New bug, .38 CD why?

[SVOboy]

Frank, cut out the BS, the guy was just asking a fair question, no need to antagonize.

[bnmorgan]

Thanks SVO, I'm accustomed to trolls.

Any suggestions for relatively easy/simple modifications that could be used to help the flow around my bug?

[Frank Lee]

Originally Posted by SVOboy

Frank, cut out the BS, the guy was just asking a fair question, no need to antagonize.

I gave a fair answer but he didn't want to hear it.

Why don't you give it a shot?

I'm done now; seacrest out.

[ShadeTreeMech]

You'll want to follow the streamlining template as closely as possible. It is below posted by another ecomodder on another thread.



Your bug has too steep of a rear angle. To reduce your rear drag you have to add to the rear of the bug. VGs etc. will *maybe* help a bit, but there is no guarantee, and it could never match what a boattail is capable of. Anything that is not a boattail is merely a compromise, as the stream lining template is the most aerodynamic shape possible for a car with a flat bottom.

Have you read on the aerocivic yet?

[ChazInMT]

Hey Morgan, I have given up on trying to "correct" anyone. If I wonder WTF with someone, I tend to do the math on the "Thanked vs Posts" part of our profile. If someone has posted 3,500 times and been thanked 200, we're looking at a 6% average for helpfulness. I figure my mission in this forum is to get thanked as many times as possible, otherwise, what are we doing?

I was looking for more information on cutting off the back end of a stream line shape. From my understanding, since a full boat tail is pretty impractical, I have seen where if you chop off the back end of it, you're not compromising the aerodynamics that much.

In other words:


All three of these shapes should have about the same CD.

I just can't seem to find where I've read this. If I'm grossly mistaken on this point, I'd like for someone to say why....with a reasoned explanation FFS.

So in my logic, you want to make the shape of your car come as close as possible to these shapes. The front is not as important. It is all in how the air comes back together after the widest point. If you keep this shape, it will create the least amount of lift and turbulence. If you flatten the top of the shape, the air will tend to leave the back with a lot of turbulence. If you try to steepen the rear angle, it will start to create a lot of "Lift Drag" as I pointed out in my Beetle pic earlier.

This is why I recommend the roof spoiler, you are chopping off the shape of your car aerodynamically while the shape is still desirable.

I think people get hung up way too much on maintaining attached flow down the back of the car to the point where it creates the lift drag. The point here is subtle. And I believe attached flow is easier to maintain than people think. It is counter intuitive that a big ol flat rear end on a vehicle can be OK just so long as the shape in front of the rear is ideally shaped, but this is what I've come to after a ton of research. And it is the only possible explanation for this:

[ShadeTreeMech]

Here's another chart for your perrusal. Chaz is close to being accurate when he says the chopped boattail has the same Cd as the full blown deal; it doesn't, but past 50%, you're really just splitting hairs.

zero------ Cd 0.488
10 %----- Cd 0.375
20 %----- Cd 0.299
30 %----- Cd 0.240
40 %----- Cd 0.194
50 %----- Cd 0.162
60 %----- Cd 0.135
70 %----- Cd 0.125
80 %----- Cd 0.118
90 %----- Cd 0.118
100 %---- Cd 0.115

And the spoiler might actually be HELPING the Cd, something about making up for the steep rear angle. Here's an article from car and driver from the 70s where they discovered adding a spoiler reduced the turbulance from the steeply descending rear glass.

I know the comment is coming, so I'll address it now. Won't a boattail ruin the lokk of the car? Well, beauty is in the eye of the beholder for certain. The Aerocivic is ugly as sin, but being able to more than double his peak fuel economy is quite impressive. Here's a thread where aerodynamics made a car look really good.



But if you don't want to go to extremes, you could try putting in a belly pan. Done right it can help quite a bit, and no one but your mechanic will know about it.

And as far as the "thanks" system goes, it's a fairly recent addition. Anyone who's been on a while will have plenty of posts, but not so many thanks due to the thanking option being less than a year old, while a newcomer will have a higher number of thanks compared to posts.

If you want to read it from the experts, read the threads from aerohead and basjoos. I'd say they're the aero experts and have covered a LOT of ground on several posts on this forum. In fact, on the botom of this page I see 5 similar threads mentioned in the similar threads section, and 4 are by aerohead and 1 is from basjoos.

[ChazInMT]

Shadetree, I don't know whether to thank you or not.....what do you mean by the last paragraph? "If you want to read it from the experts..."

The spoiler improved mileage on that hideously shaped Pinto because it had a gigantic Vortex Generator for a rear end. (If you research this shape you'll see what I mean.) You could put a 12 inch diameter log across the area above the tail lights and it would improve the airflow off that perfectly shaped, worst possible way to design a rear end. So I dunno why you bring a spoiler mounted there to the table.

