Mythbusters 10/29: Indiana Jones & Jailbreaks

[Christopher]

Indiana Jones motorcycle flip: This is a cool one, and the results weren't very surprising. When the bike was up to speed, sticking in a wooden flagpole just broke the pole. And if you look closely at the actual Last Crusade footage, that's exactly what you see in the shot of the pole going through the wheel: it breaks in two and the pieces begin to fly apart. But then we cut to the long shot of the bike flipping, and from the smoke accompanying the flip, it was immediately evident that a mortar was used to set it off.

Still, it did seem borderline plausible to me that it could work in theory -- that maybe once the wheel was stopped, the angular momentum would be transferred to the bike and cause it to flip forward. Now that I think about it, though, I see that there's no way the comparatively light wheel could have enough angular momentum to flip something as massive as a motorcycle.

Cool job recreating the shot at the end, though.

Prison breaks: Fun idea, and cool location. Poor Grant, though -- he should've thought to wear gloves. Kari had the good sense to do that, and I was expecting Tory to do so as well, but he didn't. It seemed like he might've had chalk on his hands, but that might've been just flecks of the paper rubbing off. Anyway, it's remarkable that all three methods worked. It's a testament to the power of braiding as a strength-increasing technique. No matter how weak the basic material, if you can braid it well enough, it adds up to a lot of strength.

Why did they think the hair was disgusting? It's no different from the hair on their own heads.

 

[Trekker4747]

Finishing watching it now.

I thought they were doing an entire IJ themed episode like they've done with Bond and McGyver,

Oh well. Interesting episode.

When I was a kid I was ridding my bike down a hill. My feet flew off the pedals and I couldn't get them back on because of how wildly they were moving ("kids" bike with pedal brakes). So, naturally, I deicded to put my feet on the front wheel to stop the bike.

I fliped head-over-heels over the handlebar and landed face-first into the street which has just been cheaply "sealed" with the old tar and gravel method.

I would think with enough momentum and the right conditions the same could happen with a motorbike, but it does have a lot more weight to toss.

 

[Christopher]

^^Well, it's a different situation in a number of ways:

1) A child's mass is much lower than a motorcycle's.

2) Your center of mass was higher relative to the wheel's hub, making you easier to flip.

3) Since your feet were in contact with the wheel rim, there was friction between the wheel and your body, so its forward rotation pulled you forward with it. The motorcycle's front wheel was spinning freely relative to the bike, so there was no equivalent "pull."

4) Most importantly, your bike wheel was actually spinning at the time. The myth here involves what happens to the motorcycle when its front wheel abruptly stops spinning. So it's the opposite situation.


But in any case, ow. I can sympathize. I've flown forward off a bike myself; I think what happened was that I braked the front wheel too sharply just as it hit a depression alongside a manhole cover in the sidewalk, something like that, which served as a pivot point to tilt the bike forward (since the brake shoes provided the necessary friction with the still-moving wheel) and toppled me forward onto the pavement. I had a helmet on, but it didn't protect my chin, and I got a pretty bad scrape and was bleeding. As it happened, I was across the street from a hospital at the time, so I went to the emergency room just to make sure I hadn't done any more damage.

 

[Trekker4747]

No helmet for me. Back in the day we didn't worry about such things! I ended up OK, no rusing me to the hospital or anything, some scrapes and impressions in my head from the "gravel" but otherwise I was fine and got right back on the bike.

Your explanations makes sense on the differences between the two situations.

I, too, thought their "disgust" over the hair was odd. I mean, it's just hair and certainly it had been/could be cleaned!

 

[Zachary Smith]

Sorry, but I have to disagree. I was riding my 10 speed bicycle to work when I was 17 years old--weighed about 185 LBS at the time. Work clothes were in a plastic bag hanging from the handle bars. I'd done this God knows how many times but for some reason today the plastic bag swung around and tangled in the spokes of the front tire. Tire stopped DEAD. I went up and over the handle bars and landed on my face, breaking my nose and getting a case of "road rash" on my face. OUCH!

The bike was pretty jacked up also. Bent spokes, bent front Y-frame. It was a heavy 1960's bike too--this happened around 1980 and I'd gotten the bike used about 5 years earlier and it was old then. I remember as I went up and over, the body of the bike came up with me and I considered trying to hang on to the handle bars and drop back onto the seat if the bike went back down. At the last instant I decided to tuck my head and try to roll with the fall. As I tried to do so, I managed to slam my face RIGHT into the pavement. It was a bad day . . .

