Tyre pressures 'check em'

[JohnnyV]

I know we all know the importance of correct tyre pressures but having just checked mine as I took the van out of store, thought this might be a timely reminder - Van went into store in late October with correct pressures, now just over four months later each of the four tyres had lost between 8 & 10 PSI !

So if I hadn't checked I would have been towing at 28 PSI instead of 38 - over 26% below correct pressure.

 

[Sproket]

When the vans in the garden it is easy to check & blow the tyres up if need be, in the van i keep my old/new mitchelin portable compressor, it has pre set's for the pressure, & its 12v so i can plug it in either side of the van to check before we are ready for coming home before leaving site....

 

[Gafferbill]

.......10psi could be accounted for by tyre temperature.

Could have been warm last October and cold when you recently checked them.

Comparisons always have to take tyre temperature into account.

As you say it is always a good idea to check before use!

 

[machighlander]

tyre pressures should always be checked when cold, never when warm

 

[Gafferbill]

.. Cold tyres is a relative term.

Cold tyres on a 20deg C day is not the same as cold tyres on a

-4deg C day.

 

[otherclive]

But losing 8-10 psi seems a bit extreme and defies the universal gas law of P1v1/T.....etc!!

 

[JohnnyV]

Both checks were done with cold tyres and using the same guage (actually both a 'pen' and an airline guage).

I can't really say exactly why the 8 to 10psi drop, certainly had no such drops whilst the van was in use last season, perhaps with all of the cold weather the seal against the wheel was affected? - it is just a fact, and I just wanted to sound a cautionary warning to say 'check before towing'

Jon

 

[xtrailman]

I refitted my wheels two days ago, it looked like the pressure had drop 5 or 6psi, the tyres were at about 9 degrees c, due to them being stored in the integral garage, last used the van in oct 2009, i have always found them low agter the winter lay up.

The spare had lost about 10 or 12 last checked last march 2009. So that's the loss over a year. as usual have had put 46 psi into the spare, running pressure is 42psi.

As gaffer has mentioned, i have often though ambient temperatures play a part, but "cold" tyre testing is what they say, the question is do we have a correction table to compensate for different test temperatures, if not then nobody is running with the correct tyre temperatures, makes you wonder?

 

[Gafferbill]

......I totally agree with JohnyV.

I was putting forward a possible explanation why 4 tyres on the same vehicle had each lost 8-10psi over winter storage.

My caravan tyres have 51 psi as their correct cold tyre test figure.

Stationary but exposed to direct sunlight on a warm day..... this figure has risen to 57psi.

Checked straight after a Motorway run they have been 65psi!

..... all quite normal.

 

[ProfJohnL]

I find it strangely telling that from the information we have been given that all the tyres had lost similar amounts of pressure. This makes me suspect that either the gauges used are wrong or were used incorrectly on each occasion.

Even if the measured air temperature is -5, tyres in bright sunshine will be somewhat warmer. So if they were checked In sunlight before storage, but then subsequently checked in shadow or even night also at -5, there could be some negative difference.

Other than that the tyres must have lost some air.

Just some of the other possibilities for apparent loss of pressure:

Some one deliberately deflating the tyres

Tyres that are becoming cracked with age and standing

Puncture

Damaged rims so the tyre bead does not sit correctly

Faking surface of alloy rims allowing a weep track up the wall of the rim

Poorly fitting valve assembly in the rim

Leak through the valve (loss of valve cap allowing dirt into the seal)

 

[JohnnyV]

Sorry John L, I have to refute all your suggestions - van was new last year, tyres are in excellent condition, van was in secure storage, I used the same gauges both times (as I say I use a pen type followed by an airline guage - my own!)and I check my car tyres every week so I can read a guage, and also the rims are in perfect nick and have never been curbed.

I think some of the discrepancy may be that ambient 'cold' temp was warmer in Oct that in February, however I still think an 8 to 10psi drop seems high - However all tyres will loose some air over a period of time.

The point I was making was just cautionary - Check because if you just tow away from storage and don't, you could be running significantly down on pressure.

 

[machighlander]

Cold tyres is a relative term.

Cold tyres on a 20deg C day is not the same as cold tyres on a

-4deg C day.

aye, obviously not. however, every tyre manufacturer will advise you to check pressures when cold, maybe you should let them know they have got it wrong.

 

[xtrailman]

All tyres lose pressure over time, whether new or old.

Which is why nitrogen is used, as an option.

I can't remember the technical reason for air loss, but it more or less finds its way through the rubber.

 

[ProfJohnL]

Hello JohnnyV

You cannot refute ALL my suggestions; Either you have lost air pressure or you haven't.

In which case it will be down to a probable combination of temperature difference and actual loss of air.

Or if you haven't lost pressure then it must be down to inconsistent measurement devices or inaccurate reading of devices.

Now I'm not accusing you of anything, but those are the only logical diagnosis of your reported findings.

It is most likely that you have lost air pressure, as that is a natural consequence of trying to contain high pressure in a imperfect container, but you can't ignore the possibility of measurement errors.

 

[JTQ]

Just to get the temperature thing in perspective;

With JohnnyV's 38 psi lets assume that was measured initially at 20 deg C, and later remeasured at -5 deg C; its hardly likely to be a wider span from ambient?

Well for that temperature change the cold reading would be 34.76 psi, a "loss", of just 3.24psi.

 

[Gafferbill]

Cold tyres is a relative term.

Cold tyres on a 20deg C day is not the same as cold tyres on a

-4deg C day.

aye, obviously not. however, every tyre manufacturer will advise you to check pressures when cold, maybe you should let them know they have got it wrong.

... I did not say anyone had anything wrong!

I said the temperature of the tyre affects its pressure reading and may have had some bearing on the original post.

 

[JohnnyV]

Hello JohnnyV

You cannot refute ALL my suggestions; Either you have lost air pressure or you haven't.

In which case it will be down to a probable combination of temperature difference and actual loss of air.

Or if you haven't lost pressure then it must be down to inconsistent measurement devices or inaccurate reading of devices.

Now I'm not accusing you of anything, but those are the only logical diagnosis of your reported findings.

It is most likely that you have lost air pressure, as that is a natural consequence of trying to contain high pressure in a imperfect container, but you can't ignore the possibility of measurement errors.

John L - Sorry you are correct I missed the one about loosing air - that is obviously the correct reason, and if we take out the temperature thing, then I guess its probably about 6psi??? which over four months is not that horrendous - again it was a cautionary warning only.