I do sometimes think they should leave them on until they are sure they need replacing, quite often, they put a new one right by where teh old one was and of the same size, and leave a mark on the wheel in the process.
A wheel out of balance by say 20g, may increase to a force of many Kg at speed, due to centrapetal force.
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If they put the new one on "right by where the old one was", leaving a visible mark, doesn't that show that the old one would have been in the wrong place for the new tyre?
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A wheel out of balance by say 20g, may increase to a force of many Kg at speed, due to centrapetal force.
Ah, centrifugal and centripetal force ~ one of my favourite topics of conversation. There are two misconceptions about these forces.
Firstly it is commonly believed that centrifugal force actually exists. In fact it doesn?t exist at all ~ the expression centrifugal force is merely used as a way of creating an equation in which both sides of the equation are considered to be numerically (but not directionally) equal. In other words what is commonly called centrifugal force is really just the directionally opposite of centripetal force, and this is the force radially inwards REQUIRED to keep an object moving in a circle. This force MUST exist if there is to be motion in a circle. But there is no such thing as a centrifugal force outwards that the centripetal force has to balance, as a lot of people think the equation implies.
If a centripetal force doesn?t exist then a moving object will move in a straight line. (Newton?s first law of motion says that an object will continue in a state of rest or uniform motion in a straight line unless acted on by an external force.) So? centripetal force (i.e. a radially inwards force) makes things move in a circle. Remove the centripetal force and the object then moves as it was doing at the time at which the force was removed ~ which is tangential to the circle. This leads me to the second common misconception which is that if you twirl a conker in a circle and the string breaks, the conker will fly radially outwards. In fact it continues in a straight line at right angles to the string at the time it broke. Similarly if a balance weight comes adrift it will fly off tangentially.
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L\'escargot.
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>>Ah, centrifugal and centripetal force......
"The pragmatic method in such cases is to try to interpret each notion by tracing its respective practical consequences. What difference would it practically make to any one if this notion rather than that notion were true? If no practical difference whatever can be traced, then the alternatives mean practically the same thing, and all dispute is idle. Whenever a dispute is serious, we ought to be able to show some practical difference that must follow from one side or the other?s being right."
-dedicated to the memory of John Stuart Mill from William James around 1900 and a little bit.
>>one of my favourite topics of conversation.
Quotations; one of mine.
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-dedicated to the memory of John Stuart Mill from William James around 1900 and a little bit.
Source: What is Pragmatism (1904), from series of eight lectures dedicated to the memory of John Stuart Mill, A New Name for Some Old Ways of Thinking, in December 1904, from William James, Writings 1902-1920, The Library of America; Lecture II .........actually!
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L\'escargot.
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>> A wheel out of balance by say 20g, may increase to >> a force of many Kg at speed, due to centrapetal force. >> Ah, centrifugal and centripetal force ~ one of my favourite topics of conversation. There are two misconceptions about these forces.
Fair enough, but why bring it up when CP correctly identified it as cetripetal in the first place?
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Fair enough, but why bring it up when CP correctly identified it as cetripetal in the first place?
Doh!
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L\'escargot.
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I do sometimes think they should leave them on until they are sure they need replacing, quite often, they put a new one right by where teh old one was and of the same size, and leave a mark on the wheel in the process.
The problem with that approach is that in most cases the new tyre will have different properties than the old, the high spots in the moulding will be in different places etc. After all there are an infinate number of relative positions of the tyre to the wheel, are there not.
Also, dont forget that you will have 2 weights on the wheel. One for static balancing and one for dynamic balancing. IIRC the static is done first then the dynamic. One weight may go back in the same place whilst the other may not.
Hence, trying to balance a newly trod wheel with the original weights on is rarely practical. These guys have to get the job done in a minimum amount of time, so stripping all the weights off and starting afresh is better for them, and you.
A wheel out of balance by say 20g, may increase to a force of many Kg at speed, due to centrapetal force.
Correct
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The two weights are there for both static and dynamic balance.
Or, in other words, you have to acheive static balance and dynamic balance simultaneously. If you had acheived static balance by fixing on one weight, then you would lose static balance when you add the second one.
While the wheel is spinning, the machine is solving simultaneous equations!, the static balancing equations to set the rotating force to zero, while the dynamic balancing equations to set the rotating moment to zero.
Number_Cruncher
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Just out of interest, how did they balance wheels before the computerised balancing machines were invented?
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With a spirit level. (honestly)
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With a spirit level. (honestly)
Indeed, by imposing two known imbalances on a wheel, sequentially, and measuring where it comes to rest, you can work out how much mass, and where to add it to provide static balance. I suspect there are other procedures that were used. Because this process can be done without the wheel spinning, it is termed static balance. You can tell if a wheel is out of static balance, because in the absence of brake drag, it will always come to rest heavy spot down.
Static imbalance is when there is a heavy spot so that the centre of mass of the wheel does not lie on the axis of rotation.
For dynamic balance, you have to have the wheel spinning to produce the out of balance moment to measure and then correct. A wheel that is dynamically out of balance will come to rest at random positions, because dynamic imbalance is purely a dynamic effect.
Pure dynamic imbalance is where you have a heavy spot on the inner rim side of tyre, and another equal heavy spot on the outer rim, but 180 degrees out. The centre of mass lies on the axis, so the wheel is statically balanced, but, when the wheel spins up, it tries to twist the axle alternatly in camber and steer during the rotation.
With tyres of thinner section, and slower typical speeds, this was less of an issue for older cars.
Number_Cruncher
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Thanks. Ide have never thought of using a sprit level.
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The shape of the edge of my Focus alloys is such that you can't fit balance weights on the edge. I'm pleased about this because it stops tyre fitters try to fit weights which are designed for steel wheels and which aren't compatible with alloys. I always specify self-adhesive weights on the inside of alloys, but in the past tyre fitters haven't always complied and the steel wheel weights soon fell off.
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L\'escargot.
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Some interesting replies. Having never owned a car with alloys before, I didn't know they existed until I had my puncture repaired. The annoying thing is that the mechanic scratched the inside of my wheel when removing the old weights and I suppose this will happen again when I get a tyre replaced.
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