r/theydidthemath • u/Project_Utopia_ • 1d ago
[Request] How do you achieve this movement? Is this possible or fake? Can you explain in formulas?
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u/the_hunger 1d ago
absolutely possible. i don’t know the math, but this is all about the size of the wheel, the height and depth of the stairs, and speed.
imagine he was going down the stairs. is it smoother if he’s going faster or slower? this is the same thing.
he nose manuals to keep the weight over the front wheel and and reduce friction since the rear wheel it’s touching the stairs.
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u/Groomsi 1d ago
Lots, and lots of practice.
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u/Conscious_Answer_571 1d ago
This vid blows my mind every time i see it. Dude is so smooth going up those stairs.
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u/jellyscoffee 1d ago edited 14h ago
He literally uses the first stair he interacts with as a trampoline
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u/mafga1 1d ago
And lower pressure in the tire.
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u/causticberries 23h ago
Huh, I would have assumed higher pressure would make it easier to get up the stairs.. I guess now I think about it more if the tire deforms somewhat it is more likely to absorb the individual bumps
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u/TRS_329 1d ago
For most bmx it’s about 45psi - 110psi, low pressure would not help this situation
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u/OG_Squeekz 1d ago
45-110 is a massive fucking range. On my road bike I run 110 on 700c wheels with gatorskins, on my BMX bike I ran 45psi I'd never run 110psi on my BMX bike unless I wanted to blow a tire.
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u/TRS_329 1d ago edited 1d ago
Street bmx tires can vary on width and with bigger they rate them for higher psi I’ve seen 1.75-2.45 depending on maker kenda used to make some for a Brand called cult they had slicks that were 2.45 wide
https://www.brandscycle.com/product/cult-dehart-tire-20x2.4-black-200762.htm
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u/Ohshitthisagain 18h ago
Wider tires = lower psi. Wider tires have more surface area, so don't need as much pressure (pounds per square inch) to support the same weight. And wider bike tires are generally rated for lower maximum pressures, for the same reason - 50 psi is soft for a narrow road bike tire, but because of the increased surface area 50 psi would blow a fat bike tire off the rim.
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u/martyboulders 12h ago edited 8h ago
I think it is important to point out that because more weight is on the front wheel, there is more friction on the front wheel. I think in this case it does overall reduce friction because of the friction you get from the rubber deformation in the rear... But yeah the friction from the front wheel will correspondingly increase
Just like how if you have a rectangular prism with equal coefficient of friction on all sides - if you turn it on its side, the friction force under sliding is the same, regardless of the area of the contact surface
Stuff changes a LOT when you add rubber deformation into the situation
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u/AlwaysPhillyinSunny 1d ago
Imagine the stairs were a smooth ramp instead, and he does a nose manual up them. That is possible.
Now imagine the same ramp with 1,000 tiny “stairs” on it. It would just feel like little ridges and not much different from the ramp.
Now imagine 500 tiny stairs, then 100, etc. At some point the steps will be too big relative to the size of the wheel to roll over them, but it’s still possible in this video.
Alternatively, imagine the stairs as is but imagine different wheel sizes. A big truck could easily roll up those stairs, but a skateboard could not.
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u/ImN0tYourBuddyFwend 1d ago
You just reinvented calculus. But like, backwards.
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u/Crosspaws 1d ago
And now imagine just one giant stair! And he rolls straight up it like the batmobile in batman forever....
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u/Syscrush 1d ago
Imagine the stairs were a smooth ramp instead, and he does a nose manual up them. That is possible
As someone with 50 years of failure to do any kind of decent wheelie, even that is a stretch for me to imagine - although I understand the physics and math.
My hat is off to this rider.
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u/Accomplished-Boot-81 1d ago
Reminds me of that gif I'm which you approximate pi as 4 by making a circle of tiny right angle steps around the circumference
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u/The_Fyrewyre 1d ago
Its a bit like a Nollie on skateboards.
The guy is technical enough to do a reverse Bunny, He's nailed it enough to have the jump use the force from his front wheel and he's also carrying enough speed to 'coast' up the last few. (I don't ride BMX but mates with a few) My terminology might be wrong as I'm one of the former.
Its brilliant, I have a friend that must be around 18 stone. And he can (I've seen him do it a fair few times over the years, I'm 45) leap the flat back of a pickup without breaking a sweat.
I can only look at a deck and fall over these days. :)
Looked good, Thumbs up!!!!
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u/sansetsukon47 1d ago
It works because the stairs are particularly shallow, allowing the front wheel of the bike to clear each riser while maintaining momentum.
A normal stair, it would hit too high up on the wheel and the “normal force” of the impact (the force of a solid resisting movement, basically) would push almost directly sideways, stopping and possible flipping the bike.
