Episode 154: Reverse Engineering

Air Date: November 17, 2010

Some 1970s sportscars were so badly designed that they would be more aerodynamic if the body were turned backward on the frame.

busted

Adam and Jamie first placed a small-scale car model in a water chamber to visualize the airflow around it. Whether it faced forward or reverse, they saw a large area of turbulence above the trailing end. Next, they measured the amount of drag force on the model when placed in a wind tunnel to gauge its aerodynamic character. The results were 0.34 pounds of drag forward and 0.37 pounds of drag in reverse.

In full-scale testing, Adam drove a typical 1970s sportscar through three tests: a timed quarter-mile, time to accelerate from 0 to 60 mph (0 to 97 km/h), and fuel efficiency for a 1 mi (1.6 km) course at 50 mph (80 km/h). The car’s body was then removed, turned around, and reattached to the chassis, and Adam performed the same three tests. The forward car yielded averages of 14.0 seconds in the quarter-mile, 8.0 seconds in acceleration, and 0.87 lb (394.63 g) of fuel used. In reverse, the corresponding results were 16.6 seconds, 8.66 seconds, and 1.25 lb (566.99 g) of fuel. Finally, they brought the cars up to full speed – Adam in the reversed one, Jamie in an unmodified one – then put them into neutral and let them coast to the finish line. Jamie finished the race first, leading him and Adam to conclude that the cars were more aerodynamic as designed.

If a car with a surfboard on its roof gets into a 40 mph (64 km/h) collision and stops dead, the board will be thrown ahead with enough force to punch through another car’s windshield and kill the driver.

busted

The Build Team set up a full-scale test, towing a car with a board (“board car”) into a parked vehicle (“crash car”) set in front of a crash barricade to stop it dead on impact. A third vehicle (“target car”) was placed 40 ft (12.2 m) behind the crash car. Three attempts to replicate the movie crash failed, with the board pivoting sharply down and/or getting caught in the target car’s wreckage after flying off the roof.

Small-scale tests in the water chamber revealed that the board would not generate enough lift force to keep itself on a level path, leading the team to focus simply on the impact of the board against the windshield. Tory built a rig to launch it straight ahead, using bungee cords to provide the needed force; once it was properly calibrated, the team set a target vehicle directly in front and put Buster (outfitted with a human-analog neck) in the driver’s seat. The board only glanced off at 40 mph (64 km/h), while at 85 mph (137 km/h) it penetrated partway but was stopped by the safety glass in the windshield. Having failed to replicate either the movie crash or its results, the team declared the myth busted.

(This myth is based on a scene in the film Lethal Weapon 2.)

16 Comments

  1. bob reinhardt says:

    gmc suburban 4×4 6600lb broken in half from jumping four ft jumps is it possible for that to happen I have suburban just wondering if you be interested in experimenting with the idea of breaking it in half this is probably not the right place to ask but are you interested

    • Fatso says:

      I’d be interested in the myth of whether or not punctuation exists.

  2. Stanley Penkala says:

    Reverse Engineering comments:
    The testing as performed is biased against the ‘reversed’ car, because all of the aerodynamic smoothing material under the chassis and around the wheels of the original car was removed in switching the body.
    As noted on the show, water tank flows forward and reversed were fairly similar over the top and around a model car that essentially had no flow-trapping difference to the under chassis area.
    In the show, the air dams and wheel well liners were stripped out in removing the car body. Their weight was restored in the modified car, but it does not appear that any attempt was made to ‘restreamline’ the underbody area.
    I’m not saying the reverse car would end up smoother, but I don’t think it would be as bad as your testing indicated.
    Since replacing air dams, wheel wells, etc. is impractical, I would strip out those components from the original car before establishing baseline conditions, so that the airflow under the car’s chassis is essentially the same in both forward and reversed body configurations.

    Love the show.

    • Keith Rickman says:

      I agree with Stanley but think the major difference in the results was due to less traction on the reversed car due to the weight being over the wrong wheels.
      Why was the test not done by removing the car differential and turning it over which would make all the forward gears work as reverse gears. It would take a good driver but then there is no difference in the car. Also love the show

  3. chandan says:

    Hi Team,

    I am just curious to know about the see through x-ray glasses. Is there any such thing like one we have seen in james bond movies :-). Please help me understand

  4. Andrew says:

    Hi Mythbusters. Just had a thought about the “Lethal Weapon” myth. In the clip you show, the car that is hit by the surfboard is a what appears to be a Ford Bronco and more importantly the glass breaks with the telltale shards and pieces of tempered glass. In all the tests, modern glass was used, as is pointed out by Grant. The glass did it’s job by not breaking and flexing. However, would the results be any different if tempered glass, like in the movie was used.

    • Dave says:

      Likely the glass in the movie was ‘sugar glass’ – as having real shards of glass flying about film sets ain’t so safe! It also smashes particularly easily, as you’ll know from all the bottling and window diving moments from countless movies..

