How to measure tyre pressures ?

[Peter Cee]

A bit of a nit picking question question for all you perfectionists.

If the name plate were to say (for example) 30 psi when fully loaded with persons and luggage - Ideally when should this be measured ?

I know that it is best measured with the car in the garage at 21 DegC - but should it be measured with the car fully loaded ?

If you measure it empty and then load it up, then surely the pressure will increase due to the additional weight ?

 

[snarf]

This question has been asked before and I seem to remember that opinions varied on whether the additional loading affected the pressure, it is, however, a very simple experiment to confirm this one way or another.

The ideal gas law (PV= nRT) equates the product of pressure and volume to the product of the amount gas, the gas constant and temperature. So for the case of the sealed system of a tyre the amount of gas is constant and the gas constant is, well, constant, and since any temperature variation due to additional loading is likely to be negligible the temperature is also likely to be constant hence the product of pressure and volume is for practical purposes constant. This means that the only way loading the tyre will increase pressure is if the internal volume is reduced due to loading and in my opinion I think any volume change is likely to be very small because squashing the tyre at the bottom will most probably result in an equivalent an compensating expansion elsewhere. So no I don't think loading the car will affect the tyre pressures.

 

[RSGulp]

Unless I've misunderstood your question, it seems common sense that the posted tyre pressures are set when the car is empty such as it would be in a workshop. This then sets them at the right pressure for WHEN the car is loaded up with people and luggage. It makes no sense to ensure you have available the correct number of adults, kids and suitcases all positioned in the car every time you check your tyre pressures? [&:]

 

[Lancerlot]

ORIGINAL: snarf

This question has been asked before and I seem to remember that opinions varied on whether the additional loading affected the pressure, it is, however, a very simple experiment to confirm this one way or another.

The ideal gas law (PV= nRT) equates the product of pressure and volume to the product of the amount gas, the gas constant and temperature. So for the case of the sealed system of a tyre the amount of gas is constant and the gas constant is, well, constant, and since any temperature variation due to additional loading is likely to be negligible the temperature is also likely to be constant hence the product of pressure and volume is for practical purposes constant. This means that the only way loading the tyre will increase pressure is if the internal volume is reduced due to loading and in my opinion I think any volume change is likely to be very small because squashing the tyre at the bottom will most probably result in an equivalent an compensating expansion elsewhere. So no I don't think loading the car will affect the tyre pressures.

So then, Allan, say I was overloaded with a van full of budgerigars and struggling to climb a steep hill, would banging the sides of the van to get the buggies flying, improve my chances of climbing that hill, do you think? [8|]

Regards,

Clive.

 

[vitesse]

If the van doors were open,& all the budgies had harness's fixed to the van-& if they could all be persuaded to fly forwards,maybe so,but then think of all the weight of the mirrors required to encourage the budgies to fly-on reflection ,it might slow you down.[]

 

[snarf]

ORIGINAL: Lancerlot

ORIGINAL: snarf

This question has been asked before and I seem to remember that opinions varied on whether the additional loading affected the pressure, it is, however, a very simple experiment to confirm this one way or another.

The ideal gas law (PV= nRT) equates the product of pressure and volume to the product of the amount gas, the gas constant and temperature. So for the case of the sealed system of a tyre the amount of gas is constant and the gas constant is, well, constant, and since any temperature variation due to additional loading is likely to be negligible the temperature is also likely to be constant hence the product of pressure and volume is for practical purposes constant. This means that the only way loading the tyre will increase pressure is if the internal volume is reduced due to loading and in my opinion I think any volume change is likely to be very small because squashing the tyre at the bottom will most probably result in an equivalent an compensating expansion elsewhere. So no I don't think loading the car will affect the tyre pressures.

So then, Allan, say I was overloaded with a van full of budgerigars and struggling to climb a steep hill, would banging the sides of the van to get the buggies flying, improve my chances of climbing that hill, do you think? [8|]

Regards,

Clive.

Only if the top of the van was open - otherwise it would be a closed system[] I seem to remember this was a topic of discussion on an episode of QI.

 

[snarf]

A more interesting question is if you were in a train travelling a light speed would you be able to shine a torch light onto the forward end of a carriage from the rear of the carriage?

And another interesting question follows from the claim made by astronomers that by observing distant objects we are in fact looking back in time because it takes millions or even billions of years for the light to reach us, they then extrapolate this view to say we can look back almost to the origin of the universe - the moment just after (relatively speaking) the big bang. My question is so how did we get here before the light?

 

[Lancerlot]

ORIGINAL: snarf

A more interesting question is if you were in a train travelling a light speed would you be able to shine a torch light onto the forward end of a carriage from the rear of the carriage?

Sure - it's a closed system, isn't it?

I'd answer the second............... but my brain hurts

Regards,

Clive.

 

[okellyt]

ORIGINAL: Lancerlot

ORIGINAL: snarf

A more interesting question is if you were in a train travelling a light speed would you be able to shine a torch light onto the forward end of a carriage from the rear of the carriage?

Sure - it's a closed system, isn't it?

I'd answer the second............... but my brain hurts

Regards,

Clive.

Clive's right - its a fixed frame of reference to you, the torch and the inside of the train, nothing is moving.......so from inside the train yes. An observer at right angles would see that the light never made the end of the carriage

Think about it, doesn't half bake your noodle doesn't it

Of course thats ignoring the effects of gravity and the train becomes incredibly short and tall to the observer. as does the person on the train.

