Tyre pressures + cold weather + nitrogen

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wemorgan

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A recent thread on another forum (A2OC) made me think:

Why are we told to test tyre pressures when cold and adjust accordingly, but told to check engine oil level when warm? Is this not the same theory but in reverse?

My point being, this morning it was ~0C. If I adjusted my tyres to 30psi then drove 150 motorway miles, the pressure at the end of the journey would be ~36psi, I imagine.

But what if the ambient temperature was 30C and I adjusted the pressures to 30psi then the tyres would be 36psi by the end of the journey.

Which is the correct pressure that the OEM requires?

Which leads me to: Nitrogen filled tyres will expand less than air filled. So therefore shouldn't Nitrogen tyres be filled to higher than OEM stated pressure when cold?
 
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Engine oil level is different for different engines.
On my BMW it's cold if you check with the disptick but hot if you want to check with the onboard computer.
My bikes all require oil checking with hot oil but a stopped engine.
It's simply whichever the engineers decided to do at the time.

As for nitrogen filled tyres on cars, what a load of rubbish.

The main reason for filling aircraft tyres with nitrogen is because an aircraft tyres will be at -30°C at altitude and could potentially be landing on tarmac at 50°C a few minutes later and so that temperature change combined with the MOISTURE in air could affect a significant volume/pressure change.
But the key here is the moisture, not the pressure change.

Sorry, but total waste of time on road cars.
 
I thought there were other benefits on road cars.
 
But what of my main point; why do you test tyres when cold, when you need to know the pressure when hot, since that's the operating temperature?
 
Because the tyre temp is part of the equation.

If you fill the tyre with an appropriate amount of air when cold then drive around then the carcass will flex enough to raise the temperature (and hence the pressure) to the level which the manufacturers of both the tyres and the vehicle have decided is suitable.

If you underfill the tyre then the additional carcass flexing will heat the tyre MORE, so the tyre carcass temperature will be higher but the overall pressure will still be lower or the same. In extreme cases the temperature will continue to rise without the pressure ever being high enough to stop the rise in temperature, and then the tyre carcass will self-destruct due to excess heat.

If you overfill the tyre then the carcass won't flex enough, thereby not raising the temperature of the tyre to a point where it provides optimal grip but simply wearing out the middle of the tyre and providing you with poor wear and poor grip.

So if you checked the temperature and pressure of the tyre when it was warm you'd need a chart of what pressure the tyre is supposed to be at any given temperature to know whether to put more air in or take some out.

This is also why your tyre pressure sticker usually shows two pressures for any given tyre on that vehicle - one for "normal" use and one for "high load", the latter being high speed (where the tyre will heat up more, so you need higher pressure to start with to reduce the flex which causes the heat) or heavier loads like occupants and luggage (which will again cause the tyre walls to flex more, increasing the creation of heat).

Oh my god i've written an essay! Sorry if anyone's fallen asleep :)
 
But what of my main point; why do you test tyres when cold, when you need to know the pressure when hot, since that's the operating temperature?
It's probably because it's easier to set a baseline from a known position.

When tyres are at rest and haven't been used then their temperature will be within a narrow range. When tyres have been in use and are "hot" I bet the range of temperatures and resultant pressures is much larger.

Meanwhile engines are designed for a certain operating temperature and the expected oil capacity is based on relative expansion at this known temperature.
 
When tyres are at rest and haven't been used then their temperature will be within a narrow range. When tyres have been in use and are "hot" I bet the range of temperatures and resultant pressures is much larger.

That makes sense, but even that narrow range is not so narrow, considering the range of ambient temperatures in Europe.

So "if" you assume nominal ambient is 20C and set tyre pressures accordingly, then when it is 0C and you test tyre pressures, should you set them a bit lower than OEM recommendations?
 
My tyres are all nitrogen filled. Now I'm scared to test the pressure as it will let a bit out and I'll have to pump it up with plain old air..


.. only kidding - I guess it's not such a big difference to tell what they are filled with!
 
my tyres are all nitrogen filled. Now i'm scared to test the pressure as it will let a bit out and i'll have to pump it up with plain old air..
:d :d :d
 
So "if" you assume nominal ambient is 20C and set tyre pressures accordingly, then when it is 0C and you test tyre pressures, should you set them a bit lower than OEM recommendations?
The effect of the cold weather and ground temperature might mean the tyres remain slightly colder during the journey so maybe they should be inflated to the handbook figure regardless ;)
 
My point being, this morning it was ~0C. If I adjusted my tyres to 30psi then drove 150 motorway miles, the pressure at the end of the journey would be ~36psi, I imagine.

But what if the ambient temperature was 30C and I adjusted the pressures to 30psi then the tyres would be 33psi by the end of the journey.

corrected typo in my OP....sorry for the confusion
 
nitrogen - Oh no, not again !

The word nitrogen should be banned from this forum. There is far too much urban myth about this substance. Air is four fifths nitrogen. Nitrogen does not expand more or less than air for a given change in temperature. And nor do any other (dry) gases. The only reason racing and aircraft tyres use nitrogen is to ensure the filling is dry. It's free water in the tyre that causes large pressure changes as the temperature alters.

And for the record, if you adjust an average 7 inch wide 16 inch diameter tyre to 30 psi at 20 C, then the pressure changes to 30.4 psi at 30 C, 31.4 at 40 C and 32.4 at 50 C. I doubt most car tyres get much hotter than 50 C even in summer.

And that is why no temperature need be given for the pressure adjustment - it doesn't matter because most tyre pressure gauges aren't good enough to detect the difference between 30 psi and 32 psi.

Can we now lay nitrogen to rest - forever. Please.

Geoff
 
Can we now lay nitrogen to rest - forever. Please.

Yes...

Aircraft tyres are inflated with nitrogen EASA requirement :

Nitrogen is an inert gas and will not combine with other gases should a tyre overheat and produce a volatile gas. Prior to inflation the tyre is of course filled with Air at ambient ptressure and humidity but inflation with Nitrogen reduces the percentage of oxygen and water vapour.


EASA CS 25 Specification for Large Aeroplanes:
(e) For an aeroplane with a maximum
certificated take-off weight of more than 34019 kg
(75 000 pounds), tyres mounted on braked wheels
must be inflated with dry nitrogen or other gases
shown to be inert so that the gas mixture in the tyre
does not contain oxygen in excess of 5% by
volume, unless it can be shown that the tyre liner
material will not produce a volatile gas when
heated, or that means are provided to prevent tyre
temperatures from reaching unsafe levels.


A Safety Measure but the number of injuries to Aircraft Servicing Staff due to mistakes with regulators as Hi pressure Nitrogen is used in other systems. e.g a tyre on its way to 2500 psi instead of 250psi goes bang.

In car tyres a good ruse to help people part with money.
 

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