Pressures for larger tyres

[Diff]

All the maths is fascinating etc etc, but the basic premise in the first post is correct. ie if you have a large volume tyre, it needs less pressure to support a vehicle than the same vehicle fitted with tyres of lower internal volume.

To give you an extreme example, a bicycle tyre needs about 50psi to support an adult, but only 25psi is needed in a 4x4 tyre of similar diameter to support a great deal more weight.

Yes there are factors like side wall stiffness which will have a bearing on the actual pressure difference required.

Deciding what is the 'correct' pressure when non standard tyres are fitted is always going to be tricky.

Regards,

Diff

[Turbocharger]

To answer the question, you have to go back to the fundamental laws of thermodynamics from Boyle, Hooke and their French cousin, Carnot.

The laws of thermodynamics state "you have what you have", "you can't get more than you've got" and "you can't actually break even". The First law says that you have a certain amount of energy contained in a system (the pressure in the air) and this cannot increase without energy being added, usually from a compressor. The pressure in the tyre cannot decrease without an energy transfer, but since heat energy can be exchanged with pressure energy (and indeed the balance of these products is defined by the adiabatic, isothermal or other type of compression) the pressure in your tyres will change on hot or cold days. You could inflate your tyres by trapping a volume of air in them and then heating them (closed system) but they will deflate again when they cool.

This same process means that the volume of trapped air does not change but the pressure will do so after repeated heating/cooling cycles mean that heat energy is lost from the closed system each time; the air 'fatigues' in the same way that metal sometimes does. This is why you will sometimes see cars or lorries stopped at the side of the motorway with a flat tyre. This can be combatted by checking your tyre pressures regularly and adding more air to compensate for the low-entropy air that is currently in the tyre if the pressure is low. Scientists at Michelin and Dunlop increasingly recommend emptying all the air out of your tyres and replacing it every three years, allowing the entropic heat loss process to start again. The Michelin test facility in Dusseldorf is building a library of low-entropy air (collected in low pressure glass receivers) and they are experimenting on this air to show the effects of thousands of stop-start cycles.

Energy can also be mechanically added or subtracted from this closed system. A large force applied over a small distance will give 'work done', which is a transmission of energy. This effect means that the weight of your car will push the bottom of the tyre inwards slightly, increasing the pressure of the inflated tyre. For this reason, the best mechanics at Rolls Royce and Bentley garages use air pressure gauges which are calibrated to check the tyre pressure with the wheel in the air, and old-school RAC mechanics will jack up your car before they check the pressures. Anybody else is taking a shortcut, or simply a poorly-educated cowboy.

Time for the chalk I think.

[BogMonster]

This same process means that the volume of trapped air does not change but the pressure will do so after repeated heating/cooling cycles mean that heat energy is lost from the closed system each time; the air 'fatigues' in the same way that metal sometimes does. This is why you will sometimes see cars or lorries stopped at the side of the motorway with a flat tyre. This can be combatted by checking your tyre pressures regularly and adding more air to compensate for the low-entropy air that is currently in the tyre if the pressure is low. Scientists at Michelin and Dunlop increasingly recommend emptying all the air out of your tyres and replacing it every three years, allowing the entropic heat loss process to start again. The Michelin test facility in Dusseldorf is building a library of low-entropy air (collected in low pressure glass receivers) and they are experimenting on this air to show the effects of thousands of stop-start cycles.

Is it April 1st?

Sounds like a lot of hot air to me

Going into a garage and saying "I think the air in my tyres is broken could I have some new air please" would cause gales of laughter in most workshops I should think

[glaggs]

Is this why the air in old tyres smells like fish?

ps.

"Argh

The moral of the story is:

Just put 30psi in the darned thing and it won't be flat "..........................

and, you'll get stuck in the muddy bits.

[dollythelw]

I managed to buy 4 tins of original series 2 tyre air for petal but its imperial and wont fit my current tyres - I opened the tins up to see if its all there (looks ok), are these of any use to anyone?

[Turbocharger]

If it's been used, don't breathe it. Breathing high-entropy air is a major cause of death in crowded confined spaces such as third world jails. In Ethiopia up to 75% of deaths in non-confinement jails are wrongly attributed to another cause because of ignorance. It's heart-breaking.

