JoeRX8ter

Wind resistance is related to the square of the speed so if your speed doubles the wind resistance increases by a factor of 4.

Jailbreak'74

The only way to get a real number is to put the correct equation together. You've done great, I wonder if anyone knows the formula to figure this out. You came wicked close i'm sure but there's one formula missing that would tell all. Calling all math teachers........

G8rboy

Wind resistance actually starts playing a very big role above 100mph, especially when you get to the mid 100's.

Jailbreak'74

IYO...post something that proves this otherwise we're just talking to the breeze. (i never doubt anyone when i truly don't know BTW, you could verywell be right) Hit us with documentation if you feel like it or can find it.

G8rboy

You're the only one that believes that wind resistance is not a major limiting factor to top speed, so it seems like you need to find proof otherwise.

This is just basic fluid dynamics...
http://encyclopedia.thefreedictionar...rag%20equation

Jailbreak'74

I already tried to find a chart or something and i cannot, that's why i asked. Why is almost everyone on these boards so defensive and *****-like when all we're trying to do is learn.

G8rboy

Step back and look at your responses. I'm not trying to be a *****, but when I let you know that wind resistance is a much bigger factor than you're acknowledging, you quickly replied that was just my opinion... when it's not. I bailed on aerospace engineering my freshman year because it made my head hurt... but certain aspects of fluid dynamics that I experience in real life have tended to stick with me (lift and drag when flying, drag coefficients and aerodynamics while driving, etc.).

Jailbreak'74

Until either of us inserts an intelligent article or the correct equation for the rx-8's type body style both of us are just sharing opinions. I really don't care to hear your educational stats even through they are impressive, the fact is you still don't have a number, an equation, or a write-up on the topic from a definitive source that says point blank, a car of this shape and power will be slowed by such and such MPH when going this fast with this much wind. That answer will prove who is right which I don't even care to be in the first place. People think I wanna fight on these boards when I just want to learn. (and I always state like I did here that I never tell anyone they are wrong when I don't truly know....I've made it clear I don't know the answer to this question and you're more worried about who's right or wrong about the 100 or 200 MPH opinon part of this)

If both of us cannot provide the right answer in actual tested numbers then there's nothing left to post, let someone else who might know this pick up where we left off and we can learn from them.

G8rboy

I guess I don't know what you're looking to prove here- this car is drag limited to somewhere between 150-155mph. I can't find it's frontal area calculation anywhere, but with that, combined with it's cD of .31, and taking a swag at rolling resistance you would come up with something in that range considering the power the 8 puts out.

I think you'd be surprised how incredibly difficult it is to get a car to that magical 200mph point. The Z06 vette with 400+hp and a identical drag coefficient (and probably more frontal area than the 8) can just break the 170mph point. All that extra power (compared to the RX8) just to defeat the massive amount of drag resistance to get a 15-20 mph higher top speed.

If none of that convinces you, then I don't think anything will.

Jailbreak'74

Well dude, what you just said was more informative than anything earlier. Thank you!

I will disagree the car is not drag limited to 155 though, it's been dyno'd without the gov at 170 in 6th@9k.

G8rboy

That's what drag is all about- it should be able to do 170-175mph neglecting the effects of wind resistance (since sitting on a dyno involves no drag at all), as it would theoretically be able to do in a vacuum. Only the engine and gearing specs come into play there.

Jailbreak'74

Drag/Gear limited it highest RPM in the highest gear...as fast as that sucker can spin. You're thinking of the variable drag coefficient taking that stuff into account.

Razpewton

Regardless of how this glowing debate ends, one thing is certain:

When dropped from 30,000 feet, aerodynamics is irrelevant with respects to a sports car.

I MADE THAT UP! ha ha ha ha ha ha ha ha ha ha ha :D :D

G8rboy

No- on a dyno, there is no wind resistance or rolling resistance. That measurement is the vehicle's highest theoretical speed limited only by gearing/RPM (assuming there is no ECU interference) = 170-175mph

On the road, in a real world situation, drag from wind resistance will limit the vehicle from ever obtaining this theoretical measurement.

Jailbreak'74

Correct.

kbull

Ok, I'm going to revise my previous math (slightly) in light of the debate going on here. Also, I'll put in a little more explanation. First, force = power/velocity. Take my word for this, it's a fundamental equation. We'll assume power to be maximal, ie, 238hp which is about 177.5kW. Velocity is obviously what we'll solve for. There are 2 components to the force term, frictional and (the much debated) fluid drag. Here's the crux of the problem:

Fd = (Cd*Ap*rho*V^2)/2

where

Fd = drag force
Cd = drag coefficient (0.31)
Ap = frontal projected area (more on this later)
rho = density of the fluid through which the object is travelling (Air at standard conditions is about 1.21 kg/m^3)
V = velocity of the object

The second force term, frictional force, does not depend on velocity, unlike the drag term. Therefore, we can lump this term into a constant, call it Ff. So now,

Fd + Ff = P/V

or in english,

Drag force plus frictional force equals power divided by velocity.

Solving for power yields:

Cd*Ap*rho*V^3/2 + Ff*V = P

Now we have a cubic equation to solve. I think it's safe to say that the total power dissipated by friction is less than a couple percent of the total power, ie, Ff*V = 0.02*P, or else the design wouldn't be efficient. That's probably on the high end, as the actual loss would probably be much less than that. Given this (very conservative) estimate, we have:

Cd*Ap*rho*V^3/2 = 0.98*P

Now we know everything except Ap and V. Here's where the biggest assumption must be made: Ap. Ap is related to the projected frontal area of the object in question (RX-8), but not usually equal to it. It's a value determined by experimentation. I'm going to assume a range of between 1.6 and 2.4 m^2. The high value is simply what you would get if you multiplied the width of the car by its height, so the actual area would be somewhat less than that due to the area under the car, etc. Now we'll plot the results of top speed vs. Ap over that range.

[IMG]C:/speed.jpg[/IMG]

So, the theoretical top speed of the car is anywhere from 163 to 187. We've already discussed that without air resistance, gearing allows for a top speed of 186 at redline. Keep in mind that I haven't allowed for values of Ap higher than 2.4, which it very well might be. That range was an educated guess, counting on Ap not varying too far from the actual projected frontal area. I think that about covers everything. Someone let me know if I've left anything else out that would affect what I've done here. Based on this graph and what people have observed, I'd say that a top speed of around 165 is actually a pretty reasonable guess.

kbull

Well, that certainly didn't work for inserting the image.... I'll just try attaching it. Lots of fine math there, too bad I'm not smart enough to insert a stupid picture