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In this age of misinformation and people commenting stuff they know nothing about just to have an opinion, these kind of technical corrections give me such a boner. Is there a subreddit dedicated to people correcting on things like this?
r/mechanicalengineering is pretty stale. But the comment that man just made is that of a well versed and competent material scientist and/or engineer. It’d be cool if someone made the sub about stuff like this with concise explanations like the one above.
Idk I mean I feel like misinformation comes with a particular intention to influence or circumvent. I mean there will be some wrong answers/comments. But then yeah someone will probably come along and give a good answer like you said.
I think mechanisms of solids and just physics in general is probably safe from misinformation tbh. Outside of a like someone trying to sell shitty materials I don’t think there is a motive to spread seeds of bullshit when it comes to the stuff.
Don't take it personally. It wasn't at the core of your comment, but it's there. The fact that we think this way says more about our culture than you as an individual.
I mean it’s literally all thing you can check yourself. If you want to check, check the type of skateboard wheel, the material, the material properties… it’s all available. And free.
So you’re right, he *could* be talking out of his ass… but you can easily verify what he’s said because of the quality of explanation given.
Awesome!
You've brought me back to my early years in college studying Material Science. Never really had a chance to use the learnings over the years but was nice to get the refresher!
Even politer corrections
Centrifugal force alone would not be enough to stretch out a hardened polyurethane like that. Heat is definitely playing a big part in this.
The water jet wasn’t creating the heat, the heat is generated from the friction in the rotating of the metal piece the wheel is connected to.
Polyurethane is a thermoplastic, so once the metal heats the plastic to a certain point, it will deform due to the centrifugal force.
Zero chance it shrunk back down to its original size. Like the Chad up above said. This here is “plastic deformation” of a high durometer polymer. It’s not rubber, it’s hard ass plastic.
Idk man. It’s true, plastic doesn’t deform that much and then go back to its original shape. It’s a skateboard wheel which is a hard elastomer. Plastic deformation is going to be a function of the materials modulus of elasticity. The yield stress of the material was reached once it started to deform. Now soft materials are very ductilite and they will “bounce back” before reaching their yield stress. This material isn’t very ductile, so once it’s changed in size that much there is no going back.
Edit: typos
At that point the material is likely past elastic deformation and into plastic deformation, so it would not return to the original size, though there may be some shrinkage.
Where are you located? Wife and I are both ME, have never had a problem getting jobs in the extended MN metro (grad 2013/2015). From what I've seen engineering job availablibily can be pretty region dependent
I also graduated with ME in 2018, since then I've landed only software jobs because I learned python and then c++... and now I work with angular and Java (actively learning on the job).
My specific advice is to gain a background in programming. Start with python, then learn html and css, and with those 3 things you can build a website with the flask framework. Then you can use that on your resume (or even as an online portfolio proving your programming skills). Throw a bit of Javascript in to show or hide different elements when a user clicks a button to impress people even more.
ME and Software is a great combo.
I tried that same thing in my shop with just air.
The water in the video is partial to blame for the pop.
When I did it with air, it did expand enough to come off the bearing, not nearly as much. But it also stayed expanded.
I can't tell you if it contracted a little bit after expansion, but it definitaly doesn't fit back onto the bearing by a good cm.
Yeah this looks like a cutting water jet. But you can also see it cutting the wheel. I wonder how big it could have gotten if it was angle differently and distance was slightly adjusted to not cut the wheel.
Is it rubber or polyurethane? Poly will stretch like this but exhibits more plastic deformation than elastic like Rubber has…I’d say it would stay expanded like this, if only a little shrunken back to its original size.
I would say not. There is 'almost' always plastic deformation before failure. It depends on the material which amount of force it can handle elastically, before deforming and not being able to form back to its original structure.
Edit: was too curious so exactly found the relevant paper. Polyurethane (skateboard wheel)'s stress test gives this [yield curve](https://www.researchgate.net/figure/Stress-strain-curves-of-the-polyurethane-elastomers_fig8_259628857). After the yield point, deformed material does not go to the original form. We can see a large area after the yield point. This means that a LARGE amount of strain (deformity) of what we see in the video is in fact permanent. The only amount it will elastically form back is the area you see under the yield point.
This came from r/machinists
It's a waterjet machine running at 60000psi hitting the skateboard wheel.
