Tyre Pressure Settting / Ambient Temperature

[Our Gee]

Hello all,
just been checking my Car tyre pressures and was surprised to find that all four tyres where well below the pressure I normally inflate them to. (about four to five PSI low)
The tyres where cold, infact extremely cold. The ambient temperature was hovering around freezing. I inflated all tyres to the recommended pressures of 2.1 / 2.2 bar.
Later today I drove the car on a 20 mile journey and found the ride to be extremely hard. (We motorcyclists are more likely to notice such things).
Now I am wondering, have I over inflated them or is my pressure gauge under reading in the very cold temperatures.
Any thoughts ?.

[EasyRider]

Think of them as a jar of jam. When the jam is put in the jar its hot.
As it cools down it make a vacuum inside the jar, so when you open the lid you get that popping sound and can push the middle of the lid in and out.
So as its so cold out ATM the air in side your tyre has retracted. But when your on the move and the tyres are warmed up the PSI inside the tyres increases. Try it on your bike. Take a reading before you go out, ride 20 miles and take another reading. it will be higher.

The Answer to your question is YES you have put to much air in your tyres.

[EasyRider]

Try it with a balloon. Blow up a balloon and put it in the fridge. It will get small as the air inside cools down, and the outside warm air pressure is higher than the cool air inside the balloon. Take the balloon out of the fridge and it will go back to the size it was as it warms up.

feel like i'm back at school

[David.]

Found this on the Porsche Club Great Britain forum:

I would love to find something definitive on whether to adjust tyres pressures for different ambient temperature.

20 years ago technical liaison at Michelin told me that cold pressure was (then) measured at 17° C and it would change buy 0.01 bar for each 1°C.
eg. If a tyre was 2.2 bar at 17° then at at 37° it would be at 2.4 bar. (0.01 x 20)
Similarly if was measured at 7° then that same tyre would be at 2.1 bar.

Ideally the tyres should be adjusted to the correct cold pressure in a garage at 17° and then they will be right, if you have to set them in ambient temperature that is different the cold pressure that you use should be adjusted for this.

Since then the standard temperature for "cold" tyres has been changed to 20°

Some time later another technical guy, this time at Pirelli, said that this was wrong. he said the cold temperature should be at the current ambient, and you should adjust your tyre pressures depending upon the weather. So if my tyres were correct (say 2.2) when inflated in a warm garage, but then parked outside in the cold at 0° I would have to increase the pressures by 0.2 bar to bring them back up. I have read other advice that says the same thing, adjust them for the ambient you will use them in.

Now my current car (Porsche GTS 4.0) has TPMS system, which seems pretty accurate. The pressure displayed concur with my gauges and the calibrated gauge at the Porsche Centre.

But the Multi-Function display has a function -"Fill Info" to tell you how much to adjust the tyre pressures, and the manual is clear that it is this display that should be observed, not the pressures displayed whilst driving. The manual also states that The displayed pressures take into account the tyre temperature.

Currently my tyres are at 2.2 bar, which is the correct cold pressure for the winter tyres fitted. The manual specifies cold pressure as being at 20°C. But it is cold outside - not much above 0° at the moment.

The fill info on the TPMS display says that my tyres needed to be reduced by 0.2 bar, which would reduce them to 2.0 bar at the current ambient. It also gives this information on the My Porsche app.

I guess that Porsche agree with what the Michelin guy said all those years ago.

For what it is worth, I’m going to try reducing my pressures as in the fill info; I took it out for a drive yesterday and it was quite squirmy under power, traction control lighting up, so perhaps they are overinflated?

Or, it's 0 degC.

[David.]

I have both a 12V digital air compressor and digital pressure gauge. I set the compressor to auto shut-off at the required cold tyre pressure and cross check with the gauge. They are usually within 1 psi of each other, I always go by the lower of the two. I've never adjusted the pressures for the ambient temperature, just set them at the recommended cold pressures every time. Haven't experienced any problems on motorcycles or cars. I usually check my motorcycle tyre pressures before every ride from home. With the car, it's less frequent.

[EasyRider]

IMO the pressure that matters the most to me is what pressure the tyre is when warm as that what counts when your riding or driving. If the manual says the PSi of your tyre when cold should be 36 PSi @ lets say 15 degrees outside temp. When the tyre is warm the PSi goes up. If you put 36 PSi in when the outside temp is at say 0 degrees outside the PSi with be higher when warm compaired to 36 PSi at 15 degrees outside temp. The friction between the tyre and the road when driving at 30 mph will still warm the tyre up no matter what the outside temp is. When i use my compressor to put air in my bikes i make sure the compressor is filled the night before so the air inside it has cooled down as hot/warm air will change the reading in a cold tyre.
The faster you ride or drive the higher the PSi will get, so if the PSi is not right the ride will feel hard.

[David.]

Using the above example of a tyre set at 36 psi both at 15 degC and 0 degC, in use, will the rise in tyre temperature and hence pressure be the same.

[EasyRider]

Using the above example of a tyre set at 36 psi both at 15 degC and 0 degC, in use, will the rise in tyre temperature and hence pressure be the same.

No it won't be as a cold tyre will hold more compressed air to have the same PSI when cold. so when they are both at the same temp after being used, the cold tyre will have a higher PSi.

[David.]

The physical volume of the tyre will be the same irrespective of the ambient temperature.

[EasyRider]

The physical volume of the tyre will be the same irrespective of the ambient temperature.

Not when both are at the same temp if one is filled at 0 degrees and one at 15 degrees.
It wont be much, but one will have more air when there at the same temp, and the warmer the tyres get the gap is PSi will change more and more.

think of deep sea diving and how the PSI get higher the deeper you going. that why divers get the bends if they come up to fast

[David.]

How does more air get into the tyre as you are driving along.

[EasyRider]

How does more air get into the tyre as you are driving along.

The density of warm air is less in comparison to cold air. Warm air rises up because when heat is provided, air expands. When the air expands, it becomes less dense than the air around it. The less dense warm air then floats above the denser cold air much like oil floats on water.

It's all about density.

[EasyRider]

Cold air is denser than warm air because the molecules in the air are closer together. The reason they are closer together has to do with thermal expansion, a phenomenon where molecules move farther apart as more heat energy is applied to a substance.

[EasyRider]

You put a balloon in the freezer with 36 PSi in it, as the air gets colder the PSI will Drop below 36 PSi. So you put more air in the balloon to bring it back up to 36 PSi. But when you take the balloon out of the freezer the Temp of the air inside goes up and so will the PSi as you have put more air in when cold to get it back up to 36 PSi. If you put the ballon in hot water the PSi will go up even more.

thermal expansion

Think Hot Air Ballon

[David.]

Irrespective of the ambient temperature, I set the cold tyre pressures to those recommended by the vehicle manufacturer. The pressure will rise as the tyre temperature increases with use. In the summer, on occasions, the cold tyre pressure may increase due to a rise in the ambient temperature. In this situation, the cold tyre pressure would need to be reduced before setting off.

As posted earlier, I have two tyre pressure gauges and use them as a cross check with each other. Having once bought a gauge from Halfords which differed from others I had, I went to one of their auto centres which fitted tyres. They have to have their equipment calibrated every year or replaced with new. I was able to confirm which of my pressure gauges were accurate.