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Episode 80: Big Rig Myths

  • An exploding big rig tire can decapitate a person: confirmed
  • You can drive up into a big rig trailer (i.e. Knight Rider): confirmed
  • Drafting a big rig saves gas: confirmed
  • Drafting a big rig with on a bike saves energy: plausible

The MythBusters tested three big-rig-related myths. The exploding tire myth allowed Adam and Jamie to build a deadly rubber chucker. The Knight Rider myth was a fun confirmation that you, too, could someday have a mobile garage to deploy you on the highway. There wasn't much mythbusting in the drafting myth: NASCAR drivers and bikers have take advantage of the draft regularly. The main contribution the MythBusters offered was demonstrating the advantage for their viewers -- they showed up to 39% improvement if you're crazy enough to risk your life at extremely dangerous following distances.

Exploding Tire of Death

Myth: A tire blowout from a big rig can decapitate a person in an adjacent vehicle


They couldn't find anyone to let them do a tire blowout on top of their dynamometer, so they decided to build one of their own out in a field. They chopped the bed off of a pickup truck and sunk it into the ground. The big rig was lifted so that it's rear tires were on top of the pickup truck's tires. This allowed them to spin the wheels of the big rig by accelerating the pickup truck, while a forklift allowed them to control how much weight the big rig placed on the pickup.

Buster was setup on a motorcycle adjacent to the tire.

Big rig tests

  • Shotgun test: They used a .12 gauge shotgun with a deer slug to blow out the big rig tire. Jamie hit the tire, but they didn't get the catastrophic tire failure that they wanted -- all they got was a "hisssss."
  • Flexing test: They tried to use a combination of high heat and low pressure to get the tire to blowout. "Flexing" is responsible for most big rig highway blowouts: the low pressure and high heat make the rubber wobble. They inflated the tire to 30psi (1/4 normal pressure) and heated the tire. They were able to easily get the tire to 150 degrees, but they couldn't reach the 350 degrees failure zone. To get the tire to burst, they hooked it up to a pump and tried to suddenly over-pressurize it. The over-pressurization gave Buster a gust of wind as a hole ripped opened up in the sidewall, but not much more.
  • Shrapnel test: They weren't going to get the catastrophic failure they wanted, so they decided to spin up the already popped tire up to speed to see what would happen. Rubber shrapnel and smoke came flying off in every direction, except the direction of Buster. They did get valuable data, though, on the rubber shrapnel.

Rubber chucker test

They had trouble controlling where the rubber flew in their tests, so they decided to build their own rubber chucker to fling the chunks of rubber at Buster's head. They had video of the flying debris from the previous test, so they could tune the size and speed of the chunks to be accurate.

The rubber chucker was actually very similar to their previous test rig. They placed the pickup truck on top of a trailer, wheel to wheel. Much like a pitching machine in a batting cage or on a tennis court, the spinning wheels grab anything placed between them and fling them at the target. In a test fling, they easily shot a piece of rubber at 40mph through a wooden board.

They replaced buster with a ballistics gel head that they cast around a spine. The head was stuck behind a car door window in the path of the rubber chucker. In the first shot the ballistics gel head was decapitated -- the spine they stuck inside was sticking out.


Knight Rider Garage

Myth: You can drive into the back of a moving big rig, just like in Knight Rider.

The main fear is that if you are driving at speed, as soon as you hit the ramp of the truck, you continue to drive at speed through the trailer of the truck.

Small-scale test

They drove a small R/C car backwards into a minature pickup bed on a treadmill. Instead of suddenly accelerating when it hit the big rig ramp, the car gracefully slid up into the trailer. Their conclusion was that the inertia of the vehicle will prevent it from accelerating too fast.

Full-scale test

With a rollcage, 5-point safety harness, and a helmet to protect him, Adam took on the task of driving a car into the trailer of a big rig. As Adam and stunt driver Mike Ryan drove along side to observe and coach, Adam attempted the test at 35mph and 55mph:

  • 35mph: Adam had no trouble rolling up into the big rig trailer. As his wheels hit the ramp, they slowed down and allowed the car to ease into the trailer.
  • 55mph: with rain everywhere, Adam still had no trouble parking the car in the big rig.
  • backwards: Jamie took over and backed the car out of the trailer


Drafting a Big Rig

Myth: Drafting a big rig saves fuel

They emphasized again and again how dangerous drafting a big rig is: ~3/4 of truck/car accidents are caused by person driving the car and you're driving in the blind spot of the truck. 150ft is minimum recommended following distance at 55mph, so even the 100ft test is considered dangerous.

Small-scale test

NASA let them do a small scale test to study the aerodynamics of a big rig. They captured video of smoke travelling over a minature big rig and verified that there is a low pressure area behind. They then stuck a minature car to a force gauge to study the difference with and without drafting.

