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Old 06-07-2013, 09:38 AM   #1
DancnDude
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Mythbusters - "Motorcycle Water Ski" 6/5/13

I was not all that excited about this one when the episode started because it just seems like a variation of ones of the myths they've tried before with a car and actually running on water which were both busted easily. But I was pretty shocked that it actually worked

It was impressive seeing the stunt driver go over even the shorter parts of the lake but I'm really surprised they let Jamie go for the whole thing...and he went a long ways. That was crazy!
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Old 06-07-2013, 09:46 AM   #2
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But to me Jamie didn't get the answer to his own question. That is was the motorcycle staying on top of the water due to momentum or was the power from the wheel actually extending the time and distance the motorcycle could go?
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Old 06-07-2013, 09:49 AM   #3
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So glad you started this thread.

I was also blown away by the results. I was *sure* that the bike would sink is a few feet, and they would have to go to plan b to find some way to make the myth true (like a plank underneath the water). But wow -- that was an amazing result.

I'm surprised they didn't go slightly more into the science of it. It looked like on the water, the bike couldn't keep up the same speed, and as it started to slow, the front wheel started to dip causing more drag, and eventually stopping(sinking). If they had more time, I would have like to see them attach a pair of front "skis" to support the front wheel to see how much further they could go on just the rear wheel powering through. Either way, still a surprising result.
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Old 06-07-2013, 09:50 AM   #4
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I agree.

I was thinking it was so obvious it would be busted, why would they waste our time with it.

Couldn't believe it actually worked.

So it did seem to be the 'skipping rock' effect, since he finally sank. Unless he loses so much speed on the water that his speed isn't fast enough to keep him above water.

That is something they should've addressed - What was his speed when he hit the water and then what was his speed as he crossed it.
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Old 06-07-2013, 11:14 AM   #5
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I also was surprised it worked. The motorcycle clearly can run on the water given sufficient speed, but the rear tire wasn't getting enough traction to maintain that speed, so on the longer run he eventually sank. I'll bet with some mods it could be made to run on the water indefinitely.
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Old 06-07-2013, 12:33 PM   #6
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My first thoughts were:
The myth was a motorcyclist was on a freeway, doing freeway speeds, when he couldn't make a turn and stayed straight onto the lake.

A: You wouldn't be riding a dirt bike with knobby tires on a freeway, you'd be on a cruiser or sport bike with road tires, nice and smooth.

B: I can't believe there are any freeways with a straight shot into a lake, there would have to be barriers, guardrails, something to prevent cars from sliding off.
Except maybe in a 3rd world country somewhere.

Then it was no way brand new knobby would tires be able to stay on top of the water.

Then I was when it actually did.

I wanted them to try with a lightweight sport bike with fatter/smoother tires and see what happened.
You'd have zero traction to keep up speed, but you'd probably get a farther "rock skip" than the knobbies.


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Old 06-07-2013, 12:42 PM   #7
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For the second question (whether the motorcycle could travel indefinitely over the water), it is merely a question of thrust. Traveling over the water is not greatly different than an airplane traveling through the air. You have thrust to propel the craft forward, and the craft itself needs to use that thrust to deflect enough air (water) downwards to support its weight.

The motorcycle cannot generate much thrust on the water since it lacks a propeller and the knobs on the wheels are small. Perhaps if something like fins were added to the rear wheel it could obtain enough thrust to keep going. But probably not. Maybe with rear wheel fins plus replacing the front wheel with a ski (so it does not need as much thrust to support its weight) it would be possible for the motorcycle to "fly" the surface of the water indefinitely. Even then I am not sure because the rear wheel would tend to sink unless the fins were somehow shaped and angled to push more water down on the downward stroke than they push up on the upward stroke (which I am not sure is possible). Maybe it would also need an outboard ski attached near the back wheel.

