Naaah its not. The further from the centre, the greater the distance needed to complete one revolution. A smaller wheel at a given speed will be rotating faster than a larger wheel at that speed.
What is this pin for?
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Naaah its not. The further from the centre, the greater the distance needed to complete one revolution. A smaller wheel at a given speed will be rotating faster than a larger wheel at that speed.
Okay, back to basics.... Imagine you are on a roundabout-- would it be easier to stay on the roundabout if you were stood in the centre or if you were stood on the outer edge?
AberdeenAngus
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I understand what you are saying, smaller wheel, higher tangential force etc, I just don’t agree that it causes the valve to leak.
I think the reason they put the valve on the spoke was to make it easier to get at.
Rotational force will always be the same, for a given speed, however the centrifugal force does increase with distance.
Porsche/Bridgestone had a video years ago, showing a rubber valve bending as speeds increased, compared to a metal valve which did not distort. The purpose being don't use rubber valves !
Maybe it is there, just for ease of access.
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Porsche/Bridgestone had a video years ago, showing a rubber valve bending as speeds increased, compared to a metal valve which did not distort. The purpose being don't use rubber valves !
Maybe it is there, just for ease of access.
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AberdeenAngus
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Sooo, do these bike with the valve in the spoke, have tyre pressure sensors?
Back to basics, imagine you are a dot of paint on a tyre As the tyre rolls along the road the dot will meet the road and travel round at the same speed as the road passes under the tyre, if that is 50mph then the dot will be traveling at 50mph round the tyre regardless of tyre circumference, yes a smaller circumference will spin faster but the dot will still be moving at 50mph, it is that speed that gives the outward fling, so a small wheel spinning fast will have the same 'centrafugal force' as a bigger one spinning more slowly given the same road speed.
fred_jb
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Actually there is no such thing as centrifugal force. Objects moving in a circular path experience a constant force towards the centre of rotation. Without this, as all objects do, they would continue in a straight line, just like the piece of gravel stuck in your tyre tread does when it finally works free and gets flung off to scar the front of your engine. Basically to avoid moving in a straight line an object has to experience a force towards the centre of rotation to make it continually change direction.
I couldn't do it now, but vaguely remember going through a mathematical proof of this when studying A level physics. This link explains:
https://www.physicsclassroom.com/class/circles/Lesson-1/The-Forbidden-F-Word
AberdeenAngus
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Sort of...he does have a point.
Yes, the rim is moving at the road speed regardless of the diameter of the wheel. It must do, its in contact with the road. So the same centrifugal (outward) force.
However with a smaller wheel, the tangential force on a point on the rim will be greater as the point is being pulled in a sharper angle than with a larger wheel. A small wheel has to 'work' harder to cover the same ground, as it were.
Still don't think it would be enough to make a valve leak though.
AberdeenAngus
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The inward force is the centripetal force. The outward force is the centrifugal force.
I know we could go into semantics but lets not do that.
fred_jb
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The body of the valve is continually being accelerated towards the centre of the wheel, a movement which it must comply with because it is rigidly connected to the wheel. The moving pin in the centre of the valve is however free to move, up to a point, so it's inertia will tend to cause it to try to move in a straight line and not follow the movement of the valve body, and thereby spring open. The reason that the pin does not move and allow air out once the wheel starts spinning is that there is a considerable force acting on it in the opposite direction due to the air pressure inside the tyre forcing it into contact with the valve body, and being very light, its inertia is normally not enough to overcome this force. If the air pressure was very low or the rotational speed very high, you might get close to a condition where the valve opened, but I think that is extremely unlikely.
fred_jb
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It's not semantics - there is a force towards the centre of rotation which causes the continual change of direction of an object moving in a circular path. Centre fleeing, which I believe is what centrifugal means, would be the correct name for a force acting outwards from the centre of rotation if such a force existed, but it does not.
My head hurts....
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AberdeenAngus
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The reason the valve doesn’t let air out is coz there’s a spring in the valve.
fred_jb
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Good point, though depends on the type of valve, the Presta type used on bicycles seals on air pressure alone, whereas the Schrader type used on motorised vehicles is spring assisted, though I suspect the air pressure still provides most of the force keeping the valve closed. Either way there is an opposing force preventing the valve from opening even though there is a tendency for it to do so.
Quote on Page 183 of R1250 GS Adventure Riders Manual:-
"WARNING Tendency of valve inserts installed vertically to open by themselves at high riding speeds."
"WARNING Tendency of valve inserts installed vertically to open by themselves at high riding speeds."
fred_jb
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Glad I have the GS with alloy wheels and right angle valves then! Seems like as well as loose spokes, there are other issues to be aware of.
Is it not a steering wheel then 
Bloody good job I’ve got TPS warning then for when I give my bike the beans !
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