Masraum -- your second equation corrected what your first equation missed; my bad -- I started my post before you had posted your second, I think. Sorry 'bout that.

As to your units, later, you're dead on. The force of drag does, in fact, come up in Newtons. And, as you point out, that's almost useless, because nobody measures anything on a car in Newtons. If you really wanted to relate that to your car, you could go through the torque sequence -- engine puts out 200 ft-lbf at the flywheel, which goes through the transmission. At 150, you're in 5th gear, and the ratio is ... whatever it is. You'll get a number that's the torque applied to the wheels, and it'll be like 1200 ft-lbf. Then figure you've got tires that are 18in around (or 1.5 ft), so you divide by that diameter to get 800lbf at the tires. Find a unit converstion for lbf to Newtons and you're done.

As to horsepower ... that's a unit of, not surprisingly, power. Power, unfortunately, is far more difficult to relate to anything useful. I could go through it, but it would bore the living daylights out of everyone else on the board, and I'd get shot. Or I'd screw it up and confuse everybody worse than they already are.

The bottom line -- the Mack truck trying to pass you doing 70 in a 60 zone has a frontal area of about 3 times as much, and Af is a linear term, so he's applying about 3 times as much torque to maintain that speed. That brand new sleek Audi is about 15% more slippery than you are, and Cd is a linear term, so to do the same speed, it'll cost him about 15% less torque to maintain. If you're doing 60, you'll be applying about 4 times as much torque as if you were doing 30, because the v term is squared.

As to top speed and gearing ... I'm pretty sure that the engineers at Porsche designed the thing so that the transmission redlines just at the point where max engine torque is incapable of pushing the car any faster. Or in other words, with a strictly stock setup, changing the gearing won't do you any good, because you don't have the power to go any faster anyway. If you've modified your engine, or changed the aerodynamics, you no longer have the same equation as the guys at Weissach did, so taller gearing might make sense.



Dan

Hey I just did a google search - I matched it up with the frontal area reported by the car and driver article I had. You wouldn't be the first one to tell me I'm wrong - OK - the first today. I did stay in a Holiday Inn last night for what it's worth...

http://forums.pelicanparts.com/porsche-911-technical-forum/146859-what-do-you-guys-think-chin-spoiler-track-use.html?highlight=frere

from that thread

For what it's worth here is a very interesting analysis from the site that Jeremy referenced earlier.

Analysis

Here's a basic little nugget:

- drag goes up as a square function vs speed.....
- hp required goes up as a cube function speed.

Wil Ferch

WOW - we may as well be driving a bus -
cd over .41...............

All 2003 comparables.......
BMW 7 series .29
Ford Mustang .36
Mercedes SL500 .29
Acura TSX .27
C6 Vette(2004) .28
Chrysler Pacifica .355
Toyota MR2 .35
Subaru Forester .39
Mazda MPV .34
Toyota Sienna .32
2000 Ford Econoline .37
Mitsubishi Outlander .43
Volkswagen EuroVan .36
Ford Explorer .41
Ford F-150 .404
Lincoln Aviator .41

Very cool Bill - 1,000 hp to go 250 mph in a Shelby Cobra. Wonder if the front end would get light?

These are the kinds of numbers I was hoping to see. Thanks, dudes. Keep 'em comin'.

Jeremy:
Don't get freaked out.


Cd is only *one* component that gets multiplied x frontal area, to = total drag.

---Wil Ferch

Wasn't there a Jewish German scientist last century who demonstrated that as a mass accelerates, it gains mass? And that due to this additional mass, more energy is required at higher velocities in order to achieve acceleration?

So, I guess you guys left something out.

Relativistic effects only become significant at relativistic speeds, Porsches are fast but not that fast. ;)

Super,
The magnatude of those velocities are astounding in order for the mass to increase in any significance.. it's on the order of c.. not mph.
Tim

i was just going to dive into my box of old textbooks, i guess i want my fluid mechanics book. but i remembered that you sickos love this stuff! i figured i will just sit back at let you crunch the numbers. me? my head hurts now.

cliff
p.s. remember that urban legend about a penny dropped from the empire state building? that it would blast through a person if it hit him/her in the head? all that math above, proves that is total crap.

Beware comparing Cd numbers -- different manf.s have used different test emthods -- some have even removed the mirrors to get a better Cd. Also, only manf.s who use wind tunnels with accurate compensation for moving wheels will get really accurate numbers.

But it's better than nothing. e me if you want me to post a collection (incl'ing parachutes and as I recall some birds).

No, the "Myth Busters" on Discovery Channel proved that wrong. The max terminal velocity of a penny (not in a vacuum) ranges anywhere from 35-65 mph. They figured out a way to fire a penny at that speed and verified the speed during the tests. They fired the penny at concrete, asphalt, and a dummy made with a real human skull and forensic gel. I pretty much did nothing, so they fired it at their own hands and even one of their a$$es with no ill effects. They then decided to see how fast they could fire it and see what would happen, so they rigged a hunting rifle to fire a penny, (something like 3000 mph or something like that) Even it didn't go through the dummy from about 15 feet away.

Those guys are awesome.

Interestingly enough you can (if the object is small enough) determine the max terminal velocity by suspending it from a string and driving with it held out of the window. When the string makes a 45* angle from vertical you know the terminal velocity because that will give you the resistance to the air required to surpass the pull of gravity.

Or, at least, I think thats how it works.

is that show mythbuster still on the air? i missed it. that i would like to see.

Obviously the penny needs spoilers to achieve it's true potential. ;)

and dimples.

Yep, mythbusters is still on.

Riblets