 

[Rodney Naghar]

ORIGINAL: snarf

And another interesting question follows from the claim made by astronomers that by observing distant objects we are in fact looking back in time because it takes millions or even billions of years for the light to reach us, they then extrapolate this view to say we can look back almost to the origin of the universe - the moment just after (relatively speaking) the big bang. My question is so how did we get here before the light?

I'm intrigued and have spent 3 hours looking this up (I've seen this question asked on 3 forums and it hasn't been answered once correctly) this is what I've come up with using 4-5 sources.

The singularity was not a single point in space-time, it encompassed all of space-time, and as it expanded, it continued to be so, like dots on a balloon. Galaxies that are further apart than a certain distance are separating at faster than the speed of light, because the space between them is expanding. This time dilation allows for your scenario to occur. The photons from the distant galaxy have taken more time than would be necessry for light to travel (due to time dilation) to get to earth, so we get to see light that is older than the distance in light years to it would take to get to those galaxies. (particle distance - maximum distance a particle could have travelled to reach an observer in the age of the universe)

The speed of light is only 186,000 miles/per second with respect to local comoving variables (Hubble law). It is a misconception that even though the universe is 13.7 billion years old that we can only see as far as 13.7 billion light years away, because actually, spacetime is highly curved on a cosmological scale

It is said we can't see further back than 380,000 years after the Big Bang when the CMB (cosmic microwave background radiation) first became visible from the fog of hydrogen plasma that existed before the universe cooled down enough for it to be transparent as it is now.

2 of my sources.

http://www.astro.ucla.edu/~wright/cosmo_02.htm#DH
http://en.wikipedia.org/wiki/Observable_universe

 

[snarf]

ORIGINAL: Rodney Naghar

ORIGINAL: snarf

And another interesting question follows from the claim made by astronomers that by observing distant objects we are in fact looking back in time because it takes millions or even billions of years for the light to reach us, they then extrapolate this view to say we can look back almost to the origin of the universe - the moment just after (relatively speaking) the big bang. My question is so how did we get here before the light?

I'm intrigued and have spent 3 hours looking this up (I've seen this question asked on 3 forums and it hasn't been answered once correctly) this is what I've come up with using 4-5 sources.

The singularity was not a single point in space-time, it encompassed all of space-time, and as it expanded, it continued to be so, like dots on a balloon. Galaxies that are further apart than a certain distance are separating at faster than the speed of light, because the space between them is expanding. This time dilation allows for your scenario to occur. The photons from the distant galaxy have taken more time than would be necessry for light to travel (due to time dilation) to get to earth, so we get to see light that is older than the distance in light years to it would take to get to those galaxies. (particle distance - maximum distance a particle could have travelled to reach an observer in the age of the universe)

The speed of light is only 186,000 miles/per second with respect to local comoving variables (Hubble law). It is a misconception that even though the universe is 13.7 billion years old that we can only see as far as 13.7 billion light years away, because actually, spacetime is highly curved on a cosmological scale

It is said we can't see further back than 380,000 years after the Big Bang when the CMB (cosmic microwave background radiation) first became visible from the fog of hydrogen plasma that existed before the universe cooled down enough for it to be transparent as it is now.

2 of my sources.

http://www.astro.ucla.edu/~wright/cosmo_02.htm#DH
http://en.wikipedia.org/wiki/Observable_universe

Rodney, -thanks for the links - this apparent paradox had been troubling me for a while and whilst I can't claim to undestand the explanation in detail I can follow the logic.

 

[Rodney Naghar]

Allan, trying to get my head around spacial relativity is a hopeless task. I've been reading a lot about it recently.

Have you heard of the ladder paradox? http://en.wikipedia.org/wiki/Ladder_paradox
Tom alluded to it in his post above. Very interesting.

Did you know that there are different sizes of infinity?
Have you heard of Zeno's incredible paradoxes (arrow paradox, achilles and the hare etc) seemingly crazy inferences (ie that nothing can ever move) that were only mathematically disproved quite recently.

http://en.wikipedia.org/wiki/Zeno's_paradoxes

Anyway, what was that about tyre pressures? [8D]

 

[okellyt]

Rodney, Allan

Its called cosmic inflation

A long time ago I used to know something about all this stuff.....[]

 

[tscaptain]

Nothing to do with the OP but more to do with the subsequent posts I like this little piece about light speed and observation
http://www.philforhumanity.com/Speed_of_Light.html

The budgies in the car is the same as people in an aircraft. If they all jumped up in the air together the aircraft weight (and thus flight path) would not change.

In additon to the OP about pressure versus load what about correct pressure versus speed? See here!

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=72

 

[snarf]

ORIGINAL: tscaptain

Nothing to do with the OP but more to do with the subsequent posts I like this little piece about light speed and observation
http://www.philforhumanity.com/Speed_of_Light.html

The budgies in the car is the same as people in an aircraft. If they all jumped up in the air together the aircraft weight (and thus flight path) would not change.

In additon to the OP about pressure versus load what about correct pressure versus speed?  See here!

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=72

Just spent 15 min putting the philforhumanity stuff on a spreadsheet and I can't fault his reasoning - but then I'm no longer a rocket scientist.[]

 

[Rodney Naghar]

Loved that philforhumanity stuff - fascinating, with numbers to prove it rather than just ambiguous explanations.

 

[mascanes]

Didn't know you had to be this clever to own a Porsche "¦"¦.. [&:]

 

[tscaptain]

ORIGINAL: mascanes

[FONT=calibri"]Didn't know you had to be this clever to own a Porsche "¦"¦.. [&:]

This is just the simple stuff..............[]

 

[mascanes]

Oh well, then it's back to nuclear physics and a Fiat panda for me "¦"¦"¦[8D]