# Mythbusters "Reverse Engineering" 11/17/2010



## RonDawg (Jan 12, 2006)

As a car nut myself, I had long heard the claims that the Porsche 928 was more aerodynamic going in reverse than forward. So I was pretty surprised to find out it wasn't true all along, though reverse wasn't that much less aerodynamic either.

I think the myth could have easily been confirmed or busted by simply using a wind tunnel, like they did with the small scale test, but I guess it wouldn't have made as good TV.

On the surfboard-weapon myth, I wasn't surprised at all. A modern laminated car windshield is surprisingly hard to penetrate, and with it slanted back most objects would just get deflected upward. They weren't able to penetrate it until the last test, and I think the only reason it penetrated was that the windshield sagged enough to allow the surfboard to catch the upper edge of the windshield frame.


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## Shaunnick (Jul 2, 2005)

I apologize for this being off topic, but in the first commercial break what kind of Dell netbook is that in the Windows Cloud commercial?

And for what I know is surely to come, I saw it as I was fast forwarding and backed up to take a peak. 

So anyone know.

ETA:

Never mind, I found it:

Inspiron Duo 10-inch


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## Worf (Sep 15, 2000)

RonDawg said:


> As a car nut myself, I had long heard the claims that the Porsche 928 was more aerodynamic going in reverse than forward. So I was pretty surprised to find out it wasn't true all along, though reverse wasn't that much less aerodynamic either.


I think a little common sense would've found that. If the car was more aerodynamic going in reverse, don't you think the car designers would've made it "backwards" so it performs better and gets better gas mileage? Even discounting the latter means you can put a thirstier engine in it, increasing performance even more.

Especially Porsche - I'm sure they wouldn't give up an opportunity to make a faster, sportier car if it ran better backwards. It would look funny, but I'm sure people would get used to it in time.


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## RonDawg (Jan 12, 2006)

Worf said:


> I think a little common sense would've found that. If the car was more aerodynamic going in reverse, don't you think the car designers would've made it "backwards" so it performs better and gets better gas mileage? Even discounting the latter means you can put a thirstier engine in it, increasing performance even more.
> 
> Especially Porsche - I'm sure they wouldn't give up an opportunity to make a faster, sportier car if it ran better backwards. It would look funny, but I'm sure people would get used to it in time.


The myth does have some basis. Jamie said the most aerodynamic shape in nature is the teardrop, and the 928 does mimic that shape...in reverse. The bulbous rear end is probably what gives some people that impression.


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## heySkippy (Jul 2, 2001)

Worf said:


> Especially Porsche - I'm sure they wouldn't give up an opportunity to make a faster, sportier car if it ran better backwards. It would look funny, but I'm sure people would get used to it in time.


Yeah, Porsche would never make a funny looking car!


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## john4200 (Nov 1, 2009)

RonDawg said:


> I think the myth could have easily been confirmed or busted by simply using a wind tunnel, like they did with the small scale test, but I guess it wouldn't have made as good TV.


Right, I thought the same. Also, it was not very clever to test aerodynamics with 0-60 and 1/4 miles, since a lot of the engine's energy is going towards overcoming inertia rather than air resistance.

Finally they got smarter at the end and just tested coasting speed, but even that was inferior to a wind tunnel since the coasting speed brings rolling resistance into the mix.

If they really wanted to do a road test, they should have gotten a long track and run the cars at near their top speed, say 140mph, for several miles and measured the fuel consumption. At least that way the dominant factor should be air resistance.


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## billboard_NE (May 18, 2005)

Anyone else shed a tear when they took a saw to such a fine example of a Porsche 928? I would not have felt so bad if it was a rust bucket, but a mint 928, why?

I have always wanted one after I first read about it in Road and Track circa 1982ish listed as one of their top ten something or other list.

At least it was an automatic and not a 5 speed.


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## john4200 (Nov 1, 2009)

I would not be surprised if they could sell it to a collector for more than they paid for it. One of a kind.


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## Gunnyman (Jul 10, 2003)

billboard_NE said:


> Anyone else shed a tear when they took a saw to such a fine example of a Porsche 928? I would not have felt so bad if it was a rust bucket, but a mint 928, why?
> 
> I have always wanted one after I first read about it in Road and Track circa 1982ish listed as one of their top ten something or other list.
> 
> At least it was an automatic and not a 5 speed.