As you may have guessed, most of what I'm writing here is dross, aiming to show that you can prove anything with a few numbers. Quick answer:

You need a force to hold your car off the road.

It's air the in the tyre that provides this force, using pressure. (Ignoring the sidewall stiffness for now).

The force to hold your car up is constant because the weight is constant.

Force = pressure x area

With a low pressure you need a larger area to provide this force.

Nature intervenes here and the tyre takes a 'flatter' shape so the footprint 'area' is larger.

However, once the tyre is a flatter shape it's also wobblier and harder to control from above, so the aim is to balance the two effects. 20-30psi is usually about right, unless you're aiming for mega MPG, offroad or your name is 'Jez'. The chalk method makes sure that the tyre is pressing down the same all the way across, so that it'll wear evenly.

[glaggs]

OK, I'll throw another spanner in the works. The volume of air in the tyre controls the temperature at which the tyre runs. Rubber compounds are formulated to offer the best grip/wear rate compromise at a given temperature. Too little air (pressure) too low and the tyre will over hea, which causes excess wear, tyre cooking and delamination. Too much air (pressure too) high and the tyre will not reach design opperating temp and therefore not grip efficiently. I appreciate this is not a big issue with tyres designed mainly for off road use, but anyone reading this thread and sidding with the pressure is primary impotance side of the agument should be carful if messing with road/performance tyre pressures.

ps. I worked for Dunlop for 7 years and sold tyres (motorcycle) for 10+yrs.

[Turbocharger]

No, the pressure of the tyre affects the running temperature, since a lower pressure gives a larger footprint, so the tyre deforms more as it rotates, absorbing more energy which heats the carcass up. You could argue that the additional air mass will alter the rate at which the tyre changes temperature, but since air has a relatively low specific heat capacity the volume of rubber will dominate the thermal inertia of the assembly, and in any case the large surface area and copious cooling (external) air will negate the effect of an extra 10% of air (by mass) in the tyre.

[glaggs]

As pressure is dependant on volume or mass of air in the tyre and vice verca then yes. To control or adjust the temp at which a tyre runs then the pressure is adjusted. Your owners handbook will probably recommend increasing pressure(volume) for sustained high speed driving. The danger is - run a low profile tyre at manufacturers recommended pressure n you'll be in deep pooh! The pressure needs to be increased to conpensate fo the smaller tyre volume - the question is by how much and how to work it out. The same applies with larger tyres. Run em at manufacturers recommended pressure n the tyre will be over inflatted, at the extreme this could be dangerous because of reduced contact area and tyres running at below optimum operating temp.

[BogMonster]

and, you'll get stuck in the muddy bits.

What like this?

I managed to buy 4 tins of original series 2 tyre air for petal but its imperial and wont fit my current tyres - I opened the tins up to see if its all there (looks ok), are these of any use to anyone?

I should hold on to it m8, in another 20 or 30 years that stuff will be like gold dust

[Guest otchie1]

As pressure is dependant on volume or mass of air in the tyre and vice verca then yes. To control or adjust the temp at which a tyre runs then the pressure is adjusted. Your owners handbook will probably recommend increasing pressure(volume) for sustained high speed driving. The danger is - run a low profile tyre at manufacturers recommended pressure n you'll be in deep pooh! The pressure needs to be increased to conpensate fo the smaller tyre volume - the question is by how much and how to work it out. The same applies with larger tyres. Run em at manufacturers recommended pressure n the tyre will be over inflatted, at the extreme this could be dangerous because of reduced contact area and tyres running at below optimum operating temp.

No, no, no, no and no again. Smaller tyres like bicycle tyres require higher pressures because they are exceptionally weedy in the sidewall stiffness department. You need high pressures to exert high forces on the inside of tiny surface area sidewalls. Big, manly 4x4 tyres are much stiffer about the sidewalls and have a larger surface area so get away nicely with less Newtons per square metre.

The total internal volume of the tyre can be held to be constant. You can compress more gas into that volume but the volume stays the same. There is no way, once it's compressed inside the tyre to measure how much volume your gas would occupy at stp without measuring the pressure.

