Mythbusters "tests" efficiency of dimples vs. smooth, and clean vs. muddy car

[Marc F.]

I just saw this episode for the first time. in water sharks skin has little ridges that "improving their fluid dynamics " according to the wiki. I think it sort of the same idea.

I wondered if you would want the dimples all over or just toward the back part of the car to break the vacuum?

I wondered about testing them on my wagon, anything should help. Also would you want smaller dimples or big dimples? I can see this going to vortex generators...

[Nerys]

well the aero is interesting in that smoother is better at making the surface slick.

when you make the skin of the car rough (dirt or dimples) you INCREASE DRAG and HARM FUEL ECONOMY. always.

but this statement has a gotcha that most people don't fully get.

there are different "kinds" of drag and what helps or harms one will help or harm another.

the golf ball dimples HARM parasitic drag but IMPROVE base drag.

the improvement in base drag is "greater" than the hindering of parasitic drag so you see a "net gain"

frontal area and base drag have a far greater impact on fuel economy than "parasitic drag" or "SKIN" drag.

frontal area is when the air "slams into" something. Base drag is the "suck zone" behind your car.

these FAR outshadow parasitic drag.

this is why removing a mirror or atting a kammback or a grill block will see much larger "gains" than sealing up all the cracks in your body panels or "waxing" your car.

and could actually result in WORSE performance if you REDUCE parasitic drag because you also now make the "far worse" frontal/base drag worse.

the cleaner your frontal/base drag is the more important parasitic drag becomes.

so "clean lines" are far more important to an EV1 which has a very "clean" profile than it is for a "jeep cherokee" or pretty much any other normal car on the road.

this basically comes down to do the right thing at the right time.

Golf ball dimples on your hood will do nothing but make your drag worse.

but golf ball dimples on your kamback read half your roof and your rear quarter panels will IMPROVE performance. because why they increase your parasitic drag (lowering your fuel economy) they ALSO decrease your BASE DRAG by a far greater amount which improves your fuel economy by a lot more than the "decrease" from the worsening of parasitic drag.

and that is why the golf ball dimples work.

what would be interesting is "what size" is ideal ?

[Marc F.]

Yeah, I wondered why they put them all over the car. A golf ball is spinning and has to have them everywhere.

A kammback is in the future for Tank (82 fullsize stationwagon). It would be simple enough to make one with dimples, or rather would be simpler than dimples in the wagon. Of course its going to require A-B-A-C-A testing. No Kammback, smooth Kammback, and a dimpled Kammback of the same exact design.

I have been thinking about how to make the kammback/dimples. Before I say anything I am going to do a few tests on materials that fit my range of tools.

[RiderofBikes]

i haven't read this entire thing, but the size is relative, along with how the dimples are placed?

Dimples breakthrough! - News - GOLFmagic

On the other hand it seems we have a test car already. Front smooth enough, but may need some work on the back end. Needs tail for sure though!!

[Xist]

When you guys talk about the need for ABA testing, I see references to applying the dimples, removing them, and putting them back on. Why not apply the clay, test the car, remove dimples, test the car, put the dimples back in, and test the car a third time? I understand that they are sometimes limited by time, but two sets of five one-mile runs at highway speeds should not have taken too long. Buy me a scangauge and clay and I will test according to your criteria. Buy me a gas card and I will drive as far as you tell me to!

For the record, what were the other complaints? They did not give weather information?