I actually cringed.


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## RonDawg (Jan 12, 2006)

billboard_NE said:


> Anyone else shed a tear when they took a saw to such a fine example of a Porsche 928? I would not have felt so bad if it was a rust bucket, but a mint 928, why?
> 
> I have always wanted one after I first read about it in Road and Track circa 1982ish listed as one of their top ten something or other list.
> 
> At least it was an automatic and not a 5 speed.


The problem is that if you get a "rust bucket" that will complicate efforts to reverse the body on the chassis. The separation process might cause the body to fall apart if it's rusty.

One that is in overall poor condition may not be able to do the performance tests well, or possibly may not be in running condition at all.

Where I did shed a tear is when Tom Cruise and Rebecca DeMornay turned a brand new example into a "U-boat"


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## heySkippy (Jul 2, 2001)

john4200 said:


> I would not be surprised if they could sell it to a collector for more than they paid for it. One of a kind.


LOL, no way. That car is 100% ruined. I'll bet the P-car guys at rennlist and 6speedonline are having kittens over this episode.


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## oskar de kiefte (Nov 19, 2010)

In 1992 Oskar de Kiefte get the hunch that cars are driving the wrong way around. In extreme the Porsche 928, this one looks in reverse exactly like the most ideal aerodynamic shape for above a surface. De Kiefte graduated in 1995 from the Art School HKU, with The converted Porsche silver Aero. The TV-program Mythbusters build a Porsche backwards again to test the cars top speed. 

In the beginning it was Oskar de Kieftes idea too, to test the Porsches top speed. He did test the cars top speed before he dismantled the car completely. Even the circumstances are recorded. But when he discovered that the Porsche factory already did a wind tunnel test with the Porsche 928 in the opposite direction with better results, he removed this test out of his programme.
In 1931 Norman Bell Geddes showed the World too that in an aerodynamic way cars are driving in the wrong direction. Oskar de Kiefte was astonished about the non-functional behavior of car designers. To clear this he didnt make the thousandth new car design, but completely turned around the Porsche to create a big shock.

In the past years Oskar de Kiefte build cars in different ways. Even this year he builds two cars for Happy Street, the Dutch Pavilion at the World EXPO 2010, Shanghai. These are the Square Car of John Kormeling and his own Wind Turbine Car.
De Kiefte thinks, its great that the myth he created is become an item for Mythbusters to solve. This gives his idea a new life. Oskar de Kiefte worries about the way of testing. In the first place, the Mythbusters turned around the body without changing the position of the engine to the middle. There is virtually no space left for the driver and this is no fun at top speed.
Secondly, its important to set the cars body in the right position. The backside somewhat higher or lower makes a great difference.
It must give the same shock effect nowadays as it did in the period de Kiefte present his converted Porsche 924/928 silver Aeros to the public by books and papers as well as many broadcast TV programs, like Young industrial Designers 96.

oskardekiefte.nl


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## john4200 (Nov 1, 2009)

I'm skeptical that a teardrop shape is all that great for low drag. I'd like to see the Mythbusters test different shapes for drag. They could perhaps make scale models out of wood (carving) or plastic (3D printing) and test them in their terminal-velocity rig in the lab, then test larger models in a wind tunnel. Or even drop them from tall buildings or from a helicopter.

My prediction is that the lowest drag shape would be something like an elongated double cone. Take two elongated cones (tip angle 20 degrees, say) and fasten them together, base-to-base. If you are going to drop it to test drag, you would need to weight one end so that it would fall with the correct orientation.

Other shapes that should be included for comparison would be a teardrop, a single cone, a tube with 45 degree nose- and tail-cones, and a sphere.


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## andyw715 (Jul 31, 2007)

Supposedly the upcoming Chevy Volt will be only second to EV-1 in aerodynamicy. (as far as production GM cars are concerned)


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## billboard_NE (May 18, 2005)

Some of the posts here are confusing science with Mythbusters, sometimes the paths of science and Mythbusters cross, but they are not parallel. I do enjoy Mythbusters, but I do not waste time yelling at my TV set because they are testing something in the wrong way.


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## nataylor (Apr 26, 2000)

john4200 said:


> I'm skeptical that a teardrop shape is all that great for low drag.


