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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Technical Forums > Suspension | Brakes | Chassis > Tyre Pressure and Hot Weather - Technical Question



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      06-03-2022, 06:30 AM   #1
bmw_solid
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Tyre Pressure and Hot Weather - Technical Question

Hi All,

Over the last 2 years of my ownership of my E92 330d SE, I have spent on suspension overhaul and replacement with OEM equivalent and equally spent a lot of time experimenting with tyre pressure as on BMWs, tyre pressure can basically make them drive like crap or like a dream - literally, eve 1 psi above/below or a 1 psi loss on a single wheel can wreck the whole feel.

Now, I have non-run flats on my car and I specifically went for a known budget brand called RoTalla RU01 S-Race Setula. The reason being because:
(225/35/19 & 255/30/19)
1. They are one of the quietest tyres - up there with Michellin PSS 4 @ 69dB.

2. According to some reviews and my experience with them, they are noticeably more comfortable than Kumho, Falken and possibly up there with Michellin PSS 4 - never experienced PSS 4.

3. Dry handling is not that bad either - I don't drive fast around sharp turns and going around a bend or turn at the right speed band allows the car to handle very well for an SE. If I thrash it around a corner, there is body roll but not a lot of body roll and the grippy feeling of the tyres is actually quite good.

I am running my car on 33/36 compared to the factory values of 36/39 as I think 36/39 is way too high for the car as it it too firm, suspension makes thuddy noises and tyre wear becomes concentrated towards the centre of the tyres - this can only mean one thing: run flat tyre pressures are not for non-run flat tyres

On to the technical question of tyre pressure increase:

If I fill my tyres to 33/36 at say 10 d.Celsius cold and the weather increases to 25 d.Celsius in the day time, that means 15 d.Celsius will increase the tyre pressure to ~35.25 and ~38.25 without driving the car (0.15 psi increase per 1 degrees Celsius so 0.15 x 15 = 2.25).

Now, if I get in the car (@ 25 d.Celsius weather) and start driving it for 20 minutes, will the pressure increase by a further 4 PSI or by a further 1.75 psi (2.25 + 1.75 = 4)?

In other words - a tyres pressure will increase 4 PSI after 20 minutes driving and then would stay at those pressures until driving stops. But, if we add the 15 d.Celsius temperature increase which has already increased the pressure to +2.25 PSI and then start to drive the car, would the pressure still increase by another 4 PSI?

Last edited by bmw_solid; 06-03-2022 at 06:36 AM..
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      06-07-2022, 12:50 PM   #2
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      06-08-2022, 01:17 AM   #3
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Im running 36 39 psi on my non runflats. In a normal drive, your only seeing about 4-6 psi differences between cold and hot.
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      06-08-2022, 06:37 AM   #4
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Quote:
Originally Posted by sliver 335 View Post
Im running 36 39 psi on my non runflats. In a normal drive, your only seeing about 4-6 psi differences between cold and hot.
When i filled mine to 33 / 36 cold, I went for a drive straight after for 1hr on the highway and pressure was 36/39 at the end of journey.

Im trying to find out how to ensure the cold pressures of 33 and 36 can be kept stable. Ie what is the best tempersture to fill cold tyre pressures...
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      06-08-2022, 05:04 PM   #5
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Quote:
Originally Posted by bmw_solid View Post
When i filled mine to 33 / 36 cold, I went for a drive straight after for 1hr on the highway and pressure was 36/39 at the end of journey.

Im trying to find out how to ensure the cold pressures of 33 and 36 can be kept stable. Ie what is the best tempersture to fill cold tyre pressures...
so are you trying to say you want a stable temperate of 33 and 36 warm? I don't quite understand what your trying to explain.
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      06-09-2022, 01:27 PM   #6
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Quote:
Originally Posted by sliver 335 View Post
so are you trying to say you want a stable temperate of 33 and 36 warm? I don't quite understand what your trying to explain.
I'm trying to find out what the logic is behind cold pressures... if I set my cold pressures at 20 degrees C and then the pressures increase by 1 psi due to approx 10 degrees change in temp (without driving the car) does that mean I will still get another 4 psi increase after driving off or another 3 psi (1 psi already increased in tyres)?

I.e when the temperature raises the pressure by 1 psi, is that the 'NEW' cold pressure? Because the temperature change has already pressurized the air by 1 psi and there is only so much the air can pressurize by in a tyre.... The general rule of thumb says a tyre pressure will not increase by more than 4 psi meaning it is capped off after a certain pressurization point...
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      06-09-2022, 02:30 PM   #7
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Quote:
Originally Posted by sliver 335 View Post
so are you trying to say you want a stable temperate of 33 and 36 warm? I don't quite understand what your trying to explain.
I'm trying to find out what the logic is behind cold pressures... if I set my cold pressures at 20 degrees C and then the pressures increase by 1 psi due to approx 10 degrees change in temp (without driving the car) does that mean I will still get another 4 psi increase after driving off or another 3 psi (1 psi already increased in tyres)?

I.e when the temperature raises the pressure by 1 psi, is that the 'NEW' cold pressure? Because the temperature change has already pressurized the air by 1 psi and there is only so much the air can pressurize by in a tyre.... The general rule of thumb says a tyre pressure will not increase by more than 4 psi meaning it is capped off after a certain pressurization point...
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      06-09-2022, 05:15 PM   #8
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P=F/A

PV=nRT
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      06-11-2022, 04:12 AM   #9
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Quote:
Originally Posted by Brian86 View Post
P=F/A

PV=nRT
Its more about understanding what the friction does when a tyre is spinning and what is the max psi friction can cause - I.e. why does the tyre only inflate to 4 psi after driving for 1 hour and then stay at 4 psi and not any more or less??
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      06-11-2022, 09:45 AM   #10
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Now that's getting into the laws of thermodynamics.

Try this:
https://www.amazon.com/dp/0768012414/

You're probably thinking entirely too hard about it. Just air the tires with whatever feels good when you drive it.
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