Jamie and Adam always do a much better job than the other three mythbusters. But even so, I was surprised at how badly the other three did this episode. I would have expected them to at least consult a parachuting expert. A skilled person can survive a faster fall than a "lump" by taking the landing properly, using their legs and body to absorb and deflect some of the landing force. Of course, the criminal might not know how to do that, but they should have at least mentioned the possibility. It also means that the parachute needs to be somewhat stable so that the person has a chance to use their legs to cushion the fall. A parachuting expert could have probably told them what was necessary to survive a faster fall, and could look at the high speed cameras and speed of impact and tell them whether someone could survive that. Just having Buster drop like a lump and hit the ground was a poor substitute for a skilled person landing.

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Old 06-07-2013, 01:31 PM   #8
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I think they really, really need to finally let Buster go. It's in pieces, half of it doesn't stay together anymore, and looks like hell. I know Buster is iconic to the show, but I think people will get over it. The show has plenty of money, they can afford a new modern crash test dummy.
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Old 06-07-2013, 01:40 PM   #9
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I think they really, really need to finally let Buster go. It's in pieces, half of it doesn't stay together anymore, and looks like hell. I know Buster is iconic to the show, but I think people will get over it. The show has plenty of money, they can afford a new modern crash test dummy.
Aren't they on their third or fourth Buster now? I recall past episodes where they retired one and showed that they just got a new one.
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Old 06-07-2013, 01:50 PM   #10
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I knew it was confirmed. The Nitro Circus guys have done it many times. It is a bike powered ride and not just the momentum/skipping rock effect.
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Old 06-07-2013, 01:59 PM   #11
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I knew it was confirmed. The Nitro Circus guys have done it many times. It is a bike powered ride and not just the momentum/skipping rock effect.
If you mean this:

http://www.mtv.com/videos/nitro-circ...playlist.jhtml

at about 18 minutes, then it is clearly not working as a powered ride. He has a front ski surrounding the wheel, and the rear wheel looks like it has some small fins, and yet you can clearly see that as he loses his initial momentum he begins to sink and finally cannot continue traveling. He went a long way, but clearly could not continue going indefinitely.
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Old 06-07-2013, 02:00 PM   #12
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Aren't they on their third or fourth Buster now? I recall past episodes where they retired one and showed that they just got a new one.
They've intermittently added in other dummies, but the one that the three were using for the drop test was AFAIK the original Buster.

A while back, Jaime and Adam made a big production out of making a new Buster that had breakable "bones" made of balsa wood, but very shortly after stopped using it without much fanfare. My guess is that the process to un-do all of the ties to keep replacing "bones" was taking too much production time. More recently, Jaime and Adam have started to use the sim-cadaver for the same purpose. The problem with both is that they didn't have room for instrumentation, like the accelerometer.

I don't think the show has ever actually gone and gotten a new crash test dummy, at least not that I've ever seen.
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Old 06-07-2013, 02:02 PM   #13
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If you mean this:

http://www.mtv.com/videos/nitro-circ...playlist.jhtml

at about 18 minutes, then it is clearly not working as a powered ride. He has a front ski surrounding the wheel, and the rear wheel looks like it has some small fins, and yet you can clearly see that as he loses his initial momentum he begins to sink and finally cannot continue traveling. He went a long way, but clearly could not continue going indefinitely.
My mistake. It was a few years ago when I watched that. But I did believe it would be confirmed at the beginning of the Mythbusters episode.
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Old 06-07-2013, 02:08 PM   #14
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Looks like there have been three Busters:

http://en.wikipedia.org/wiki/List_of...members#Buster
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Old 06-07-2013, 02:30 PM   #15
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Don't forget "Baby Buster" from a few weeks ago
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Old 06-07-2013, 02:34 PM   #16
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So glad you started this thread.

I was also blown away by the results. I was *sure* that the bike would sink is a few feet, and they would have to go to plan b to find some way to make the myth true (like a plank underneath the water). But wow -- that was an amazing result.

I'm surprised they didn't go slightly more into the science of it. It looked like on the water, the bike couldn't keep up the same speed, and as it started to slow, the front wheel started to dip causing more drag, and eventually stopping(sinking). If they had more time, I would have like to see them attach a pair of front "skis" to support the front wheel to see how much further they could go on just the rear wheel powering through. Either way, still a surprising result.
Having seen that trick done on Top Gear with a high powered 4x4 I was guessing that it wouldn't work for a bike unless it was a very rare one with a powered front wheel.

