Tonight on Discovery Channel, Mythbusters looks at aerodynamics.
2, n=1, tests they show in the previews is whether or not a dirty car is more fuel efficient than a clean car and WHAT HAPPENS WHEN YOU DIMPLE A CAR!
that’s ridiculous … my personal experience has taught me that my car accelerates faster, corners more crisply and is more fuel efficient when washed/waxed/detailed. I’m also pretty sure that a layer of Turtle Wax Ice interior polish makes me better looking and raises my V02 Max by 17%.
and red IS the fastest color.
11% increase. pretty significant advantage with the dimpled car.
Who needs dimples? I went to the unveiling of the new Porsche Panamera over the weekend at the Universal Studios Backlot. The car has a spoiler that changes shape at three different speeds, 50 mph it comes out, 80 it gets bigger, and at 127mph, it changes its angle. Dimples cannot do that.
Interesting to note that the 850 lbs of clay did not affect fuel efficiency when the car was already up to speed. I guess weight really doesn’t matter.
Interesting to note that the 850 lbs of clay did not affect fuel efficiency when the car was already up to speed. I guess weight really doesn’t matter.
As noted on the show, the extra weight would affect fuel economy during acceleration so I imagine that overall mpg is either unchanged or worse. It would depend on the ratio of city and highway driving.
One thing they didn’t mention/consider is the ground effects the 800+ lbs. of clay may have produced. If the car was lowered by the weight then there would could possibly be less air traveling beneath the car to create drag, resulting in better fuel economy. Just a hunch of mine that their results are a little skewed.
Both tests had the same amount of clay on (or in) the car. So even if the clay lowered it enough to better the aerodynamics, it was the same across the two tests and doesn’t matter.
Both tests had the same amount of clay on (or in) the car. So even if the clay lowered it enough to better the aerodynamics, it was the same across the two tests and doesn’t matter.
Ah! I missed that part as I was half watching, half on the PC.
Thanks for the clarification.
Didn’t see the show…
What speed were they testing at? And I wonder what the advantage would be if they slowed to 40km/h.
by far, the coolest part of the test was when they put the plastic model in the water channel with the fluorescent dye.
i’m not allowed to nitpick the show any more, so that’s all i have to say. 
by far, the coolest part of the test was when they put the plastic model in the water channel with the fluorescent dye.
i’m not allowed to nitpick the show any more, so that’s all i have to say.
lol
by far, the coolest part of the test was when they put the plastic model in the water channel with the fluorescent dye.
That was cool. I must say, I was a bit surprised at the outcome. I thought that there might be a tiny increase, but not 11%. But you just have to wonder what the “catch” is. I mean, car manufactures take these things to wind tunnels don’t they? Surely this has been thought of before? Maybe it’s an aesthetic thing. But, since that eye-sore called a Prius has been so successful perhaps they will add dimples, knowing that greenies will buy ugly.
I also wonder what material they would need to use?
First, you need to make dimples that are structurally sounds. Second, you need a material that can tolerate rain, gunk, and crap sitting in the bowl for extended period of time without corroding.
Wow, when are they going to dimple actual bikes?
I should patent that before Cervelo does. Damn
11% increase. pretty significant advantage with the dimpled car.
Actually, dimpling sheetmetal would be pretty simple in the stamping process, and is often stronger structurally than flat sheet metal.
What speed were we talking again? I always thought that dimples had a better affect on the laminar flow at higher speeds (spinning golf balls). 11% seems a bit high.
I will say that I’ve personally seen a 4-5 knot increase in top speed (around 200knot range) of aircraft when cleaned and waxed versus a leading edge covered with bugs, and top surfaces covered in soot/dirt/dust.
Dimpling introduces turbulence, thus delaying the onset and magnitude of flow separation on bluff bodies (e.g. a spherical golf ball). For bodies that are already streamlined, such as an airfoil, the advantages to dimpling become trivial.
As of now my car is no longer hail damaged, it is dimpled for improved aerodynamics.
I’m thinking I saw a Lexus ad maybe 5 years ago- it had a dimpled plate on the bottom of the car- supposedly to smooth air flow- and make the ride quieter. They took the car, flipped it upside down, and showed how smoothly the air flowed by showing a jet of smoke or something along those lines…
Makes sense- as far as laminar flow goes… Why wouldnt they continue that thought process? it was probably a 400lb plate of sheet metal they screwed into the bottom of the vehicle- probably reduced fuel efficiency greatly in city driving… maybe that idea busted- anyone know?
Just found this
and this
One thin to keep in mind is that air drag is very scale and speed sensitive. The key variable is the Reynolds num, which is a func of vehicle size and speed- and results from different Reynolds mums can absolutely not be applied to others. This is why scale
models don’t work very well in wind tunnels, or have to be rested at odd speeds to create similiar Reynolds mums. So if this is true, it’s very hard to extrapolate
let’s talk about dimple theory here- q
want to address a few misconceptions:
a) golf bAlls are dimpled Bc they don’t know ahead of time which side is the front. Objects that have a distinct front ( cars, bikes, helmets, water bottles) should have sideways ridges not dimples.
B) the point of dimples is to seperate laminar flow from sticking to the back of smooth objects and slowing them down. This happens to be important if two conditions are met: 1) the flow must Already be undisturbed by other bumpy things (this is why dimpled waterbottles are dumb- air already disturbed by time it gets to bottle) 2) the speed/ size rsship must be in a narrow range. It happens that golf balls And baseballs fit both criteria- but very few othe things do- cars are wrong speed and bumpy enough in front ( grille anyone), bike wheels are wrong speed, as are bike helmets.
So in summary: bumps rarely work, they should be ridges not bumps, scale testing doesn’t work well, and golfballs are an exception not the rule.
Clyde