 

[Roger Wilco]

^^^
Yeah, I very strongly feel, that the Mythbusters screwed up the testing of that particular myth; everybody who has ever ridden a bicycle knows for example, that using the front wheel brakes as an emergency brake is a very bad idea. It is very easy to block the wheel on a bicycle, which essentially is what happens when you put a stick in the spokes, like they did in this myth. (and I think the weight allocation on a normal bicycle is similar to the cross-country bike they used in that myth). And when that happens at a sufficient speed, you go flying over the handle in no time. I'm not only surprised that they even bothered testing something that obvious, but that they got the wrong result too.

I think the crucial mistake might have been that the dummy biker didn't have a grip on the handle; in other words, he didn't try to keep the bike in lane like any human biker would do in such a situation. That's the reason the bike just flipped over, going the path of lowest resistance instead of continuing its forward motion.

And also contrary to the Mythbusters' expectation, I would have been positively shocked if the hair-rope wouldn't have worked, I was ready to complain about Kari's lack of rope manufacturing ability already (I wasn't sure about the toilet paper though).

 

[Christopher]

Sorry, but I have to disagree. I was riding my 10 speed bicycle to work when I was 17 years old--weighed about 185 LBS at the time. Work clothes were in a plastic bag hanging from the handle bars. I'd done this God knows how many times but for some reason today the plastic bag swung around and tangled in the spokes of the front tire. Tire stopped DEAD. I went up and over the handle bars and landed on my face, breaking my nose and getting a case of "road rash" on my face. OUCH!
...I remember as I went up and over, the body of the bike came up with me and I considered trying to hang on to the handle bars and drop back onto the seat if the bike went back down.

Remember what I said above -- your center of mass was above the hub of the wheel. That means the center of mass of the system [you + bike frame] was above the pivot point. So the forward momentum of that system translated into a rotation around the pivot point. But that just caused you and the bike frame to rotate forward around the stationary wheel. It didn't cause the whole bike, wheel and all, to jump up 20 feet into the air.

And as I also said already, the frame of a bicycle is much lighter than the body of a motorcycle. The heaviest part of the system is you, the rider. A motorcycle is heavier, so the center of mass of the system is lower down, making that kind of forward flip of the cycle's body less likely.

There's also the relative speed and the nature of the obstruction to consider. In your case, in my case, the front wheel actually did stop dead. In the movie scene, that was pure fiction; putting a wooden flagpole in the wheel of a motorcycle travelling at 40-50 mph would cause a temporary, minor disruption in its rotation.

Also, Buster was wired onto the bike to stay in place. Otherwise, if he'd just been sitting loose on the bike (and somehow been able to stay upright), he would've been flung forward when the bike stopped, just like you and I were. That's just Newtonian physics. But that's still a totally different situation from the upward flip that the movie showed. What happened to you, me, and Shrekker4747 happened because of our forward momentum and the effects of leverage. There wasn't enough upward momentum to send us flying straight up like in the movie, or we might not be here having this conversation.

The myth that was busted wasn't that obstructing the front wheel will cause a crash, because obviously it will. The myth was that it would launch a bike vertically. And that was conclusively busted, and is clearly impossible in terms of basic physics.

 

[Scout101]

Problem with thinking about bikes and motorcycles the same way is that they just don't work the same way. With a bike, you use the rear wheel to slow down and stop, and any kid knows that if you use the front one, you're screwed.

With a motorcycle, you DO use the front brake, almost exclusively. There IS a rear brake, but it's by your right foot, and you mostly only use it at a red light or when braking in a real emergency. Something like 20% of the braking power in the rear, and the other 80% of the braking power is in the front.

The front of the motorcycle is just a big shock, with the fork tubes absorbing the difference as the bike 'dives' forward because of the brakes. It's absorbing the momentum that sends you ass over teakettle when you're on your BMX instead...

 

[Christopher]

Problem with thinking about bikes and motorcycles the same way is that they just don't work the same way. With a bike, you use the rear wheel to slow down and stop, and any kid knows that if you use the front one, you're screwed.

Well, I didn't know that when I had my tumble, and I think I was in my early 30s at the time. I kinda figured it out right afterward, though.

The front of the motorcycle is just a big shock, with the fork tubes absorbing the difference as the bike 'dives' forward because of the brakes. It's absorbing the momentum that sends you ass over teakettle when you're on your BMX instead...

Ahh, that explains it. So it wouldn't pivot forward even if the circumstances of the bicycle accidents were recreated. Cool, thanks for the info.