[BennyDunks]

Hey knowledgeable folks, I got in the car the other day (when it was real cold) and got a check tyre pressure notification - I navigated to the tyre pressure monitor and saw the rears were a little bit down on the recommended.

So I topped them up at the garage, then I noticed that the front and rear recommended had changed slightly.

My question is how does this work and what is causing the car to change the recommended pressures?

(not my picture, stole it from Google for reference)

[Charlie (Wessex)]

Pressure reccos change with temperature.

[BennyDunks]

Originally Posted by Charlie (Wessex)

Pressure reccos change with temperature.

I assumed that, but is that based on tyre temperature? Or outside temperature?

[HighlandPete]

Originally Posted by BennyDunks

I assumed that, but is that based on tyre temperature? Or outside temperature?

Tyre temperature. The idea is to be at the optimum working pressures at any given tyre temperature.

[paulockenden]

Originally Posted by BennyDunks

I assumed that, but is that based on tyre temperature? Or outside temperature?

The temperature as measured by the pressure sensor in each tyre.

[BennyDunks]

This might be a stupid question but sitting on the drive at 3 degrees and then driving on the motorway at 70mph isn't there going to be a big variance?

[HighlandPete]

Originally Posted by BennyDunks

This might be a stupid question but sitting on the drive at 3 degrees and then driving on the motorway at 70mph isn't there going to be a big variance?

Yes, there will be a difference and the TPMS should be giving the recommended pressure for a given tyre temperature.

[Scottish0177]

Originally Posted by HighlandPete

Yes, there will be a difference and the TPMS should be giving the recommended pressure for a given tyre temperature.

Surely recommended tyre pressure is for a cold tyre that hasn't been driven on yet

I understand that tyres will heat up when used, and i understand that the idrive will show temp and associated pressures for those temps but in reality its seems pretty pointless to give temp recommendations as you are not going to stop a journey and top up pressures because they change or advise that they have changed, and even if you did you would be putting air into or removing air from a hot tyre mid route.

unless i'm missing something, willing to be suitably informed

[bmnut]

Originally Posted by Scottish0177

Surely recommended tyre pressure is for a cold tyre that hasn't been driven on yet

I understand that tyres will heat up when used, and i understand that the idrive will show temp and associated pressures for those temps but in reality its seems pretty pointless to give temps as you are not going to stop a journey and top up pressures because they change or advise that they have changed, and even if you did you would be putting air into or removing air from a hot tyre mid route.

unless i'm missing something, willing to be suitably informed

Me too!
I always thought the posted recommended pressures related to 'cold' settings?

[xenon]

The posted settings are related to cold settings so, for example, at cold it says your pressure is 2.6BAR and the recommended is 2.6BAR. After you've been out for a blast and the tyres and air within is warm, the pressure may now be 2.7BAR. The iDrive knows this and therefore adjusts the recommended pressure that it shows you to suit. If it didn't, the recommended would always be different when warm and people would get confused. In this way the "recommended" pressure takes in to account the variance by temperature so it always shows the recommended pressure at the particular temperature you're looking at.

[HighlandPete]

Originally Posted by Scottish0177

Surely recommended tyre pressure is for a cold tyre that hasn't been driven on yet

I understand that tyres will heat up when used, and i understand that the idrive will show temp and associated pressures for those temps but in reality its seems pretty pointless to give temp recommendations as you are not going to stop a journey and top up pressures because they change or advise that they have changed, and even if you did you would be putting air into or removing air from a hot tyre mid route.

unless i'm missing something, willing to be suitably informed

I agree the whole temperature/pressure information is confusing to many users.

BMW clearly state RDCi system is a giving the optimum pressure for the temperature displayed:

Unlike the tire pressure sticker, the electronic tire pressure label permanently monitors the nominal pressures taking into consideration the current temperatures. This means that it determines and displays the optimum tire pressure at any temperature.

Assuming you set the pressure when tyres are cold, the RDCi system will use the current ambient temperature, (both tyre temperature sensors and outside ambient temperature sensor should be the same), it has the baseline to monitor the pressure as the tyre warms up.

BTW, BMW is not suggesting adjusting on route, more like regular tyre pressure checks when cold and adjusting to recommended cold setting pressures.