Any - Boyles law and tyre pressures - Brit_in_Germany

Oh dear, HJ seems to have got a bit confused here

www.honestjohn.co.uk/askhj/answer/75465/is-putting...a-

I thought the only possible advantage of pure N2 as opposed to using air was that the oxygen might degrade the rubber (to a small extent). It has nothing to do with the temperature dependece of the pressure which will be true for all gases.

Any - Boyles law and tyre pressures - skidpan

I thought the advantage was the fact that tyres filled with Nitrogen had a smaller pressure variance with temperature which is why they use Nitrogen in F1 (and probably other formula).

Irrelevant in the real world since we don't drive fast enough or corner fast enough to get the tyres that hot.

Mine are filled with 100% air and always have been. If its good enough to breath its good enough for my tyres.

Any - Boyles law and tyre pressures - Wackyracer

Air is 78% nitrogen to start with, couple that to the fact that oxygen molecules can escape through rubber quicker due to it small molecule size. After a few years you'll have at least 90% nitrogen in your tyres.

Any - Boyles law and tyre pressures - RobJP

For HJ (if he reads this) :

All gases, whether mixtures or pure gasses (air being a mixture) obey what are called the 'Gas laws'.

Basically, if the number of moles of the gas (so X amount of air/nitrogen/whatever) stays the same (a system with no leaks), and the volume stays the same (a tyre is close enough to this as to be ignorable) then the pressure exerted on the walls of the container is directly related to the temperature in Kelvin (kelvin is simple to calculate, it's 273 + the temp in centigrade. So 0C = 273K, 100C = 373K)

So, if you started one morning from cold, and your tyres were at 10 C (283 kelvin) and 30psi.

Twenty miles later, your tyre temperature is at 40C (not the air temp, but the tyre temp).

40C = 313 K

So the new tyre pressure will be (313/283) * 30 = 33.2 psi.

The same holds true whether your tyres are filled with air, nitrogen, carbon dioxide, or any gas OR mixture or combination of gasses.

As someone has already pointed out, air is 78% nitrogen, not the 85% you quote.

Any - Boyles law and tyre pressures - Cyd

Air is 85% Nitrogen anyway, but the other 15% expands and contracts according to temperature. So a tyre filled 100% with Nitrogen should remain at the same pressure both at the beginning and end of a journey, whereas the pressure in a tyre filled with air may have risen maybe 3PSI.

Oh dear, oh dear, oh dear. Even many who failed Physics O level can probably see how wrong that that is.

I started losing faith in HJ some years ago when I started to notice a deterioration in the quality of both questions and answers in the Telegraph supplement. This, though, is rock bottom.

A favourite catchphrase of Montgomery Scott's comes to mind ......

Any - Boyles law and tyre pressures - RobJP

Air is 85% Nitrogen anyway, but the other 15% expands and contracts according to temperature. So a tyre filled 100% with Nitrogen should remain at the same pressure both at the beginning and end of a journey, whereas the pressure in a tyre filled with air may have risen maybe 3PSI.

Oh dear, oh dear, oh dear. Even many who failed Physics O level can probably see how wrong that that is.

I started losing faith in HJ some years ago when I started to notice a deterioration in the quality of both questions and answers in the Telegraph supplement. This, though, is rock bottom.

A favourite catchphrase of Montgomery Scott's comes to mind ......

Don't slate someone, educate them. The former serves no purpose. The latter means they may be in a position to pass on the knowledge themselves in the future.

For all we know, HJ may well have left school with no (or minimal) qualifications. He certainly left school back in the day when it wasn't at all uncommon - my wife's late father did so, but went on to build a very successful business buying, repairing and selling tractors, even becoming a main agent for a couple of brands in the late 1980s.

Which is better - to laugh at someone for their lack of knowledge on a subject, or to try to help them understand that subject ?

Any - Boyles law and tyre pressures - RT

Don't slate someone, educate them. The former serves no purpose. The latter means they may be in a position to pass on the knowledge themselves in the future.

For all we know, HJ may well have left school with no (or minimal) qualifications. He certainly left school back in the day when it wasn't at all uncommon - my wife's late father did so, but went on to build a very successful business buying, repairing and selling tractors, even becoming a main agent for a couple of brands in the late 1980s.

Which is better - to laugh at someone for their lack of knowledge on a subject, or to try to help them understand that subject ?

That's quite reasonable for friends, relative and casual aquaintances - but not someone answering technical questions about cars on a car forum or on the motoring section of a major newspaper - when doing so from a position of authority.

