Boyle's law - tyres

[GSmonkey]

You set your tyre pressures cold, end of story.

However, for a bit of indulgence: say at 20degC (293K) you set the pressure at 2.5bar at the rear.

P1V1/T1 = P2V2/T2. Assume (*) V1=V2

P2=P1T2/T1

Temp/ Pressure Change from 20degC
-20 92.67%
-10 96.34%
0 100.00%
10 103.66%
20 107.33%
30 110.99%
40 114.65%
50 118.32%
60 121.98%
70 125.64%
80 129.30%
90 132.97%
100 136.63%

(*) But, if you're getting really anal about things, obviously the volume of the tyre does change with circumstances. With a change in temperature of the tyre carcass the tyre will expand increasing the volume in the tyre and decreasing pressure.

Increasing speed not only increases the temperature of the carcass, but also puts a large centripetal force on the tyre again increasing its volume (albeit you'd have to correct for the tyre walls straightening)

Another thing you've forgotten about is atmospheric pressure. Depending on the tyre of pressure sensor you may have to correct.

All above is probably very interesting to Rossi's race engineer.

As I'm not the type of guy to put tyre warmers on my bike in a morning before work, I think I'll stick to checking my tyre pressures once a week in the garage, without first correcting for either ambient temperature or atmospheric pressure.

Wasn't biking meant to be a relaxing hobby?

 

[London Mole]

Michelin man's values...

... on my GSA's Anakee are 41 PSI front and 41 PSI back!!!

Why would the tyre manufacturer stamp the wrong values on its own products???

 

[(RIP) Bin Ridin]

N2 matters where the trips are long and temps are high. Tyre life is a big economic factor running 600 tractors and trailers here, and we are thinking hard about N2. Tyres run at +150C, and cold pressure setting is hard to do - they don't get cold very often.

 

[Blackal]

Increasing speed not only increases the temperature of the carcass, but also puts a large centripetal force on the tyre again increasing its volume (albeit you'd have to correct for the tyre walls straightening)

That's centrifugal force that does that - Centripetal force tends toward the centre of curvature and is very small in magnitude compared to the outward Centrifugal force.

Al

 

[FastFred]

Thanks guys. I started a good thread here. I've ordered some temperature and pressure monitors....
Hopefully when I solve the problem I'll be able to rear wheel steer a bit better in the wet, cold and icy conditions

 

[Blackal]

Thanks guys. I started a good thread here. I've ordered some temperature and pressure monitors....

Opinions will vary...................

Hopefully when I solve the problem I'll be able to rear wheel steer a bit better in the wet, cold and icy conditions

You'll be looking down at the readouts - when you hit the tractor

Al

 

[GSmonkey]

That's centrifugal force that does that - Centripetal force tends toward the centre of curvature and is very small in magnitude compared to the outward Centrifugal force.

Al

There is only one force in operation, that is centripetal force - the force applied to an object to make it change velocity and go round in a circular motion rather than in a straight line. "Centrifugal Force" is a fictional force - only the reaction to centripetal force.

Don't worry, it took Newton a few years to work it out as well!!!

 

[swissrob]

airlines use it because it is law. As an inert gas it wont do anything in the case of a brake fire, also because it is dry and inert it will not corrode the wheel casting.

as above just ride the feckin thing

 

[Boxerboy55]

Couple of petrol heads at my work were banging on about the benefits of replacing the air in their tyres with pure Nitrogen.

They had actually paid a guy to do it.

Pissed myself laughing.

Mr Barnum would have been proud of that one.

Last month Slo-Fit stung me for filling my daughter's new car tyre with N2 without pre-warning me. I thought they were having a laff, but unfortunately I didn't read the bill details till after I got home.

And the funny thing was (ha-ha not) they had NOT filled my 2 new business lease car tyres with N2 earlies the same day. Now they are tossserrrrrrs!

 

[peterh]

London Mole posted

Michelin man's values...
... on my GSA's Anakee are 41 PSI front and 41 PSI back!!!

Why would the tyre manufacturer stamp the wrong values on its own products???

If marked on the sidewall I believe those are not target values but the maxima the tyres should be inflated to. Certainly not the values Michelin UK gave me.

 

[Kremmen]

Do whats i do with me tractor .... fill thems with water.