(Well, you could weigh it but that's a different question and could lead to people weighing their tyres to calculate the volume from the combined density which you'd need a big bath to work out in the first place and then she'd complain....damn dirty great jeep taking over the house again. It's not a jeep FFS. Oh never mind. Just don't forget to wash the mud off first.)

The tyre temperature thing is again sidewall related, too much flex in them walls and you will be enrolled on Project Pearshaped. Of course, too much flex comes about from insufficient force holding them out which derives from low tyre pressures.

I once spent a few unproductive months trying to convince an American 'engineer' that lbs of weight DID NOT measure mass but rather the force of gravity acting on the mass. He never did get it. He had google links too but i didn't read those either.

Still, I'll carry on pumping up me tyres and you can pour air in from buckets to add more volume.

Volume..hmm...you're not confusing tyres with stereos are you?

[Guest otchie1]

I managed to buy 4 tins of original series 2 tyre air for petal but its imperial and wont fit my current tyres - I opened the tins up to see if its all there (looks ok), are these of any use to anyone?

Imperial air :hysterical:

[glaggs]

Sorry but no. Volume of air could be measured as its introduced into the tyre. The more gas put in, the higher the pressure. Yes the internal volume of the tyre is constant but volume of air changes proportionally to the pressure. Its not easy to measure the volume so we measure the pressure. However it is important to understand that on a given vehicle, fitting bigger or smaller tyres, you can't assume that running the manufacturers recommended pressure will be OK. The air volume supports the vehicle, thats the fact, the way of checking correct volume is by measuring the pressure. I wasn't looking to debate this, just to find an easy calc link!

[Turbocharger]

Jez - are you rotating the tins on the shelf to ensure even wear?

I seem to remember a certain forum member making a compressor out of refrigerator parts. He was having cornering problems, his car would enter a roundabout and then refuse to leave it. After months of confusion he emptied the air out of his tyres and blew them back up again on a petrol station forecourt - it turned out that the fridge compressors were making left-hand rotational air, when everyone knows that car tyres need right hand (clockwise) spinning air. That's why Jaguar moved away from left-hand-thread wheel nuts holding the nearside wheels on on their pre-war cars - same problem.

Apparently you can completely balance out the handling of your car with a old clockwork compressor and racers on the Nurburgring will see better lap times with this technique - this doesn't spin left or right, and is known as 'wind up air'.

[BogMonster]

it turned out that the fridge compressors were making left-hand rotational air, when everyone knows that car tyres need right hand (clockwise) spinning air.

only in the Northern Hemisphere, down here it goes the other way and if you put right handed air in the tyres they will implode

[Hybrid_From_Hell]

I have just got off the phone to Proffessor Stephen Hawkins and asked him about the situation above

He said

Staarpphooooooooonnnninggggmeeeeeeeee

Gettttaliffeannnnpurteireinnit

HTH

Nige

[FridgeFreezer]

Damn, I just got rid of a load of imperial air when I sold the 9.00x16's, the metric air isn't as good you know. Jez - is your air BS or Whitworth? The S1 boys will jump at the chance to buy original Whitworth air as it needs a special compressor and is very hard to come by.

[Guest otchie1]

left-hand rotational air,

:hysterical:

[Guest otchie1]

Sorry but no. Volume of air could be measured as its introduced into the tyre. The more gas put in, the higher the pressure. Yes the internal volume of the tyre is constant but volume of air changes proportionally to the pressure. Its not easy to measure the volume so we measure the pressure. However it is important to understand that on a given vehicle, fitting bigger or smaller tyres, you can't assume that running the manufacturers recommended pressure will be OK. The air volume supports the vehicle, thats the fact, the way of checking correct volume is by measuring the pressure. I wasn't looking to debate this, just to find an easy calc link!

Enough, enough already, my sides hurt.

[glaggs]

I knew there was something out there that supports the facts as I understand them.... check this link out you non-believers!

http://www.4x4mag.co.uk/novfeatures/novtyres.htm

[FridgeFreezer]

Putting twice as much air in the same space is doubling the pressure, not the volume sorry but that article may be correct in its methods but wrong in terminology.

