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Tyre pressures (again!)
userBrian Ramsden
Posted: 19 January 2005 12:35 PM
Subject: Tyre pressures (again!)

One aspect of this subject which has not been covered in the forum is the effect of teperature changes on tyre pressure. My tyres are required to operate at close to their maximum (65psi) pressure. I found that, after inflating them to the correct pressure before leaving home and then driving of to the sun, they would then (even when "cold") be over this limit. I therefore checked with Michelin and they gave me the following information:-

The recommended temperatures are correct for an ambient temperature of 63ºF/17ºC. This also applies to the maximum permitted pressure.
On van tyres an ambient temperature change of 1ºC gives a change of 0.29psi. 1ºF gives 0.16psi. Car tyres are so small the effect is insignificant.

thus, if your recommended presuure is 65psi and you are setting out from england at an ambient teperature of 40ºF you should set the pressure to 61psi. If you then journey to the sun (Morrocco?) where the ambient temperature is say 90ºF, if you check your pressures they will have increased to 69psi. Even though this is above the maximum permitted presssure, they should not be reduced, but kept topped up to 69psi. Michelin did advise that, if you are topping up the tyres when the ambient temperature is high, add the air a little at a time and allow it to reach the same temperature as the tyre, before adding more.

Conversly, if you go winter skiing and check your tyre pressures when the pemperature is, say,-5ºC, don't inflate them back up to 65psi, leave them at 58psi.

Perhaps I'm the onlu one who is a touch paranoid about tyre pressures, but I found this information helpful
Posted: 19 January 2005 2:32 PM
Subject: Tyre pressures (again!)

Most people run their tyres too soft.
Modern tyres have a large safety margin for pressure. How many times do you see lorry or van drivers reduce pressures when unladen then increase when laden? Come on!

Under pressure running will result in increased tyre temperature build up and decreased miles per gallon.

Over pressure results in a smaller footprint on the ground and increased fuel economy.

I tend to stay on the high side as its a camper not a racing car.

Posted: 19 January 2005 8:43 PM
Subject: Tyre pressures (again!)

I totally agree with Clive. After some 15 years driving winter & summer in a large delivery van and many (40) years before driving a W-I-D-E rande of vehicles
Inflate the tyres to the manufacturer figures. Winter & Summer.
With my delivery van to lower the pressuses as you say would have resulted in rolling on corners when under full load conditions (which was almost every day)
Racing cars are different and they use higher pressures. Do you change tyres when it rains ?????????
Dont know where you got your tyre pressure gauge from, but it sounds to be more accurate than those you get at garages & deffinitely more so than those slide out things you buy at car shops
userMel E
Posted: 19 January 2005 11:24 PM
Subject: Tyre pressures (again!)

Our Daewoo Matiz was fitted with temp and pressure gauges on each wheel because, on tow, its the safest way to be sure all is OK behind the motorhome.

The normal front tyre temp rose to about 28C in winter and up to 40C in summer. The clever device calculated the appropriate pressure for the actual temperature and displayed both this and the actual temp. This avoided the problem of the pressure alarm going off simply because the tyre was hot.

I.E.: Hot tyres, higher expected and required pressure. so no problem!
userBrian Ramsden
Posted: 20 January 2005 10:16 AM
Subject: Tyre pressures (again!)

I think both Clive and Pete have missed the point. I was not suggesting running at other than the recommended pressures, only pointing out that, when the AMBIENT temperature is high, the recommended pressure is higher and when the ambient temperature is low, the recommended temperature is lower. Whatever pressure you set your tyres at, if the outside temperature drops, your tyre pressures will drop, but there won't be any difference in the handling, due to the increase in the density of the air at the lower ambient temperature. So, if you have settled on a particular pressure as giving a comfortable ride at normal ambient teperature, then for the same ride quality you need to take into account the actual outside temperature when you "top up" your tyres - thats if it is extremely hot or cold.

Brian Ramsden
userMike Chapman
Posted: 20 January 2005 1:00 PM
Subject: Tyre pressures (again!)

Hello Brian,

You are not alone, I also care about getting tyre pressures correct. This is not helped by the lack of any sensible information in the vehicle and coachbuilders handbooks. I will stay off that subject.

You mention ambient temperature but radiant temperature is also important. Tyres being mostly black pick up radiant heat from the sun. The temperature of the tyres on the sunny side of a vehicle can be considerably higher than those on the shaded side. Like me you have probably seen drivers checking their tyre pressures in this environment and reducing the pressure of the heated tyres. A good case for shielding tyres from the sun.

Your recommendation is right. DO NOT adjust the pressures of hot tyres, or very cold tyres, unless it is absolutely essential and then with great care. The best advice is to drive carefully and check the tyres, when "cold", at the earliest opportunity.

If travelling outside the UK in mountainous regions then altitude will also play a similar part to temperature.

May I point out that under inflation is more critical than over inflation as when the tyre heats up excessively the structure of the tyre can be seriously damaged and this is not always evident unless it is removed and inspected. Over inflated tyres seem to pick up punctures more easily.


Mike Chapman
Posted: 20 January 2005 3:47 PM
Subject: Tyre pressures (again!)

Hi Mike Chapman

I have measured the difference between tyres In the sun & shade, when in the Spanish winter sun

There was just over 5 psi difference

Any shade will do to reduce the effects of the sns rays: An old ground sheet, Paper or cloth sack, or you could buy some of those plastic or waterproof covers that the continental travellers use.
It doesn't seem to matter if you drive off, as the pressure drops rapidly (we stopped to check again after a few Killo meter to check)
Posted: 20 January 2005 5:10 PM
Subject: Tyre pressures (again!)

Thank you all. I think i've got it! Inflate them when its cold,let them down when its hot. When they are hot and cold (sun on one side)let them up and down,and only drive the hot side downhill and the cold side uphill. Stop every mile or two to check again.
Sorry to be flipant on what i recognise as a serious subject but I couldn;t resist.
Posted: 21 January 2005 12:50 PM
Subject: Tyre pressures (again!)

Dont forget the effects of altitude!
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