Pretty much by definition, a teardrop shape will be the most aerodynamic (presenting the least resistance to the object moving through a fluid). This is precisely why falling rain drops are this shape. They want to take the path of least resistance. Taking any other shape would require some force to hold it, and the only force in the water is a tiny bit of surface tension.

Now the precise geometry of the most efficient teardrop is going to be determined by a number of factors, including the viscosity of the fluid and the speed of the object, of course.


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## john4200 (Nov 1, 2009)

nataylor said:


> Pretty much by definition, a teardrop shape will be the most aerodynamic (presenting the least resistance to the object moving through a fluid). This is precisely why falling rain drops are this shape. They want to take the path of least resistance. Taking any other shape would require some force to hold it, and the only force in the water is a tiny bit of surface tension.
> 
> Now the precise geometry of the most efficient teardrop is going to be determined by a number of factors, including the viscosity of the fluid and the speed of the object, of course.


No, I do not think so. The shape a raindrop makes is an equilibrium, for a uniform density fluid, due to air resistance and surface tension.

But when looking at drag for non-uniform density solid shapes, you have more flexibility. For one thing, it does not have to be of uniform density! You can weight one end so that it is stable. A uniform density double-cone would not fall in the proper orientation for low drag. But if you weight it, then it could.

Also, the front of a raindrop could never maintain an elongated cone shape with surface tension. But a solid elongated cone could have lower drag than the rounded teardrop shape.

So no, you cannot say that "by definition" a teardrop shape will have the lowest drag of any possible shape.


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## nataylor (Apr 26, 2000)

john4200 said:


> So no, you cannot say that "by definition" a teardrop shape will have the lowest drag of any possible shape.


I assumed when were were talking "shapes" we meant regular solids of uniform density and volume. Weight or density has nothing to do with the aerodynamics of a shape. It might play a roll in the orientation of an object in a fluid in a gravitational field, but when we're talking about pure aerodynamics, it can be totally ignored.

And while we're qualifying things, let's limit ourselves to sub-sonic travel through a fluid.

For reference, here's an illustration giving the relative percentage of flat-surface drag induced by various nose-cone shapes:










You'll see that it in, in fact, the parabolic shape fo the teardrop that contributes the least amount of drag.

Also, two cones, affixed at their bases, will have a transition at the center of the unified shape that will induce drag along the the remaining length of the object. That means, in general, moving as much of the volume as close to this transition as possible, while still maintaining a streamlined flow results in the least drag.

This is why rain drops form a teardrop shape and why you'll find most sub-sonic aircraft have parabolic shaped noses rather than pointed ones.


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## john4200 (Nov 1, 2009)

nataylor said:


> I assumed when were were talking "shapes" we meant regular solids of uniform density and volume.


Name me a car that has uniform density! 



nataylor said:


> For reference, here's an illustration giving the relative percentage of flat-surface drag induced by various nose-cone shapes:
> 
> You'll see that it in, in fact, the parabolic shape fo the teardrop that contributes the least amount of drag.


Thanks for the link to the chart, that is interesting.

But apparently those general shapes do not behave the same when they are slightly modified from the ideal, since the rear of the car the Mythbusters tested looks more parabolic than the front, and yet the scale model had more drag backwards than forwards.

Also, the explanation you gave for a teardrop shape is a myth. Falling raindrops are NOT teardrop shaped! They are roughly spherical but with a flattened bottom. And a hamburger bun shape is definitely NOT the lowest drag shape.


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## heySkippy (Jul 2, 2001)

Ideal aerodynamics for a car are far more complex than just what is the most efficient in the air. A great aero design also needs to stay on the road.

I submit that the fastest production car in the world is state of the art in aerodynamic design. Not exactly a teardrop, but definitely not a pair of cones.


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## john4200 (Nov 1, 2009)

heySkippy said:


> I submit that the fastest production car in the world is state of the art in aerodynamic design. Not exactly a teardrop, but definitely not a pair of cones.


Not even close to a teardrop. Closer to a backwards teardrop.


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## nataylor (Apr 26, 2000)

As heySkippy said, there's a big difference between idealized aerodynamics and real-world engineering of a vehicle that has to stay on the road, corner, and accelerate.


