Tire Pressures w/Full Load; +2psi Up Front?

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by Rokeby, Jul 5, 2009.

  1. Rokeby

    Rokeby Member

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    Later this week I'll be making a 1300 mile ramble from Merlin to
    Maine and back, with the load maxed out. There is the general rule of
    having 2+ psi in the front tires than the rears. IIRC, this because of
    the typical weight bias being to the front of the car due mechanicals,
    etc, at that end.

    Loading up the car will put most of the total load -- 4 adults and
    luggage -- in the aft half of the car. I can easily believe that as a
    result, there could be no forward weight bias.

    Does this mean that I should be running with equal tire pressures front
    and rear? :confused:

    I've read that the +2 psi in fronts also helps minimize/forestall oversteer.
    Would that by itself be reason to keep the 2psi f/r differential? :noidea:

    FWIW, I'm running the OEM Turanzas; 27k miles, 42f/40r, plenty of
    tread, BT rear brace and shock tower brace.
     
    #1 Rokeby, Jul 5, 2009
  2. firepa63

    firepa63 Former Prius Owner

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    Keep the tires 2 psi higher in front regardless of the load. I'm also heading to Maine later this week...
     
    #2 firepa63, Jul 5, 2009
  3. happy2472

    happy2472 New Member

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    What is the ecact tire pressure ratings for all the tires? hot or cold psi?
     
    #3 happy2472, Jul 25, 2009
  4. tochatihu

    tochatihu Senior Member

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    Here's what I'd do: Take a pressure gauge that you have reasonable confidence in accuracy. Set the fronts cold at max sidewall, rears at 2 psi less. Sometime during the day when the tires are fully heated (by flexing friction), measure the pressures hot. If this is more than 10% higher than the cold, the tires are overloaded and there is not much you can do about it, other than shedding payload. But you could take the hint to drive conservatively.

    If you are within the 10% increase and don't like the feeling of hard tires, next day run them a few psi lower. Do the hot psi comparison again. You can continue to decrease the psi on subsequent days if you like, but don't go down to to pressures that give greater than 10 % psi increase when hot.

    Setting the front/rear pressure differential to give the same hot psi % increase may not compensate exactly for the slip angles (the reason for the stated 2 psi differential) but it should be pretty close for a particular loading and load distribution.
     
    #4 tochatihu, Jul 26, 2009
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