Physics talk

[bogan]

So, a 10mm hole in a 1000mm steel sheet gets larger if you heat the steel?

Well, my oven isn't that big, and neither is me torch, so it would depend entirely on where the heat was applied. In fact in some cases I think you could get the hole (if it were quite a bit biger than 10mm) to shrink.

a bike doesn't get heavier, but the physics involved means the tyre shrinks.

How so? With nice wide sticky bike tyres you'd have both the centrifugal force, and the heat-pressure trying to push out the center, counteracted only by the extra weight transfered to the rear wheel, and that only applied to a few percent of the circumference at any one time.

Anyone happen to know what the maximum lateral G a bike can pull in a corner is? I bet you'll be surprised how low it is.

I think its around 1.2 for sticky road tyres, though the motogp guys get up around 1.5 iirc.

[Gremlin]

Hey. I never started a new thread. I just took the original thread off topic. Id rather just have an infraction than be responsible for this thread.

Yours was the second post that took it off at a tangent, but more comprehensive and better to start a thread... This discussion needed it's own thread, it was off all on it's own.

However, you are always welcome to an infraction if you want?

[mossy1200]

you're entirely right.

a bike doesn't get heavier, but the physics involved means the tyre shrinks. also a few guys mentioned drags cars and I believe the original discussion was in a car/truck to do with cruise control/limiters....

the front wheel will get lighter during acceleration due to a torque at the back wheel.

the race car wheel video is an analogy, there aren't any videos on youtube with a motorcycle wheel at speed which show a representation of what I'm describing to you.

braking yes, also cornering on a cambered corner.

I see we have our own physics thread now, I was considering starting one so next time someone talked about centrifugal force I'd just link them to the thread.

Anyone happen to know what the maximum lateral G a bike can pull in a corner is? I bet you'll be surprised how low it is.

I only started talking about a bike tyre ballooning after speeds exceed 200kph.
Theres also many more factors such as original pressures. Eg drag car expansion is excessive due to tire design to grab the road on take off then expand. This giving traction and torque followed by reduced resistance on road followed by increased gearing. All important in the drag scene.
A 2% increase on a steel belt road bike tyre is a smaller amount but can effect speedo reading.
I believe that most bikes on both front and rear wheel would not have any or much more downforce at speed with avergae joe riding it. most in my opinion would see a reduction of downforce.

[mossy1200]

Yours was the second post that took it off at a tangent, but more comprehensive and better to start a thread... This discussion needed it's own thread, it was off all on it's own.

However, you are always welcome to an infraction if you want?

Im good for a while thanks. When I get within 1 infraction of losing my KB licence do I get a warning letter?

[Gremlin]

Im good for a while thanks. When I get within 1 infraction of losing my KB licence do I get a warning letter?

In KB land you have to do impressive things to lose your licence (and then it's permanent). Instead, you just lose features, more and more as your points climb...

[Akzle]

Well, my oven isn't that big, and neither is me torch, so it would depend entirely on where the heat was applied. In fact in some cases I think you could get the hole (if it were quite a bit biger than 10mm) to shrink.

sorry brogan. factoid is factoid.
the atoms around the hole are packed as tight as they're gonna get. 'twill expand outward. to freedom.

[mossy1200]

In KB land you have to do impressive things to lose your licence (and then it's permanent). Instead, you just lose features, more and more as your points climb...

That didnt take long. You just infracted me for something else.
Could be worsa. Bad gwama and spellin mistakes could be infracturiZed also.

[bogan]

sorry brogan. factoid is factoid.
the atoms around the hole are packed as tight as they're gonna get. 'twill expand outward. to freedom.

Sorry Greeny, the textbook only goes so far once you get into the real world.
If you have a bit of flat bar and heat it, it'll expand out each side right. So if you have a big disc with a large hole in the center (this can be thought of as a curved bit of flat bar) and heat only the sides, they will expand similarly to the flat bar, going inside and outside. the top and bottom will of course get further apart, but the minor diameter will get smaller. Its only when you heat the whole (not the hole) thing the shape stays the same, and the length increases the circumference and therefor diameter.

