What actually makes a bike turn?

[sv-wolf]

The force of the car door accelerating you to the centre of the turning circle has no particular name as far as I am aware, except that it is the centripetal force transmitted to your body by mechanical forces within the structure of the car.

That would be centrifugal. Because it is not something that is truely measurable as it is not a direct force we are told that it does not exactly exist. My point through all of this was not that centrifugal force (or an objects desire to travel in a straight line) is overcome by the lean that is the direct result of application of the bars.

Moving your body from one side to the other will have the same effect, but it's much slower. And WAY less responsive.

It can't be 'centrifugal' since it acts towards the centre of the turning circle, not away from it.

I didn't quite understand your question first time, but I've reread it and the penny has just dropped. This might answer your question.

You can analysie the forces on a materail object in many ways depending on what you wish to achieve. In a relative universe all forces have to be analysed in relation to some fixed point. Two obvious ways are to analyse the forces acting internally within an object, ignoring the relationship of that object to its environment or to analyse the forces acting externally on it from its environment.

If you analyse the external forces acting on a turning car and its passengers, treating them as a single object, then the accelerating force (the centripetal force) has no balancing force. if it had a balancing force, as I said, there would be no resultant force and therefore no turn.

If you analyse the companent forces within a turning car and specifically on two bodies within it, the car body itself and you as passenger, then where you are thrust against the car door, yes there is an equal and opposing force to centripetal force. This is called the Normal force. The force that the car door exerts on you (the centripetal force) is equalled by the Normal force which you exert on the door. This has to be the case because if there were a resulting force between you and the car wall you would accelerate across the car.

But this is viewing the situation locally, ie between two bodies, you and the car. It ignores the relation of the car to the road. As I said, every analysis of forces is relative to some defined point.

So, if you consider the forces both on the car and on the two bodies comprising the car you have as follows: one force P, the centripetal force acting on the car body (as transmitted from the tyres), another equal force P which the car body transmits to you, and a third force - P, this is the normal force which you exert back on the wall of the car. The Normal force has to take a negative sign because forces are vector qunatities which vary according to direction as well as intensity. Add these three forces together: P + P - P and you get a net resultant force of P. This is the resultant centripetal force acting on the system as a whole.

Phew!

If you still believe in the existence of centrifugal force then I suggest you go down to you local bookshop or library and look up any text on mechanics or applied mathematics of High School or Pre-University level which deals with rotational force.

[oldnslo]

So this matters to the reality of riding a motorcycle how? Are you advocating micro-analysis of every motion performed in the act of turning? Actually thinking of every detail involved during the act of turning would probably send a rider off the road crashing into the woods. Generally, most people come hard-wired to cope with the requirements of turning a two-wheeled, inline-tracked vehicle. Notice any 5-year old on a bicycle without training wheels.
But, this thread might be just an answer to a question that should never have been asked.

[sv-wolf]

No offense, but I'll stick with NASA on this one. All semantics aside.

http://observe.arc.nasa.gov/nasa/space/ ... index.html

Just read this Ron. if you scan the whole article page by page you'll see that what they call the centrifugal force is actually a force only experienced internally to a turning system. Ie, the reference frame is the car itself. What they call a 'centrifugal force' is no more than a Normal Force as applied to the internal condition of a rotating object. The 'centrifugal force' mentioned in this article is useless for defining rotational motion from an external reference point, ie the road or the earth, and that is why it is a loose and sloppy way of talking.

it also confuses people. It has caused Sev to try and apply this so-called force to a rotating object when viewed from an external frame of reference. As a result he is convinced that two mutually cancelling forces (centripetal and centrifugal) which are equivalent to no force at all can turn his bike round a corner.

Within the engineering community this kind of loose talking may well suffice since everone has a deep understanding of the vectors involved, but it is no more than a piece of jargon. I doubt whether any mathematicians would countenance it.

[Sev]

No, that's just what you think I'm saying.

It's two different forces one competing against the other that working together hold us up and turn us. Like I'd said, while Centrifugal cannot actually be called a true force it is a term used to describe the action of objects appearing to be pushed to the outside of a turn as a result of the acceleration in a straight line. This is what keeps the bike upright and travelling in a straight line.

This is what must be overcome through the lean to turn. If you didn't have that force acting in opposition to the lean you'd just fall over. Try it on a stopped bike.

I'm sorry if I pulled in other examples :rolleyes:

[txbadboy]

heres an experiment that should answer some questions...

take a coin or anything nicely round, resembling a wheel. roll it and notice when it slows down, it leans to the side and begins turning....and it does this without handlebars. i believe this happens because as the "wheel" leans, the surface area is reduced and velocity forces it to pick up the slack by spinning faster. like on a wheel chair, spin on wheel faster than the other, it will turn.

did that make sense to anyone else?....

