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AdeTuono
3,752 posts
96 months
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Getragdogleg said: AdeTuono said: Getragdogleg said: It's not b  ks but if you can't grasp it then don't worry about it. FFS, what's the matter with you. You originally said 'remember that the wheel is stopped still at point of contact with the road and is travelling at twice the speed of the forward motion at the top.'. Just explain to me, in English, how one part of a wheel can be static while another part of the same wheel, at the same time, is moving. If you can't get your head round the simple fact that it's impossible, then there's no hope for you. The rotation will be the same speed all the way round. I am not arguing that, I am saying that in terms of forward motion of a car there are some odd things going on with a wheel. Firstly it is joined at the car through the centre of rotation, that means the top of the wheel when viewed from the side will be travelling forward in realation to the observer at twice the speed the vehicle travelling at. The part of the wheel in contact with the road is still rotating with the rest of the wheel but in relation to the observer and indeed the road it is not moving. The best analogy of this sillynees was provided further up by the guy who posted about the tank on caterpillar tracks. the sections of track that touch the ground are not moving, the sections on top are going forward at twice the speed of the vehicle because they are above the centre of rotation. The tracks are still rotating though, the rotational speed does not vary, the linear speed the observer at the side will see is where to oddities occur. I have had fun here ! Well, that's totally different to what you originally posted, and not analagous with the tank tracks at all. If the centre of the wheel (the rim) rotated inside the tyre, as the drive sprockets rotate inside the tank track, you may have had a point.
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Getragdogleg
3,665 posts
52 months
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It is exactly the same, the rim has nothing to do with it, the centre of the axle is the point of rotation.
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deeen
4,070 posts
114 months
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Ok.
1) Drive the car forward at a steady 10 mph in first gear. Note the revs. Stop the car.
2) Lift the car using a crane, so the car remains roughly level.
3) Engage first gear, maintain steady "10mph" revs.
a) Would you agree that the tops of the driven wheels are moving forward? How fast?
b) Would you agree that the bottoms of the driven wheels are moving backwards? How fast?
c) And the whole wheel is rotating?
If your answer to (b) is 10mph, continue. If not, go back to (1).
4) Drive the crane forward at 10mph
d) How fast are the tops of the driven wheels moving now?
e) how fast are the bottoms of the driven wheels moving now?
f) Is the whole wheel still rotating?
If your answer to (e) is zero, continue. If not, go back to (1)
Put the car down (crane still doing 10mph). What happens?
5) Continue the thread with a different question, because all the idiots are stuck on (1).
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CommanderJameson
20,647 posts
95 months
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deeen said: Put the car down (crane still doing 10mph). What happens? It takes off, obviously.
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R300will
3,598 posts
20 months
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There seriously needs to be a 'can of worms' icon to post here.
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1point7bar
1,091 posts
17 months
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deeen said: Ok.
3) Engage first gear, maintain steady "10mph" revs.
a) Would you agree that the tops of the driven wheels are moving forward? How fast?
b) Would you agree that the bottoms of the driven wheels are moving backwards? How fast?
c) And the whole wheel is rotating?
If your answer to (b) is 10mph, continue. If not, go back to (1). But an hour later the wheel bottoms wouldn't be 10 miles away. You are using a description of linear velocity to describe rotation thus your construct of "10mph" revs.
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walm
3,441 posts
71 months
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Since the car is going 10mph, the wheel will also be rotating at 10mph.
(It has to lay down 10 miles of rubber in an hour in order to transport the car!)
So if you sit in the car looking at a point on the outside of the tyre:
- At the top it is going 10 mph one way.
- At the bottom it is going 10 mph the other way.
- At either side it isn't going anywhere left-right, just up and down.
From the point of view of the road everything is exactly the same but MINUS 10mph.
Once every rotation a point on the tyre will be stationary and once every rotation it will be +20mph.
Forget the up and down motion of points on the tyre for a moment.
Imagine you were in a truck running back and forth. The truck is driving along at 10mph and your top running speed is 10mph.
As you run to the front of the truck your top speed will be 20mph relative to the ground (your running speed plus the speed of the truck).
As you run backwards, at top speed you will be stationary relative to the ground (your 10mph backwards offset by the truck moving forwards).
That is all that is happening to the bits of tyre.
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AdeTuono
3,752 posts
96 months
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Getragdogleg said: It is exactly the same, the rim has nothing to do with it, the centre of the axle is the point of rotation. None so blind as those that will not see.
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Tyre Tread
6,317 posts
85 months
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AdeTuono said: None so blind as those that who will not see.
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AdeTuono
3,752 posts
96 months
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Tyre Tread said: AdeTuono said: None so blind as those that who will not see. Sorry! 