 

[Engineer]

The whole Nitrogen v Air thing is bollox. We went through this ages ago, it's a myth that Nitrogen is any better - It makes bugger all difference.

Now someone will come along with some pseudo science and disagree and we can go through the whole thing again

 

[RickG]

The whole Nitrogen v Air thing is bollox. We went through this ages ago, it's a myth that Nitrogen is any better - It makes bugger all difference.

Now someone will come along with some pseudo science and disagree and we can go through the whole thing again

I must have missed it first time around, is this like an oil/tyre thread now?

In my opinion there's no point in nitrogen in bike road tyres compared to air as in everyday use the other variables encountered are more significant. Fundamentally however, what is the right pressure anyway? Can we actually define this to a level of accuracy that warrants the concern of the gas used inside? How accurate is your pressure gauge anyway? etc etc...

In a GS for everyday use, nitrogen is therefore unlikely to be perceptibly 'better' in your tyres than air

As far as the non-pseudo science goes, in terms of pressure change with temp' the differences between the coefficients of expansion of nitrogen vs air are small. However, water vapour in the gas does increase the coeffcient more significantly . Nitrogen (or air) from a bottle is often 'dry' and therefore more temperature stable.

In some situations (like racing, or it seems trucking) the difference IS important and to maintain the optimum performance users will use dry nitrogen instead of local atmosphere. In racing, tyre temps can reach temperatures much higher than on small road vehicles. The greater the range of temp, the greater the pressure variation so any means of reducing the latter may often keep the tyre working at it's best or prolong it's life

In everyday use there's of course another very big variable. Us. How many of us sitting reading this cr*p I'm writing know exactly what pressures are in our tyres right now (this excluses smart-ar*ses with fancy read-outs on their bikes)? When did you last check?

 

[Engineer]

I must have missed it first time around, is this like an oil/tyre thread now?

In my opinion there's no point in nitrogen in bike road tyres compared to air as in everyday use the other variables encountered are more significant. Fundamentally however, what is the right pressure anyway? Can we actually define this to a level of accuracy that warrants the concern of the gas used inside? How accurate is your pressure gauge anyway? etc etc...

In a GS for everyday use, nitrogen is therefore unlikely to be perceptibly 'better' in your tyres than air

As far as the non-pseudo science goes, in terms of pressure change with temp' the differences between the coefficients of expansion of nitrogen vs air are small. However, water vapour in the gas does increase the coeffcient more significantly . Nitrogen (or air) from a bottle is often 'dry' and therefore more temperature stable.

In some situations (like racing, or it seems trucking) the difference IS important and to maintain the optimum performance users will use dry nitrogen instead of local atmosphere. In racing, tyre temps can reach temperatures much higher than on small road vehicles. The greater the range of temp, the greater the pressure variation so any means of reducing the latter may often keep the tyre working at it's best or prolong it's life

In everyday use there's of course another very big variable. Us. How many of us sitting reading this cr*p I'm writing know exactly what pressures are in our tyres right now (this excluses smart-ar*ses with fancy read-outs on their bikes)? When did you last check?

We could make this an oil type thread and keep it going for ages

I am sure that a little water vapour in the air would make it behave less like an 'ideal gas' and hence not follow 'ideal gas equation' perfectly, but by how much? Guess I might have to dig out my old Physical Chemistry books and spend some time doing some sums - if I can be arsed!

 

[teamwimp]

don't forget centirfugal growth when riding at high speed!

 

[Engineer]

don't forget centirfugal growth when riding at high speed!

We can forget that - well I am going to

 

[London Mole]

London Mole posted



If marked on the sidewall I believe those are not target values but the maxima the tyres should be inflated to. Certainly not the values Michelin UK gave me.

Cheers.

So if the BMW manual states fully loaded with pillion rear should be 42PSI, the're putting us at risk???...

[I'm confused.com!!!]

 

[peterh]

Why not try asking Michelin? Here's a contact form:

http://www.michelin.co.uk/michelinuk/en/more/contact/20061205150539.html

The pressures that tyre makers recommend are not always the same as those in the bike manual. Mostly, but not always.

 

[AberdeenAngus]

P'raps we could have an emote for when someone is getting a bit too anal.
A picture of a big rusty shuriffs badge would do

 

[Packer]

Will nitrogen improve the tyre life on my forklift trucks?

They use solid tyres but is there a modification that would allow me to fit them with nitrogen?