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Mythbusters Johnny Lightning Experiment

Created by dave. Last edited by dave, 13 years and 109 days ago. Viewed 14,812 times. #2
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Mythbusters Johnny Lightning Experiment

Now I'm a big fan of Mythbusters. When it comes to blowing up cement mixer trucks, I'll defer to their expertise every time. But occasionally they will hit on a topic where I might know something about the subject, or even one which I might have done some of my own research.

This week the re-run Mythbusters episode included the myth that a certain brand of toy car was faster down a quarter mile than the real thing. The Mythbusters interpreted this to mean they needed to find a quarter-mile incline, set up some track for the toy car, get a real car, and race both cars down the hill under the influence of gravity.

After watching the episode I had a number of problems with the experiments as presented, most notably in the area of the toy car's track dynamics; in the past I have >>expressed an interest in such things.

Note that while the car used in the episode was the Johnny Lightning series, my comments here will mostly be about Hot Wheels, since that is what I have the most experience with.

The Lab Tests

First things first: you are not permitted to go out and play in the field before doing your labwork. The Mythbusters indulged in a couple of laboratory experiments, one designed to test the wheels of the toy, and the second to measure air resistance.

The rolling test was interesting, designed to see if the wheels of the toy car would come off at high speeds. There were a couple of cracks at this, the last (and deemed successful) involved holding the car in a shoe placed over a spinning tyre on a road car. They spun the car up to 85 miles per hour and declared it a success.

There are a couple of problems with the design of this last test:

  • the shoe was designed so that the rear wheels were protected by the shoe. If the rear axle was going to fail, the shoe would keep the wheel more or less located about the axle, possibly dynamically repairing the failure.
  • quibble: while the Datsun 280Z that the Mythbusters used clearly showed in excess of 80 miles per hour on the speedometer, I wonder how fast the wheels were going. As the car has a rear differential, you can know for certain that the transmission (which is where the spedometer is connected) is going the indicated speed; and you know that the _average_ speed of the two rear wheels is the indicated speed; however, there was no attempt to see what the other wheel on the car was doing.
  • in judging the experiment a success, the Mythbusters claimed that there was no visible damage to the car's wheels. The failure point for these cars is most likely to be at the axle/wheel interface, or the axle/base interface, as that is where the most motion is likely to happen. And you can't really inspect those without pulling the whole thing apart both before and after the test in order to compare them.
The second test was designed to see what the maximum windspeed the cars would sustain. There are many problems with this test.
  • three words for you: _rolling road windtunnel_. Conducting the test with a static "road" surface changes the dynamics of the airflow going around and under the car. Similarly, the small diameter of the tube meant that the surfaces of the tube were always going to interfere with windflow. Also one might wonder how "clean" the air being forced into the tunnel is or if it has a bunch of embedded vortexes in it which will cause the car to bounce.
  • the angle of the windtunnel was much steeper than the later tests (including, most importantly, the actual run on the road). Since the only thing tugging on the cars is Sir Issac Newton, the steeper the windtunnel, the more force the cars are being pulled down with, the higher speed wind the cars will tollerate. To wit: place any toy car on a level track, and blow on it; unless your car is extremely unfriendly to the track, you'll push it backwards with much less effort than the 70 to 80 mph claimed in the test.
  • I'm not even sure how blowing air at the cars actually proves anything. Airflow resistance is going to be some function of mass (heavier cars get pulled down harder), tunnel angle (steeper angles mean the cars get pulled down harder) , the toy's drag co-efficient (more drag means the car is more likely to get blown out), and airspeed in the tunnel (higher speed == more drag); the biggest factors are going to be the mass of the toy and the angle of the tunnel. I'd have to hear an argument and maybe see some math.
Track Basics

Next our heros set up a section of track in the lab and raced some cars down it. Again, there is the issue of the incline of the ramp in the lab vs. the actual incline in the field.

So before we go any further, we need to ask: what makes a car go well on the track? As you may have seen in my linked article, I identify five areas of concern which may impact the quality of the car's run on the track:

  • The mass (or perhaps more specifically, the density) of the car. All things being equal, a more massive car will have more inertia with which to fight the forces of drag which will build up in the axles and wind resistance and track-border resistance. This is true in many of the gravity-powered competition cars; I recall my Kub Kar days where the winners were cars which had been hollowed out and filled with dense material. The most blatant examples of this "massing up" had several large metalic bolts in the front and rear which were allegedly "tail pipes" or "headlights" (yah right).
  • The width of the car on the track (and by extention, the length of the car). Wider (and longer) is better because it reduces the maximum angle that a misdirected car will impact the edge of the track. This reduces the force imparted into the car during the redirection (ie when it bounces off the track), which both reduces the amount of drag the redirection imparts on the car, and reduces the possiblity that the car will ride up the edge of the track and bounce away out of play.
  • Front and/or Rear Overhang. Cars with insufficent clearance under the front or rear of the toy will drag on the surfaces of loops and/or "turbo curves", resulting in a dramatic loss of speed.
  • Track Width (by which I mean the automotive track width, the distance between the wheels on the car). Again, wider is better. Many of the cars I have which have decorative fenders covering part of the wheel have shortened axles to acommodate a shorter track; this inevitably results in an axle which will wobble more than a wider axle. This is possibly some function of the distance between the axle mounts on the car's base; a wider track means more distance between the axle mounts, reducing the axle's tendancy to wobble due to manufacturing variances.
  • Roll Quality. The sad fact is that not all cars roll equally well. Possibly due to some manufacturing flaw (casting or assembly), some wheels do not rotate freely or are slighly off due to a bent axle. I have some examples of multiple copies of the same car (same manufacturing year and everything) which vary hugely in terms of roll quality.
From these factors we can perhaps look at an idealized rolling toy.
  • it has good mass
  • it has a good width and length
  • it has a fender design which minimizes resistance against the track
  • it has a wide wheel track
  • it has good roll quality
I'd like to briefly talk about the third quality of our idealized rolling toy: good fender design which minimizes resistance against the track. This is a variation of the front/rear overhang observation. In our idealized rolling toy, the track resistance is going to come from being bounced back and forth from side to side on the track (tyre/track rolling resistance is going to be practically non-existant). So to idealize the rolling, we have to minimize the resistance somehow. The best way is to ensure that the toy only goes down the center of the track and never interacts with either side of the track. This is extremely unlikely over any distance greater than about 6 inches. Therefore, we should try to minimize the resistance when the car encounters one side or another.