[Dave-]

I only started talking about a bike tyre ballooning after speeds exceed 200kph.
Theres also many more factors such as original pressures. Eg drag car expansion is excessive due to tire design to grab the road on take off then expand. This giving traction and torque followed by reduced resistance on road followed by increased gearing. All important in the drag scene.
A 2% increase on a steel belt road bike tyre is a smaller amount but can effect speedo reading.
I believe that most bikes on both front and rear wheel would not have any or much more downforce at speed with avergae joe riding it. most in my opinion would see a reduction of downforce.

Yup, and at speeds above 200 you'll start seeing the tyre shrink.

There are many many more factors, right down to the elasticity of the steel in the belts.

yes a 2% increase would, but it doesn't increase.

And you're right with the last remark too, bikes dont produce down force because the wings smack the ground in a corner and the down force in the corner does whacky things to the centripetal forces because you're leaning the bike over.

How so? With nice wide sticky bike tyres you'd have both the centrifugal force, and the heat-pressure trying to push out the center, counteracted only by the extra weight transfered to the rear wheel, and that only applied to a few percent of the circumference at any one time.

Asked and answered.

Well, my oven isn't that big, and neither is me torch, so it would depend entirely on where the heat was applied. In fact in some cases I think you could get the hole (if it were quite a bit biger than 10mm) to shrink.

We assume the heat is evenly distributed.

and no the hole will expand.

read this: http://hyperphysics.phy-astr.gsu.edu...mo/thexp2.html

Also think about cold welding. and heating things up, then slipping them over things and waiting for them to cool down.

[bogan]

Asked and answered.

err, again, how so?

The extra weight is what, 1kN max. Total centrifugal force (4.6kg tyre doing 54m per second) is 50kN by my reckoning (though maybe I've got some units mixed up?). Obviously only around 1.4kN (3%) is directly acting against the weight at one time, but that is enough, plus the momentum of the rest as it comes around makes it fucking plenty.

We assume the heat is evenly distributed.

Why? is your oven that big?

[pzkpfw]

You crack me up. You link the very information that shows that you dont understand what cetripetal force is.

Wrong.

Gravity from the mass of the Earth and the Moon attract each other.
The centripetal force is what is stopping them from coming together.

Wrong. With respect to the Earth, the Moon would really like to just continue off in what would appear to be a straight line - see "velocity" in the diagram linked below. (At least for a while; it'd still be bound by the Sun). The force of Gravity (calling it a force is kind of debatible too) from the Earth, towards Earth, is what makes the Moon continually fall towards Earth instead of continuing in that straight line - and away.

(The Moon also attracts the Earth of course, which is why they actually both orbit the mutual barycentre.)

The centripetal force is outwards and not in.

Wrong. Where in those links to mainstream science is that stated?

e.g. see this diagram on that page on centripetal force. Note the direction of the arrow. http://en.wikipedia.org/w/index.php?...ram.svg&page=1

The balance of the mass and the cetripetal force is what maintains the distance as an orbit.

Rubbish. (In the context of your explanation).

The other example is the roller coaster. The carts are on the inside of the loop. Your theory would have the cart on the outside being pulled into the centre. Im feeling safer on my roller coaster than yours.

No, you don't understand.

At any moment in the loop, if the loop vanished, the cart would continue off in a straight line (not quite straight, it would fall due to Earths gravity).

What makes the cart continue in the loop, is the force applied inwards on the cart by the track.

The force applied by the cart to the track (outwards), is the fictitious centrifugal force.

The faster you revolve an object through rotation the higher the outwards G forces that act on the mass in rotation.

Wrong.

The faster you rotate an object, the more the bits of it want to fly off in a straight line. Thus the more force you need to apply to keep it all rotation.

The tyre wishes to leave the centre point and expands with the limiting factor being the strength of the construction.
Turn it fast enough the tyre becomes taller and skinnier.

That's what you'll see, but you describe the cause incorectly.

You need to read your link again im afraid.

Nope. You do.

[Dave-]

err, again, how so?

