Setting Tyre Pressures

Discussion in 'Wheels and Tyres' started by boraboy, Feb 18, 2017.

  1. boraboy Forum Member

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    I often see requests asking what tyre pressures do people set on their cars? and it always makes me laugh.

    I would not criticize the person asking the question as I think if you dont know then they should ask but this is the wrong question to ask.

    Then there are those who say You should have them at xx, thats what I have in mine and that makes me mad.

    There are so many variants, the main one being what temperature the air within the tyre is, but also:

    Whats the outside temp
    Has the car been driven when they were checked
    How accurate is the pressure gauge being used
    Whats the atmospheric pressure and does the gauge being used take that into account
    What is the car setup, i.e. suspension, brakes, etc

    When it comes to temperature, has the car been left with one side in the sun and the other in shade or has it been garaged before.

    All these mean that no one can say what anyone else should be setting their tyres to because thats what they do.

    Anyway, gripe over.

    There is a train of thought that is also being implemented by German manufacturers where they use the tyre temp to determine what pressure the owner should fill his tyres to (Tyre pressure assist).

    This works on the idea that the cold inflation pressure should be set at 20C, so if the temperature of the air within the tyre is lower, then the set pressure should be lower than cold inflation and if the air is hotter than 20C then the set pressure is higher than the cold inflation pressure.

    This allows the owner to drive his car, stop at the garage and set the tyres to the right pressure for the tyre temp and not set a warm tyre to the cold inflation, (Which is also why people have trouble their TPMS lamp).

    The formula to work out the tyre pressure assist is:

    [​IMG]

    So for my car (cold inflation of 2.7bar), say the tyres are at 30C and an atmospheric pressure is 0 bar gauge (1 bar absolute), they should be set to:

    [​IMG]

    [​IMG]

    Set pressure = 2.7 x 1.034

    So the Set pressure @30C = 2.792


    As an example of this, last week we were testing a car at high speed on an oval and (200 and 240kph) and the first time we set the tyres to the cold inflation when the tyres were at 3C, by the time they had warmed up they were well over pressure and the grip went down. The tests following this, we set the pressure using this formula and the tyres stayed at a sensible pressure and gripped a lot better.

    Obviously this is generally for road cars, race cars/track days tend to get the tyres a lot hotter and set them to cold inflation, which is then maintained by driving on them hard.


    So what are peoples thoughts on this, please discuss.
     
  2. jamesa Forum Junkie

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    Following comments are made oblivious to optimum performance / levels of grip etc ...

    The `simple` answer for the majority of road users is to follow the owners manual and treat the noted `cold` as being ... not driven and at ambient temperature.

    I believe that vehicle manufacturers set their recommended pressures conservatively high, to prevent tyre overload. Most tyres have a sidewall marked, max cold inflation pressure of 50psi.

    (My Michelin PS Cup 2 tyres come with a recommended track cold and hot setting, the cold setting is 7psi below the recommended road cold setting).

    My > 15 year old cars` handbooks do not refer to any setting temperature other than `cold` and instruct not to take pressure out (I assume that this is to cover high ambient temperatures). Attention is also drawn to higher pressures for higher vehicle loads.

    My 4 year old car mentions `cold` as being 20`C and to not take pressure out.

    I use the cold set pressures whatever the ambient is, given our wonderful UK conditions.

    My new in `91` Mk2 16v Golf GTI came with > 60psi set pressures :o , dealer admitted that these were for `transportation only`. I contacted the tyre manufacturer Continental who confirmed this and said that no damage would have been incurred.

    .......

    For the oval (banked ?) test ... were the pressures set differently for each side of the car, given the difference in applied (banked ?) loading ?

    ......

    My thoughts on the `tyre pressure assist` for road users is ... too complicated.
     
    Last edited: Feb 18, 2017
  3. boraboy Forum Member

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    That's the max pressure (although tyre fitters often go over that), cold inflation pressure I refer to being the manufacturers recommended pressure for the road





    That's interesting and kind of why I started this, I hadn't seen this in any of my books although mine is a 2010 car




    This something I thought about with this system, on a winters day at 0C my pressures would be 2.5bar so 200mbar lower and depending on how I drove, may not reach 20C and not 2.7bar. With the roads we have here that could lead to damage from potholes

    The manufacturers I know of using this have it shown on the cluster to guide the driver so not so complicated for them but I take your point, and strange you mention your michelins cos these manufacturers use Michelin and I beleive that's where this has come from, but dont quote me on that ;)
     
    Last edited: Feb 18, 2017
  4. jamesa Forum Junkie

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    The reference to 20`C ... is not in the handbook, it is on the `door jamb` (and in brackets) with the tyre size options / pressure settings.

    The reduction in pressure due to cold temperatures is compensated for by the conservatively high, recommended cold set pressure ... my view.

    Pressurising with Nitrogen can help offset the issue of air`s moisture content affecting (reducing) pressure in cold conditions.

    The Cup 2`s are a special case, them being a two compound track focused tyre ... there is an accompanying four page leaflet on use / inflation tips. Set pressure temperature is not mentioned.

    ....

    In another thread by RobT ... Radical SR8 ... he posts a link to Hankook Competition Tyres (http://www.mk-racing.cz/download/hankook_katalog.pdf). This provides further insight into the tech. of these `bits of rubber` ;)
     
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  5. boraboy Forum Member

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    Probably another one of the reasons they tell you to check pressures weekly
     
  6. jamesa Forum Junkie

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  7. boraboy Forum Member

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    Interesting but in my veiw it isnt quite correct and my reason for thinking this is because it says that it uses the standard gas laws with a constant volume which would be a solid container whereas a tyre is not a solid container and expands as the pressure rises, even more the wider the tyre is due to the large flat surface of the tyre which will balloon.

    Theres not alot of difference and most likely wouldn't make any difference in the real world but should be considered
     
  8. jamesa Forum Junkie

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    Agreed ...

    Useful as a reference point and got me thinking after checking the front tread depths on my car that has the cold set (20`C) in the tyre pressure table.

    For the current ambient ref. that table I am cold set 2psi high ... the tyre tread wear pattern indicates that I am underinflated BUT the design of the Michelin Pilot Sport 2 is such that there is less depth on the inner and outer sections of the tyre from the outset. Not to worry then ... :)

    Anyways thanks for raising the point ... have always been fascinated by this `stuff` [:[][:-B]
     
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