If you look closely at the picture of the Olds 442, you will notice that the wheels on the car are not cylinders -- they are truncated cones. The car only actually contacts the track at a extremely thin point for each wheel. (This, incidentally, is why tyre/track rolling resistance is practically non-existant). It also means that the outside edge of the tyre is raised. If the bodywork of the car is designed such that the outer edge of the tyre is going to contact the track during the interactions, the fact that this part of the tyre is rolling with the car with approximately the same speed relation as the track will minimize the drag experienced. The edge of the tyre will "roll over" the edge of the track much better than the fender of the car will.

How The Custom Cars Stacked Up To The Theory

So how did Jamie and Adam's custom cars stack up to this criteria?

Adam's car reflects the first rule of competitive gravity-driven cars, that being mass rules. His car has good length and width, good track, and I presume at least fair roll quality.

I'd almost suspect that Jamie did some playing with the cars before making the design decisions he did, since his car fits very well into our categories. His wheel design is a bit more advanced than the simplified wheels that Adam used. And while I suspect his body design was inspired by aerodynamic concerns, I don't think that airflow management is important on such a small scale.

Head to head in the lab, Adam's car smoked the competition. This is because in gravity games, Mass Rules. However in the field, the results were different, for reasons which were entirely predictable in hindsight...

Out In The Field

So out in Taho, our heros have assembled a quarter mile (1300+ feet) of this blue dual-lane race track. Once assembled, they quickly run into a couple of problems:

  • the heat of the day is making the track get longer, causing it to buckle
  • the toy cars won't make it the whole way to the bottom of the hill
There's not much you can do about the first problem; plastic is going to expand and you have to deal with it.

The second problem is caused mostly by the connections between track segments. If they don't line up (or if the connection is counter to the flow of the car) then you are risking a collision between the car and the end of the track segment. At low speeds this is merely going to be a high amount of speed-reducing friction; at high speeds, the energies involed are more likely to influence the car's trajectory into the air and change the car's orientation (ie cause the car to tumble out of control). Now any eight-year-old who's played with track for more than about fifteen minutes knows this, although he doesn't necessarilly know it in those terms; all he knows is that at a certain speed, the car is more likely to leave the track at certain locations.

Adam's custom car clearly suffered from this as both test runs we saw on TV resulted in the car leaving the track at the same point. Interestingly enough, the front fender design of Jamie's car appears to compensate for this effect -- instead of forcing a collision between a square front edge and a square track end, the rounded front deflects the car back onto the track, minimizing the effect of the collision. I think that this single point is probably the reason why Jamie's car was so successful overall.

The effect on the toy car from these junctions would be similar to placing 8-inch high, three-feet long speed bumps in the path of the viper; something which would probably retard the car's progress down the hill somewhat. And actually the low front end of the viper would make it unsuitable for such a run; you'd need something like my Legacy just to clear the speed bumps. Talk about "some cars not suitable for some sets!"

Clearly we can talk about an idealized track in the same way that we can talk about our idealized rolling toy. Such a track:

  • would be resistant to side-to-side and up-and-down motion;
  • would not have any joins;
  • would be flat; and
  • would be straight.
The track used in the experiment approximated these requirements to varying degrees. The video shows that the track would flex back and forth when the car went down it; it had long segments (good) but the segments still had to be joined together (bad); it was as flat as the road surface (fair); and due to the side-to-side flexing, the "straightness" of the run was academic.

Adam gave Jamie some on-camera guff for straightening the track prior to some of the early runs, but a straighter track yields a faster run.

From all of this, we can see that in order to maximize the speed of our toy car, we need a car that is close to our idealized rolling toy; however the toy also needs to have an idealized track, or at least a design which can cope with the failings of the track. All things considered, Jamie's car matches these criteria the best.

What all of this means is that the toy car will accellerate faster due to a lower rolling-resistance at low speeds, but the limitations of the toy, and more specifically the track, will be the limiting factor on the top speed of the toy.

Other Notes

According to the page on Racing Grooves, the original idea for the episode revolved around the maximum possible speed of Hot Wheels cars. Unfortunately for whatever reason Mattel did not permit them to use Hot Wheels names or cars; however it does explain some of the labwork the team did before going out in the field. It also explains the build-your-own-car competition that Adam and Jamie indulged in.

I have seen on the Discovery Channel's message boards a comment saying that the Johnny Lightning cars are inferior to Hot Wheels cars; since I don't own any Johnny Lightning cars, I can't comment. However, the Hot Wheels snob in me believes it.

One quibble I read on the Discovery Channel message board revolved around the scale car's time only being applicable to the scale difference; ie that the toy car's time over 1/64th of the distance would be compared to the full-sized car's full distance. I think this silly and completely defeats the point of playing around with the toys. The object of the exercise isn't to rig the competition so that one side or the other will win; the object of the exercise is to play with some toy cars and have fun.

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