[sv-wolf]

So this matters to the reality of riding a motorcycle how? Are you advocating micro-analysis of every motion performed in the act of turning? Actually thinking of every detail involved during the act of turning would probably send a rider off the road crashing into the woods. Generally, most people come hard-wired to cope with the requirements of turning a two-wheeled, inline-tracked vehicle. Notice any 5-year old on a bicycle without training wheels.
But, this thread might be just an answer to a question that should never have been asked.

Hey John. Some people might be interested in the mechanics of their bike's motion, some might not. The analysis of rotational motion probably doesn't relate to the reality of riding a motorcycle, but then neither, for many of us, do discussions about the minutiae of engine construction or the history of the Vincent motorcycle company. These things are just part of a whole spectrum of bike-related subjects from which people choose their interests. Let's not legislate what people should or should not get interested in.

[sv-wolf]

No, that's just what you think I'm saying.

It's two different forces one competing against the other that working together hold us up and turn us. Like I'd said, while Centrifugal cannot actually be called a true force it is a term used to describe the action of objects appearing to be pushed to the outside of a turn as a result of the acceleration in a straight line. This is what keeps the bike upright and travelling in a straight line.

This is what must be overcome through the lean to turn. If you didn't have that force acting in opposition to the lean you'd just fall over. Try it on a stopped bike.

I'm sorry if I pulled in other examples :rolleyes:

My point is simple. You cannot have your cake and eat it. You cannot in on the one hand say that 'centrifugal force' is not a true force and on the other hand claim that it balances a 'true' force. Only another 'true' force can do that. (if something is not a true force then it is not a force.) Your thinking is very muddled on this Sev. Put it this way, how would you quantify and calculate with something that is not a 'true force'

The bottom line is that 'centrifugal force' cannot turn anything. According to the definition given by Ron's NASA guys it is just a terminological inexactitude for a Normal force (which is a 'true' force BTW) and which by definition has no turning power since it never does anthing other than oppose and cancel out another force. Without a force, nothing can turn, since it's natural motion is in a straight line. Argue this one with Newton.

As far as needing another force to balance your lean is concerned, you are thinking purely in terms of static objects. When things are in rotational motion you are in a whole differenjt ball game. In any case there is a counterbalancing force. This is the centripetal force at the tyre and the inertial condition of the bike acting through the centre of gravity. Mathematically these forces are quantitatively in balance. You don't need some additional mythical force to explain what happens.

[Sev]

I
Never
Said
Centrifugal
Force
Makes
You
Turn

I said it acts in opposition to the turn, which is technically true, even according to what you said. As it is the bike trying to carry on in a straight line!

[sv-wolf]

I
Never
Said
Centrifugal
Force
Makes
You
Turn

I said it acts in opposition to the turn, which is technically true, even according to what you said. As it is the bike trying to carry on in a straight line!

I haven't had so much fun in years.

Another muddle. If centrifugal force opposes the turn, why does the bike continue to turn? Do you mean it opposes the lean? Two different things.

Look, your previous post seems to suggest you are trying to view a three dimenstional dynamic system as if it were a two dimensional static one.

Consider this

Static situation. Place a vertical object of a piece of paper. While the object's centre of gravity lies within its base it stays upright. Push the object so that it's centre of gravity falls outside its base, and it falls over. A force acting horizontally against the direction of lean will steady it and maintain it statically in the lean. Agreed?

Dynamic situation. Place a vertical object on a piece of paper. Push it till its centre of gravity lies outside its base. As it begins to fall, pull the paper in the direction of its fall. If you do this right, the leaning object will continue to lean without falling and a dynamic stability will be achieved. Where is your necessary balancing force now? There is no rotation here, so no possibility of your very wonderful centrifugal force. The only additional movement here is in the direction of the lean, not against it.

When you are dealing with dynamic situations, especially those in three dimensions, simple notions of mutually opposing forces in two dimensions will not suffice to explain what happens in the real world. And once you begin to resolve forces in three dimensions you are in a different ball game althogether.

You just don't need the notion of 'centrifugal force' to explain how a bike turns. The concept just muddies the waters. I understand your reluctance in this. The situation is a complex system of dynamic and constantly changing vectors, not easy to visualise, and partly counter-intuitive. But a cornering bike is not a static object in two dimensions, so the simple principles of statics don't apply.

What's your maths like, Sev? It is years since I was at university playing with equations relating to rotational movement, so I will need a little time to remember the details, but if you are game I will think them through and post a full mathematical demonstration here for you to consier. Are you up for it?

[niterider]

The more I read on here I am almost convinced that the only two forces here are cetrifugal pull and the gravitational pull. This is all I have gotten from all these writing. An object with small wheel would act the same way as the bike with big wheels. I have very little Ed. in physics just trying to make sense of all this. Even a person running around a corner has to compensate for the pull of both forces.
Just one more thing. How does a kids scooter the kind with the little bitty wheels stay up right while riding? Is it the cetrafugal force of the wheels?