Cd and blunt front ends

[texanidiot25]

The Trans-am nose on my firebird is a fairly blunt area too. I'll get better pics tomorrow, but it's basically a slightly V-shaped wall:


And the T/A kit gave the firebird a .32 Cd stock. Compared to the .36 I think the regular Firebird had. And some variations of the third gen Camaro had .29 Cd I believe (you can find this info on thirdgen.org under the camaro or firebird tech data link).

One trick my dad used on his '74 Camaro (with the later Z/28 "rubber bumper") back in his bracket racing days, was a set of lexan covers for both the head lights and turn signals, and it made some significant changed in both the car's ET and trap speed. It was a consistent high 11 second car on street tires and pump gas... And it drove to the track, and drove home, walking all over many trailer queens...


Very similar to his set up. Had a boxy hood scoop though.

[Cd]

I painted one of those '74 Camaros in high school. ( Have hated the smell of Bondo ever since. )
At the time, I thought G.M. was just being cheap when I saw that the lower half of the grille was 'fake'.
Now I know why it was closed off.

Actually, Hot Rod modified a T/A in the mid '90s ( or was that late 80s ) to run at Bonneville. ( The red Gale Banks car ) They lowered the car and put on moon disks. The .Cd instantly dropped down to .25 - .28 or so.
It's been decades since I read the article, but I still remember that it dropped into the twenties, because I was confused about that blocky front end on the car.
( Then as now )

[Cd]

MarkW365 : So I'm still so confused.
If a sleek rounded shape leads to better aero, how can a brick with rounded edges give the same .Cd ?

[i_am_socket]

I think it's not just about the point of impact but how it travels beyond that point. A long flat smooth brick will do better than a rounded shape that breaks up the flow later on.

[texanidiot25]

Originally Posted by .Cd

I painted one of those '74 Camaros in high school. ( Have hated the smell of Bondo ever since. )
At the time, I thought G.M. was just being cheap when I saw that the lower half of the grille was 'fake'.
Now I know why it was closed off.

Actually, Hot Rod modified a T/A in the mid '90s ( or was that late 80s ) to run at Bonneville. ( The red Gale Banks car ) They lowered the car and put on moon disks. The .Cd instantly dropped down to .25 - .28 or so.
It's been decades since I read the article, but I still remember that it dropped into the twenties, because I was confused about that blocky front end on the car.
( Then as now )

Banks does trucks primarily as far as I know... Which isn't much about him I'll admit, lol. I'll try and dig it up.

True the nose is blocky, but I guess when the air does hit at high speeds, the area right infront of the bumper creates a cushion of air, just directly ahead of the bumper (inches or so) that the incoming air blows over. Similar to the air flow bubble of a truck bed, where slower moving air directs the incoming air. But that's all an uneducated guesstimation. These cars have an underbody tray and air dam in the nose to help with Cd. I'd wager (Flows air around the tires too), and the center of the air damn scoops air for the radiator (how the nearly grill-less 3rd and 4th gen car's stay cool... loose that and watch the temp climb through the roof).




^air damn, though mine is missing the left side



But over all the car does retain a slick profile, and low ride height. Bullet shaped mirrors and such keep everything but the antenna close to the car with the exception of tires, which stick out in an aggressive wide-stance, even on a stock wheel/tire combo:


My car had these when I got it... 90s alloys

[aerohead]

Originally Posted by .Cd

MarkW365 : So I'm still so confused.
If a sleek rounded shape leads to better aero, how can a brick with rounded edges give the same .Cd ?

Sometimes vehicles which appear slick are not.As for the front,if you can keep the flow attached to the point of maximum cross-sectional area,you're 40% there.And this can be achieved with 3% of vehicle- width- radius at top and sides.It doesn't look aero but it works.The real challenge to drag reduction occurs behind the point of maximum cross-section.I did an installment on vehicle fronts.Larry Shinoda's development of the Trans Am's front end is in there.It's very clean.With speed limiters removed,these cars can approach the top speed of the Corvette.Klemperer's 1922 brick "minivan" has Cd0.16.A brick can be okay,fastbacks are the lowest drag,but only if all the rules are followed.

[texanidiot25]

Originally Posted by aerohead

I did an installment on vehicle fronts.Larry Shinoda's development of the Trans Am's front end is in there.It's very clean.With speed limiters removed,these cars can approach the top speed of the Corvette.Klemperer's 1922 brick "minivan" has Cd0.16.A brick can be okay,fastbacks are the lowest drag,but only if all the rules are followed.

Links to your installment?

[JJW]

The thread has progressed somewhat but I have a question which is somewhat related to the original question. I got to talk to an aerospace engineer once when I was a kid, and I asked how a rounded shape could be as aerodynamic as a cone shape (I think I said "more pointy" at the time) on the nose of model rockets. His response was that the round shape presented the same amount of area to the oncoming air as the pointy one, and slower than the speed of sound, they both did about as well to push the air to the sides of the rocket body.

Can anybody elaborate on if my memory from when I was about 11 is any good? If that is correct, and knowing a bit more now, it seems to me that the blunt shape does the job of getting the air attached to the sides via less surface area, and therefore less viscous drag than a long pointy cone-like shape which would have the same cross sectional diameter. (I'm a software engineer, not a mechanical engineer, so please excuse any confused terminologies!)

[texanidiot25]

Possibly this weekend I'll grab an old magazine article I have at my home, I think from hotrod mag of an 89 NASCAR Monte Carlo vs. a Lemans Porsche LMP car. Despite the Monte Carlo having a near stock-car body and obviously a boxier car, was able to reach higher top-speeds and had less of a Cd. thanks to it's smaller frontal area.

[MazdaMatt]

Cd is area-irrelevent, right? Cd*A is what calculates out the drag coeff, right? Cd is simply a product of shape and surface imperfections, so a 1/25 scale model should have the same Cd, but a much smaller Cd*A.