Tyre Pressures
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Thread: Tyre Pressures

  1. #1
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    Default Tyre Pressures

    Just looking over the specs...

    Road Runner is supposed to have
    Front 1.5 Bar (21PSI)
    Rear 2.0 Bar (29PSI)

    Thats pretty low!
    But that'd be for TRX's.....

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    Comments on what i "should" run?

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    1000+ Posts alan moore's Avatar
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    I run 32 F 37R on the Alpine and 22F 32 R on the 750, but you do not have quite the extremes of weight distribution that the Alpine and 4CV do.
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    When I did a defensive driving course, the instructor said basically to ignore the 'comfort biased' tyre pressures in car manuals. He reckoned 36psi front 34 rear on the 205. Seemed to know what he was talking about so tried it ---> Heaps more grip with better ride quality, more noticably on mid corner bumps and ruts.

    Hope all this helps!
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    Real cars have hydraulics DoubleChevron's Avatar
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    My car handles far better with as much air as I can get in the front tyres (I've tried 40psi for a while) and 28psi in the back tires. Handled really nicely but rode like a bloody truck... So I dropped the front back to 34-36psi.

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    Is the psi dependent on tyre size at all ?

    On the 205GTIDrivers forums, a lot of people seem to be using 30psi all round, but here, everyone seems to be using anywhere between 35-40psi.

    Why the difference ?

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    Fellow Frogger! Dr_Pug's Avatar
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    Quote Originally Posted by lordvader
    Is the psi dependent on tyre size at all ?

    On the 205GTIDrivers forums, a lot of people seem to be using 30psi all round, but here, everyone seems to be using anywhere between 35-40psi.

    Why the difference ?
    Contrary to popular belief, tyre pressure is not determined by the type of tyre or its size but upon your vehicle's load and driving application i.e. speed

    Tyre pressures should be checked when the tyre is 'cold', as pressure increases as the tyre becomes 'hot'.

    Take the "cold" reading and check them against the recommended tyre pressures from your placard.

    However, In cold temperatures the passage of a cold front may produce a dramatic change in surface air temperatures, for instance from +12° C to -32° C in the course of 6 hours. If a car is parked outdoors in these conditions, the reduced temperature will lead to a proportional lowering of pressure in the car's tyres (ideal gas law, Note 1.G). The pressure is proportional to the absolute temperature, given in degrees Kelvin. For instance, the tyre pressure would drop from a normal 207 kPa (30 psi) to 175kPa, ie (273 - 32) x 207/(273 + 12). This will make the tyre appear slightly flat at 25 psi.

    Heavy loads or towing puts an extra strain on your tyres. So if your vehicle is fully loaded with passengers and luggage, the general rule is to add 28kpa (4PSI or 4lbs).

    Frequent braking, or driving fast over a rough road, or carrying a heavy load may raise the temperature of air within the tyre 40° C or more above the ambient air temperature. In this case the same tyre pressure would be raised to about 34 psi [236 kPa, ie (273 + 52) x 207/(273 + 12)]. This contributes to sudden tyre ruptures.

    At high speed, (defined as driving at 120km/h for over one hour), your tyres will wear out twice as fast as when you drive at 70-80 km/h. If your tyres are under-inflated by twenty per cent tyre life can be reduced by thirty per cent. The rule here is to add 28Kpa (4PSI) from your Minimum Compliance Plate Pressure. Don’t inflate your tyres above 40 psi or 280 kPa. When the tyres get hot from driving, the pressure will increase even more.
    04' GTi 180

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    sans witticism SLC206's Avatar
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    Where'd you paste that from

    There is no way I'd drive around at 39 psi in the 180, in fact I wouldn't describe its placarded pressure (35 psi) as a "comfort-based "rating at all
    Regards,

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    Quote Originally Posted by SLC206
    Where'd you paste that from
    There is no way I'd drive around at 39 psi in the 180, in fact I wouldn't describe its placard pressure (35 psi) as a "comfort-based "rating at all
    Doesn't matter what tyre pressure you have driving on Parrachatta road !!! ... But yes I tend to be on the higher end of the scale 35/36 ..

    Car manufacturers tend to recommend a balance between comfort and handling, often opting for the comfort side of things.

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    Quote Originally Posted by SLC206
    Where'd you paste that from
    Lets see

    Pressure affected by temperature, was from my 2nd year physics book and the rest from a friend who works at BJ's head quarters.

    I didnt know I had to provide a reference list for my information feels like im back at uni again
    04' GTi 180

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    sans witticism SLC206's Avatar
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    Quote Originally Posted by Dr_Pug
    Lets see

    Pressure affected by temperature, was from my 2nd year physics book and the rest from a friend who works at BJ's head quarters.

    I didnt know I had to provide a reference list for my information feels like im back at uni again
    I wasn't having a go at you - I was just thinking that it looked like a Microsoft answer.
    Regards,

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    It looks like you're trying to set your tyre pressures.

