[local_issues]

Ding ding ding! We still don't know how bikes work, we have a pretty good but incomplete model for lift on airplanes, and no one has the faintest why Tylenol works.

We should figure that out! But we can definitely keep using all the applications until then. (Except for Tylenol, we keep learning how bad that stuff is)

EDIT: It won't let me post more, so here's the answers to responses.

For sure!

https://www.cbc.ca/news/science/science-of-cycling-still-mys...

http://www3.eng.cam.ac.uk/~hemh1/gyrobike.htm

https://www.newscientist.com/article/mg22730370-400-how-does...

It's a fun little fact. We know a ton about BUILDING bikes, which is a more useful tool anyway

[ummonk]

Nonsense. There's nothing incomplete about our model for lift on airplanes.

[aeternum]

Oh really, so why do the Navier-Stokes equations work?

[ummonk]

If you're asking how to apply the Navier-Stokes equations: apply boundary conditions

If you're asking how to derive the Navier-Stokes equations, use continuum / conservation principles.

[dkqmduems]

Conservation of momentum?

[bandrami]

There's no a priori reason conservation of momentum would apply; for that matter in 0-angle-of-attack aviation it doesn't.

Maybe to put it differently, the "Newtonian" model of flight comes out of assuming CoM applies, which we know isn't universally true.

EDITING: of course it always "applies" in the sense that there is a definable system in which total momentum will be unchanged, the point being that there is no guarantee that's a system in which the plane gets lift greater than its weight

[coderenegade]

A symmetrical airfoil at zero angle of attack doesn't generate lift because it doesn't turn the fluid. A non symmetrical airfoil will generate lift even at zero AOA because air has viscosity and the lack of symmetry causes it to turn around the shape. When you bend fluid flow, COM comes into play.

[mthomasmw]

Do you have a link handy for "we don't know how bikes work" ?

[SkyBelow]

I'm assuming they mean the balance mechanism, and specifically what allows us to balance. How much is it the rider shifting their weight, how much is micro steering adjusts as we move forward, how much is the gyroscopic forces of the wheels, how much of it has to do with the angle of the handle bars to the wheel verse the center of weight.

That said, I'm guessing this one is well understood by experts, but more complex than someone would assume at first glance, and many who have some understanding likely have an incorrect or at least incomplete understanding of how balancing works.

[reaperman]

We have self-balancing bicycles. We clearly know it well enough to replace the human with a computerized machine.

Maybe we don’t know how humans use the bicycle but we know how bicycles balance, we can write programs to balance them physically.

0: https://youtu.be/Ya7iacmVjUM

1: https://youtu.be/2Z67NkvXIF4

[acters]

You are correct, a lot of forces are canceling out on the long term(instead of instantaneously) it can easily be manipulated into increasing periods of unbalance in one direction until a point is reached then a separate mechanism is used to force it to a balanced state. Conservation of energy is always in effect. Gyroscope effects that bike wheels can be added with other separate gyroscopes. Thus, a self righting bike. The effect called precession is understood well enough.

How a human is able to manipulate it is simply by using the force of gravity from shifting their weight(moving the center of gravity). However, the movement of the center of gravity has to be perpendicular to the wheels axle. The steeper the angle of attack the wheel has to the ground, there will need to be an exponential increase in distance to move the center of gravity. Once the wheel is parallel to the ground, there will be an undefined distance needed to move the center of gravity.

[BizarroLand]

I wonder what learning the specific system by which bikes are balanced would teach us about the world or human beings?

It's such a banal thing to be so fascinating.

[hinkley]

We don't know until we know.

The smart phone is the culmination of understanding a million facts about materials sciences (applied and theoretical), some of which were obvious, some of which were non-obvious. Starting from a transistor you could see from across the room down to ones you can't even see with a magnifying glass.

[doctorwho42]

It's the reason I got a degree in physics, if you have good professors - discussions like this cause you to break down the problem quite quickly in your head in a working model. Think force diagrams, but with a ton more math backing it up.

I actually find the ice skate a better example than a bike. We have all the physics solved for bikes, it's a complicated system but so is everything in motion. Hence we assume a spherical cow for the sake of the problem.

