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February 13, 2014 12:25 PM   Subscribe

Tire Ski Jump [slyt]
posted by mannequito (26 comments total) 13 users marked this as a favorite

Technically a double, but the link in the old one is dead so hopefully this can stay up. Because people need to see this again.
posted by jbickers at 12:34 PM on February 13, 2014 [2 favorites]

Fun enough but before I clicked on it I was half expecting some lunatic to be curled up fetally inside the tire being rolled down the ski jump.
posted by aught at 12:35 PM on February 13, 2014 [3 favorites]

Yeah I saw it was a double only after posting, but without the link being automatically flagged it was hard to search for.
posted by mannequito at 12:40 PM on February 13, 2014

Stay for the tractor tire. I promise it's worth it.
posted by Pogo_Fuzzybutt at 12:53 PM on February 13, 2014 [5 favorites]

That may be my favorite use of Tsubasa wo Kudasai ever.
posted by darksasami at 12:59 PM on February 13, 2014

Damn, that's funny. I like how the dude's wearing a white coat cuz, you know, science.
posted by Mister_A at 1:49 PM on February 13, 2014 [5 favorites]

TIL tyres are nowhere near as good at ski jumping as people.
posted by Decani at 2:01 PM on February 13, 2014

It is not said nearly often enough, but thank you, Japanese entertainment industry, for being awesome.
posted by Joey Michaels at 2:21 PM on February 13, 2014 [4 favorites]

I joined metafilter to talk about the physics of this last time it was posted
posted by Ned G at 2:35 PM on February 13, 2014 [3 favorites]

(If anyone wants to know, the smaller tires go further, as they aren't wasting energy since they have a smaller moment of inertia, so they waste less energy on spinning around, and put it into going faster instead)
posted by Ned G at 2:39 PM on February 13, 2014 [2 favorites]

Umm, sidewall flex must play a part in that equation, Ned G. The stiffer the sidewall, the more vertical energy component transferred to the tire, thus a longer jump.
posted by Purposeful Grimace at 3:18 PM on February 13, 2014

I'm not sure the results actually bear that out, Ned G. The largest tyre breaks the ramp (likely impeding its distance potential) while still landing only four meters short of the winner.
posted by The Confessor at 3:19 PM on February 13, 2014 [1 favorite]

do I need a college degree to get this job
posted by Brocktoon at 3:46 PM on February 13, 2014 [1 favorite]

The Confessor: "I'm not sure the results actually bear that out, Ned G. The largest tyre breaks the ramp (likely impeding its distance potential) while still landing only four meters short of the winner"

And the winner was the dump truck tire, which I think was the the second largest. And the smallest tire was 5th place.
posted by team lowkey at 3:51 PM on February 13, 2014

If I somehow stumble my way into billions of dollars, I'll blow at least some of it on a ski jump or a tall building and drop stuff all damn day. (I'll schedule a MeFi meetup so everyone can play H-O-R-S-E with their choice of tire/projectile).
posted by Turkey Glue at 4:16 PM on February 13, 2014 [1 favorite]

They should have rebuilt the ramp and re-ran the test... I thought the Japanese were more methodical than that.

Mythbusters needs to do this one.
posted by MikeWarot at 4:23 PM on February 13, 2014

Huh, you guys are right, the results don't seem to match up. It's getting late here, so I'm gonna head to bed, but I'll do some maths tomorrow at work and see if I can predict the distances, and see where I go wrong (bet it's bloody resistive forces. It's always bloody resistive forces)
posted by Ned G at 4:49 PM on February 13, 2014 [1 favorite]

Mo momentum, Mo problems
posted by Muddler at 6:06 PM on February 13, 2014

I was honestly horrified that the crew was willing to attempt to "catch" the tractor tire. It was hard to guess the weight of it from looking, but considering it broke the ramp, and the ramp is designed specifically to launch man-sized objects (~100kg) with a safety margin (x2), I think a guess of 200kg is not unreasonable. By the time it finished crashing through the barriers, it was still probably moving at 50km/h. Bus and truck tires will frequently kill bystanders unlucky enough to get hit by a stray one. That would be like being hit by a medium size motorcycle (and rider) travelling at city speed limits, which could easily kill someone.

After the small tires went down, and I saw the special rig they had set up for the tractor tire at the top, I thought to myself: "Surely they've cleared all the crew downrange!"

The laugh track going as it rolled past the 'catchers' by about a meter was the kicker.

By comparison, how would Mythbusters have likely set up such a stunt?
posted by WaylandSmith at 7:02 PM on February 13, 2014

The ramp may have been weakened by the other tires.
posted by Brocktoon at 7:20 PM on February 13, 2014 [1 favorite]

I seriously can not believe this has not been posted yet:
posted by Mei's lost sandal at 8:10 PM on February 13, 2014 [3 favorites]

You don't have to! It was.
posted by ceribus peribus at 8:31 PM on February 13, 2014

WaylandSmith: the tractor tire was 220kg, or at least that's what the clip's little graphics lists. So pretty good guess.
posted by cirhosis at 9:26 PM on February 13, 2014

Mythbusters needs to do this one.

Small scale model test.

posted by jeribus at 4:28 AM on February 14, 2014 [1 favorite]

It seems that I'll often prefer to do 5 minutes worth of mental callisthenics than 10 seconds of paying attention to what I was watching…
posted by WaylandSmith at 4:01 PM on February 14, 2014

I've just had a chance to look at this properly, and basically I was as wrong as is possible when I said you want the smallest wheels possible. If you're making a car then that's true, as you don't want any mass to be located in the wheels, since it wastes energy to make them spin. However, in this situation you don't have that choice.

When I tried to work out the kinetic energy of the wheel in a very simplified way, by modeling the tyres as hoops with all the mass at the edge it turns out that K.E doesn't depend on the radius at all. That's because there are two effects dependent on r; the first is that increasing the radius increases the moment of inertia, so it makes it more energy intensive to spin the wheel round. However, increasing the radius also makes the wheel bigger, so it doesn't have to spin as fast to travel the same distance, which exactly cancels out the other effect.

When the wheels are modeled as tubes rather than hoops, the K.E is slightly dependent on the ratio (inner radius)/(outer radius) which makes sense: the smaller the inner radius the closer the mass to the centre of the wheel so the moment of inertia is smaller, and the larger the outer radius the faster the wheel can travel whilst spinning at a slower rate.

If anyone's interested the formula I got is K.E = 1/2 m v^2 (1 + (1+r^2/R^2) / (8 pi^2)) where r is the inner radius and R is the outer radius of the tyre.

Equating this with potential energy lost (= mgh) gives a launch velocity independent of mass and only weakly dependent on radius, so it's going to be the things I didn't account for: air resistance, rolling resistance etc that settle it. On this front you'd expect the bigger tyres to do better since the energy they lose will be a smaller percentage of their total.

Finally, I wondered if the big tyre had gained an advantage by smashing through the launch ramp, as although it obviously cost a bit of energy to do, I thought it might have given itself a better launch angle. However, I worked out the optimum and (if Wikipedia's quote of a 32 degree landing slope is correct), it turns out that the optimum launch angle is 29 degrees, which looks similar the slope of the kicker ramp on the video.
posted by Ned G at 10:26 AM on February 16, 2014

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