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Getragdogleg
3,665 posts
52 months
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walm said: Since the car is going 10mph, the wheel will also be rotating at 10mph.
(It has to lay down 10 miles of rubber in an hour in order to transport the car!)
So if you sit in the car looking at a point on the outside of the tyre:
- At the top it is going 10 mph one way.
- At the bottom it is going 10 mph the other way.
- At either side it isn't going anywhere left-right, just up and down.
From the point of view of the road everything is exactly the same but MINUS 10mph.
Once every rotation a point on the tyre will be stationary and once every rotation it will be +20mph.
Forget the up and down motion of points on the tyre for a moment.
Imagine you were in a truck running back and forth. The truck is driving along at 10mph and your top running speed is 10mph.
As you run to the front of the truck your top speed will be 20mph relative to the ground (your running speed plus the speed of the truck).
As you run backwards, at top speed you will be stationary relative to the ground (your 10mph backwards offset by the truck moving forwards).
That is all that is happening to the bits of tyre. Thank you, I thought I was surrounded by people who simply cannot imagine that a tyre lays rubber down and picks it up again.
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Getragdogleg
3,665 posts
52 months
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AdeTuono said: If the centre of the wheel (the rim) rotated inside the tyre, as the drive sprockets rotate inside the tank track, you may have had a point. I missed this last night due to tiredness, I am not being rude or anything but seriously, The drive sprockets on caterpillar tracks rotate and interact with the track via teeth that keep the track and sprocket still in relation to eachother, so the sprockets are exaclty like the rim of a car wheel acting on the tyre, there is NO slip at all on a sprocket to track system. So, it is EXACTLY like a tyre except the area in contact with the ground is very long and therefore shows the lack of movement between contact patch and ground much better than a tyre, the contact area of which is small and hard to observe. I am not blind, please look again at this as you are mixed up about what I am saying and making yourself look silly. Yes the tyre is always rotating reletive to itself and the observer. The forward motion of the vehicle means that from a side view the bottom of the tyre is not moving on the ground as it touches, it can't be, otherwise there would be wheelspin or a skid going on, the top is going at twice the speed of the vehicle. Please, look at it again, close your eyes and see what some of us are saying.
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Ayahuasca
16,051 posts
148 months
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1point7bar
1,091 posts
17 months
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An object that rolls against a surface without slipping obeys the condition that the velocity of its centre of mass is equal to the cross product of its angular velocity with a vector from the point of contact to the centre of mass,
v = R ù
Where ù is the angular velocity.
The contact patch still displays angular velocity when the relative linear velocity to the road is zero.
We are talking about two different things.
Linear velocity/speed of rotation.
MPH/RPM
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Getragdogleg
3,665 posts
52 months
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1point7bar said: The contact patch still displays angular velocity when the relative linear velocity to the road is zero. SO to an observer who is measuring the speed of the vehicle in linear MPH the bottom of the tyre is stood still on the road. Therefore the top of the tyre at 12.o.clock is moving forward at twice the linear speed of the vehicle. except it has angular forces that are acting on it to push the car forward and in terms of RPM the tyre is rotating at the same speed all the way round the contact area. Like I said, it's an interesting head bender.
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GTIR
19,072 posts
135 months
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Holy socks, is that argument still going on!
It's like the conveyor belt discussion but more pointless.
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V8mate
34,915 posts
58 months
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Getragdogleg said: SO to an observer who is measuring the speed of the vehicle in linear MPH the bottom of the tyre is stood still on the road.
Therefore the top of the tyre at 12.o.clock is moving forward at twice the linear speed of the vehicle.
except it has angular forces that are acting on it to push the car forward and in terms of RPM the tyre is rotating at the same speed all the way round the contact area.
Like I said, it's an interesting head bender. An explanation with moving images hereThe bit about the part of a train going backwards when the train is going forwards is brilliant!
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TheEnd
12,077 posts
57 months
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Yea, so do we all agree that we know how a wheel works?
Next one is fun... why is ice slippy?
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Getragdogleg
3,665 posts
52 months
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woo hoo ! cleverphysclipssite said: The point at the bottom of a rolling wheel is instantaneously stationary. The axle is moving forward at speed v so, from the geometry shown, the top of the wheel is moving forwards at 2v. We can also understand this in terms of relative velocities. Pretty much what I said to start this entertaining bun fight.
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R300will
3,598 posts
20 months
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TheEnd said: Yea, so do we all agree that we know how a wheel works?
Next one is fun... why is ice slippy? Because the surface is quite smooth, although if you slide across it on your stomach fast enough it can take your skin off so it's not that smooth. trust me.
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