Tyre pressures and pressure loss.

[Thingy]

Following on from the TPMS thread in another section, I got to thinking about caravan tyres and pressures. In all we have had seven caravans, some from new, others used. Invariably, before each trip of the year every single tyre on all of those caravans has had to have pressure added, sometimes by as much as 7 or 8 psi. I have always put this down to the fact the caravan has been standing for months without really understanding the mechanics as to why it happens, but then....................

During lockdown my wifes car has been standing for months, as long as the caravan has, just because itsa a pain to move one car to get the other out. On checking its pressures, not a single psi has been lost from any tyre. Similarly, on the car in use, nothing is ever lost. I cant recall a tyre losing pressure on any of my cars that wasnt either due to a damaged rim or duff valve.

The only thing I can imagine makes the difference is that caravan tyres are usually holding much more pressure than a car tyre.

 

[RogerL]

There may be a quality difference in the wheels used on caravans, particularly alloys.

 

[JTQ]

I can't recall ever having tyres, cars, trailers and caravans that don't, over time lose the odd psi.
They also reflect changes in ambient temperatures summer to winter.

Whilst the higher pressure will motivate more air volume loss through porosity, the fact the higher pressure at the same time stores more energy, effectively negates a real difference in % psi drop.

I had one steel wheel where micro cracking in the weld affected zone caused quite large drops, but these were recalled when the issue was finally identified.

 

[Thingy]

I can understand the odd psi loss with some tyres, I am assuming we are talking about the porosity of the tyre compound, but a loss of 7 or 8 psi over say six months is a lot imho. Does this porosity equate also to the quality of the tyres as I have long been of the opinion that the transit style tures fitted to caravans dont match up to the quality of the premium grades, i.e. Continental et al.

 

[JTQ]

I can understand the odd psi loss with some tyres, I am assuming we are talking about the porosity of the tyre compound, but a loss of 7 or 8 psi over say six months is a lot imho.

Not really, it is osmosis, this is one of many LINKS you can find on line, this one, albeit American talks of "tires", but we know what they mean.

This and others talk of 1-3 psi a month, so even at the better end your 6 months is looking at 6 psi drop, a figure I think typical though I usually correct it long before that.

My caravan tyres are Goodyear but Cargo tyres with Transit type vehicles being their target market, though they ideally suit our heavy 1900 kg, SA van. These I see in no way as a budget tyre, but very high quality. [ Hymer factory fit].

Osmosis, in this case can be reduced by inflating with nitrogen, it passes slower than the oxygen content in air, not something I feel inclined to invest in, for the modest issue it reduces.

 

[figbat]

To be pedantic, it is diffusion rather than osmosis. Yes, oxygen molecules are a touch smaller than nitrogen molecules but the difference is not much and not enough to support the profit-chasing phallacy that nitrogen is required for road applications.

I’d agree that it will be a combination of higher pressures and different wheel and tyre constructions. Besides, 7 psi out of 60 is the same as 4 out of 33, which is a not-uncommon car pressure loss.

 

[JezzerB]

We find our caravan tyres lose almost exactly the same as our cars-or gain if the weather warms ie you pump up in the winter and a sudden warm snap appears-pressures seem to jump at least 1 psi. BUT our Buc is a TA and therefore running lower than our neighbours single axle Coachman at 48psi-our Minis are around 42psi so not much difference-you could well be right

 

[JTQ]

To be pedantic, it is diffusion rather than osmosis.

Thank you and I accept you are right, though it is what I picked up today some of our friends over the pond refer to re tyre pressure loss through the "rubber".

I, as originally posted, simply think of it as a form of porosity, though, was aware on a very micro scale, just that checking online it gained the name "osmosis". On refection I realise I knew that was used for migration of solvents.
Thanks.

 

[Beachball]

My caravan had the correct pressure but my car had to be pump up a little bit . but they are fine now

 

[Hutch]

2 psi per month is normal, and also depends on the type of valve cover you have, the valve core is not the seal the valve cap is, a plastic cap doe's not not seal.Go for a cap with a o ring inside.

 

[Darren.H]

Rubber is porous
Better quality tyres are less so.
Basically every tyre will loose pressure

Read this a few weeks back, our pushbikes tyres were all almost flat over winter, had a Google as to why,