of-the-napkin calculation. (Hence 6 feet instead of 5.whatever.) Note I also did not acoount for any of the first and second order drag terms or the high-speed second order tyre hysterisis (which is part of the third order power usage as well), or a bunch of other things.

I used the 1 meter height to account for the fact that the car is not the full 50 inches tall along it's entire length. The stagnation zone on the front (as you have shown) only really extends at little above the bottom of the winshield, which I measure at ~35 inches on my car or pretty dang close to 1 meter.)

As other people have pointed out the numbers I used for top speeds may not be right. (I'm not going to try the experiment to find out, but if someone has verified data, I'll be happy to redo it with other numbers.) Using some of the other numbers thrown out here I get numbers in the 0.40 range....

It's worth noting that my numbers done with ~10 minutes work and wild estimates are well inside the error margin of the CFD model you did which took much longer. By the way, which turbulence model, and which boundary layer approximation did you use? (I honestly curious, I do CFD for a living too.)

Lastly 1.2 or 1.3 kg/m3 depends on who's standard atmosphere chart one uses and if one rounds up or down. (I used ACS's because it was handy and rounded up, sue me)

Thanks for the feedback,
Eric