Edited by RT on 03/03/2017 at 13:52

Any - Boyles law and tyre pressures - daveyjp

The science behind nitrogen in car tyres is as convincing as the science behind magnets on fuel lines!

Any - Boyles law and tyre pressures - Cris_on_the_gas

It's because Nitrogen is lighter than Oxygen. However as air is effectively 80% Nitrogen and 20% Oxygen. Ignoring the CO2 and other trace pollutants. The theory is that this means a reduction in the unsprung weight of the vehicle and hence improved handling. I can't see this making a huge difference although.

As stated above all gases expand at same rate according to pressure and temperature and according to Boyles Law, Pressure, Volume and Temperature is a constant.

Edited by Cris_on_the_gas on 03/03/2017 at 14:53

Any - Boyles law and tyre pressures - focussed

Oh dear - you've all missed the point, including our resident, vociferous, scientist.

The whole point in inflating tyres with pure nitrogen is that the tyres will stay at the inflated pressure almost indefinitely.

I'll leave it up to our resident scientist to explain why.

Any - Boyles law and tyre pressures - Vitesse6

Oh dear - you've all missed the point, including our resident, vociferous, scientist.

The whole point in inflating tyres with pure nitrogen is that the tyres will stay at the inflated pressure almost indefinitely.

I'll leave it up to our resident scientist to explain why.

No don't be shy, tell us why you believe this to be the case

Any - Boyles law and tyre pressures - RobJP

Oh dear - you've all missed the point, including our resident, vociferous, scientist.

The whole point in inflating tyres with pure nitrogen is that the tyres will stay at the inflated pressure almost indefinitely.

I'll leave it up to our resident scientist to explain why.

The theory behind it is simple - Nitrogen molecules are approximately 25% larger than Oxygen molecules, so should therefore be less able to leak 'through' the rubber of the tyre by a process called permeation. Oxygen will permeate roughly 3-4 times faster than nitrogen will through a typical tyre.

However, that all completely ignores the ABSOLUTE rate of permeation. If the tyre is really well constructed as to allow virtually no permeation, then the fact that the oxygen permeates faster than the nitrogen is irrelevant. The oxygen will permeate at a tiny rate, and the nitrogen at an even smaller rate.

We've got 3 cars. A shogun, a Z4 and the 325d. I check tyre pressures at least monthly, and check the tyre visually pretty much every day - it's not even something I think about, it happens when I walk out of the door on a morning.

I've never had to put air in the tyres on the Shogun or the Z4 yet. The Z4 is approaching 2 years old, and we've had the Shogun over 3 years (we put a set of A/T tyres on it when we got it, the old road tyres were useless on fields). The 325d I've had to top up once in the 3 years we've had it - it was a leaking valve.

So that's 12 tyres - all reputable makes - that don't suffer any noticeable permeation (oh, they're all filled with air, in case you were wondering)

I can imagine permeation being a problem on dheapo 'ditchfinder' tyres. But on well constructed tyres I reckon I lose more air through checking the pressures than I do via permeation.

Any - Boyles law and tyre pressures - focussed

Gottit in one - it's the size of the nitrogen molecules! They don't get to permeate through the tyre carcass 'cos they is too large to find their way through the rubbery molecules.

Any - Boyles law and tyre pressures - scot22

Wish I could understand all of this, and its level of importance.

Any - Boyles law and tyre pressures - RT

Wish I could understand all of this, and its level of importance.

Don't worry Scot - this isn't one of the things you need to know about cars.

Any - Boyles law and tyre pressures - RobJP

Wish I could understand all of this, and its level of importance.

It should be one of the last things you ever worry about. The physics isn't hugely complicated - but I would say that, I'm a scientist. So virtually any sort of mathematics is second nature to me. I tried to explain it in layman's terms in my earlier post, but I'm not a teacher by any means, so my explanation may well be terrible. If so, my apologies.

In fact, I forgot. There's a fourth set of wheels / tyres - the winters that I use on the 325d.

When I took them off last April, I checked the depths, for any cuts and bulges, and the tyre pressures. They were all at 32 psi. They then sat in the old stable/workshop. In November, I put them back on, checking the pressure before I did so. Still 32psi. They're Pirelli Sottozero runflats.

Any - Boyles law and tyre pressures - galileo

Rob, your explanation was spot-on, I wish the present education system gave more emphasis to Physics, Chemistry and Maths and a bit less on some othe rsubjects. Fewer people would then fall for scams and snake-oil adverts.