But ice skates... Now that's a funky one. Why do ice skates works? Ice skates aren't sharp bladed, they actually have flats. Ice is not slippery, it's when something is on ice in between our shoes and the ice that cause it to be slippery. Some people think it's the localized pressure of the blade that causes ice to locally melt. Hard to really wrap your head around. But it works :)

[jibe]

Ice skates aren't sharp bladed

Not sure how you meant that, but ice skates are sharp. Each edge of the blade is sharpened by grinding a hollow out of the center.

https://weekendwarriorshockey.com/how-sharp-should-my-skates...

[SkyBelow]

>We have all the physics solved for bikes, it's a complicated system but so is everything in motion. Hence we assume a spherical cow for the sake of the problem.

This depends upon the question one is trying to answer. If one is trying to create a bicycle that can self balance, that involves considering different factors compared to trying to determine why certain injuries result in a person losing the ability to balance on a bicycle while others do not. Is the focus the bicycle or the human?

[ezconnect]

I thought it was not just about ice skating but the question of why is ice very slippery is the hard part to explain.

[hughw]

e.g. What keeps bicycles balanced with or without a rider is still an active area of research, and even the seemingly basic idea that, for a bicycle to be self-stable, it needs to turn the handlebars into the fall, has not yet been proven.

[*]https://ciechanow.ski/bicycle/

[IshKebab]

"Active area of research" is quite different to "we don't know how they work".

We know how they work. We might not have fully characterised the stability conditions, but that's not the same thing.

[hughw]

Cmon, man

[salty_biscuits]

You can write out the equations of motion for a bicycle that will very accurately predict the dynamics. You can put these equations into a numerical simulation and predict motion very accurately. You can change the parameters of the model and do simulations with high confidence. Just because there isn't some neat little equation that says exactly what each parameter change is going to do (without doing the simulation) doesn't mean that we don't understand bicycle physics. It's a silly line of reasoning. Those articles are hyperbolic.

[ddalex]

You are contradicting yourself:

> You can write out the equations of motion for a bicycle

> there isn't some neat little equation that says exactly what each parameter change is going to do

I mean, if you have the equations you can see what each parameter is doing !

[babypuncher]

I think their point still stands, even if "we don't know how bikes work" is a flowery exaggeration.

[IshKebab]

But then we don't know how anything works...

[DonHopkins]

Yet we still have Insane Clown Posse refrigerator magnets.

https://www.insaneclownpossemerch.com/collections/insane-clo...

Scientists Try To Teach ICP Fans How Fucking Magnets Work

https://metalinjection.net/av/scientists-teach-icp-fans-fuck...

[babypuncher]

But their point is we don't need a 100% complete scientific understanding of something in order to engineer it into something cool or useful.

[hinkley]

As a bit of an aside, most people don't know how bicycle wheels work. There's a whole section in https://en.wikipedia.org/wiki/The_Bicycle_Wheel that talks about how they actually work. It's not tension at the top, it's compression on the bottom.

[denimnerd42]

what do you mean it's not tension at the top? did you misspeak? bicycle spokes are solely under tensile forces. they can't support compressive forces at all. i'm a hobbyist wheel builder and a once upon a time professional bicycle mechanic during hs & college.

if you want to test this take nearly all the pretension out of your spokes and sit on your bike. feel which ones are taught and which ones are completely loose. or just go to walmart. those bikes hardly have any pretension in their wheels.

[wiredfool]

This dialogue is reminiscent of rec.bycycles.tech arguments with Jobst, ca 1993.

A bike wheel is a linear elastic system, that can be thought of as a superposition of a uniformly set of tensioned spokes as one state, and a set of spokes in compression in the loaded zone (bottom of the wheel) as the other state. So long as the superposition of the two states obeys the limiting conditions (i.e. spokes in tension) they can be analysed separately.

The size of the loaded zone is related to the relative stiffness of the spokes (axial) and the rim (bending), and can be calculated using beam on elastic foundation methods. For typical rim/spoke combinations, this is approximately 4 spokes.

Outside of the loaded zone, spoke tensions essentially don’t change.

[hinkley]

Hey do you happen to recall, did Brandt prove this theory with a tensiometer or did I just hallucinate that?

[wiredfool]

Pretty sure he did, but I don't have r.b.t. archives. He was definitely a proponent of them, preferred a specific brand/style, and would have easily been able to do the experiment.

[hinkley]

And thread. Thereby proving my point: Almost nobody knows how bicycle wheels actually work.