The extra weight is what, 1kN max. Total centrifugal force (4.6kg tyre doing 54m per second) is 50kN by my reckoning (though maybe I've got some units mixed up?). Obviously only around 1.4kN (3%) is directly acting against the weight at one time, but that is enough, plus the momentum of the rest as it comes around makes it fucking plenty.

*deep sigh*

a 250kg bike weighs 9.87 * 250 = 2467.5N = 2.5kN

centrifugal force doesn't exist, what you're calling centrifugal force is actually a tangential force involving the mass of the tyre and the constant change in acceleration from the rotation of the wheel, if you read this thread, by the end of it you'd know this.

I'm not sure where you got 3% from or what it has to do with anything.

but to answer you question.

the tyre gets smaller because as the tyre connects with the road it deforms, it takes some amount of time for the tyre to undeform and 1 rotation of the wheel isn't long enough at +200kph so before the tyre can reform. It's forced to deform again and the radius of the tyre is reduced.

yes my oven is big enough. or you can just leave the 1m^2 plate in the sun.

[pzkpfw]

...

Here's a test - swing an object around you on the end of a string. At some moment, let go of the string. Look at the object at the moment you let go. Look at what direction the object goes in. Think about what forces were acting on that object before and after letting go.

The key point, is that the object will keep going in the direction it was going at the moment of letting go.

It won't go off in a direction directly away from you.

Say you are swinging it clockwise (seen from above) around you, and let go when the string is due South. The object will fly off West - not South.

This is because at (just before) the moment of letting go, the object is travelling West, and the centripetal force (towards you) is acting North, to keep the object moving in a circle. {90 degrees before that, the object was East of you, moving Southwards, and the centripetal force you are applying by the string is acting West}.

If the centrifugal force were real, and acting away from you, you'd expect that object to fly off Southwards.

[bogan]

*deep sigh*

a 250kg bike weighs 9.87 * 250 = 2467.5N = 2.5kN

centrifugal force doesn't exist, what you're calling centrifugal force is actually a tangential force generated by the mass of the tyre and the constant change in acceleration from the rotation of the wheel, if you read this thread, by the end of it you'd know this.

I'm not sure where you got 3% from or what it has to do with anything.

but to answer you question.

the tyre gets smaller because as the tyre connects with the road it deforms, it takes some amount of time for the tyre to undeform and 1 rotation of the wheel isn't long enough at +200kph so before the tyre can reform it's forced to deform again and the radius of the tyre is reduced.

Yes but the rear has half that to begin with, add another 100kg from the front.

Centrifugal force is simply a description of a perceived force equal and opposite to the centripetal force. I bet people in space stations would just think down was directly away from the center of rotation; I bet when they were designing space tables they would have to withstand a downwards force "generated by the momentum and rotation force which is equal and opposite to the centripetal force at any given time"; and I bet those space dudes would get tired of writing all that and just come up with a shorthand notation for it, oh if only there was some word already in use that most people understood immediately
The fact you don't use it in simple physics calculations is irrelevant.

the 3% is three percent of the circumference with a 50mm contact patch. If the rotation isn't long enough for the tyre to deform one way, tell me how 3% of that time is long enough for it to deform the other?

[Spuds1234]

Here's a test - swing an object around you on the end of a string. At some moment, let go of the string. Look at the object at the moment you let go. Look at what direction the object goes in. Think about what forces were acting on that object before and after letting go.

The key point, is that the object will keep going in the direction it was going at the moment of letting go.

It won't go off in a direction directly away from you.

Say you are swinging it clockwise (seen from above) around you, and let go when the string is due South. The object will fly off West - not South.

This is because at (just before) the moment of letting go, the object is travelling West, and the centripetal force (towards you) is acting North, to keep the object moving in a circle. {90 degrees before that, the object was East of you, moving Southwards, and the centripetal force you are applying by the string is acting West}.

If the centrifugal force were real, and acting away from you, you'd expect that object to fly off Southwards.

We covered this and I explained it in detail although it is a wall of text and I dont think many people will read it. Now we are onto expansion of a motorcycle tyre at what I assume is to be the following speeds 100kph, and 200+kmh so we can work out what affect this will have on the speedo reading or something.

I think thats how this all got started?