  5. Scott says:

    I noticed that the tires were spun longer on the start in the reversed position, this coupled with what Stanley said will affect the result.

  6. frank leon says:

    Ijust wanted to tell you guys that Ireally love your show.keep up the good work guys.

  7. Phiza says:

    If you look closely at the slow mo replay the motion of the surfboard through the windshield in the final test was not halted by the windshield itself but by part of the trigger mechanism on the surfboard hitting the front edge of the roof of the car.

  8. Tom Murphy says:

    Did anyone else notice it was a windsurfer board on the car in the movie (there was a sail one the roofrack also) not a surfboard at all. It did not have a turned down nose and would have been a lot heavier that a surfboard.

  9. Ryan Winborn says:

    Mythbusters,
    I love your show. Honestly you would produce a few shows a year if you we’re required to do a FEMA
    failur modes and affects audit on every myth. Basically I am saying it is not realistic to think tank every myth.
    The whole crew does a great job of getting most of the
    Issues addressed. The 9## sport car episode disappointed me a bit. The removal of specific under penting components and inner fenders was necessary and reviewed. I honestly think the addition of free weights to compensate for lost weight was a feeble attempt for a group as technically minded as yours.
    The loss in aerodynamics from removing these items has a huge affect on wind drag. Specific German auto companies have ha extensive experience with air cooled mid-mounted engines which require an undercarriage specific to direct cooling air to the correct areas. They have I figured out.
    A chin spoiler on some sports models mounted at the from most lower edge of the front bumper cover at 90 degrees to the road surface will reduce the coefficient of drag .01.

  10. John says:

    After taking a quick look at the math, I must say I think there may have been other forces at play in the results of the “Reverse Engineering” myth tests.
    The difference in drag coefficients in the small scale tests was roughly nine percent in favor of the forward facing body. No problems there. However, although the 0-60 test reflected this nine percent difference, the quarter mile time was a difference of approximately 19 percent, over double. And they recorded an increase of fuel usage of almost 44 percent!
    With all the major mechanical work that was required to pull off the feat of turning the body around, I think it would be very easy to see that perhaps the drivetrain may have been damaged or altered in such a way that would hurt the car’s performance. The 44 percent difference in mileage is too significant to ignore especially when considering the much smaller change in drag.
    The tests may also have been faulty. Even with a small amount of fuel to measure, a one-mile loop with stops leaves far too much room for error in the driving style, regardless of how hard they tried to drive the same way. A more appropriate test would be both cars, side by side, same speed, down a longer stretch of road, say twenty miles. This would reflect the aerodynamics of the car more accurately than start-stop short distance tests would.
    I am also left wondering how they know these two cars are the same mechanically. Let’s face it, the 928 is roughly thirty years old, and expecting two cars to have aged identically through three decades is naive at best. Perhaps one had lost more horsepower than the other. Perhaps suspension and drivetrain parts were more worn or had more resistance than the other. These would easily account for the 0-60 and quarter mile time differences. The only way to know the initial differences in the cars before even starting to test aerodynamics would be to use a wheel dyno to compare horsepower and drivetrain loss numbers.
    Along the lines of worn out parts, the final test, while the most accurate, could have been skewed as well. Worn out wheel bearings, different brand/model/treadwear of tires and differences in tire pressures all have a significant effect on the overall resistance to motion a car has. The way to counter this would be to install new wheel bearings and tires to both cars to ensure they are to the same specs and in the same condition.
    And finally, as I have read in other comments, the underbody aerodynamics play a huge role as well. The myth was to test if the body of the car was more aerodynamic going the opposite direction of the way it was designed to go. Now the mythbusters did a phenomenal job of remounting the body the what appears to be the correct ride height, body gaps look godd, etc. However, if Porsche were to have released the car themselves, they would have never released it without fender liners, underbody aeros, etc. No manufacturer would.
    Mythbusters is a fantastic and intelligent show, but I feel like someone slacked on this episode pretty hard. Would love to see them do a retake on this one!

  11. Valery says:

    Four boys. What I love about it: recycling cehotls and toys. (Don’t know what I would have done if the last one had been a girl!) What I like about it: Don’t have to directly deal with the increasingly early sexualization of girls (skimpy cehotls, provacative behavior, etc.). And having four of one sex has allowed me to specialize. I probably know a lot more about boys and their problems and tendencies than I would if I’d only had one or two.What I don’t like about it: Incredibly stupid and ignorant people who ask, So, you gonna keep trying for a girl?

  12. Trina says:

    To avoid those US data roaming rates, you can use WiFi to hop on the net when you need it. Depending where you’re tnaevllirg in the US, finding a Starbucks can be much easier than an open WiFi signal. With a Starbucks registered card, access to WiFi is free in their stores. You can register your (free) card in Canada and it will work there. A note of caution: making a connection is not as simple a process as one would hope. Give yourself some time the first time you try this in the US.

  13. Matthew Goodwin says:

    I’ve just watched the surfboard slow mo and it does look like the board is stopped by the launch hook. Think that might need a revisit.

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