Numpty tyre pressure question
Discussion
Numpty question I grant you, but I keep getting conflicting advise, so thought what the hell, I'll get some more from here 
When checking tyre pressures, you do it when the tyres are cold right? So on a day like today, at 5°C, my readings match that of the car at 2.0 bar front and 2.3 bar rear.
However, when I go for a drive, the temperature in the tyres of course increases quickly and I soon fine the tyre pressures at 2.2 bar front and 2.5 bar rear.
Is this ok, as the label in the door states something about 20°C, but rarely do see ambient temperatures such as those from a cold start to check.
When checking tyre pressures, you do it when the tyres are cold right? So on a day like today, at 5°C, my readings match that of the car at 2.0 bar front and 2.3 bar rear.
However, when I go for a drive, the temperature in the tyres of course increases quickly and I soon fine the tyre pressures at 2.2 bar front and 2.5 bar rear.
Is this ok, as the label in the door states something about 20°C, but rarely do see ambient temperatures such as those from a cold start to check.
Edited by mdianuk on Sunday 2nd April 10:56
Does not matter, I bet 90% cars over winter were 5 psi down as no one checks pressures bar us nerds.
I have a pyro and set that way if the centre is hotter after a run I let air out, it's quite simple then and no need for what psi should I have my tyres at type forum questions, because as you have found out depending on temp and length of journey tyres can be 10psi out any way.
Sitting on the work run at 20 mph is not like that Sunday 7am B road blast. When tyres are sitting at 40degrees.
The Porsche guide is set to allow a psi gain or drop so tyres stay ok for longer, as with all oem setting, geo etc one can improve on said setting if one is anal enough to do so.
I have a pyro and set that way if the centre is hotter after a run I let air out, it's quite simple then and no need for what psi should I have my tyres at type forum questions, because as you have found out depending on temp and length of journey tyres can be 10psi out any way.
Sitting on the work run at 20 mph is not like that Sunday 7am B road blast. When tyres are sitting at 40degrees.
The Porsche guide is set to allow a psi gain or drop so tyres stay ok for longer, as with all oem setting, geo etc one can improve on said setting if one is anal enough to do so.
Tyrepressure advice is to give the tire a deflection , that wont give to much heatproduction so the rubber reaches a to high temperature and hardens. Once this hardening is done , there is no way back and every bending of the rubber by the deflections of every segment of the tyre per cycle , gives damage to those hardened parts , and at a sertain moment blowing tyre.
The car-maker determines a pressure for the speed of tyre ( same as car) and the load on it.
Asuming they do this right , it is meanth for 18 to 20 degr Celcius,
When driving at this outside tyre so ambiënt temperature , at about speed of 55m/90km/h the tyre inside air will get about 40 to 45 degr C, and by that the pressure of tyre rises , wich gives lesser deflection so lesser heatproduction a cycle.
Cooling down of rubber of tyre is only dependent on temperature differences between rubber and surounding rubber, rubber and in and outside tyre air, and temp difference between in and outside tyre air, and not on speed.
Now warm pressure of tyre is hard to determine, because what speed and howmuch has it already cooled down when you measure it. Also severe braking can heaten up the tyre inside air , by the heat of brakes transported trough the rimms to the tyre inside air.
Cold pressure is easyer to determine acurate, and is when temp inside tyre = temp outside tyre, that simple.
But what is the outside tyre temp, and tyre inside can be heatened up by the sun shining on the black rubber.
outside temp is pretty acurate to determine using a thermometer.
So you only need to measure the pressure cold, and know the outside tyre temp to re calculate it for 18/20 degr C.
When its colder outside> lower tyrepressure> more deflection > more heatproduction a cycle.
But also more temperature differences > more cooling down of tyre> so rubber stays below a critical temp when it hardens.
When its warmer outside> higher tyrepressure> lesser deflection> lesser heatproduction a cycle.
But also lesser temperature differences> lesser cooling down of tyre rubber> rubber hottest temperature the same so still below critical temp.
My estimation is that in a range of outside tyre temp of between -10 degr C and +40 degr C , the warming up and cooling down is in balance , so the rubber reaches the same temperature.
So if you calculate the measured pressure back to 18 to 20 degr C , you can adjust the tyres pressure for that.
I made a list with Excell and will show it here.
It assumes the tyre not to expand or schrink with pressure chanche.
Greatings from a Pigheaded Dutch Selfdeclared Tyrepressure-specialist
Peter
The car-maker determines a pressure for the speed of tyre ( same as car) and the load on it.
Asuming they do this right , it is meanth for 18 to 20 degr Celcius,
When driving at this outside tyre so ambiënt temperature , at about speed of 55m/90km/h the tyre inside air will get about 40 to 45 degr C, and by that the pressure of tyre rises , wich gives lesser deflection so lesser heatproduction a cycle.
Cooling down of rubber of tyre is only dependent on temperature differences between rubber and surounding rubber, rubber and in and outside tyre air, and temp difference between in and outside tyre air, and not on speed.
Now warm pressure of tyre is hard to determine, because what speed and howmuch has it already cooled down when you measure it. Also severe braking can heaten up the tyre inside air , by the heat of brakes transported trough the rimms to the tyre inside air.
Cold pressure is easyer to determine acurate, and is when temp inside tyre = temp outside tyre, that simple.
But what is the outside tyre temp, and tyre inside can be heatened up by the sun shining on the black rubber.
outside temp is pretty acurate to determine using a thermometer.
