Tyre Pressures (from 996 register)

[mok924lux]

ORIGINAL: Keggers

I noticed in Richard's running report that his new tyre pressure monitoring system shows that the pressures are 3psi lower at 2 to 3 degrees than at 20 degrees.

It has been very interesting to see what is happening to the pressures and temperatures, especially in this cold weather, as the tyres warm up. It has made it easy to get the pressures absolutely spot on, being able to set the pressures once the tyres are at 20 degrees C "" the recommended temperature. As you can see from the display, this works out about 3psi lower when the temperature is 2-3 degrees.

So, would this suggest that we should probably all be filling our tyres to 3psi below the recommended levels when filling them at cold at the moment? That means I would be going for 33psi rather than 36psi.

As most of you know me I thought you might appreciate what when through my head when I saw this post:

An interesting conundrum: Doyle, Charles and Avogadro spent a lot of time developing their gas laws but basically for an ideal gas PV/T = constant, where P is pressure, V is volume, and T is temperature. Thus if the temperature goes down, for a fixed volume of gas, the pressure must fall to maintain "the constant".

However, Porsche (and tyre) engineers know this so does this mean that tyres perform better at lower pressures in the cold (and by the gas laws; at higher pressure in warm weather) or do we all need an adaptive tyre pressure system that maintains constant pressure whatever the temperature?

I suspect that the answer to your question from a physics point of view is yes, one should fill slightly low when tyres (not atmospheric conditions) are cold and vice versa. However, the important temperature is the gas in the tyres so now we have to consider the ratio of the volume of gas still in the tyre versus the volume of gas entering the tyre; since the gas entering the tyre will be at atmospheric temperature and the gas in the tyre will depend on tyre temperature.

To do this, we must first out work out the volume of gas in the tyre, its temperature and pressure in order to calculate the "constant" in the ideal gas law and then "¦

Sometimes I do wish I hadn't study physics.[8|]

 

[3.2_fordy]

Matthew
This is sad,[&o] you can tell if you have pressure if you give them a kick before setting off[]

 

[mok924lux]

[]

Well, to be honest that's all I do.

 

[ashley]

ORIGINAL: mok924lux

Sometimes I do wish I hadn't study physics.[8|]

So do I Matthew, so do I[]

 

[petric]

hi mathew most tyre centres now have gas instead of air stays constant temp/pres, havent used it my self but always run all my cars at less than recomended pres ps your wheel is laid here finished thanks peter