    Would you like to:

    • Slide off the road sideways into a bush
    • Bounce out of the seat over expansion joints
    • Purchase the MS-Tyre handling and ride update (Microsoft reserves the right to remove your wheels at any time)
    • Acheive a nice balance of ride, wear and comfort
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    Quote Originally Posted by SLC206
    I wasn't having a go at you - I was just thinking that it looked like a Microsoft answer.
    no, that would be:
    "there's a network problem"

    #)(*$#( MS...
    (yah, i'm a network engy)

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    Chris

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    Fellow Frogger! Dr_Pug's Avatar
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    Quote Originally Posted by SLC206
    I wasn't having a go at you - I was just thinking that it looked like a Microsoft answer.
    I guess sometimes i come accross as textbook type of responder, cant help it all those years of essays does it to you.
    04' GTi 180

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    damn, in my car (due to the low profile tyres) I run about 42-43psi when cold, the pressure comes up to about 45-46psi when they warm up.
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    I would say Dr_Pug's answer is a logical & informed one about tyre pressures. Over the years no doubt with improvements in tyre design & materials I note that manufacturers have been recommending higher inflation pressures. The 504 when released all those years ago was to have its tyres (175R14) set at 22 PSI front & 26 PSI rear, or thereabouts. Nowadays commonly manufacturers recommend pressures in the thirties.
    It is important from many angles - tyre wear, safety etc - but also because the tyre is one vital component of the suspension & vehicle dynamics & acts like a spring. So the ptimal tyre pressure will depend on the particular vehicle characteristics. As you would know I suppose the pressure setting in the tyres at the front & rear will have a definite influence on whether the car understeers or oversteers excessively. I don't see the point of running settings that are excessively high (38 to 40 plus), especially on cars where the manufacturer is recommending a much lower pressure. It is saying that the manufacturer, with all its design & engineering expertise, does not know what it is doing - which is clearly ludicrous.
    Generally I would agree that adding 3 PSI or so to each tyre for either higher sustained speeds(100 kph or more) or loadings is sensible - but pumping the tyre up to rock hard is rather stupid.
    Cheers
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    It depends on what you want to achieve Nate.

    If you want fuel economy and your tyres to last a long time run around 38psi.

    On the road you'll get maximum grip around 30-35psi with cold tyres.

    Hitting a race track you'll find that running 35+ psi cold will result in 40+ psi hot and you'll end up with no grip.

    I wouldn't go less than 28psi for track use on road tyres though because you'll kill the rubber compound with the heat generated.

    I'd try 32F 35R and see how that goes.

    If you aren't getting enough grip when driving hard, put it down to 30F 33R.

    My guess takes into account your weight distribution and vehicle weight, they are the two factors that dictate what pressure to run. Its nothing to do with tyre size.
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  18. #18
    Veni Vidi Posti 68 404's Avatar
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    Quote Originally Posted by n b j
    damn, in my car (due to the low profile tyres) I run about 42-43psi when cold, the pressure comes up to about 45-46psi when they warm up.


    34 psi all round is a good average pressure for doing everything.

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    FWD cars should have slightly higher pressures at the front.

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    Its RWD & Mid engine

    I think its also got to do with the amount of rubber on the road compared to the weight...
    It might be better to run lower pressures, because its 900kg on 225 tyres?!?

    I went for a run on the weekend, ran 24 front (instead of the 21 reco'd), and 28 rear (whats reco'd), and it rode quite well!
    I might put the rears up a little, maybe run 25's & 30's.

    The car was a lot softer on the ride & handled without issues (not that i'm anywhere near pushing it!)

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    Nate, apparently the Lotus Elise guys run about 34F 36R. They are an MR layout like your car and probably weight a bit less.
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    Quote Originally Posted by ClioF1
    Nate, apparently the Lotus Elise guys run about 34F 36R. They are an MR layout like your car and probably weight a bit less.
    Sure about that? From memory they require a much lower pressure, like 23-24 psi for a series 1 and around 26-28 psi for a series 2.

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    Quote Originally Posted by macquered
    Sure about that? From memory they require a much lower pressure, like 23-24 psi for a series 1 and around 26-28 psi for a series 2.
    You're probably right if its in relation to the placard, I'm just going off what some of the guys run when driving hard.

    Edit - Yep its 23F 27R on the placard. Elises have a 39/61 weight distribution.

    The Clio's placard states 33F 30R and damn its soft running those pressures. I ran 37-39 in the front when the tyres were new and I now run 35 as they are nearly due for replacement.
    Last edited by ClioF1; 22nd February 2006 at 11:43 AM.
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    1000+ Posts brenno's Avatar
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    I'd suggest Lotus know what they are doing when they suggest tyre pressures for a cut throat sports car.

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    Maybe the placard is for cold pressures on the assumption that you're about to thrash it to within an inch of its life.

    On a related note -- is it possible that some of these recommended pressure are for setting the tyre pressure with the tyre off the car, and the pressure would then increase when the weight of the car goes on? That's the only reasonable explanation I can think of for the 26F 28R recommended pressures on a GS. 26 in the front is waaaay too soft. I run 30 minimum...

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