New toy: E61

Simon · 14 May 2009 at 23:18

Cd is only one factor of drag, the other is frontal area!

RoysonBobson · 14 May 2009 at 23:25

Simon said:
Cd is only one factor of drag, the other is frontal area!

Cd is a measure of drag incorporating skin friction and form drag (pressure difference due to frontal area and rear shape). It also takes into account the fluid the body is in (for cars, air) as well as the speed etc

clv101 · 14 May 2009 at 23:27

RoysonBobson said:
Show me a 500mph 200mpg car and I won't care what its Cd is

Ha, there was me wishing for the moon on a stick!

clv101 · 14 May 2009 at 23:28

Simon said:
Estates are often more aerodynamic than the saloons

Really? I thought the lamina flow breaks down at the flat rear, chewing up energy in the process?

Jonnycoupe · 14 May 2009 at 23:29

RoysonBobson said:
Can you support that? I'm curious to see which 'supercars' have a lower Cd than the M5.

I doubt the OP E61 will have a lower Cd than most modern 'supercars'

Why not?

Its not generating lift as much as spoilers and wings will degrade aero efficiency. Dont forget the frontal area aswell. You need to compare CdA between cars.

Jonnycoupe · 14 May 2009 at 23:30

clv101 said:
If so, I bet they don't have a flat back. That's what'll kill the BMWs aerodynamics.

My car has a flat back...

Look up Kamm, although the BMW doesnt really taper so thats not really applicable here I guess.

Jonnycoupe · 14 May 2009 at 23:32

clv101 said:
Really? I thought the lamina flow breaks down at the flat rear, chewing up energy in the process?

Lamina flow doesnt exist on cars.

Look at the solar cars for laminar flow attempts, you cant even put stickers on those things as the step seperates the airflow.

Demon · 14 May 2009 at 23:34

Having now driven the 535d Sport, I'd love to get a go in an E90/1 M5..

The one thing that annoys me with the E39 M5 is the amount of work it needs when you really press on.. Turn DSC off and as with all E39's rear end grip evaporates over bumps mid-bend and you are always fighting to get the balance on the limit, turn DSC on and it castrates the car... Great on a track, a bit poor on UK b-roads.

The E90 535 in comparison is considerably better in this regards, much more sophisticated, much better composure, I'd wager it'd be quicker A-B for most people then the older M5, and I can only imagine the E90/1 M5 to be even better.

MikeHunt79 · 14 May 2009 at 23:35

Oh god, didn't know estates could be this crazy, now all you've gotta do it try getting 200mph+ with some 8x4 plywood strapped to the roof!

Simon · 14 May 2009 at 23:36

Car looks nice, needs a wash in some pics. Can't believe someone keyed the boot

RoysonBobson said:
Cd is a measure of drag incorporating skin friction and form drag (pressure difference due to frontal area and rear shape). It also takes into account the fluid the body is in (for cars, air) as well as the speed etc

Basically Drag = Cd x A.

RoysonBobson · 14 May 2009 at 23:46

Simon said:
Basically Drag = Cd x A.

No. Cd is a way of comparing drag forces on a variety of similar shapes in similar flows of similar speeds. It is a dimensionless number, expressing how the flow is retarded as it passes the object in question.

The accepted definition of Cd = Fd / (0.5*rho*A*v^2)

Where Fd = Drag force, rho = density of fluid (usually taken as constant), v = velocity of object relative to fluid (or vice versa) and A = reference area (for the purposes of cars this is the frontal area).

The Fd will consist of the skin friction force (the air 'sticking' to the car as it passes) and of the form drag . Form drag is a result of flow separation as the fluid passes the object. Separation will lead to a negative pressure difference from the front to the back, which becomes a force when it acts over the area of the body (F = Pressure * Area).

/Topic

Simon · 14 May 2009 at 23:53

Yes as I said

Fd = Cd x A

The only variable is speed, once you hit a certain speed Fd = engine force and the car is at top speed

RoysonBobson · 14 May 2009 at 23:57

Simon said:
Yes as I said

Fd = Cd x A

The only variable is speed, once you hit a certain speed Fd = engine force and the car is at top speed

Did you not read what I wrote.

Think about the units. Cd has no units. Fd has units of force, and A has units of m^2.

Therefore, on a purely dimensional ground what you said doesn't make sense. More details in my previous post.

Demon · 14 May 2009 at 23:58

RoysonBobson said:
No. Cd is a way of comparing drag forces on a variety of similar shapes in similar flows of similar speeds. It is a dimensionless number, expressing how the flow is retarded as it passes the object in question.

