OT How does a bicycle turn right when leaned right?

Have you seen Razor's new Ripstik? The front end is the opposite of a bicycle front end. The caster is leaned backwards. Search Youtube for Ripstik demonstrations.

I understand that if you lean a caster forwards or backwards, the wheel will try to keep itself pointed straight ahead. But I don't understand what happens when you lean to the right (or left). I've read that the reason a bicycle wheel turns to the right (when leaned right) is to help put the bicycle back in its lowest position.

But, getting to the point... When I put a rear caster on a Ripstik that is leaning forwards like a bicycle caster does, when the Ripstik is leaned to the right, that back wheel is going to turn to the left. Anybody disagree?

Thanks.

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Reply to
John Doe
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Post this question in rec.bicycles.tech. Theres a few guys over there that will understand the mechanics of this.

Cheers

Reply to
Martin Riddle

,snipped>

When a bicycle wheel is upright, the important part, the contact patch, forms a rolling cylinder. When the wheel is leant to the right, this becomes a slice of a cone and so rolls in a circle.

IOW, when leant to the right, the circumference of the left hand side of the tyre rubber on the contact patch is greater than the right hand side.

This is particularly obvious on motorcycle tyres, which have a more or less circular cross section.

Cheers

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Syd
Reply to
Syd Rumpo

It happens because of the rake angle of the front forks.

Reply to
DecadentLinuxUserNumeroUno

There was a Scientific American article--I think it was a guest Amateur Scientist--about trying to make an unrideable bicycle. Reversing the caster on the front forks did it.

Cheers

Phil Hobbs

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Reply to
Phil Hobbs

I am surprised that no one has mentioned that the plant (bike) has a "zero" in it's right half plane control loop. Robots riding bikes may prove difficult.

Cheers, Harry

Reply to
Harry Dellamano

Maybe referring to "The stability of the bicycle" April 1970 by David Jones.

I agree with that stuff. It's not some cone shape, but it's also not just the caster angle. If you roll a hula hoop, it will turn in the direction that it leans, just like a bicycle front wheel. And the faster you go, the more stable it is.

There are at least two forces at work, the caster angle and gyroscopics. If you lean the caster to the front or to the back, the wheel will tend to stay straight. But when you lean, the caster effect, because of gravity, would turn the wheel in the opposite direction. There just isn't a whole lot of castor effect on a bicycle front wheel. However, there is lots of castor effect and little gyroscopic effect on a Razor Ripstik.

Standing still, if you lean a bicycle to the right, the front wheel will turn to the left.

Reply to
John Doe

If you roll a hula hoop or a bicycle wheel slowly enough, any gyroscopic effect becomes insignificant. It will turn right if it's leaning right because of the conical rotating contact patch.

You can do this by holding a bicycle wheel at an angle to the vertical while pushing it forward as slowly as you wish. The rolling cone thus formed will naturally describe an arc. Try it.

Basic things first, then gyroscopes, rake and trail.

Cheers

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Syd
Reply to
Syd Rumpo

In my experience it just falls over.

How does a hard plastic hula hoop deform into a conical shape?

The conical deformation is a theory in inline skating, too.

I have tried it, in fact. I'm dying to make an inline skateboard. Many months ago, I tested the conical theory. I put five wheels inline under a board. Even using easily deformed inline skate wheels, the thing simply did not turn by leaning. Even when leaning and at the same time putting my weight to the back like we do when skating.

The likely real method that an inline skate turns is because the skater generates rotating force. Leaning back puts the pressure on fewer wheels, making the rotating axis shorter. Also, since the wheels are pliable, the turning force applied causes the skate to turn as the wheels grip and regrip the pavement while moving. That and some sliding.

Reply to
John Doe

I do not believe that such a pole exists, The bicycle riding robots already exist. Just youtube for it.

?-)

Reply to
josephkk

No, I mean you are wheeling it slowly, holding its axle at an angle from the horizontal while you push it forwards. It will describe an arc unless you force it not to, in which case the rubber will have to slip.

The contact patch must have a non-zero size. If it's leaning, one side will have a greater diameter than the other.

Think of a ball on an axle. If the axle is held at other than horizontal, as you push the ball forwards it will turn left or right if not physically constrained. There was in fact a wheelbarrow based on this (the 'Ballbarrow', I think, invented by Dyson). It doesn't matter how little downward force (weight) you apply, the geometry will make it turn.

It's more than a theory. The diameter of a bicycle tyre is greatest around its centre line, the diameter decreases as you move away from the centre line. If the wheel is not vertical then its contact patch is essentially conical.