Without power to the front wheel it can't 'climb' out of the water as well, if it ever sinks enough to slow its rotation it'll just plow in. I'm really surprised how well a rear wheel powered bike did.
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Old 06-07-2013, 02:50 PM   #17
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Having seen that trick done on Top Gear with a high powered 4x4 I was guessing that it wouldn't work for a bike unless it was a very rare one with a powered front wheel.

Without power to the front wheel it can't 'climb' out of the water as well, if it ever sinks enough to slow its rotation it'll just plow in.
Do you have a link to that?

I do not see how a wheel could "climb" out of the water, since a wheel is, you know, round. Whatever water it pushes down on the front of the wheel, it will push up on the rear of the wheel. You don't see a paddlewheel starting to climb out of the water on a paddleboat.

For no particular reason (it is not relevant to this discussion but it is funny to look at):


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Old 06-07-2013, 05:00 PM   #18
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Having seen that trick done on Top Gear with a high powered 4x4 I was guessing that it wouldn't work for a bike unless it was a very rare one with a powered front wheel.

Without power to the front wheel it can't 'climb' out of the water as well, if it ever sinks enough to slow its rotation it'll just plow in. I'm really surprised how well a rear wheel powered bike did.

Richard Hammond's Iceland Buggy Trip - BBC
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4x4 at 3m20s
4x4 vs snowmobile at 4m50s
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Old 06-07-2013, 08:22 PM   #19
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Richard Hammond's Iceland Buggy Trip - BBC
You need to upgrade your Flash Player

4x4 at 3m20s
4x4 vs snowmobile at 4m50s
Thanks for the link.

That run looked impressive. But I'm not sure what to make of it. It seemed like he was maintaining at or near his initial speed even near the end of the run, so the 4 wheels with special treads seem to produce a decent amount of thrust. As long as it can keep going fast enough, it can utilize the skimming / skipping effect that the motorcycle in Mythbusters used (but could not maintain sufficient speed).

But I still wonder how long that 4x4 could have kept going. Maybe it would slow down and sink if it tried to do, say, twice the distance we saw. Too bad they did not discuss how far it could go beyond the 500m they demonstrated (or did they and I missed it?).
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Old 06-07-2013, 10:35 PM   #20
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Wheels stay on top of water for 2 reasons. Horizontal momentum that keeps the wheel skipping on the surface, and a powered wheel can climb vertically whenever it starts to get too deep.

Thus that 4x4 zips along the water as well as it zips along the gravel bank. The motorcycle with the unpowered front wheel can only go as far as a skipping wheels can travel.

Having already seen those hydrophobic 4x4s before, I knew the bike would be able to run on the water quite a ways.
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Old 06-08-2013, 01:45 AM   #21
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...a powered wheel can climb vertically whenever it starts to get too deep.
How do you know? Are you seriously telling me that if you submerged a heavier-than-water wheel by 1/3 of its diameter and then started it spinning quickly that you think the wheel would rise to the surface?

I find that hard to believe, since, as I already mentioned in this thread, wheels are round, so the back pushes down even if the front pushes up.

Note also that air can be considered to be a fluid, albeit with a lower viscosity than water. So, would you claim that a powered wheel can climb into the air? Heck, why build airplanes? Just put some windscoops onto the wheels of our cars and we can fly away!

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Old 06-08-2013, 02:22 AM   #22
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I agree.

I was thinking it was so obvious it would be busted, why would they waste our time with it.

Couldn't believe it actually worked.

So it did seem to be the 'skipping rock' effect, since he finally sank. Unless he loses so much speed on the water that his speed isn't fast enough to keep him above water.

That is something they should've addressed - What was his speed when he hit the water and then what was his speed as he crossed it.
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Having seen that trick done on Top Gear with a high powered 4x4 I was guessing that it wouldn't work for a bike unless it was a very rare one with a powered front wheel.

Without power to the front wheel it can't 'climb' out of the water as well, if it ever sinks enough to slow its rotation it'll just plow in. I'm really surprised how well a rear wheel powered bike did.
I would have liked to have see a graph of the bike speed vs. distance traveled from the water's edge. My guess is that the bike starts to slow down immediately on contact with the water and continues to lose speed until it sinks. The distance it travels is mostly determined by the initial speed as it leaves the ground. I bet. no matter the initial speed or distance traveled, it sinks at the same speed each time.