  • 7 car lengths: 21% drag reduction
  • 10ft: 60%
  • 6ft: 80%
  • 2ft: 93%

Full-scale test

Freightliner lent the MythBusters one of their new Cascadia big rigs, which they are billing as the most aerodynamic big rig on the market.

Mike Ryan, Hollywood stunt driver, was there to educate Grant on the ins and outs of drafting. Also on-hand was Andrew Smith, test engineer, who helped them hook up a computer to the fuel injection system to accurately measure the fuel consumption.

  • 55mph control: 32mpg
  • 100ft: 35.5mpg, 11% improvement
  • 50ft: 38.5mpg, 20%
  • 20ft: 40.5mpg, 27%
  • 10ft: 44.5mpg, 39%
  • 2ft: 41mpg, 29%

The fuel economy actually dropped at 2ft. Andrew Smith's theory was that at 2ft, Grant got nervous with the throttle as it was difficult to maintain that 2ft gap.


Bonus myth: Drafting with a bicycle

Online bonus myth clip

They setup Tory with a heart rate monitor and had him peddle twice down the road at 20mph. The first time way by himself, the second time he got to draft the big rig. Tory was able to coast behind the big rig, barely pedaling, whereas his heart rate averaged 166 without the help of a draft.



about the beheading with the truck tire One why would you be sideways behind a truck wouldn't it hit the wind sheild and not the side window
As for the death ray do you suppose they had the knowledge to make a magnafieing glass and used that to focus the bean from the sheilds I know they had crystal back then But if Archameaties could have made on to catch the sun bouncing off the polished sheilds couldn't that work???

I thought the thing about the car being sideways behind the big rig was bogus too.
Fortunately I think that even if you ARE unfortunate enough for a semi to lose a tire right in front of you, the tire fragments are most likely going to deflect off of your windshield.
If you're beside it, however, you may have to react quickly as the truck could end up in your lane (I've seen it happen).

I didn't watch this episode, but I thought all of these were givens.

Exploding tires have been killing people for years (ask any tire shop), though the odds are very unlikely if you're following the vehicle, since you've generally got a good bit of protection (either a laminated windshield--aka bulletproof glass sans a few layers--or a helmet).

The driving into a trailer is extremely obvious; if you've ever driven from dirt/gravel onto asphalt with the tires spinning without defying physics and launching yourself forwards 50 feet, you've proven this one true.

Drafting is a duh; however, I constantly see the mistake of "drafting a semi", when in fact you could draft a cheetah for mild gains. I know for a fact that two sports cars drafting each other from Albequerque, NM to Tucson, AZ can increase gas mileage by around 10%. I find it interesting that they didn't try 0 ft though; it's not hard at all, and should net you better gains.

Also, everybody talks about the risk from the truck driver not seeing you. This is bogus. The only danger is the limited reaction time from following too closely (I won't argue that this isn't a danger, though it's less dangerous than following something else, like a sports car). The place where it's important for the driver to see you is when you're *beside* him, because then he can actually hurt you. When you're behind him, any car with brakes in decent repair should easily outstop a truck, so even if he tried to hit you, he should fail (unless it's a problem of reaction time).

I don't understand how they consider bicycle drafting merely plausible. This is how all of the human-powered 200+ mph runs I've ever seen got up to speed.

Tailgating accidents always involve reaction time.

So its ridiculous to say tailgating isn't dangerous. The larger the vehicle being tailgated gives the tailgater less visibility to the road ahead. The main factor that makes it dangerous is increased reaction time of the tailgater to hit the brake. With all the negative influences in vehicles (radio, cell phones, conversations between passengers, two-way paging/texting) that can affect the tailgating driver, its obvious that they wouldn't be staring(in anticipation of braking) at the brake lights of the vehicle ahead. Furthermore, after a while of tailgating, a tailgater's comfort level tends to increases and they will be further distracted by other things. Thus increasing braking time.

When driving for more than a few minutes at a constant speed, most drivers don't continue to hover their left foot over the brake pedal. This also increases reaction time when braking.

Most big rigs/trailers tend to drive with enough stopping room ahead of them based on what they see ahead and the amount of visibly clear road, but when smaller vehicles cut them off or emergency issues arise then all they can do is hit the brake.

If the rig happens to stop due to another on coming big rig in its way(head on) both going the same speed, then the tailgating car would be severely impacted due to it hitting a brick wall at full speed (provided direct head-on impact at the same speed both rigs would come to a quick stop with alot of head on damage).

Big rig drivers will sue tailgaters for financial damages for their damages and delays to their job/business when involved in rear end collisions.

I wouldn't want to tailgate just to save 10%, 20%, 30% of my gas for whatever amount of time directly behind the rig. It pennies compared to the cost and risk involved.

Cars stop a lot fast than trucks. The recommended stopping distance used to be 1 car length/10 mph.

as far as the myth of the truck tire decapitating its not just the rubber of the tire. its not common now but the common truck tires were on split rims that the lock ring could go flying as well and can be deadlier than the rubber tire chunks