[denimnerd42]

Well. I wish we could sit in front of a bicycle wheel and discuss it. Because I have a feeling we are shooting arrows at different targets. As a mountain biker I'm more interested in what happens when an extreme amount of load is applied to the wheel, not the model with assumptions applied. Definitely a difficult concept to discuss with only text. Anyways.. glad to have a good discussion with you about bicycle wheels. Don't find many people like you. :)

[IshKebab]

Nah they both understand it. One of them is just arguing to show off.

[hinkley]

That book I linked has another name, “the wheel building bible”. Jobst Brandt earned an obituary in Bicycling magazine including quotes from his friend Tom Ritchey (one of the original mountain bike makers). Jobst was a bike fanatic and a mechanical engineer.

Bike spokes are not loose, they’re under substantial tension. Bolts, I just learned a couple weeks ago, work in the opposite way. A tightened bolt compresses the two pieces of metal together, and when you tug on them, the bolt doesn’t stretch more. The tension instead first cancels out some of the compressive force on the two pieces of metal, before the bolt ever feels more load.

Conversely, all the spokes on the wheel are under tension. When you put the wheel on a surface and push down, the compression cancels out some of the tension on the bottom of the wheel. Cancel out all of the tension, and the wheel turns into a potato chip if you don’t reload it exactly, perfectly on axis. IIRC, none of the prior models or theories for how a spoked wheel works could adequately explain how potato chipping happens. His does.

I used his book to build half a dozen wheels or so and the information it contained to fix many more.

[denimnerd42]

i own the book.. and also The Art of Wheelbuilding - Gerd Schraner

of course in a properly built wheel usually all the spoke are under tension...

i was just demonstrating the fact that the spokes on the upper half of the wheel are supporting the hub and are under greater tension than the bottom ones, the spokes on the bottom half of the wheel should remain in tension, but only through the fact that they are already under tension applied during the building of the wheel.

the fact that the wheel works by tension of the spokes becomes obviously apparent when you start to remove the pretension and then the spokes will feel loose on the bottom half. of course you'd never want to ride a wheel like that because it will quickly become out of true.. just like a walmart wheel.

[hinkley]

I think one of us needs to reread that book, because he emphatically denies that tension at the top of the wheel increases. It’s tension at the bottom that decreases.

> of course in a properly built wheel usually all the spoke are under tension...

No, a properly built wheel all of the spokes are always under enough tension you can bounce a penny off them. Always.

[bradyd]

They even make spokes that are rope

https://berdspokes.com/pages/technology

[hinkley]

I wonder if those have less or more breakin issues compared to spokes. With a spoke you have to reset the angle of the bend to the shortest distance between the hub and the rim. But with cables they have to settle in along their entire length.

Doesn’t change the answer. They’re still compressing. They’re just pretensioned.

[wiredfool]

There's a lot more creep early on, which is compensated for by a staged tensioning over the course of a few days. They may require the spoke holes in the hub to be radiused, which can be a warranty issue between you and the hub manufacturer. Windup is controlled by a flat on the small bit of spoke used for threads.

They're ok (i.e., made it through Tour Divide with no issues), done well they're certainly better than badly done steel spokes, but it's not clear if the best builds are better than the best steel builds.

[denimnerd42]

you sound highly confused, what is compressing? the rim? the spoke?

[matsemann]

Not to appeal to authority, but I wrote my thesis on bicycle wheels. You got it the wrong way around.

[hinkley]

And have you read Brandt’s work?

https://news.ycombinator.com/item?id=36891231

The world is full of papers that are wrong. Including maybe the one this whole thread is about. It’s okay, it happens. Science doesn’t find right or wrong, though a lot of people think so. It finds more wrong and less wrong.

[hatsunearu]

Yeah, that's complete crap. Bike dynamics is well researched and the instability and stability mechanisms are numerous but well understood.

[intrasight]

We don't know how photosynthesis works but we know a ton about GROWING things. And life as we know it depends on photosynthesis.

[dotnet00]

How photosynthesis works at a cell machinery level is high school level biology.

[intrasight]

That's not an understanding of how it works

[dotnet00]

I disagree. It very much is an understanding of how photosynthesis works, it isn't a complete understanding sure, but an understanding nonetheless.

Unless you're going to argue that we don't know how an MRI works because we don't fully understand the physics behind superconductivity.

[MVissers]

I thought we had a pretty good understanding of this up until the molecular and electron level.

Am I missing something?

[intrasight]

My daughter did her research in this field while an undergrad, and explained to me that photosynthesis is not yet well understood. Some of the theories of the mechanism are discussed here: https://physicsworld.com/a/is-photosynthesis-quantum-ish/