Yes, a powered front wheel would have been an assist, at least as long as the bike is moving forward.
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Old 06-08-2013, 04:02 AM   #23
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Old 06-08-2013, 04:52 AM   #24
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If it were a real experiment, then have Jamie do the run multiple times to get the distance, then see what happens if he squeezed the clutch as soon as he got to the point where the water is more than a foot deep. Then you could see if momentum at that point was the reason it stayed up or if it was really powering over the water.

I'd also be interested in what the front tire was doing if we could zoom in with the high speed camera. (Already deleted it). It is still spinning at 60mph, or just skidding across the water? If a tire can paddle the water for propulsion then water zooming under a freewheeling tire should be spinning from the water.
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Old 06-08-2013, 09:22 AM   #25
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How do you know? Are you seriously telling me that if you submerged a heavier-than-water wheel by 1/3 of its diameter and then started it spinning quickly that you think the wheel would rise to the surface?

I find that hard to believe, since, as I already mentioned in this thread, wheels are round, so the back pushes down even if the front pushes up.

Note also that air can be considered to be a fluid, albeit with a lower viscosity than water. So, would you claim that a powered wheel can climb into the air? Heck, why build airplanes? Just put some windscoops onto the wheels of our cars and we can fly away!
I don't think it would climb if evenly submerged; but in an asymmetrical submergence, for example where it's raising a bow wave, or you hit a small wavelet in the water, I think the additional torque a powered wheel has over a free-spinning one helps it climb up, instead of dig into, to that front wave.

That would help the vehicle maintain speed and keep it from digging in.


Also, more powered wheels has another probably larger benefit (which I should have realized and brought up earlier). As you mentioned in an earlier post it's hard to get thrust with a wheel pushing on water; in effect it's a very low friction surface. However much power you've got there's an effective limit to how much thrust any single wheel can generate. Adding a 2nd powered wheel should roughly double your traction since you can transmit the engine's torque to the water through roughly twice the contact area. That should directly translate into better ability to maintain speed (although I guess if the engine was low enough power that one wheel could handle 100% thrust then adding a 2nd wouldn't help here)
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Old 06-08-2013, 10:13 AM   #26
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I don't think it would climb if evenly submerged; but in an asymmetrical submergence, for example where it's raising a bow wave, or you hit a small wavelet in the water, I think the additional torque a powered wheel has over a free-spinning one helps it climb up, instead of dig into, to that front wave.

That would help the vehicle maintain speed and keep it from digging in.
I'm still skeptical of the `wheel climbing the wave` explanation. I already explained the main problem I have with it, but your "asymmetrical submergence" theory can address my initial refutation, so here is another. If you have a wheel spinning in water, it kicks up a lot of water spray, as anyone who has ridden a fenderless bike through a puddle well knows. To be accurate, the wheel throws the water up and backwards, but my point is that there is a significant upwards component to the momentum of the thrown water. If the wheel throws water up, then there is a corresponding downward force on the wheel (basic action-reaction, conservation of momentum, the rocket effect, whatever you want to call it).

So, even in the asymmetrical submergence case, the back of the wheel is clearly generating some negative lift. If the wheel has large knobs (fins, paddles, whatever you want to call them), then it can also generate positive lift in the front. But I find it hard to believe that any wheel would generate net lift (i.e., the lift in the front exceeds the negative lift generated at the back) in the water.

But I do not think either of us can prove our case with a thought experiment. Perhaps we should write a letter to Mythbusters asking them to test it? They could start with a small-scale paddle wheel and see if they can generate net lift by spinning it fast enough.