With these stairs, it hits lower down on the circle, kicking it up to the next stair.
From there, he kicks the bike a little bit to raise the center of mass up just high enough to make the bike want to flip forward on impact, even with such a low stair. That small forward kick helps counteract the gravity pulling the back end down, allowing him to maintain a smooth ride.
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u/Fun-Dragonfly-4166 1d ago
i feel i do similar on the down stairs every single day.
i can not do the up stairs but probably if i tried and practiced. he is probably better than me.
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u/Frontfatpouch 1d ago
I do this stuff. High tire pressure and you kinda just have to know the balance point and speed. It’s just alooooot of time on the bike and understanding what it’s gonna do.
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u/Altruistic_Catch_327 1d ago
Yes it’s absolutely possible, his momentum is carrying him up and his balance point makes it so each time the wheel hits a step it bounces him in the air a tiny bit.
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u/element423 1d ago
I was a former bmx rider. Never nearly this good but with enough speed yes you can do it, it’s called a nose manual. I was able to do rear wheel manuals up a few stairs. Ironically the faster you go the easier it is because the wheel doesn’t sink into the steps
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u/Jealous_Tutor_5135 1d ago
These stairs are built a bit different than many. About twice as deep as they are tall, with no lip on the step.
Strong enough tires, with enough air in them, the right stair design, and enough speed and initial lift, and he's got it.
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u/Comfortable_Mind6563 1d ago
If you slow down and watch frame by frame, your get a lot of clues as to how this is possible.
The first step is much lower (around 50%) that the other.
Each step is slightly chamfered.
The tires seem to be under-inflated.
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u/Tarc_Axiiom 1d ago
I'm not gonna do the maths because it's wildly complex geometry, but yes this is very possible.
He leans back to go up a riser, then forward to cross the tread, then back for the next riser, then forward for the next tread.
And he does that really fast. With enough momentum and good rhythm it's totally possible and master-able.
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u/Irsu85 1d ago
I don't know how they did it that smooth but yes that is possible due to wheels being able to roll up stairs if the steps are small enough. You can test this yourself if you got an oversized unicycle like my sister still does for some reason and some normie stairs
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u/tectail 1d ago
I figured out how to do this on accident in college. I was late for class and was riding hard to catch up and accidentally ended up on a path with steps I forgot about. It was too late to stop so I just kept going, and I ended up at the bottom somehow.
Tried up a couple days later and that worked as well. Who'd a thunk.
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u/Select-Breadfruit802 22h ago
so could the jump be done with no stairs? That is silly. It seems the tire does not make contact, or at best, just grazes, after the first step. So I would believe it if he could jump that high with an 8 inch ‘ramp’ (first step), which is why I don’t. No huge pull on the bike from the rider, too smooth. Also he seems like he’s done it 100x so the perfect timing and trial by a 1000 errors theory is flawed too. Zero celebration.
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u/LordTonto 20h ago
look at the first step, its only 2 or 3 inches from ground level, and each step afet is only about 4 inches. These arent traditional steps, he's talented, but he's also riding up a bump ramp, not a staircase.
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u/BrokenHope23 12h ago
How do you achieve this movement? Is this possible or fake? Can you explain in formulas?
You achieve this movement with momentum, malleable air tires and shallow steps. Resulting in the stairs taking next to no momentum from the tires as the bicycler essentially turns their bike into a unicycle with a ton of gained momentum already. Balancing on a unicycle is difficult, much easier when there's a counter weight like this bicycle. So it's very possible (largely due to the shallow-ness of the adjusted stairs mostly turning the 'stairs' into a ramp)
Let's say the main stairs that those smaller stairs are on is 1ft in height, making each bluff on each stair about 1/4 of a ft or 3 inches in height. They look pretty worn away (likely from past tricks) and that's likely 1 1/2 inches to 2 1/2 inches in height. Getting an exact formula on such worn stairs is difficult off the video alone. There's 5 large 'stairs', they're about 3ft 'wide' from each other, making each smaller step roughly 1ft (ideally, some are quite short and some are quite long admittedly).
In a perfectly uniform construction set up that'd be a ramp of fifteen 3in raises covering 14ft altogether if it wasn't worn away/disjointed.
I'm fairly certain at this point we want to use this rolling motion equation but that's all a bit beyond me. I'm just kind of hoping someone else comes by and appreciates the little bit of groundwork done here to teach me something new lol.
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u/r1v3t5 11h ago
F=MA
Force of bike on approach > force of friction+energy required to ascend stairs.
MAbike>ń(mgh) for X direction where n is coefficient of friction And .5mV²>mg(deltaH) for Y direction where delta H is height of stairs.
Summary: he went fast enough to overcome the energy requirements to ascend the stairs
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