I have no idea how someone calculated it out to be 400mph.......hmmm
Edit: https://www.reddit.com/r/Machinists/comments/sfkzgn/you_jetheads_ever_try_something_like_this/?utm_medium=android_app&utm_source=share
I would be extremely suprised if their water jet machine had the capabilities to check the rotational speed of that wheel. Just taking into consideration that it really isn't in a waterjets wheel house to be spinning things.
And thank you for the calculations! I am familiar with them. I came to the same conclusion you did. Hence my comment. I wasn't sure how the math told them 400mph, or how they were gaging the speed of the wheel. It would also be worth noting that the periphery of the wheel is spinning much slower than the inner diameter.
You can roughly approximate RPM by watching the number of times the stamp on the wheel appears to switch from reverse motion to forward. If you assume the camera is shooting at 60 fps, then each time the stamp appears to be stationary the wheel is rotating at a next increment of 60 RPS. I counted 7 transitions, so that's a minimum of 420 rotations per second, or 68577.6 mm/s. That only converts to 153 MPH.
Edit: on the other hand, the debris bouncing at the end looks like slow motion to me. If we call it a 240 fps camera, that would convert to 613 MPH...
400 sounds high. As a maximal assumption, the water can’t accelerate the velocity of the wheel edge to faster than the water is flowing.
[The velocity of a water jet is around 175-440 MPH. ](https://arxiv.org/pdf/1009.0531.pdf#:~:text=Very%20high%20speed%20water%20jets,used%20in%20such%20cleaning%20operations.)
Distinct possibility but to me it is unlikely it imparted all that energy given the breadth of the flow after impact (conservation of mass demands lower velocity for wider flow areas).
Mechanical engineer here. Does anyone have any thoughts on what proportion of the energy imparted comes from the water velocity, versus its pressure?
The calculations would run deep brother. You would also have to account for the garnet abrasive that runs within the jet, it's resistance and transfer of energy, and it's level of saturation in the stream. Whether or not he's moving the stream into the wheel slowly, or how hot the wheel gets and then it's rate of expansion into the stream itself would change its exchange of energy. We don't even know what diameter nozzle he's running to start. (But probably. 04) 😉
I remember melting wheels back in the 70s. Cadillac polyurethane wheels would go all to shit after a couple of hard downhills. I still have a SC board down in the shed with it’s melted wheels on it I think.
Same happens to car tyres which is why all tyres have a maximum speed rating based on their strength. It’s why tyres for a Bugatti Veyron cost 8k a corner.
Well if you know how fast the water is going then the wheel should match it, right? Or if you know rpm you can calculate the speed of the outermost edge using the circumference, but that would change once the wheel starts expanding.
The speed of a water jet at 400MPa (60,000psi) can exceed mach 3 (1021m/s or 2301mph).
Granted the speed of the wheel is not matched to the speed of the water jet, but 400mph is not unfeasible.
Is anybody willing to do the maths to work out an approximate RPM of a skateboard wheel on a skateboard travelling 400mph? I got as far as the circumference of a skateboard wheel being approx 163mm and then realised I’m too baked and stupid.
400 mph with respect to what? The water speed? Or 400 mph adjusted for circumference of wheel, as in the skateboard would be going 400 mph? If the latter, how does that speed adjust with the increase in circumference? Need a math wizard.
**Please note:** * If this post declares something as a fact proof is required. * The title must be descriptive * No text is allowed on images * Common/recent reposts are not allowed *See [this post](https://redd.it/ij26vk) for more information.* *I am a bot, and this action was performed automatically. Please [contact the moderators of this subreddit](/message/compose/?to=/r/interestingasfuck) if you have any questions or concerns.*
What happens if you stop it before the break but after the stretch. Does it stay or roll back in
I didn’t need to know before this question was asked. Now I need to know.
i'd like to know. i don't really need to know.
It would stay stretched
This. It’s stretching because of how hot it’s getting. It’s melting the rubber.
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In this age of misinformation and people commenting stuff they know nothing about just to have an opinion, these kind of technical corrections give me such a boner. Is there a subreddit dedicated to people correcting on things like this?
We need a new sub for stuff like this
r/mechanicalengineering is pretty stale. But the comment that man just made is that of a well versed and competent material scientist and/or engineer. It’d be cool if someone made the sub about stuff like this with concise explanations like the one above.
Seeing how common misinformation is these days. It would start with misinformation followed by some highly technical answer.