I understand that boat tailing the VW Bug would help a lot. There is no denying that. You can keep posting about it as a solution to the problem Morgan has put before us here, but you do so at the risk of violating his "relatively easy/simple modification" request I seem to remember him saying. I think he's looking to avoid having the "cut his car in two, throw away most of the rear half, then, rebuild it so its 10 feet longer and tapered" modification here, tempting as it may sound.

Why do you not feel a roof spoiler mounted at the back of his roof above the rear glass as I suggested would work brilliantly???

Truth be told, I'm seriously contemplating trying to improve the aerodynamics on my new car by doing the same thing. The only thing I see as an issue is the air coming off the c pillar wrapping into the area of the rear windshield. But I suppose in theory, if the rear air is creating a more dramatic low due to the lift, the stagnant are created by the roof spoiler would be higher pressure.



I'll close the door on long trips.

[aerohead]

Originally Posted by ChazInMT

Shadetree, I don't know whether to thank you or not.....what do you mean by the last paragraph? "If you want to read it from the experts..."

The spoiler improved mileage on that hideously shaped Pinto because it had a gigantic Vortex Generator for a rear end. (If you research this shape you'll see what I mean.) You could put a 12 inch diameter log across the area above the tail lights and it would improve the airflow off that perfectly shaped, worst possible way to design a rear end. So I dunno why you bring a spoiler mounted there to the table.

I understand that boat tailing the VW Bug would help a lot. There is no denying that. You can keep posting about it as a solution to the problem Morgan has put before us here, but you do so at the risk of violating his "relatively easy/simple modification" request I seem to remember him saying. I think he's looking to avoid having the "cut his car in two, throw away most of the rear half, then, rebuild it so its 10 feet longer and tapered" modification here, tempting as it may sound.

Why do you not feel a roof spoiler mounted at the back of his roof above the rear glass as I suggested would work brilliantly???

Truth be told, I'm seriously contemplating trying to improve the aerodynamics on my new car by doing the same thing. The only thing I see as an issue is the air coming off the c pillar wrapping into the area of the rear windshield. But I suppose in theory, if the rear air is creating a more dramatic low due to the lift, the stagnant are created by the roof spoiler would be higher pressure.



I'll close the door on long trips.

The science tells us that the Koenig-Kamm roofline is the hot ticket,whether chopped at any convenient length for 'practicality'.or do the full monty,running it out to 80% ( the physical limit due the ground clearance ).
Short of that,things can be done as palliatives to deal the flow separation and keep construction more simplified.
With respect to the Pinto,the spoiler affected a place for the flow to re-attach to,and in so doing created a locked-vortex circulating above it,which is known to allow the outer flow to 'skip' over,almost as if it were a solid surface.
Had Car and Driver filled the area in as Chrysler did with the Airflow they would have achieved even better economy.Although,for a single piece of aluminum they did okay.Which I think is the spirit behind the 'simple' spoiler.
Your idea for the roof-mounted spoiler is money in the bank.I'll bet you a meal on it.
Mercedes has used them to good effect on their Autobahn stormers.
Ford's patented bi-wing spoiler would be even better,with Ernie's wing down below it.
Going back to the chopped tails,you can see from the table that there is a 28% difference in drag between 50% and 80% tail.
At 55 mph that's 14% mpg.
At 70 mph that's 16% mpg.
At 80 mph that's 28% mpg.
I'd never try it with a cantilever tail and it's the reason I settled on the trailer solution.
Kamm and Korff recommended chopping at 50% frontal area,which is not the same as 50% of the roofline slope.
Unless the sides of the car begin their inward taper at the same point as the roof begins to drop,the roof might have to be extended further in order to reach 50% frontal area.
Kamm's 'best ' car was only Cd 0.23 ( same as the modified Airflow ),so you can see that half of the drag remained even in his car.
I'd build that roof spoiler!

[ChazInMT]

Thanks a Ton Mr. Knox. And thanks for sticking to your guns in the Cinderella thread a few months back too, because of that, I've come to understand most of what I know as good aero science. You forced me into my own "Aha Moment" when I realized that by striving for utilizing the template by keeping it under the wheels, it makes it the most efficient shape for the subject size vehicle. I still firmly believe that attached flow is maintained if the template is used to analyze individual parts of the cars, but if the element being analyzed drops under that "under wheel main size template", it starts creating this unwanted lift drag that I've been blabbing about in this thread, and although it isn't as horrible as uncontrolled flow separation, it is not ideal. Duz that make any sense??

My suspicions are confirmed in my mind by most of the modern shapes of vehicles that are being produced these days. I think things have come a long ways in vehicle aero design in the last 5 - 7 years. The new Subarus in particular have me saying "Dayyyyyum!" every time I see them.

So Again, 1000 Thanks! You are The Man!!

[bnmorgan]

So if I understand right, the roof spoiler IS the way to go (money in the bank being a positive thing?? I prefer silver in hand, but...i'm paranoid like that)

That being said.....fabricate or buy?
Too bad swapping to the electric turbo-S spoiler would be such a bear. I can get my hands on one