Look at it this way - if you pump a tyre up to 10psi and dunk it in a bath full of water, X amount (volume) of water will come out.

Pump it up to 20psi and do the same and a very similar volume will be displaced. The volume has not changed, the pressure has.

Since I subscribe to the theory of intellectual osmosis I will now stop reading this thread.

[glaggs]

No you are confusing the internal area of the tyre with the amount of gas compressed into that space. If your theory was correct putting air into a tyre would be like putting water in a glass, By doubling the volume of air you will also increase the pressure. As I said earlier Iwasn't trying to start a debate, these are facts and I think simply explained in the link. I was hoping to maybe find a tyre technician on the forum who could add to the collective knowlage, not argue proven and tested science!

Keeping it simple - Larger tyre = larger space in side it. To inflate it to the same pressure as the original tyre you will have to add more air ie a greater volume of air. But your truck will now handle like a person I'm not that keen on. Let air out and reduce the pressure to one equal to a figure as calculated in the article and your truck will handle OK again. Take it off road and let more air out so it'll grip on the mud etc.

Air volume supports the vehicle FACT! - not air pressure.

[Guest otchie1]

I knew there was something out there that supports the facts as I understand them.... check this link out you non-believers!

http://www.4x4mag.co.uk/novfeatures/novtyres.htm

Nope, that guy starts of by talking about the area of a tyre when he means volume and then measures it in cubic feet. He is confused. His results may be right but then Columbus thought he'd found India when he crashed into America...his theory was wrong but he still found land.

I took the trouble to chase up the other links you had and they have similar problems with the basics. What these people are actually trying to talk about is the mass of air contained within a fixed volume at a given pressure. They are then trying to recalculate the correct pressure for their new bigger tyres by assuming that if they use the same amount of mass of air in their bigger tyres as was in their smaller tyres then all will be good. So, they work out the mass of air (but call it volume or area depending on their degree of incompetence) in their small tyres, ratio it to the bigger tyres, come up with a lower psi figure and assume it 'must be right'.

As pointed out many times this ignores the twin realities of tyre design and physics. Gravity acting on your vehicle tries to deflect the sidewalls of the bit of the tyre between the wheel and the road. Some force has to stop that happening and that force is resultant from the pressure of the air contained in the fixed volume of the tyre.

Yon matey 4x4Mag expert, after regurgitating all that volume nonsense then gives a formula that completely ignores it. His calculation takes the maximum load capacity of all four tyres and expresses the weight of the truck as a fraction of it (if max load capacity of all tyres is 6 ton and truck weighs 3 ton then you get 1/2 meaning you are at half max load of tyres). He then simply multiples that against the maximum pressure of the tyres to arrive at the correct psi. This may give a 'good enough' figure for most cases but would require a linear relationship between tyre load and the required pressure which I strongly doubt given the inverse proportionality of most things gassy. It also ignores the possibility of a minimum or starting pressure for those tyres. You'll also note that nothing in his equation mentions volume (or even mass ).

But enough; me and my O & A level in Physics, my C&Gs, my BTEC in motor vehicle engineering and my one-exam-away-from-a-BSc will continue to rely on air pressure. You, two Google hits and an American who gets volume and area confused can believe that fairies do it.

[Turbocharger]

^^^^ what he said. More mass of air in the same volume of tyre = higher pressure. I concede it is a fact that this high-pressure air would occupy a larger volume at room temperature & pressure but it's not helpful or useful to try it.

Glaggs - if you're desperate for a formula I'd take the max load fraction approach for a starting point but on your way back from pumping your tyres up I'd buy some chalk.

Otchie, if you need a BSc you can borrow my MEng for a bit until you get your own (or will swap for a calibrated tyre pressure gauge).

[glaggs]

Sorry stand corrected on mixing up the terms volume and mass ref the air inthe tyre. Still maintain its the 'mass' of air that is the crucial factor in bearing the load. Tyre construction has an influence but only a samll one. The industry talks in terms of air volume not mass, sorry. Applying the calculation from the link I need 17psi per tyre based on a gestimation of vehicle weight, which sounds about right. Off to buy some chalk and see if we were close?