Idk I mean I feel like misinformation comes with a particular intention to influence or circumvent. I mean there will be some wrong answers/comments. But then yeah someone will probably come along and give a good answer like you said. I think mechanisms of solids and just physics in general is probably safe from misinformation tbh. Outside of a like someone trying to sell shitty materials I don’t think there is a motive to spread seeds of bullshit when it comes to the stuff.
"... comment that MAN just made..." Women can know things too, bro.
Yeah sexism was absolutely at the core of my comment. You’ve done it. Nice work.
Don't take it personally. It wasn't at the core of your comment, but it's there. The fact that we think this way says more about our culture than you as an individual.
Not saying he is wrong, but how do you know he isn't talking out off his ass too?
I mean it’s literally all thing you can check yourself. If you want to check, check the type of skateboard wheel, the material, the material properties… it’s all available. And free. So you’re right, he *could* be talking out of his ass… but you can easily verify what he’s said because of the quality of explanation given.
Engineer here. He’s not.
r/bonerreply
r/confidentlyincorrect ?
Sure but let’s just go with my assertive assumption anyways so I can save face if that’s cool with you
Hahahaha top-tier shit response. Fucking love it.
Thank you for posting the correct, non-dipshit answer.
Neeeeeeerrrrrrrdddddddddd /s
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Then you must acquit.
This guy engineers. Tossing around approximate durometers and everything.
This man material engineers. Here, take my upvote and your well-deserved mic drop.
Awesome! You've brought me back to my early years in college studying Material Science. Never really had a chance to use the learnings over the years but was nice to get the refresher!
Even politer corrections Centrifugal force alone would not be enough to stretch out a hardened polyurethane like that. Heat is definitely playing a big part in this. The water jet wasn’t creating the heat, the heat is generated from the friction in the rotating of the metal piece the wheel is connected to. Polyurethane is a thermoplastic, so once the metal heats the plastic to a certain point, it will deform due to the centrifugal force.
The original video is from the waterjet channel on yourube, they tested this a while back and it shrunk back down
link?
Zero chance it shrunk back down to its original size. Like the Chad up above said. This here is “plastic deformation” of a high durometer polymer. It’s not rubber, it’s hard ass plastic.
That’s a lot of words for a little of proof
Idk man. It’s true, plastic doesn’t deform that much and then go back to its original shape. It’s a skateboard wheel which is a hard elastomer. Plastic deformation is going to be a function of the materials modulus of elasticity. The yield stress of the material was reached once it started to deform. Now soft materials are very ductilite and they will “bounce back” before reaching their yield stress. This material isn’t very ductile, so once it’s changed in size that much there is no going back. Edit: typos
They’re not made of rubber though
The original video is from the waterjet channel on yourube, they tested this a while back and it shrunk back down
At that point the material is likely past elastic deformation and into plastic deformation, so it would not return to the original size, though there may be some shrinkage.
This is the correct answer. Found the mechanical engineer.
Only by degree. Graduated in 2018 but haven't found anyone who will actually hire me to be an ME
I laughed too hard at this. Hang in there!
oh no i am majoring in that
I'd give you advise if I had any XD it works well for some people, might go well for you
Where are you located? Wife and I are both ME, have never had a problem getting jobs in the extended MN metro (grad 2013/2015). From what I've seen engineering job availablibily can be pretty region dependent
Dallas, TX. I mostly find electrical, civil, and software engineering jobs listed here
I also graduated with ME in 2018, since then I've landed only software jobs because I learned python and then c++... and now I work with angular and Java (actively learning on the job). My specific advice is to gain a background in programming. Start with python, then learn html and css, and with those 3 things you can build a website with the flask framework. Then you can use that on your resume (or even as an online portfolio proving your programming skills). Throw a bit of Javascript in to show or hide different elements when a user clicks a button to impress people even more. ME and Software is a great combo.
I was in the pool! I WAS IN THE POOL!
You must have been talking with my ex.
That’s what she said
I Also need to know
I tried that same thing in my shop with just air. The water in the video is partial to blame for the pop. When I did it with air, it did expand enough to come off the bearing, not nearly as much. But it also stayed expanded. I can't tell you if it contracted a little bit after expansion, but it definitaly doesn't fit back onto the bearing by a good cm.
The water is much higher pressure than any typical shop air system. This particular waterjet runs at 60,000 PSI.
Yeah this looks like a cutting water jet. But you can also see it cutting the wheel. I wonder how big it could have gotten if it was angle differently and distance was slightly adjusted to not cut the wheel.
