Tyre pressures.

[Hutch]

Here is a good one for the forum , before I contact the tyre manufactures, changed the caravan tyres last year as out of life, being safe date wise, , my caravan a 2013 Coachman 560, states tyre pressure of 65 psi. Which I keep the tyres at. On the tyres it states Max Pressure should be 65 psi. After travelling down from Newcastle to Milton Keynes, yesterday decided to check the tyre pressures, when the tyres are hot, 68.5 psi.
So should I inflate the tyres to 61.5 when cold. Making the tyre pressures when hot 65psi, Bearing in mind Our caravan is always up to its Max payload. When towing.

 

[Damian]

Nope, don't change the cold tyre pressure, keep at 65psi.
The tyre makers will have factored in the increase in pressure as the tyre gets warm.

 

[Thingy]

The whole guessing game would be wildly innacurate. The temerature on different days, speed, weight, road surface would all comine to raise the pressures by a different amount on a daily basis. Guessing correctly on Monday would undoubtedly be way off on Tuesday.

 

[Dodger524]

Tyre pressures are quoted with the tyre temperature at 20 deg C.

Pressure will change with temperature.

 

[ProfJohnL]

It was always drummed into me that you should check tyre pressures when the tyre was cold, because as Dodger was told us the pressure in the tyre will increase with temperature. As Thingy points out, the temperature of the tyre is affected by a range of factors, but probably the most significant is the friction heating caused by the flexing of the tyres side walls when rolling.

As Damian points out, the tyre manufactures are well aware of this characteristic, and will build into there tyres an allowance to cover the normal increase in running and environmental temperatures.

In a normal road vehicle, the majority of heat generation come from the flexing of the sidexwall when rolling, if a tyre is overloaded or is running underinflated, the side walls will deflect more, this will generate more heat and that combination of physical movement and temperature will do more damage to the tyre wall compound, potentially leading to premature failure.

There have been many debates on the forum about selecting the correct tyre pressures with some quoting values to a hundredth of a Bar. But the reality is it's very very unlikely any consumer has access to a pressure guage that can accurately read a tyre pressure to that precision. It's most likely that none of our gauges can do better than +/- 0.1 Bar. Consequently there will be a modest variation in real inflation pressures depending on the gauge used.

Given that pressure accuracy can't be guaranteed, and the fact that the pressure will vary with temperature, how accurate can we be expected to be? My guess is that allowing for the inherent variations between gauges, a measurement with a tolerance of +/-0.2 Bar (approx +/- 2psi) is about as good as we can get.

What is arguably more importantl is that all the tyres on a vehicle should be checked by the same gauge, to create a consistency of measurement.

 

[Icaru5]

the tyre manufactures are well aware of this characteristic, and will build into there tyres an allowance to cover the normal increase in running and environmental temperatures.

I presume you actually mean “their tyres” ....

 

[Damian]

Is there any need whatsoever to make a posting like this?
Of course it should have been "their" but we all make simple mistakes at times and this forum does not give prizes for perfect spelling or grammar.

It does though take drastic action over certain other aspects such as trying to demean another poster for an innocent mistake.

 

[waffler]

To continue the pressure topic. When we bought our first coachman in 2009 my electric pump wouln't go to 65 lbs so off to car spares shop. Offered a cheap, medium or expensive pump. Chose medium- off to van and then realised the guage was analogue and flicked like mad do bought a pressure guage. Now of course the total cost was more than the expensive pump!!!!! (didn't tell the boss!). The guage was impossible to use so took it usual tyre shop for a lesson. They couldn't use it either so back to the shop, a well known chain. On this I went back to spares shop and confessed. Paid the difference and went off with the expensive one which lasted me until last year. The guys in the shop enjoyed the humour, you know what I mean. So now when there's a choice the wife reminds me .........