I don't know much about skateboards, but if the wheels were fixed in a line, say like a bike with the steering welded straight, then canting them over would have the effect of losing grip and skidding to some extent. The wheels would want to describe an arc, but would be physically constrained from so doing.

Dunno, but as I said, simple things first. Set a bike wheel rolling along on its own and it will turn one way or t'other depending on which way it starts leaning, and not because of rake, trail or gyroscopes, but because of geometry. As it slows down and leans more, so the radius of the arc it's following decreases until grip is lost.

Cars screech as they corner because their essentially cylindrical tyres are being forced around an arc and grip can't be maintained, even at low speeds. Bikes lean into corners and don't normally screech because their circular cross-section tyres form conical contact patches appropriate to the radius of the turn.

Cheers

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Syd
Reply to
Syd Rumpo

A RHP Zero does not prove insurmountable, just some heads up and maybe lower loop bandwidth. A little Goggling yielded this;

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I also have a garage door with a RHPZ in its control loop, it must bepulled first before pushed open. That non intuitive $hit is all over the place. Cheers, Harry

Reply to
Harry Dellamano

I don't know of any studies that prove the conical theory. And as I said, my own experiment proved it wrong. And there's no way you're going to get a conical contact surface out of a rolling hard plastic hula hoop that turns in the direction it is leaning. And, again, the faster a cycle travels, the more stable it is. That's obviously due to gyroscopics. And anybody who has tried to lean a gyroscop should notice that it turns.

Good luck with your theory, here on Earth.

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Syd Rumpo  wrote: 

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Reply to
John Doe

Just geometry and observable reality. A hard plastic hula hoop on a hard floor starts to turn but very quickly loses grip as the sideways force overcomes friction and slips on to its side. A soft, more easily deformed rubber tyre does much better and rolls around in ever decreasing circles till it loses grip.

Experiments with counter-rotating wheels on bicycles have demonstrated that the gyroscopic effect is small. You could look that up. The Ballbarrow example proves steering by deformation of the wheel (ball) into a cone. You could look that up too.

It's not magic. A flat surface - the contact patch - describing an arc must have a conical shape unless it is being forced to slip.

Cheers

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Syd
Reply to
Syd Rumpo

As a "wheel" of any kind begins to "lean over" at an angle other than perpendicular to the surface it is on, the front edge of that "wheel" begins to "step" out a turn that is described at other than the angle the perpendicular rolling wheel describes.

When said wheel is on a perpendicular fork the arc described is small. If the fork is angled forward, a "caster angle" and a "camber angel" is introduced into the geometry and the arc is described with a greater voracity.

what this says is that the angled fork capitalizes on the arc a tilted wheel gets imparted upon it.

For a regular street tire, the contact patch is usually a nice ellipse. during a turn... a 'leaning' turn, (which they all are)it rolls a bit over toward the side of the tire tread and the contact patch actually enlarges from the centrifugal pressure. I do not know what the shape becomes. The conical reference sounds to me like a car tire with a wide contact patch, which would turn a bit conical as the turn takes place.

Were one to turn such a wheel without any caster or camber (perpendicular), a turn takes place due to the edge of the wheel contacting the ground first pushing the whole assembly in that direction.

WITH caster and camber said wheel will cut away at a more aggressive angle, and the way it contacts the ground is the reason.

On a bike, once the lean starts the aggressive angle change, a centrifugal force even gets imparted and the bike will follow that track all by itself, even setting the steering angle to its optimal position.

The fork rake angle, and the offset the front axle has with reference to the axis the fork "steers" in are what set up the aggressiveness of steering 'push' which gets imparted when the bike is leaned to one side.

This is why some bikes are easier to ride hands free than others. The rate of change is too fast on some and the corrections come too slow.

Reply to
DecadentLinuxUserNumeroUno

What a load of bollocks! Anyway, what are you doing here, Nymbecile? Can't you find some bogs to clean? Be a good girl and run along now.

Reply to
Pomegranate Bastard

Cut the guy some slack. He's cleaned up his act amazingly--may we all follow that example!

Cheers

Phil HObbs

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Dr Philip C D Hobbs 
Principal Consultant 
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Reply to
Phil Hobbs

AKA pavement princess?

Only found in red light districts;-)

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Reply to
Fred Abse

BikeTalkKTH2006.pdf

I missed that you are talking about a Zero rather than a Pole. Big difference.

?-)

Reply to
josephkk

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