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Also, more powered wheels has another probably larger benefit (which I should have realized and brought up earlier). As you mentioned in an earlier post it's hard to get thrust with a wheel pushing on water; in effect it's a very low friction surface. However much power you've got there's an effective limit to how much thrust any single wheel can generate. Adding a 2nd powered wheel should roughly double your traction since you can transmit the engine's torque to the water through roughly twice the contact area. That should directly translate into better ability to maintain speed (although I guess if the engine was low enough power that one wheel could handle 100% thrust then adding a 2nd wouldn't help here)
I'm with you here. As long as we avoid the "powered wheel climbing the water argument", I can accept that it is possible for a heavier-than-water vehicle to travel over the water indefinitely, provided it generates enough thrust with "paddle wheel(s)" to offset the drag of water resistance (plus air resistance) as it skims over the water. Then the explanation that the wheel "climbs" the water is not necessary -- it is just the wheel skimming the water due to the high speed of travel.
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Old 06-08-2013, 12:20 PM   #27
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I'm still skeptical of the `wheel climbing the wave` explanation. I already explained the main problem I have with it, but your "asymmetrical submergence" theory can address my initial refutation, so here is another. If you have a wheel spinning in water, it kicks up a lot of water spray, as anyone who has ridden a fenderless bike through a puddle well knows. To be accurate, the wheel throws the water up and backwards, but my point is that there is a significant upwards component to the momentum of the thrown water. If the wheel throws water up, then there is a corresponding downward force on the wheel (basic action-reaction, conservation of momentum, the rocket effect, whatever you want to call it).

So, even in the asymmetrical submergence case, the back of the wheel is clearly generating some negative lift. If the wheel has large knobs (fins, paddles, whatever you want to call them), then it can also generate positive lift in the front. But I find it hard to believe that any wheel would generate net lift (i.e., the lift in the front exceeds the negative lift generated at the back) in the water.
Firstly, look at the vectors. The upward component to horizontal component ratio is inversely related to the speed of the wheel. IE, a wheel moving along the surface at 100 MPH will produce a rooster tail that isn't that high but extends back quite a distance. But as the wheel slows to 10 MPH, the vertical component has overtaken the horizontal component an you get air sprayed almost straight up.

The angle shown by the rooster tail is an indication of the angle of the wake behind the wheel, starting from the bottom of the wheel. You can see by the following image that the water at the rear of the wheel may not be touching water for nearly as long as the forward part of the wheel. This would result in greater upward lift then downward lift at higher speeds.



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I'm with you here. As long as we avoid the "powered wheel climbing the water argument", I can accept that it is possible for a heavier-than-water vehicle to travel over the water indefinitely, provided it generates enough thrust with "paddle wheel(s)" to offset the drag of water resistance (plus air resistance) as it skims over the water. Then the explanation that the wheel "climbs" the water is not necessary -- it is just the wheel skimming the water due to the high speed of travel.
The spin helps the skimming by climbing the water in front. It works similar to a flat ski shape. It won't work if it gets too deep, but it will keep the wheel up if it's already there.
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Old 06-08-2013, 01:37 PM   #28
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The angle shown by the rooster tail is an indication of the angle of the wake behind the wheel, starting from the bottom of the wheel. You can see by the following image that the water at the rear of the wheel may not be touching water for nearly as long as the forward part of the wheel. This would result in greater upward lift then downward lift at higher speeds.
How do you know? The lift at the front could still be less than the negative lift at the rear. The forces at the front and the rear are created by different types of moving water. You cannot tell which force is greater just by looking at it.

As I said, the way to settle the question is to test it. Maybe if Mythbusters gets a letter from each of us, they would test it.
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Old 06-08-2013, 02:28 PM   #29
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I think they really, really need to finally let Buster go. It's in pieces, half of it doesn't stay together anymore, and looks like hell. I know Buster is iconic to the show, but I think people will get over it. The show has plenty of money, they can afford a new modern crash test dummy.
Something disturbs me about seeing Buster referred to as "it."

My reaction was something like, "How mean. Buster's a 'him'. A distinct part of the Mythbusters crew. He has feeli... no, wait. Oh yeah, I guess LoadStar's right."

But it still bothers me.
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Old 06-08-2013, 03:47 PM   #30
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Keep in mind that with forward momentum, there will be a higher pressure area on the front of the spinning wheel and a lower pressure area on the back of the spinning wheel. Thus, the downward motion at the front of the powered, spinning wheel can create more traction and lift against the water than the drag created by the upward motion of the rear of the wheel.
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