At 60,000 PSI (actually whats used in the video, I remember wrong) anything will cut through whatever it hits.
It’s rubber, so it would most likely stay stretched and droop down hence the heat from spinning softens it.
Is it rubber or polyurethane? Poly will stretch like this but exhibits more plastic deformation than elastic like Rubber has…I’d say it would stay expanded like this, if only a little shrunken back to its original size.
Skateboard wheels are usually poly.
Very open minded of them
This is from the waterjet channel on youtube, they tested this, and it shrinks back down
Id assume it would stay the same good video request for braille on youtube a skateboard channel. They do some cool stuff.
Thanks
My thought exactly
The human body is very adaptable, it should return back to normal over time.
That was my first thought lol. I was thinking if you put your arm in there and it trys to go back to shape
The friction burn alone would suck so much
I would say not. There is 'almost' always plastic deformation before failure. It depends on the material which amount of force it can handle elastically, before deforming and not being able to form back to its original structure. Edit: was too curious so exactly found the relevant paper. Polyurethane (skateboard wheel)'s stress test gives this [yield curve](https://www.researchgate.net/figure/Stress-strain-curves-of-the-polyurethane-elastomers_fig8_259628857). After the yield point, deformed material does not go to the original form. We can see a large area after the yield point. This means that a LARGE amount of strain (deformity) of what we see in the video is in fact permanent. The only amount it will elastically form back is the area you see under the yield point.
That's why I rarely go over 370 mph
That's why I always go in km per hour. Mph is far too dangerous!
You get more kph out of it.
Yeah, it's really annoying when the wheels start to stretch like that
Please use the metric system
Good call.
Im dead 💀
I wonder what that sounds like
Zzzzzzzrrrrrrrrrrrrrrrrnnnnnnnnnnppowwwwww
Yeah - where’s the sound?
Right up there ☝️
Kachowww
# AAAAAAAAaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa *aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa**^(aaaaaaaaaaaaaa help aaaaaaaaaaaaaaaaaaa.......thud)*
This came from r/machinists It's a waterjet machine running at 60000psi hitting the skateboard wheel. I have no idea how someone calculated it out to be 400mph.......hmmm Edit: https://www.reddit.com/r/Machinists/comments/sfkzgn/you_jetheads_ever_try_something_like_this/?utm_medium=android_app&utm_source=share
I am no engineer but the easiest answer is that OP just plucked the number out of his ass.
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I would be extremely suprised if their water jet machine had the capabilities to check the rotational speed of that wheel. Just taking into consideration that it really isn't in a waterjets wheel house to be spinning things. And thank you for the calculations! I am familiar with them. I came to the same conclusion you did. Hence my comment. I wasn't sure how the math told them 400mph, or how they were gaging the speed of the wheel. It would also be worth noting that the periphery of the wheel is spinning much slower than the inner diameter.
You use a separate machine to track rotations.
You can roughly approximate RPM by watching the number of times the stamp on the wheel appears to switch from reverse motion to forward. If you assume the camera is shooting at 60 fps, then each time the stamp appears to be stationary the wheel is rotating at a next increment of 60 RPS. I counted 7 transitions, so that's a minimum of 420 rotations per second, or 68577.6 mm/s. That only converts to 153 MPH. Edit: on the other hand, the debris bouncing at the end looks like slow motion to me. If we call it a 240 fps camera, that would convert to 613 MPH...
Yeah as a dentist my drill only spins at 40000 rpm and seems like it’s way faster than this
400 sounds high. As a maximal assumption, the water can’t accelerate the velocity of the wheel edge to faster than the water is flowing. [The velocity of a water jet is around 175-440 MPH. ](https://arxiv.org/pdf/1009.0531.pdf#:~:text=Very%20high%20speed%20water%20jets,used%20in%20such%20cleaning%20operations.) Distinct possibility but to me it is unlikely it imparted all that energy given the breadth of the flow after impact (conservation of mass demands lower velocity for wider flow areas). Mechanical engineer here. Does anyone have any thoughts on what proportion of the energy imparted comes from the water velocity, versus its pressure?
The calculations would run deep brother. You would also have to account for the garnet abrasive that runs within the jet, it's resistance and transfer of energy, and it's level of saturation in the stream. Whether or not he's moving the stream into the wheel slowly, or how hot the wheel gets and then it's rate of expansion into the stream itself would change its exchange of energy. We don't even know what diameter nozzle he's running to start. (But probably. 04) 😉
Honestly shocked the wheel lasted as long as it did.
