MYTHBUSTERS prove golf ball dimple theory on cars works!!!

[TDeKon]

I too enjoy the show and am often impressed with their test methods. However, as one with experience in this area, there are several concerns I have with this test that I would like to share.

1. There was no repeatability study. I would like to see how the results repeat without changing any parameters.

2. Adding 800 lbs of clay without dimples does many things to the efficiency of the vehicle and yet their results showed no difference. Those would be increased frontal area (increasing drag), increased rolling resistance due to the increased tire loading , and decreased ride height due to suspension compliance (decreasing frontal area and usually decreasing drag).

3. No atmospheric conditions were stated. During one of the earlier tests, it was obvious that there was a noticable breeze that I did not see later. During EPA coast down testing, we often repeat coast-downs up to 10 times and throw out certain 'flyers' based upon a very complex test procedure and average the rest. We also equip the vehicles with a pitot tube to determine the true relative wind speed and direction. Additionally, during performance vehicle coast-downs, we closely regulate the transmission and rear end temps (within 5 degrees F), tire pressures, and brake drag (using positive brake piston retraction springs).

4. Their method of recording fuel usage was not what I would call scientifically robust. The fact that they used a manually triggered switch to cycle between two fuel tanks 10 times over the course of 5 total miles leaves a lot of room for error. I also question the repeatability of the scale used to weigh the fuel, as well. The difference between 26 and 29 mpg over 5 miles is .124lbs of fuel (@ 6.25lbs/gal) or about 2 oz. They may want to look at the SAE Supermileage rules as they run vehicles continuously for 9.6 miles and use one fuel tank (no switching back and forth), which is weighed at the beginning and end of a run (on a certified calibrated scale with high resolution in the range needed).

I don't really want to get into the aerodynamics of the concept, as the golf ball is dimpled for it's intended usage, which is much different than a vehicle's. Also, if you ask Lexus why their underbody panels are dimpled, you will find that the dimpling stiffens the large flat panels and changes their natural frequency to something in a less audible range. Their advertising and marketing guys just like the cool image of 'blowing smoke' (rather appropriate) over the panels. You also do not see dimpling on aircraft, but very smooth surfaces (in most cases), for a reason.

With all of that being said, I can add clay to almost any vehicle and decrease the coefficient of drag, often substancially, by sharpening training edges of the car and softening leading edges. I can also roughen the surface in sensitive areas to promote flow reattachment or delay flow separation. I use a wind tunnel to very accurately determine the effects of the changes and typically not coastdown or on-road or on-track testing because I can control or eliminate most of the variables and scientifically arrive at a conclusion with a very high confidence level.

Terry

[robfrey]

I think more work needs to done in this area. I would have been easy for the myth busters to go though a full tank of fuel on the highway to see if it repeats. I would have liked to see that. I think there is something here but I'm not sure how it would apply to our sport IE dimples all over or just certain places. I know that my buddies who race boats sand their hulls with 400 paper with an orbital sander and they pick up speed over a smooth waxed hull.

[SPARKY]

we cross hatched gelcoat on glider wings with 400 to help stop water beading in showers and to keep the flow laminar just a little farther back

[Dakzila]

  Maybe this theory is connected to the fact that most "fast women" have dimples in their cheeks? (Sorry, it's Friday).

[bbarn]

Dimples, laminar flow, vortexes... The aviation world is full of all kinds of tips on how to manage airflow. Back in the day, the big hot new item in the market were vortex generators. Basically, they are small "wings" or tabs that get mounted on the airfoil that cause controlled turbulence to the flow of air over the surface of the wing. Reference: http://www.avweb.com/news/reviews/182564-1.html If you want boring technical information its available, if you just like to look at the pictures like me, there are enough here to give you a fundamental understanding.

Dimpling a golf ball evenly is required because a golf ball does not have a fore and aft end, it will tumble in an unknown orientation when struck. (Maybe some day I'll share a video of a golf ball being fired from my cannon, when it does have a fore and aft end it behaves much differently!...but I digress). Therefore, you need the symmetrical pattern on the surface so that the trailing vaccum, which is oriented on the opposite side of the direction of flight, can be effected by the result of the dimples regardless of the orientation of the ball. Someone in this thread was correct in saying that you don't need to dimple an entire car to get the benefit of a tighter more accurate airflow. It is all about where you generate and land the vortex for the desired effect. Perhaps "dimples" isn't the only shape we should be focusing on...Rob, you still want do that streamliner project? I have some more ideas to add to the list!

[robfrey]

About that liner, ooooh yeah! I am going to try to get a little engineering done everyday.

[wobblywalrus]

This topic lends itself to modeling full-scale vehicles in a wind tunnel.  A perfect doctoral project at a university.  Great fun for all involved.

[Papi]

I wonder if a vehicle not being completely spherical would achieve the same effect?
Somewhat doubtful.....

[pookie]

Hi OTTO, Looks like someone is going to put your"golf ball' theory to the test, at El Mirage this weekend. Dennis Varni has a streamliner with a "golf ball" finish on the exterior. If you stand 3 to 4 feet away the exterior looks dull but as you get closer you can see the the "dimples" in the finish. At first I thought it was a bad case of"orange peel" in the paint job, as Dennis only had the bottom pieces of the streamliner body attached.. When I ran my hand over the finish I was feeling a rough finish.. I went to Varnis' pit area, to see the top sections and sure enough same type of finish.. Since the streamliners engine is a rare purpose built[I understood only 18 were ever built] the liner may not be the perfect test of testing the aero, but it does show that someone else is thinking outside the box............ or not.   Mike R.

[Stan Back]

I looked under the hood of my Chevy truck and it didn't look like that engine.  More cylinders and less valves (?).

Stan