The accepted definition of Cd = Fd / (0.5*rho*A*v^2)

Where Fd = Drag force, rho = density of fluid (usually taken as constant), v = velocity of object relative to fluid (or vice versa) and A = reference area (for the purposes of cars this is the frontal area).

The Fd will consist of the skin friction force (the air 'sticking' to the car as it passes) and of the form drag . Form drag is a result of flow separation as the fluid passes the object. Separation will lead to a negative pressure difference from the front to the back, which becomes a force when it acts over the area of the body (F = Pressure * Area).

/Topic

So, transposing,

Cd((0.5*rho*A*v^2) = Fd
and
(0.5*rho * V^2)Cd*A = Fd

Comparing two cars in the same air at the same speed we can ignore the (0.5*rho*v^2) as these become a common constant, that leaves you with a comparitive equation of
Fd = Cd * A?

(No idea, but perhaps this is why the generalisation of Drag = Cd x A is often used)?

Of course the generalisation only gives you a dimensionless/unitless value, but it is simplisitically comparable?

Jonnycoupe · 15 May 2009 at 00:01

RoysonBobson said:
Cd is a measure of drag incorporating skin friction and form drag (pressure difference due to frontal area and rear shape). It also takes into account the fluid the body is in (for cars, air) as well as the speed etc

Ill pull this in light of your last post....

Cd does not take any account of speed or the fluid. Its a constant its whole purpose is to be independant of those two factors; they are part of what you have already stated anyway in terms of establishing Drag anyway. I think we're all reading from the same page though no need to digress this further.

Thats one annoying scratch on the boot :mad:

Jonnycoupe · 15 May 2009 at 00:03

Demon said:
(No idea, but perhaps this is why the generalisation of Drag = Cd x A is often used)?

Exactly, you just compare CdA's, everything else is the same if you are doing a comparison.

Drag factor = CdA I guess.

Simon · 15 May 2009 at 00:03

I rearranged your formula in simple way (took out the constants)
All my point was (without getting to technical) was that the frontal area of a car is just as important as the Cd when determining top speed.

Rearrange it so that V is the subject of the formula and replace Fd with power

This thread has showed the difference between someone who understands a subject and someone who learns to answer questions in an exam

Demon · 15 May 2009 at 00:04

Jonnycoupe said:
Exactly, you just compare CdA's, everything else is the same if you are doing a comparison.

Drag factor = CdA I guess.

I think that's the key thing, Drag Factor, not Drag Force..

RoysonBobson · 15 May 2009 at 00:06

Demon said:
So, transposing,

Cd((0.5*rho*A*v^2) = Fd
and
(0.5*rho * V^2)Cd*A = Fd

Comparing two cars in the same air at the same speed we can ignore the (0.5*rho*v^2) as these become a common constant, that leaves you with a comparitive equation of
Fd = Cd * A?

(No idea, but perhaps this is why the generalisation of Drag = Cd x A is often used)?

/Grumble.

That's not very rigorous, but then again as an engineer rather than a mathmatician I should be happy with that sort of explanation

And you are correct, in the sense that the whole point of Cd is to compare different cars in the same flow where the only variable is area.

However, when wanting to work out the force in a specifc case you must remember the 0.5*rho*v^2 term, otherwise your forces would be very small and you could get 1000's mpg and 0-60 in 0.001 seconds and top speeds beyond the speed of sound etc.

Simon said:
I rearranged your formula.

All my point was (without getting to technical) was that the frontal area of a car is just as important as the Cd when determining top speed.

Yes I agree but it's misleading unless you're equating between cars and cancelling. Sorry if I seem like I'm being picky.

Simon said:
Rearrange it so that V is the suject of the formula and replace Fd with power

Watch your dimensions again here, Power = Force * velocity

Simon said:
This thread has showed the difference between someone who understands a subject and someone who learns to answer questions in an exam

I apologise for taking this way off topic. With regards to fluids - I like to put myself in the latter camp: learning only what's needed for the exam, hence why my explanations may be lacking a little in clarity.

Simon · 15 May 2009 at 00:09

RoysonBobson said:
Watch your dimensions again here, Power = Force * velocity

Power as in engine power converted to a force (which is the driver of top speed in a car assuming sensible gearing). Admittedly I called it power when it is infact the force you will be using. Manufacturers don't publish force of their car's unfortunately

All i was trying to say is that Cd isn't the only thing to consider when you want to know how fast a car will go!