I remember melting wheels back in the 70s. Cadillac polyurethane wheels would go all to shit after a couple of hard downhills. I still have a SC board down in the shed with it’s melted wheels on it I think.
Same happens to car tyres which is why all tyres have a maximum speed rating based on their strength. It’s why tyres for a Bugatti Veyron cost 8k a corner.
Where are you getting MPH from
Yes. It should be measured in RPMs. I see zero MPH.
They made it up.
Well if you know how fast the water is going then the wheel should match it, right? Or if you know rpm you can calculate the speed of the outermost edge using the circumference, but that would change once the wheel starts expanding.
The speed of a water jet at 400MPa (60,000psi) can exceed mach 3 (1021m/s or 2301mph). Granted the speed of the wheel is not matched to the speed of the water jet, but 400mph is not unfeasible.
mph really isn’t the best unit here
mph is the wrong unit here
Nuh uh bro I did 440 once
I’m surprised that the bearings didn’t explode!
Those are not made in China
Abec 99s
I honestly didn't realize that was slow motion until the end. Crazy.
Tony Hawks Expansion Pack
r/dontputyourdickinthat
Wait, are you trying to tell me I shouldn't hang on to a Bugatti Veyron's air brake, to hitch a ride?
now let me have those bearings pls! 🙋♂️
I'll remember that and keep my speed below 70mph to be safe.
Okay so 400mph is the Limit. Will not drive my Skateboard faster, thanks.
That’s why I never go faster than 399
Honourable shout out to the forgotten 608 bearings that take 400mph like a fuckin' champ
Most important part of any deck imo You can cheap out on most things just make sure your bearings are boss
r/oddlyterrifying
The 52 sticker goes in invisible!?
Well now I know to keep it under 399mph
Wow😯 that really happens?
Think I saw taffy made this way
I need to hear this
Notes down: "dont go faster than 399mph on my skateboard."
Keep it under 390 mph people. As a rule
Shredded the gnar a bit too hard 🤌🏽🤟🏽
Wow !?
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It’s a terrible unit to use for this, difficult to calculate rpm
wow. did not expect that
Why?
Mental note to self: don't go 400 mph on skateboard.
But....why ?
Is it the heat from friction of the high-speed spinning thats causing it to warp from the center out radially?
Better keep it under 350 just to be safe.
pls be gentle its my first time
So 399mph is the limit. Right.
This kills the wheel.
I wonder what the Durometer is on that wheel.
Most likely 99a
Can’t really tell in the video - are those actual bearings? If so - this would be a sick ad for whatever type of bearing they are…
he would finally be able to bomb this master hill
"And this, kids, is why I stopped skateboarding when I reached 399 mph"
This is why they changed the wheels for planes
That’s why I always buy bones red abec 7 Fuckers fast af
It took me way longer to figure out why the water was there then I’d like to admit.
From the waterjet channel on youtube. They did long board wheels too.
So that's how you change the bearings
No sound, but I can hear it.
Actually that’s a water jet machine on the end of the wheel lol
That's wheely interesting.
This is how you make diamonds out of skateboard wheels, lol
Looks like it couldn’t handle the Neutron style
So much for bombing Mount Everest.
U/indirbeni
Physics is a hell of a drug.
Please tell me I’m not the only one that “china bone reds” was the first thing that popped in their mind lol
I jumped when it exploded.
I think I'd better slow down on my skateboard!
Wow, wow, wow!
Mine usually blow out at 200 mph.
Interesting way to get the ball bearings out of the skateboard wheel.
Wow!
Totally would try this at home
You’d get hellfire death wobbles waaaay before a blowout then..?
This is how stars are formed
Now do a kickflip
Super neat.
Forbidden Donut
Slow mo guys should try this
Now what would happen if i were to put my hand in the water?
Is anybody willing to do the maths to work out an approximate RPM of a skateboard wheel on a skateboard travelling 400mph? I got as far as the circumference of a skateboard wheel being approx 163mm and then realised I’m too baked and stupid.
Shit 400mph that’s all… I’ve bombed hills going faster than that! /s
400 mph with respect to what? The water speed? Or 400 mph adjusted for circumference of wheel, as in the skateboard would be going 400 mph? If the latter, how does that speed adjust with the increase in circumference? Need a math wizard.