[alancalverd]

Which is fine, until you remember that you can fly a stunt plane upside down indefinitely...

Only if it has a symmetrical wing cross section or you are prepared to add a lot more thrust (engine power) than right way up.

You can measure the Bernouilli lift in a wind tunnel, or realise that it is the same phenomenon as the venturi suction that makes the carburettor ice up and stop the engine working when you are having a bad day (yes, sadly, we still use carburettors in lots of small planes.)

There are lots of compromises in wing design, but nobody would go to the trouble of making the cross section asymmetric for slow flight, if it didn't provide more lift than a symmetric wing.

[alancalverd]

It turns out that what keeps a bike upright is... the rider.

Funny, that. You can get off and push the bike away from you, and it stays upright for quite a while with nobody on it!

[Bored chemist]

It turns out that what keeps a bike upright is... the rider.

Funny, that. You can get off and push the bike away from you, and it stays upright for quite a while with nobody on it!

Funny that- you take the big heavy weight off the top and it becomes more stable.
"for quite a while"
Not nearly as long as it stays upright when the rider is on it.
What was your point?

you are prepared to add a lot more thrust

They are.
And, in those circumstances the "classic" explanation of how planes fly should lead to the plane power-diving into the ground.
But the plane doesn't do that.
And that's because the "theory" is, at best, not the whole story.

[alancalverd]

But the plane doesn't do that.

because the key element that determines lift, for all aerofoils, is the angle of attack - the angle between the wing chord and the direction of travel. It's all in the textbooks, and if you use an inverted aerofoil with a negative AoA it does indeed produce  downthrust. But don't tell the people who design cars, who think it's entirely due to magic.

The art of inverted flight is to maintain the appropriate AoA . The more symmetrical the aerofoil, and the smaller the angle of incidence  (the angle between the wing chord and the fuselage reference line) the easier this is, because your horizon reference changes less. You can find plenty of images of planes like the Extra or Pitts, which are designed for sustained inverted flight, compared with say a Cessna 172, with a high-lift (very asymmetric) wing and no taste for such silliness.   

[alancalverd]

Not nearly as long as it stays upright when the rider is on it.

So we put a big heavy weight on the bike (i.e. a rider) and it stays upright? It's remarkably difficult to keep a bike upright when it's stationary, but no problem when it's moving quickly, and you can steer it by leaning, hands off, which rather suggests an inbuilt dynamic mechanism. Big motorbikes weigh a lot more than the rider, who can't shift very much, but they also stay upright, and have angled front forks. One wonders why.

I thought you had spent your best years in Oxford - hasn't bicycle technology reached the midlands yet?

[Bored chemist]

It's remarkably difficult to keep a bike upright when it's stationary

That's because you fall over sideways, but you can't easily move a stationary bike sideways to compensate.

hasn't bicycle technology reached the midlands yet?

Oxford was awash with bikes when I was there- about 35 years ago- and I imagine it still is.
But it's not the technology that we are addressing, it's the understanding. People rode also horses before they had any meaningful understanding of how a horse worked.

So we put a big heavy weight on the bike (i.e. a rider) and it stays upright?

Yes, though, if the big heavy weight stops actively controlling the system, they fall over.
That pretty much nails the point.
What keeps it upright is careful direct control by the user.

You pretty much say so yourself...

you can steer it by leaning

It is clearly you that does the control,.

In any event, it can't be the gyroscope effect.
People have made bikes with contrarotating wheels to cancel that out, and they are still rideable.

[Bored chemist]

because the key element that determines lift, for all aerofoils, is the angle of attack

I know that.
You know that.
It's more or less my point.
The textbooks all bang on about Bernoulli, but it's the "pushing the air down" that actually keeps you up.

[Halc]

So we put a big heavy weight on the bike (i.e. a rider) and it stays upright?

Yes, though, if the big heavy weight stops actively controlling the system, they fall over.
That pretty much nails the point.
What keeps it upright is careful direct control by the user.

Alan is right. A bicycle (with or without this heavy brick on the seat) will stay upright as long as it maintains enough speed to do so, which is indefinitely in a frictionless environment. A bicycle is designed to not fall over on its own while at speed. It is the angle of the fork and the offset (the fork is curved, not straight) which does this.

You pretty much say so yourself...

you can steer it by leaning

It is clearly you that does the control,.

It is you that can choose where the bike steers, something the heavy brick isn't going to do. But it's not you that keeps it upright. You can fall asleep on a bike and assuming you're sufficiently strapped to it, it doesn't fall over.

In any event, it can't be the gyroscope effect.

No, I agree it's not that.