So you only need to measure the pressure cold, and know the outside tyre temp to re calculate it for 18/20 degr C.
When its colder outside> lower tyrepressure> more deflection > more heatproduction a cycle.
But also more temperature differences > more cooling down of tyre> so rubber stays below a critical temp when it hardens.
When its warmer outside> higher tyrepressure> lesser deflection> lesser heatproduction a cycle.
But also lesser temperature differences> lesser cooling down of tyre rubber> rubber hottest temperature the same so still below critical temp.
My estimation is that in a range of outside tyre temp of between -10 degr C and +40 degr C , the warming up and cooling down is in balance , so the rubber reaches the same temperature.
So if you calculate the measured pressure back to 18 to 20 degr C , you can adjust the tyres pressure for that.
I made a list with Excell and will show it here.
It assumes the tyre not to expand or schrink with pressure chanche.
Greatings from a Pigheaded Dutch Selfdeclared Tyrepressure-specialist
Peter
Basically OP (though I feel I know you as mdianuk through your informative GT4 OPC state of play posts), you're right. Tyres warm up when you drive and it's the cold pressure that counts. IMO even a short trip to a gas station to use their airline/gauge messes things up so use your own tyre pressure gauge and pump on your drive. And every week.
But, if the weather gets warmer since you did them last, check the pressures and consider whether you want to let some air out. And vice versa if it gets colder. For me, running with under-inflated tyres is a sin.
ETA: Anything else anyone says is b
ks.
But, if the weather gets warmer since you did them last, check the pressures and consider whether you want to let some air out. And vice versa if it gets colder. For me, running with under-inflated tyres is a sin.
ETA: Anything else anyone says is b
ks.Edited by Maxym on Sunday 2nd April 21:52
Never let out air when its warmer outside !!!!! The tyre needs that higher pressure to give it lesser deflection , so lesser heatproduction, because the cooling down of tyre is also less because the temperature differences are less.
When its colder outside , you can highen up the pressure to adviced, to give it less deflection so less fuel consumption.
But for savety , so no overheating of rubber, its not needed, see for that my former answer.
Once read that tyres are tested to can stand a pressure of 2 to 3 times the pressure needed for the maximum load up to 160km/99m/h. This is called in the official European formula of ETRTO , that I once got hold of and went running with, the reference-pressure , but will call it AT-pressure furtheron , because on LT and trucktyres this pressure is given on sidewall something like this "maximum load xxxx lbs/kg AT yyy psi/kPa( cold) .
This is not the maximum cold pressure as they only give on Standard load and XL/reinforced/Extraload normal car tyres.
For those AT pressure is SL 2.5 bar/250kPa/36 psi in EUR system with some exeptions to lower.
American system ( P tires) SL 35 psi/2.4bar/240kPa, and as far as I know no exeptions.
For XL// EUR AT 2.9bar/290kPa/42 psi( again some exeptions), USA( P-tires) AT 41psi/2.8bar/280 kPa.
For higher speed then 160km/99m/h the AT pressure must be highened up with a system depending on speedcode of tyre, to the official system( also got hold of that at the same time of the formula), but determined my own system of changing the Loadindex 1 step for every 10km/6,5m/h different speed, lower speed> higher loadindex, higher speed> lower loadindex, then use this maximum load belonging to that new loadindex in the formula to calculate the pressure.
When its colder outside , you can highen up the pressure to adviced, to give it less deflection so less fuel consumption.
But for savety , so no overheating of rubber, its not needed, see for that my former answer.
Once read that tyres are tested to can stand a pressure of 2 to 3 times the pressure needed for the maximum load up to 160km/99m/h. This is called in the official European formula of ETRTO , that I once got hold of and went running with, the reference-pressure , but will call it AT-pressure furtheron , because on LT and trucktyres this pressure is given on sidewall something like this "maximum load xxxx lbs/kg AT yyy psi/kPa( cold) .
This is not the maximum cold pressure as they only give on Standard load and XL/reinforced/Extraload normal car tyres.
For those AT pressure is SL 2.5 bar/250kPa/36 psi in EUR system with some exeptions to lower.
American system ( P tires) SL 35 psi/2.4bar/240kPa, and as far as I know no exeptions.
For XL// EUR AT 2.9bar/290kPa/42 psi( again some exeptions), USA( P-tires) AT 41psi/2.8bar/280 kPa.
For higher speed then 160km/99m/h the AT pressure must be highened up with a system depending on speedcode of tyre, to the official system( also got hold of that at the same time of the formula), but determined my own system of changing the Loadindex 1 step for every 10km/6,5m/h different speed, lower speed> higher loadindex, higher speed> lower loadindex, then use this maximum load belonging to that new loadindex in the formula to calculate the pressure.
Edited by Jadatis on Monday 3rd April 10:07
Sorry old chap. My last sentence might have been influenced by a very good dinner with some vinous accompaniments, and wasn't aimed specifically at you. Nevertheless I didn't understand your first post and am none the wider after the second.
You seem to be saying that if you are inflating, say, four tyres that have just been put on their rims that you would do so using different pressures in the depths of winder compared with high summer. And that you'd ignore the manufacturer's recommendations.
Is that correct?
You seem to be saying that if you are inflating, say, four tyres that have just been put on their rims that you would do so using different pressures in the depths of winder compared with high summer. And that you'd ignore the manufacturer's recommendations.
Is that correct?
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