Optimal Car Shape
 

I have read many times that the teardrop shape is the ultimate for car aerodynamics. I assume that reflects a car with two people sitting side by side, because the ultimate projectile shape is a cylinder with points on both ends. Artillery shells use a gas generator to virtually create the rear tail. Now, I can see it being pretty tough to make a car in that form. Have two wheels and outriggers that fold out only at low speeds? If you put wheels out all the time you compromise the shape. I suppose you could also approach it like a catamaran and have two connected together. Be a great way to get privacy from the kids! I apologize for my limited drawing skills. Couldn't seem to get the picture url link to work in this post.

I wonder if the much lower speeds seen by cars accounts for the teardrop shape making sense?

I'd like to see where someone did an aerodynamics study to show that the teardrop shape is the most efficient. The teardrop shape would create a high pressure area in front of it as opposed to cutting through like a cylinder with tapered ends.

if you look at the fuselage of an airplane, it has the same shape. you have to look at it from a different angle though. tha aptera tapers from top to bottom and bottom to top. airplanes taper on the sides more than the top. of course all of this going front to back.

I have heard that the optimum angle is a 10 degree grade or slope.

*edit* also if you had to taper in the front and the back, the vehicle would have to be extemely long to have any room inside.

Sure would. That's why thought the catamaran arrangement would make more sense.

I'm mostly just shooting the breeze. Besides, I like to think on a problem before checking out the established research. Otherwise, you can fall into the trap of not considering things that haven't come before.

Actually, what I usually do is this:

If I think the problem has a very optimal solution, I just go straight for the standard solution. If I think there's still room for improvement, I do the brainstorming first, researching second approach.

As cars are a compromise of aerodynamics with practicality, it's far from cut and dry.

I once spent a week reading research for hydrogen as the ideal jet fuel. I had no possible use for it, but was living in the same city as my alma mater and had free access to their science library. Those were the days!

Ever hear the joke about the difference between engineers and physicists?

A physicist will spend all day deriving a formula he could've looked up in five minutes. An engineer will spend all day looking for someone to tell him a formula he could've derived himself in five minutes.

I try to walk somewhere in the middle, but tend towards the physicist end a bit (ironic since I'm an engineer).

Seems like some posts have gone missing in this thread...weird.

I talked to my uncle who has a lot of wind tunnel testing knowledge and he agreed that the shape I was discussing would work pretty well (assuming the weird configuration could be made practical for the powertrain and passengers), but when I did a little measuring to see what a catamaran style car would look like it became evident pretty quickly that it's just not wide enough to warrant it, at least for four occupants. Planes and boats, sure. I once saw a race car that almost had this approach, as it just had a cockpit bubble on the driver's side. You'd have to do something similar: two people on one side, and the trunk and engine on the other or something. Very odd configuration indeed with better options (center hump, for example) particularly if you're comfortable with three wheels. If I had some 3d software I'd have done up a quick model, because the resulting vehicle is pretty amusing in its impracticality. Figuring out how to make a powertrain and balance work was fun.

In the 1952 there was a land speed record attempt by Piero Taruffi in a twin boom vehicle (catamaran shape) called a TARF I think.

An interesting design study for those interested.

Some details and a cut away drawing here:
https://jalopnik.com/photogallery/dblbullet/1003234992

Cheers , Pete.

That's much closer than the car I saw! Thanks! Looks pretty wide, but I failed to find actual dimensions anywhere. Engine counterbalancing the driver, which is what you'd expect. Neat stuff. Taruffi's Italian, so it'd be interesting to know what prompted his decision to sit on the right, since they drive on the right in Italy. Lanes and other traffic on his test run aren't probably an issue, I guess. :) That cutaway is actually of a different vehicle, FYI.

I found a short article about it. Also, here's a thread about other similar vehicles. It should've occurred to me that in the pure speed arena where general practicality is less of a concern this would've been tried.

I'm sure that NASA has data on the lowes drag shapes........

Since car design is a compromise between utility and aerodynamics it comes down to more than just the simulations. It's more the process of trying to make a weird shape work that I find interesting.

There's some NASA data on aerodynamics in my Sig Meta Thread...

Sorry, Sludgy, I mistook your point. I'll take a look.

Gustave Eiffel, 1903 https://www.allbusiness.com/professio.../290566-1.html

the shape isn't what he expected either, just what empirical observation pointed to. I'll see if I can find the ball studies, it explains it well. teardrops may be ugly, but they work.

Here's the part in that relating to aerodynamics:

Eiffel really put emphasis on transcending mathematics and understanding things on an intuitive level. As an engineer I try to do the same and think our schools should teach math and science with a direct application method to reinforce it. I jokingly call it the "go-kart teaching system", because I know as a school kid I'd have loved to be part of a team to design and build one. You learn trigonometry when you need trigonometry, etc. Obviously you'd need a bunch of different projects for students to gravitate to, carefully chosen to include a useful assortment of skills and theory.

Here's an interesting example of a very low drag car that looks nothing like a teardrop . . .

Mercedes Bionic Car

Cool! Has a Cd of .19. I did a quick search and a teardrop based vehicle should be somewhere in the .13-.15 range (please correct me if I found bad sources). Of course, the Bionic has a much less limiting shape.

I keep looking at the bionic and wondering if I can get Marvin close(r) to that shape.

Messing with the window shape sounds hard.

Looks like an Insight mated with an xB and gave birth to a minivan.

Yeah, some of the mods would be more inclined towards making the air take the same path they take around the Bionic rather than faithfully replicating the shape... What I mean by that is that in certain areas, like probably the roof, I'd try and use a deflector to make stable vortices that round out the shape, rather than adding a bodywork humpback.

Another option for airflow control to try and force the air to flow round a profile different to the actual surface shape is similar to that artillery shell method, and I believe may be used on some classified munitions and projectiles. This would be high voltage charging to either push air away at certain points or to pull it back in.