[Bored chemist]

bicycle (with or without this heavy brick on the seat) will stay upright as long as it maintains enough speed to do so,

It is a relief to know that bike crashes can't happen and that any toddler can ride a bike because all they need to do is provide motive power.

No need for any complex controls, so obviously the people who build cycling robots are fraudsters, and the crashes are staged.


Of course, it's a simple matter to prove this- you can use an electric bike.

[alancalverd]

it's the "pushing the air down" that actually keeps you up.

If you do the measurement in a wind tunnel you find that the pressure increase under a fairly simple aerofoil accounts for about 30% of the lift force at optimum AoA, and the pressure decrease over the upper surface, about 70%. That's why spoilers are fitted to the upper surface. A single-element aerofoil (Rogallo wing, sail) shows a smaller difference, and a flat surface will generate compression lift even if the upper surface is completely stalled.

[alancalverd]

the people who build cycling robots are fraudsters

Not entirely, though they are certainly not contributing much to the sum of human happiness.

The stable capture range of negative feedback is fairly limited for a bike. Too much tube rake will make it difficult to turn and will absorb a lot of energy in the feedback mechanism, so gross movement of the rider is required to induce or oppose large tilt angles.  https://www.bing.com/videos/search?q=motorbike+tube+angle&&view=detail&mid=3733E9AC47545AC580F83733E9AC47545AC580F8&&FORM=VDRVRV explains it very nicely.

[hamdani yusuf]

This video claims that we still don't know how bicycles work.

But somehow we can build self balancing bike.

[alancalverd]

This video claims that we still don't know how bicycles work.

Interesting point. When a politician says "we" he means "you". When a journalist says "we" he means "I".

Images of penny-farthings and the like can be misleading because the front tube rake can be quite small - as little as 0.5 degree will confer some stability because the trail distance (from the front tube - ground intercept to the point of wheel contact) is large and, if the bike has solid tyres, fairly constant.

[Bored chemist]

explains it very nicely.

Wow!
It seems the terminology for rake has changed compared to the page I posted earlier.
https://www.cam.ac.uk/research/discussion/opinion-how-does-a-bike-stay-upright-surprisingly-its-all-in-the-mind

They now call it a "fork offset".

Are you labouring under the impression that the video actually explained why it was  easy to fall off a Raleigh Chopper which has a small trail?


Chopper.jpg (383.21 kB . 1139x915 - viewed 336 times)

[Bored chemist]

It's a bit beside the point.

Q "What is the actual explanation of the gyroscopic effect?"
A "Actual physics."

[alancalverd]

Offset is the distance the axle is ahead of the contact point. Reduced offset improves the response to intentional steering, increased trail improves stability.

Problem with small wheels is to get enough trail for hands-off stability without making the bike unsteerable.

Chopper instability probably involves seat tube angle and wheelbase  too - there's far more information out there than I can be arsed to sum up!

[Bored chemist]

Which bit of
 

why

did you not understand?
When you answer, be sure to take account of the video hamdani yusuf posted which points out that you can even have a negative trail and still ride the bike.

[evan_au]

People have made bikes with contrarotating wheels to cancel that out, and they are still rideable.

People investigating bike stability have also made bikes with vertical forks (or even tilted-back forks) and they are still rideable - but it takes more practice, more effort and failures are more frequent.

I imagine that a bike with reversed steering (or wearing glasses that reversed left and right) would also be rideable - but you would have to unlearn a lot of childhood experiences before you could learn to ride it.

[hamdani yusuf]

People have made bikes with contrarotating wheels to cancel that out, and they are still rideable.

People investigating bike stability have also made bikes with vertical forks (or even tilted-back forks) and they are still rideable - but it takes more practice, more effort and failures are more frequent.

I imagine that a bike with reversed steering (or wearing glasses that reversed left and right) would also be rideable - but you would have to unlearn a lot of childhood experiences before you could learn to ride it.

Youtube also has videos about it.

[evan_au]

bike with reversed steering

The US Space Shuttle had a strange quirk that when gliding at hypersonic speeds, the effect of the left/right control surfaces was reversed, compared to the handling at lower speeds (and compared to all other aircraft).
- Because it was "fly by wire", they reversed the high-speed steering in software
- But I do wonder about the handling in the transition zone, where anything you did with the control surfaces would have no effect...   Maybe it just flew like a paper dart...

Knowledge ≠ Understanding

I don't think riding a bike is a matter of knowledge or understanding - it is learned reflexes, at a subconscious level.
- Yes, you do engage knowledge and understanding when trying a new task
- But it eventually becomes learned at the subconscious level, and you can handle routine conditions without thinking about it (or while thinking about something entirely different)
- This is especially common with so-called "muscle memory", which is encoded in neurons well outside your frontal cortex (where our conscious thought resides)