But:
(1) experiment trumps expectation
(2) drag coefficients are VERY sensitive to details of the object.

so… I still think your thing here is cool.

Thanks for the insight Mike. And hey, OH-IO! I’m an OSU grad myself.

I’m no rocket scientist, but I am a nuclear physicist and a professor at Ohio State University. I’m teaching an undergrad course this term, and will direct them to your page. Very nice work!

Everything Charlie says about drag is correct. The thing of course is that while that formula captures *most* of the velocity dependence of the force, the drag coefficient itself depends on velocity, for nontrivial reasons. (I know, you guys both know this. I just have to say *something*.)

Very cool.

To be fair, I didn’t do a whole lot of testing/racing with the dimpled car. I should do a ton more races versus the non-dimpled car to see if results are consistent. Like I said in the article, there is a lot of room for error the way I did things.

I explored weight distribution too a few weeks ago (check out the article under the Racing section) but haven’t really been able to do any testing with it. Again, it’s one of those cases where I would have figured the scale would negate efforts but maybe the dimple experiment suggests otherwise.

However, in your thoughts about looking for aerodynamic cars…I wouldn’t change my habits based on this experiment. Hot Wheels are so variable when it comes to performance, even from the exact same model of car. A car that looks (and could be) aerodynamic may suck in a race because it’s too light, the axles are weak, the wheels wobble, chassis is narrow.

Albeit, the slick/aero cars *look* faster, I’ve found that they rarely are. But finding a good racer as a base, then applying science (like the dimples) might just be the combination to get the fastest car. Hmmmm…

F(d) = -1/2pv^2AC(d)u

F(d) is the force of drag,
p is the density of the fluid
v is the velocity of the object relative to the fluid (air, in this case)
A is the reference area,
C(d) is the drag coefficient (a dimensionless parameter, e.g. 0.25 to 0.45 for a car)
u is the unit vector indicating the direction of the velocity

Also, I would think that dimpling would lower the mass of the dimpled car, and that mass is one of the primary determinants of the speed of a gravity-powered vehicle like a Hot Wheels car. If two cars are identical except for mass, the one with greater mass has more potential energy, if both cars are at the same starting height.

But your results speak for themselves; the dimpled car out-performed the un-dimpled one, in spite of its lower mass.

If aerodynamics makes a big difference in a Hot Wheels’ performance, I’m wondering if I should be looking for exceptionally aerodynamic models when I purchase them in stores, if I’m after performance?

And yes, if you haven’t guessed, I’m a Rocket Scientist. Or at least, that’s what I tell the neighborhood kids that play with my Hot Wheels. It’s almost true, since I work at NASA.

— Charlie