Any - Boyles law and tyre pressures - kiss (keep it simple)

Older alloys tend to leak more than the tyres. I would be keen to know if Nitrogen helps at all in this case.

Any - Boyles law and tyre pressures - RobJP

Older alloys tend to leak more than the tyres. I would be keen to know if Nitrogen helps at all in this case.

Well, out 4 of the sets of wheels I refer to 3 sets are less than 4 years old. The set on the Shogun are, I assume, from when it was registered - 2007. Though I believe they stopped making those in 2006, so they could be 11 years old.

Even so, hard metals of thickness like wheels aren't going to allow any gas to permeate through - much like a dive cylinder (and you can get those in aluminium or steel), they'll hold contents at extremely high pressure until there is a physical failure.

SWMBO is a dive cylinder tester (IDEST qualified), and we're both scuba divers - she's also an instructor, one of her many talents. We've both seen cylinders that have been left for several years, fully filled with air to 200+ Bar (that 4500+ psi), with no or minimal loss of contents.

Any - Boyles law and tyre pressures - Stanb Sevento

Just a small point. Alloy wheels of any age can leak through porosity. Alloys are in the main are cast from liquid metal and liquid metal contains disolved gases, when the liquid metal cools and starts to form the solid crystal structure the gas is expeled to the crystal boundries. In sever cases this can build up and form gas inclusions that join together making a porus material. Casting in a partial vacuum reduces the gas content and cooling faster reduces the time gas has to migrate to the crystal bounderies and gives smaller more isolated gas inclusions. Simples

Any - Boyles law and tyre pressures - RobJP

Rob, your explanation was spot-on, I wish the present education system gave more emphasis to Physics, Chemistry and Maths and a bit less on some othe rsubjects. Fewer people would then fall for scams and snake-oil adverts.

Blame my teachers, I had a bunch of complete enthusiasts. The physics teacher constantly had a water pistol on his desk, woe betide someone who wasn't paying attention.

The chemistry teacher was known to mix up thermite, take it out onto the back yard, and light it with some magnesium ribbon on top of steel girders - just to demonstrate what powdered iron ore and aluminium can do. That was always fun.

Any - Boyles law and tyre pressures - Andrew-T

Gottit in one - it's the size of the nitrogen molecules! They don't get to permeate through the tyre carcass 'cos they is too large to find their way through the rubbery molecules.

As another 'trained scientist', I suggest that permeation of rubber by air, oxygen, nitrogen or almost any other gas except helium or hydrogen, will be insignificant compared with losses at the bead or via the valve.

There are a few 'urban myths' included above. Filling tyres with nitrogen instead of air will have a tiny advantage in reducing internal oxidation of rubber, especially if the tyres run hot, as in a performance car - when there might be an extra tiny advantage in non-flammability.

Also the mass of the gas in a tyre will make a negligible contribution to the unsprung weight, especially as it isn't rigidly attached to anything.

Any - Boyles law and tyre pressures - John F

Well, you've all either missed the point even though most will realise what is going on here.

The point here is that advice to put nitrogen in tyres is a yet another flagrant wheeze of elements of the motor trade to con the uneducated punter into spending more money from a position of false 'authority' using cod science. No doubt they are also persuaded to buy new tyre valves even if high mileage frequent changers.

Any - Boyles law and tyre pressures - scot22

Excuse me drifting off the thread but education is the most important non family thing in my life. I had an 'education' which neglected science and practical subjects. I have argued since 1972 that schools should be concerned about education youngsters to enjoy a fulfilling and worthwhile life. We have an analogue curriculum for a digital age.

This is one of the reasons why I have a dislike/mistruct of politicians.

Apologies for the diversion - should be in general discussion.

Any - Boyles law and tyre pressures - skidpan

When they fill your tyres wityh their expensive nitogen do they evacuate all the air out of the tyre first?

If not then the tyre is not filled with nitrogen, its simply got more nitrogen in it than it would have but its still got all that nasty oxygen and other gasses present in the atmosphere.

Any - Boyles law and tyre pressures - ExA35Owner

It's because Nitrogen is lighter than Oxygen. However as air is effectively 80% Nitrogen and 20% Oxygen. Ignoring the CO2 and other trace pollutants. The theory is that this means a reduction in the unsprung weight of the vehicle and hence improved handling. I can't see this making a huge difference although.

My calculations based on the size of the tyres on a Fiesta and on presure of 2 bar, is that nitrogen would make the unsprung weight a but under 3 grams less per tyre. I can't see this being detectable!

