Different front & rear tire pressures

Discussion in 'General Motoring' started by Dave Botsch, Jan 16, 2005.

  1. Dave Botsch

    Dave Botsch Guest

    The subaru manual for the '05 Impreza Sport wagon says to keep the front
    tires at 32 psi and the rear tires at 29 psi. I've always heard that
    keeping the front and rear tires at different pressures is bad and even a
    difference of 3 psi can greatly, greatly increase the liklihood of
    hydroplaning and such.

    Even of more interest is that all 4 tires were measured this morning at
    30.5 psi. I haven't touched the tire pressure since the car was bought a
    few months ago, so, this is how it came from the dealer.

    Can anyone offer some insight?

    Thanks!
     
    Dave Botsch, Jan 16, 2005
    #1
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  2. Dave Botsch

    JD Guest

    The STi recommends 36 in the front, and 31 in the rear. The Subaru tech who
    works on it (and also races) told me that it is to balance the car properly
    and reduce understeer. The camber angles are different on the front and
    rear wheels to improve handling. In addition, most of the weight (the
    engine) is in the front, so the increase in front tire pressure is intended
    to reduce the spring rate and balance the car overall.

    They told me that matched front and rear pressures is not bad on Subarus in
    general, but the car will have a tendency to understeer. Many dealers want
    that since understeer is much easier to drive out of than oversteer. If
    there is too much pressure in the front tires, the car will oversteer
    though.
     
    JD, Jan 16, 2005
    #2
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  3. Dave Botsch

    TG Guest

    Tires are supposed to sit on the road a certain way...too much air and the
    center of the tread is weighted more so the center wears faster and there is
    also less traction. Too little air and the edges of the tread wear plus
    sidewalls squirm and the tires run hot. Since the weight on the front and
    rear axles is different the air pressure has to be different as well.
    My pickup exaggerates this. The engine, trans, and cab are all on the
    front and an empty box is on the rear...I need to run 35 in the front and
    only 28 in the rear to get good tire wear and handling. TG
     
    TG, Jan 16, 2005
    #3
  4. Dave Botsch

    y_p_w Guest

    These are the same numbers as the tire sticker for my '04 WRX
    (P205/55-R16 tires). Many front-engine cars come with similar
    recommendations for more pressure in the front tires, as they
    have more weight on them. I'd imagine the recommendations for
    a car with a near 50/50 weight split (think mid-engine) might
    be different.

    Hydroplaning risk is a combination of tread depth, tread design,
    tire pressure, tire width, and weight on the axle. I've even
    heard that the risk of hydroplaning (for front-engine cars) is
    reduced if tires with deep grooves are kept on the rear. However,
    that conflicts with the usual desire to rotate tires such that
    they will be replaced as a set of 4.
    Strange things happen.
     
    y_p_w, Jan 17, 2005
    #4
  5. Dave Botsch

    hippo Guest

    My rule of thumb for years has been rwd, start with 2psi higher in the
    back, fwd 4psi higher in the front,. awd, equal pressures all round; then
    set pressures to match your driving style and how you want the car to
    handle from there. Obviously don't exceed the tyre manuf recommended
    pressures though. If you find a setting you like, accept any higher tyre
    wear that results as a penalty for enjoying yourself! Years back we ran an
    Austin 1800 with 42f, 26r and almost completely removed any understeer -
    mind you, the resulting liftoff oversteer was rather entertaining,
    especially on a wet road ....
    For what it's worth, my 91 Liberty (Legacy) wgn ran 32f, 35r under most
    conditions for 200000+kms and my 99 OB ltd runs 36 all round & is
    reasonably neutral. Cheers
     
    hippo, Jan 17, 2005
    #5
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