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Tire pressure...

Discussion in 'E39 (1997-2003)' started by JAMAICAN TWIN TURBO, May 26, 2009.

    JAMAICAN TWIN TURBO guest

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    Why does the manual for my 2003 bmw 530i say I need 33psi in the front wheels and 41psi in the rear wheels? That sounds crazy...i went too Town fair Tire and they suggested I do 33psi in front and 33psi in rear. Which one sounds right too you guys? I've noticed the car turns alot smoother with the Town Fair Tires recommendation. Thanks

    T

    vtx guest

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    There should be a plate on the driver's door frame that lists different pressures for different tire sizes and car load.

    I've driven mine at 41psi front/45psi rear since February 2003, but that's for wide 18" rims, yours are probably narrower - thus the recommendation for 33/41.

    Another question is whether your wheels are staggered (rear wider than front) - that would be a good call for having higher pressure.

    Between BMW manual and "a shop" I would go with the manual, and augment it with a liberal dose of discretion.

    Are you sure that what you call "smoother" is not actually "softer"? It depends on personal preferences, of course, but "soft" is a last thing I would like my car to be.

    There are two dangers to underpressured tires - first, blowups from overheating (rubber bends repeatedly on approach and takeoff from the road) and second, so-called "snakebite" punctures - and possibly bent rims, too.

    Somewhat overpressurized tires give you less rolling resistance (hence, higher MPG) and firmer feel of the road (if that's your thing).
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    leland

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    Tire Pressure

    What are the recommended pressures on the driver's side door jam?

    vtx guest

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    Depends on the model and tire size, if I get your question right.
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    leland

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    Tire Pressure

    I match the tire size (front and rear) to the guide on the plate. Mine have the pressures for various size tires under load and normal conditions. I use the normal condition pressures.

    JAMAICAN TWIN TURBO guest

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    On mine it says 33front 41rear. My wheels aren't staggered as I do not have sports package. So you guys think I should keep my psi @ the manual recommendations instead of town fair tire or firestone recommendations? What do all you guys keep your psi at? Thanks for the help.
    -T
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    Brian A

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    As I understand tire pressures (and I am just paraphrasing things I've read; I'm no expert or professional), it's a balance between ride quality and technical performance.

    The manufacturers recommend tire pressures that tend toward high ride quality and understeer. This means they specify the lowest pressures that are safe and they specify higher pressures in the back to help with the bias toward understeer.

    Since cars have become so low maintenance and self-diagnosing, many people neglectfully let tire pressures drop substantially before refilling them (ie they are not checked until the next oil change). I am sure this has caused the trend for any shop that touches your tires to now fill them several psi above the door jam pressure (the maximum tire pressure BTW is dictated by the tire design and is indicated on the tire itself by the tire manufacturer).

    Unless you are running your car at autocross or track, understeer is an absolute non-issue. A little understeer is your friend.

    My answer? I tend to keep my tires at couple of psi above the door jam pressures and won't go below them.

    (At autocross everything changes: I run my fronts at 40 psi and my rears at 37.5 psi as compared to the recommended pressures on the door jam of my car which are 28 psi front and 30 psi rear.)

    M3Driver guest

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    I always used the ones in the owner's manual when we had our '00 528i. The ones on the door were very high on purpose due to the Firestone litigation of a few years ago (Ford Explorer + under-inflated tires = oops). BMW didn't want to get caught in that trap. At least that is story I always heard.....

    BIMMIR guest

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    I'd start with the door jamb.
    But, here's a trick I heard recently, and it might have been from one of you?

    Put what you think is the right pressure. Then, draw a chalk line across your entire tire across the treads. Drive a few miles and return and check the chalk line. If the middle of the chalk line is worn off, you are overinflated. If the edges are worn off, underinflated.
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    Dmarque

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    Great Response!

    Sure sounds like a practical response. Any driving instructions ....not sharp cornering for example?
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    Brian A

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    You can't meaningfully mark or measure the contact patch itself using chalk. It would all rub off within a couple of revolutions regardless of whether your tires were under, over or normally inflated.

    You might be thinking of marking the sidewalls for autocross, which is an entirely different thing. There, you are assessing the amount of roll-under of the sidewall caused by the high cornering forces.

    vtx guest

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    Well, by that book I'm running severely overinflated. Desert climate, no chalk is necessary, track is clearly visible - half an inch of clear undriven space on each side.

    However, when I look at the thread wear pattern - nothing of that shows, the wear is uniform. So the conclusion I make is that overinflated on a flat road equals inflated just right when cornering.

    Pressure I maintain in my tires is the one specified for max load, even though I usually drive alone or with one passenger.
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    letterman

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    Related question re tire pressure

    This borders on the lazy man's approach, but here goes - does anyone use the valve stem caps that turn red if your tire pressure goes too low? I have seen them advertised pretty frequently of late, and was considering getting a set for my wife's car (as she doesn't usually go out of her way to unscrew caps, get the gauge out, etc.).

    Thoughts?
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    John in VA

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    She would still have to pay attention to the valve stems to know if air was low. I'm too cheap to buy something like this. Just check the pressure on a monthly basis - put it on your calendar to do the first weekend of each month.
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    CRKrieger

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    I have a set on my Jeep - partly because my wife pays no attention to them, so I took them off her car. But they're kinda' handy to see at a quick glance if your pressure's a bit low. Mine have a piston kind of thing that's usually green and reveals a yellow or red sleeve through a clear cap if they're below 35 psi.

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