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## vertigo235 (Oct 27, 2000)

nataylor said:


> As heySkippy said, there's a big difference between idealized aerodynamics and real-world engineering of a vehicle that has to stay on the road, corner, and accelerate.


Not to mention the driver has to be able to see and also be comfortable.


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## MarkofT (Jul 27, 2001)

nataylor said:


> As heySkippy said, there's a big difference between idealized aerodynamics and real-world engineering of a vehicle that has to stay on the road, corner, and accelerate.


And the fact that only 3 sides of the vehicle need to interact with the air. They do their best to get air out of the 4th side.


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## doom1701 (May 15, 2001)

MarkofT said:


> And the fact that only 3 sides of the vehicle need to interact with the air. They do their best to get air out of the 4th side.


Is the fourth side in your equation the back, or the bottom? Since it exists in 3d space, a car really has 6 sides (or the box that would contain it has 4 sides).

For the myth, I think it was pretty much busted in the small scale, but they still went ahead with it. I don't think the methodology was quite reliable for the full scale test. It would have been better to find some way drive the car in reverse, rather than reversing the body.


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## john4200 (Nov 1, 2009)

doom1701 said:


> It would have been better to find some way drive the car in reverse, rather than reversing the body.


No, that would have been worse. Then you are introducing different gear ratios into the issue.


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## doom1701 (May 15, 2001)

john4200 said:


> No, that would have been worse. Then you are introducing different gear ratios into the issue.


OK, how about I put it this way--it would have been better to find some way to drive the car backwards. Somehow flip the transmission (not sure if that is feasible, but I'm not a car guy) so that you just reverse the direction that the tires spin. Driving in reverse "gear" would obviously be out of the question, but it seems like cutting the car into pieces and tacking it all back together introduced too many variables.


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## heySkippy (Jul 2, 2001)

Yeah, reversing the car body was more of a stunt than anything else. Quite entertaining to watch though.


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## john4200 (Nov 1, 2009)

doom1701 said:


> OK, how about I put it this way--it would have been better to find some way to drive the car backwards. Somehow flip the transmission (not sure if that is feasible, but I'm not a car guy) so that you just reverse the direction that the tires spin. Driving in reverse "gear" would obviously be out of the question, but it seems like cutting the car into pieces and tacking it all back together introduced too many variables.


Even if that could be done, it is still not a good idea. If you just "flip the transmission" then you would be driving a car in reverse at 100mph or whatever. With the turning wheels in the rear, and all the other FWD/RWD tuning out of whack.

Reversing the body was the best that they could do, if we assume they had to do a road test. If they were willing to forego a roadtest, then the best thing would be a full size windtunnel test.


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## Jonathan_S (Oct 23, 2001)

RonDawg said:


> On the surfboard-weapon myth, I wasn't surprised at all. A modern laminated car windshield is surprisingly hard to penetrate, and with it slanted back most objects would just get deflected upward. They weren't able to penetrate it until the last test, and I think the only reason it penetrated was that the windshield sagged enough to allow the surfboard to catch the upper edge of the windshield frame.


On that last test notice that the tip of the surfboard broke off immediately. So what really impacted the window and dug in at the frame was the metal plates that they'd sandwitched the board between to hold the bungee attachment hook. An unmodified board would have been lighter (less energy) and almost certainly would have done even less damage than the already non-fatal impact they demonstrated.


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## alansh (Jan 3, 2003)

From what I've heard the "jellybean" shape is probably the most aerodynamic, given practicalities of car design.


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## mattack (Apr 9, 2001)

billboard_NE said:


> Some of the posts here are confusing science with Mythbusters, sometimes the paths of science and Mythbusters cross, but they are not parallel. I do enjoy Mythbusters, but I do not waste time yelling at my TV set because they are testing something in the wrong way.


I understand your point, and agree with you _to some extent_. However, the reason they do a reasonable number of "redo" shows is BECAUSE many of us _are_ expecting them to do reasonable scientifically accurate tests _and_ be entertaining.


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## alansh (Jan 3, 2003)

From XKCD:


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## heySkippy (Jul 2, 2001)

On the subject of that poor Porsche, I found it odd that it ran a 14 second flat 1/4 mile but could only manage 8 seconds 0-60. Those numbers don't jibe.


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## YCantAngieRead (Nov 5, 2003)

alansh said:


> From XKCD:


Okay, that made me laugh. Awesome.


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