Any - Boyles law and tyre pressures - RobJP

Right, how much does the air in a tyre weigh ...

Assuming a 205/50 tyre on a 16" wheel (its roughly an average sort of size for a 'normal' wheel/tyre combination).

The approx volume of the internal of the tyre is 37 litres.

A cubic metre of air weighs (roughly) 1.3kg

Assuming the tyre is filled to 2.2 bar, it will take (37*2.2) = 81 litres of air to fill it.

1 cubic metre = 1000 litres.

So the air in the tyre weighs roughly (81/1000) * 1.3kg = 100g.

The air in 4 tyres therefore weighs 400g - the same as about 0.5 litres of petrol

Basically, the weight of air in the tyres can be ignored. It's as close to zero as makes no difference.

Similarly, changing out the oxygen part for nitrogen makes such little difference as to make it irrelevant - especially as we're already talking about an irrelevant number.

Edited by RobJP on 04/03/2017 at 19:02

Any - Boyles law and tyre pressures - nortones2

Naturally, the Americans have been subject to the N sales line. Some testing has apparently shown minor benefits in pressure retention over the 90 day test duration. There might be small benefits from avoiding oxidation of the tyre interior using N, but all in all it's no substitute for regular pressure checks! www.nhtsa.gov/DOT/NHTSA/NRD/Multimedia/PDFs/Crash%...f

Edited by nortones2 on 04/03/2017 at 20:17

Any - Boyles law and tyre pressures - Manatee

I agree that the pressure increase related to temperature change will be the same for oxygen or nitrogen, at least while both remain in a gaseous state, but I would do the calculation slightly differently.

I think you should use absolute pressure as well as absolute temperature. At "zero" pressure the tyres actually have 1 bar of pressure in them. Add 2.2 bar and you have a total starting pressure of 3.2bar.

So in your example below:

So, if you started one morning from cold, and your tyres were at 10 C (283 kelvin) and 30psi.

Twenty miles later, your tyre temperature is at 40C (not the air temp, but the tyre temp).

40C = 313 K

So the new tyre pressure will be (313/283) * 30 = 33.2 psi

assuming that atmospheric pressure is 14.5psi then the starting pressure of 30psi above ambient is actually 44.5psi. So the new measured pressure will be (313/283)*44.5-14.5 = 34.7psi.

Similarly, regarding the weight of air in a tyre, at 2.2 bar above ambient you will have 3.2 "normal" volumes of air in there at 2.2bar above ambient, or c 118 litres weighing about 150g.

That's just my logic, I'm not a physics teacher or anything.

Edited by Manatee on 05/03/2017 at 19:40

Any - Boyles law and tyre pressures - RT

I agree that the pressure increase related to temperature change will be the same for oxygen or nitrogen, at least while both remain in a gaseous state, but I would do the calculation slightly differently.

I think you should use absolute pressure as well as absolute temperature. At "zero" pressure the tyres actually have 1 bar of pressure in them. Add 2.2 bar and you have a total starting pressure of 3.2bar.

So in your example below:

So, if you started one morning from cold, and your tyres were at 10 C (283 kelvin) and 30psi.

Twenty miles later, your tyre temperature is at 40C (not the air temp, but the tyre temp).

40C = 313 K

So the new tyre pressure will be (313/283) * 30 = 33.2 psi

assuming that atmospheric pressure is 14.5psi then the starting pressure of 30psi above ambient is actually 44.5psi. So the new measured pressure will be (313/283)*44.5-14.5 = 34.7psi.

Similarly, regarding the weight of air in a tyre, at 2.2 bar above ambient you will have 3.2 "normal" volumes of air in there at 2.2bar above ambient, or c 118 litres weighing about 150g.

That's just my logic, I'm not a physics teacher or anything.

Your logic is fine - absolute zero temperature and pressure - so add 1 bar to the pressure and add 273.15 C to the temperature - simples!

Any - Boyles law and tyre pressures - dadbif
For crying out loud, just pump them up and check the pressures every week....
Any - Boyles law and tyre pressures - bolt
For crying out loud, just pump them up and check the pressures every week....

I was waiting for a reaction like that, why make something so easy, complicated.

Still, as long as someone has learnt something here:)

Any - Boyles law and tyre pressures - SteVee

fascinating (I mean that sincerley).
The best thread I've ever read on tyre pressures. I think i now understand why my tyre pressures might change from one season to another and why I shouldn't be bothered about filling up the tyre with Nitrogen.

I've learnt something - so thanks to all contributors.

 

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