Mythbusters Matchbox car

[Xist]

Is there a sticky about why we say that golf ball dimples would not work on cars? I know that this was explained many times before I joined the site and many times since. It seems like we usually tell them to use the search function and then someone explains it anyway.

So, please excuse me for bringing it up.

Jserman28 mentioned it and I found a picture of the Mythbuster car.

And this:

Mythbusters golf ball car, Hot Wheels style | Redline Derby Racing

He took two identical Matchbox cars, weighed them, ran them ten times down his track, drilled 126 holes into the one that lost six races, weighed it again, added enough clay to the bottom to bring the weight back to where it was, and then raced them again.

The car with dimples, which had been slower, won 8/10 races.

I would not say this was "scientific," I imagine there were many problems. I wish that he had gotten the whole thing on video. Then he could have gotten times and could have said how much faster it was and we would have more reason to believe him.

I did a little searching regarding pinewood derby cars and found this:

Derby Talk - View topic - How come no one puts aerodynamic "fenders" on cars?

Some guy stating that the Reynold's Number for a pinewood derby car indicated dimples would not do anything.

And this:


He said that it went slower than he expected.

Someone said that pinewood derby cars went 8 MPH, someone else stated twelve, and a third person wrote thirteen, but everybody believed that the speed was far too low for aerodynamics to matter.

Except this guy:

Pinewood Derby Stories and Photos from Maximum Velocity: 2/20/11 - 2/27/11

He tried a technique discussed in a booklet that he read. Those pins are supposed to be turbulators. He did A-B testing with ten runs each:

With the turbulators, the car ran a 2.503 average time with a standard deviation of .003. Without the turbulators, the car ran 2.504 with a
standard deviation of .002. The one thousandth second difference is
less than the standard deviation, so it is not significant. The test
was re-ran with a similar result.

I have thought about Pinewood Derby since joining these forums and thought that we had a better idea of how to aerodynamically shape a car than most people, but then I remember that the fastest cars usually had the smallest frontal area.

Would dimples make an aerodynamic shape worse?

If I ever get to talk to my nephew again, I might mention this as a possible science project for his school. Mythbusters busted by ten-year-old boy!

[aerohead]

*1st,we have to qualify what we mean by aerodynamic,as any thing that moves through the air,or air moves over it is aerodynamic.
*If we're talking about streamlined full-size cars,then,dimples would hurt,as they would increase turbulence.
*For non-streamlined,full-size cars,dimples in the right places could conceivably lower drag by modifying the boundary layer to mitigate separation,by acting as a crude turbulator.
*But if you have a non-steamlined car and you want to modify the boundary layer to mitigate separation,there are structures which perform this task much better than dimples.
*A dimple is perfect on a golf ball since it's the only shape which always looks the same to the air no matter the ball's orientation in the flight path.
*On an automobile,the direction of travel for the body is 'fixed' (the car is not spinning) and therefore can use specialized architectures which would never work on a golf ball.
*On a car,you'd have a choice of nubs,trip strips,grooves,a variety of vortex-generators,mini chines,and sub-boundary layer thickness structures for boundary layer energizing.

[mcrews]

Originally Posted by aerohead

*1st*A dimple is perfect on a golf ball since it's the only shape which always looks the same to the air no matter the ball's orientation in the flight path.
*On an automobile,the direction of travel for the body is 'fixed' (the car is not spinning) and therefore can use specialized architectures which would never work on a golf ball.
.

Best layman explanation I have head on the golfball. WELL DONE

[Sven7]

I don't remember what the actual numbers would be, but the smaller the scale you get, the faster it has to go to get reliable results. I figure before you could get reliable results from a Matchbox car, it would break the sound barrier and melt the tires at their axles.

Interesting way to test it, but I really would not trust the results.

[Xist]

Originally Posted by Sven7

I figure before you could get reliable results from a Matchbox car, it would break the sound barrier and melt the tires at their axles.

If you can make it blow up, then it would be an appropriate tribute!

[aerohead]

Originally Posted by Sven7

I don't remember what the actual numbers would be, but the smaller the scale you get, the faster it has to go to get reliable results. I figure before you could get reliable results from a Matchbox car, it would break the sound barrier and melt the tires at their axles.

Interesting way to test it, but I really would not trust the results.

If a MATCHBOX car was 1-50th-scale,they'd require 50X a normal critical Reynolds number velocity of 1:1 scale,or,50X 20 mph,or 1,000 mph for the wind tunnel.
And then compressibility effects would ruin the whole thing.Only a pressurized tunnel could do the work.

[Xist]

Oh boy. I was just thinking that if I made a track five times as long, on a ramp, I could get a 65 MPH pinewood derby car, but it would not be scientific.

Just fun.

Like Mythbusters!

What if you laid a hundred feet of PVC pipe on a good hill, wide enough for the car to roll, sealed as long as you can measure the final speed before it hits a bean bag of bag's worth of cotton balls.

What kind of pressure would you need?

[UltArc]

Is that car coming or going, with the pins? Would the same car be faster if run backwards? As in, if the front is closer to us, it looks more like a dragster. If the rear is pointed to us, it is more like a Dymaxion.

Aerohead, wouldn't the air moving over it as it is stationary be considered aerostatics, rather than aerodynamics?

[Sven7]

All you need for Pinewood Derby is wheel alignment and powdered graphite in the axles.

As for making tiny toy cars go really fast, strap a CO2 cartridge or Estes rocket onto it. You still won't get reliable results, though!

[aerohead]

Originally Posted by Xist

Oh boy. I was just thinking that if I made a track five times as long, on a ramp, I could get a 65 MPH pinewood derby car, but it would not be scientific.

Just fun.

Like Mythbusters!

What if you laid a hundred feet of PVC pipe on a good hill, wide enough for the car to roll, sealed as long as you can measure the final speed before it hits a bean bag of bag's worth of cotton balls.

What kind of pressure would you need?

You'd have to build a road on the floor of the pipe to create the test section,and size the pipe such that the model was no more than 5% of the test section cross-sectional area.
And you have to know the rolling resistance for the tires/wheels beforehand.
As to the pressure,you'd want to talk to NASA or an organization like that.