What actually makes a bike turn?

Message

Sev · #21 Unread post by **Sev** » Mon Oct 24, 2005 7:18 pm

Every action has an equal and opposite reaction. So basically Centrifical force is the force that pushes back against Centripital force. Otherwise the bike or car would go over the otherway.

It doesn't actually exist of itself, but it is used to explain why things move to the outside of a turn.

sidecarguy · Tue Oct 25, 2005 3:50 am

wow thsts confusing!!}

now try body turning a bike w/asidecar!!

there it is proven useless only the steering works!!

Sev · #23 Unread post by **Sev** » Tue Oct 25, 2005 5:13 am

sidecarguy wrote:wow thsts confusing!!}

now try body turning a bike w/asidecar!!

there it is proven useless only the steering works!!

That's like trying to turn a car by leaning over in the seat. The extra wheels provide WAY to much stability.

ronboskz650sr · Tue Oct 25, 2005 7:08 am

I wonder what it is that makes a centrifuge so effective at separating solids out of liquids? Or for simulating flight g-forces, by placing g forces on a human being. I wonder how they induced vertigo in us at flight school to demonstrate the effects of head movement in a turn, by just using a centrifuge with a cockpit in it? I always thought the centrifugal force seemed pretty real, as my helmeted head slammed over to the wall during the vertigo demo. Just magic, I guess.

#25 Unread post by **sv-wolf** » Tue Oct 25, 2005 8:10 am

Sevulturus wrote:Every action has an equal and opposite reaction. So basically Centrifical force is the force that pushes back against Centripital force. Otherwise the bike or car would go over the otherway.

It doesn't actually exist of itself, but it is used to explain why things move to the outside of a turn.

It doesn't exist of itself, Sev, because it doesn't exist, period. You can't explain a physical event by reference to a non-existent force. On a cornering bike, centripetal force is not balanced by some mythical centrifugal force but works together with a turning couple comprised of the weight of the bike acting vertically downwards through its centre of gravity and the normal force acting vertically upwards through the contact patch at the wheels.

A force is a vector quantity defined in terms of the mass of the object on which it acts and the acceleration it produces along the line of the force. A bike or any object which is not being acted upon by external resultant forces will move in a straight line and with a constant velocity. If you want to make a bike turn, you must apply a resultant force in the direction you want it to turn, that is towards the centre of the turning circle. That force is centripetal force and on a turning bike it is supplied as friction at the contact patch. If that centripetal force were balanced by an equal and opposite 'centrifugal force' the two forces would cancel each other out, there would be no resultant force and the bike would not turn. Instead it would continue in a straight line.

'Centrifugal force' is a leftover concept from pre-newtonian physics. If, for example, you swing a bucket round your head on a rope, the bucket appears to tug outwards. However, the bucket is not pulling 'outwards' it is trying to move along a straight line that lies at a tangent to the circle in which it is being forced to turn. No resultant force is required to make it do this. This fact is gven by one of the basic laws of Newtonian physics ("if a body is at rest it will remain at rest and if it is in motion it will continue to move in a straight line and with a constant velocity unless acted upon by external resultant forces"). If you suddenly let go of the bucket, it will revert to its straight line motion along a tangent to the turning circle. What it will not do is accelerate away from the centre of turn (as would be expected if it were being acted on by some centrifugal force).

On your first point, note that Newton said every action has an immediate and opposite reaction. He did not say that every force has an equal and opposite 're-force'. The action of the centripetal force acting between the bike and the ground at the contact patch is to move the bike towards the centre of its turning circle across the surface of the earth: the reaction is to move the earth away from the bike (minutely of course, because of the much greater mass of the earth)

ronboskz650sr wrote: I wonder what it is that makes a centrifuge so effective at separating solids out of liquids? Or for simulating flight g-forces, by placing g forces on a human being. I wonder how they induced vertigo in us at flight school to demonstrate the effects of head movement in a turn, by just using a centrifuge with a cockpit in it? I always thought the centrifugal force seemed pretty real, as my helmeted head slammed over to the wall during the vertigo demo. Just magic, I guess

No not magic, but Newtonian physics. Just as with a bike, the 'natural' motion of a particle in a centrifuge is in a straight line, tangential to the circle in which it is being turned by centripetal forces. I am less familiar with the complex forces involved in fluid dynamics, but the principle is exactly the same. A simpler example is a fairground rotor. You stick to the wall of a rotor not because some strange centrifugal force is accelerating you outwards away from the centre of the turning circle, but because a centripetal force (the Normal force at the wall) is at each moment pulling you out of your natural tendency to move in a straight line tangential to your turning circle.

