The dimples on a golf ball reduce air-drag resistance, so why don’t we put dimples on planes or cars? Perhaps because it’s ugly, or that cars are much bigger and than golf-balls, so we expect the effect of skin effects to be smaller. Finally, a Reynolds number analysis suggests that dimples on cars should increase drag, not reduce it.

In 2009, the Mythbusters decided to test the conjecture. Hosts Jamie Hyneman and Adam Savage ran careful gas usage tests on a Ford Taurus that was first covered with smooth clay. They drove the car repeatedly (5X) on a track at 65 mph (about 100 km/h), and measured “slightly over 26 mpg,” 9.047 l/100km, a respectable value. They then carved dimples into the clay to simulate the surface of a golf ball. See picture at right, and put the removed clay into the trunk so there would be no decrease in weight.

They then drove the dimpled car over the same course, five times as before at exactly 65 mph, and found the car got 14% more mpg, 29.6 mpg, or 7.946 l/100 km. See video excerpt here. They considered it their most surprising Mythbuster episode.

As it happens, dimples had been put on some production cars, even before the episode. They are just located underneath where most people don’t see them. The VW “Golf” had dimples even before the episode, and the Porsche Cayman GT4 does today, see picture above left, but most experiments find little or nothing. Car dimples are typically smaller than those used on Mythbusters, so that may be an explanation. Dimples have been found to help on soccer balls (the stitching acts as the dimples), and bicycle wheels (less advantage).

The graph at right shows the source of confusion for cars and the great advantage for golf balls. It’s a plot of the drag coefficient for smooth and dimpled golf balls, as a function of the Reynolds Number, where N_RE = Vdρ/µ. In this formula, V is velocity, d is the diameter of the car, ball or whatever, ρ is the density of the fluid, and µ is viscosity. N_RE can be thought of as the ratio of the inertial to viscous forces acting on the object. It’s a way of describing the combined effects of speed and size for different objects in motion.

We see, above, that dimples reduce golf-ball drag by more than 50%, but only at speeds/ Reynolds numbers that are much lower than for normal cars, N_RE between about 4×10⁴ and 3.5×10⁵, as are typical of golf balls during play. A typical car at 65mph will have a N_RE.CAR = 3×10⁶, suggesting that there should be no advantage for dimples, or possibility a disadvantage, that dimples should increase drag. A side note one sees, above, is that it is only the dimples on the front of the golf-ball that reduce drag: other dimples do nothing. If one were to add dimples to high-speed trains and airplanes I’d suggest them only on the front, so far I have not seen them.

I think that the Mythbusters did a good job with their experiments, and find their 14% improvement significant. So why do so few other cars see and advantage. One thought I had was to note that the Ford Taurus is a remarkably round car, providing ample space for front dimples to help, most cars today are more angular. I also note that the production cars have smaller dimples, as on the Porsche, above. Then again, the Mythbusters folks may have made some non-obvious experimental error.

Robert Buxbaum, January 4, 2024. An important side issue in this is that Google’s AI was awful, a handicap in researching this article. It lies continuously and convincingly, and did so here. I’d asked it for the year of the episode, and the AI lied, and said 2012. I asked for the type of car, the AI said an SUV, and it gave a misdescription of the tests. Lying AIs appear as villains in science fiction, e.g. HAL of 2001 A Space Odyssey, now in real life.