Centripetal force, because it is a real force tends to produce an acceleration in the direction it acts. So any rotating body has an acceleration towards the centre of its circle. You need calculus to demonstrate this mathematically, but it is a fundamental principle of mechanics.

Sev · #26 Unread post by **Sev** » Tue Oct 25, 2005 9:16 am

SV-wolf wrote:Centripetal force, because it is a real force tends to produce an acceleration in the direction it acts. So any rotating body has an acceleration towards the centre of its circle. You need calculus to demonstrate this mathematically, but it is a fundamental principle of mechanics.

Not to argue to much, but if I'm accelerating towards the inside of my turn in a car why do I get pushed against the outside door?

This is the result of the door technically pushing inwards against me. While I being technically heavier then the part of the door I am pushing against should be moved to the outside through displacement . Because both the door and I do not actually move towards or away from the center, there must be a force acting in opposition to the door trying to move through me into the center of the circle. What is this called?

Just like me pushing against a wall, if the wall does not move I have done 0 "work." But I have exerted force against the wall, and the wall has pushed back with equal force hence no work has been done.

#27 Unread post by **sv-wolf** » Tue Oct 25, 2005 10:05 am

Sevulturus wrote:
SV-wolf wrote:Centripetal force, because it is a real force tends to produce an acceleration in the direction it acts. So any rotating body has an acceleration towards the centre of its circle. You need calculus to demonstrate this mathematically, but it is a fundamental principle of mechanics.
Not to argue to much, but if I'm accelerating towards the inside of my turn in a car why do I get pushed against the outside door?

This is the result of the door technically pushing inwards against me. While I being technically heavier then the part of the door I am pushing against should be moved to the outside through displacement . Because both the door and I do not actually move towards or away from the center, there must be a force acting in opposition to the door trying to move through me into the center of the circle. What is this called?

Just like me pushing against a wall, if the wall does not move I have done 0 "work." But I have exerted force against the wall, and the wall has pushed back with equal force hence no work has been done.

The simple answer, Sev is that you need a real force to act on you to turn you away from straight line motion. The centripetal force acting on your turning car will be through traction at the wheels. Traction/friction is a real, identifiable, measurable force. That force is then be transmitted to every compenent of the car by a variety of mechanical contact forces within the structure of the car. It is finally transmitted to you through the friction between the seat you are sitting on and your butt. However you are not a rigid entity and your centre of gravity lies above the frictional force, so, in simple terms, while your butt is being acted upon directly by a frictional force, the upper part of your body is not. It has an inertia. The upper part of your body will initially, still persue its straight line motion, and the subjective impression is that you will be thrown against the side of the car. If the centripetal forces acting on the car are very great - and the frictional forces acting between you and the seat are insufficient to match them, you and the object will slide bodily towards the side of the car which is on the outside of the turning circle.

At the point when you hit the door, the centripetal force at the car door (transmitted mechanically through the frame of the car from the contact patches) then acts directly on you, forcing you out of your straight line motion and you then also move with the car in its turning circle. If the door lock were suddenly to give and the door to open, that force would be removed and you would immediately resume your straight line motion tangential to the turning circle and go skidding down the road.

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.

ronboskz650sr · Tue Oct 25, 2005 10:55 am

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

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

niterider · Tue Oct 25, 2005 10:55 am

When I go around a curve I don't lean to make the bike turn, I lean with the bike as I sit upright on the seat but at the same angle that the bike has with the road. The only riders that lean away from thier bike into the turn are riders that are running at excessive speeds around curves. If one is to lean without turning the handle bars, I really believe that the bike and rider would hit the dirt. I am not going to try it to prove my point.

Sev · #30 Unread post by **Sev** » Tue Oct 25, 2005 11:38 am

SV-wolf wrote: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.