Tire Pressure Recommendation for Winter

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by Tideland Prius, Dec 6, 2009.

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  1. Tideland Prius

    Tideland Prius Moderator of the North
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    Hey, just looking for some suggestions with regards to winter tyre pressures. Obviously i can't use the factory setting because it's not the same tyre.

    I'm currently running Nokian Hakkapeliita R winter tyres. They're set at 42/40 corrected at +20°C/68°F. (IOW, at the present -20°C/-4°F, they're probably at 34/32). The tyre is rated for 50 something PSI so I have plenty of "space" to work with.


    Is it better to run them at a higher pressure or lower pressure? I suppose in deep snow, lower is better but most of the time, the roads will be plowed so I'll be driving on solid pavement (or at worse, compact snow). With the higher PSI rating, is it best to pump the pressures back up to 42/40 at -20°C/-4°F? or leave them at 34/32?
     
    #1 Tideland Prius, Dec 6, 2009
  2. cossie1600

    cossie1600 Active Member

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    Higher, less contact patch, just don't go into the 50s or 60s like the nut job who wants to save .2 mpg
     
    #2 cossie1600, Dec 7, 2009
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  3. Bob64

    Bob64 Sapphire of the Blue Sky

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    The "Max sidewall" rating is not the psi burst pressure of the sidewall. Rather it means that it will support the maximum specified load at that pressure, but if you go above that pressure, it won't support any additional load.

    source: edmonds's myths #10
    Top 10 Automotive Mechanical Myths and Misconceptions


    I don't think tire pressure affects the contact patch too much unless your way under-inflated.
     
    #3 Bob64, Dec 8, 2009
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  4. Tideland Prius

    Tideland Prius Moderator of the North
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    So correct them back to 42/40 at the OAT then? Alright. I probably won't do it outside so i'll have to calculate it from 20°C again.
     
    #4 Tideland Prius, Dec 8, 2009
  5. richard schumacher

    richard schumacher shortbus driver

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    What's to calculate? Fill them to 42/40 and you're done.
     
    #5 richard schumacher, Dec 8, 2009
  6. swich2mac

    swich2mac Go Hard or Go Home!

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    Is that 42-front and 40-rear?
     
    #6 swich2mac, Dec 9, 2009
  7. Tideland Prius

    Tideland Prius Moderator of the North
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    Temperature. The underground garage is at 20°C. Outside, it's -20°C. I would lose 6PSI if I were to set 42/40 in the garage meaning I'm running the car at 36/34.

    I do not know about you but I have no intentions of filling up a tyre outside in -20°C weather.
     
    #7 Tideland Prius, Dec 9, 2009
  8. RodJo

    RodJo Member

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    "Always check your owner's manual for the recommended tire pressure for your vehicle." (edmonds's myths #10
    Top 10 Automotive Mechanical Myths and Misconceptions) In context, the myth is discussing carrying heavy loads in an SUV, in which case the tire pressure may need to be increased to the max sidewall pressure, which could actually be the recommended pressure for that load.

    Contact patch is related to PSI, regardless of tire pressure -- it's in the "square inch" part. Increasing tire pressure decreases the area needed to carry the load, whereas decreasing tire pressure increases the area needed to carry the same load.

    The recommended tire pressure is a compromise of various factors. I've provided a more detailed discussion elsewhere, but on average, a 1 psi increase in tire pressure causes a 1.2% decrease in tire adhesion on pavement. (Improved Automotive Suspension Testing Apparatus)

    While it is risky to compromise your tire adhesion by overly inflating the tires, winter conditions can change some of the parameters. A couple of extra PSI might be helpful on snow by putting more weight down per square inch (think the opposite of wearing snow shoes), thus helping your tires get to the pavement. So it may make sense to sacrifice a little dry pavement adhesion in exchange for better traction in snow.
     
    #8 RodJo, Dec 9, 2009
  9. FireEngineer

    FireEngineer Active Member

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    Unless your going through a lot of deep fluffy snow and need floatation above it, go to max sidewall PSI. If your going to fill in your heated garage just add a couple of PSI more and then forget about it.

    Wayne
     
    #9 FireEngineer, Dec 10, 2009
  10. Tideland Prius

    Tideland Prius Moderator of the North
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    k. I'll bump them back to 42/40. I won't go max sidewall because at 42/40, the ride's stiff enough for me.
     
    #10 Tideland Prius, Dec 12, 2009
  11. wvgasguy

    wvgasguy New Member

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    comment deleted
     
    #11 wvgasguy, Dec 13, 2009
  12. FireEngineer

    FireEngineer Active Member

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    All that padding from the layers your wearing in the cold and your worried about your bottom getting bruised?

    Wayne
     
    #12 FireEngineer, Dec 14, 2009
  13. Rhino

    Rhino New Member

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    You can't figure this out by calculations. You have to collect empirical data. The reason is that the tires are not constant volume. The volume probably shrinks as pressure is lost so your PV=nrT sort of equations would not work.
     
    #13 Rhino, Dec 14, 2009
  14. Tideland Prius

    Tideland Prius Moderator of the North
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    I wasn't calculating actually. I was just using the rule of thumb (1 PSI for every 5°C/9°F) for a quick estimate. I really don't want to check the tyres when it's -30°C outside (or -40 as it was yesterday)


    I do not know how your streets are like but ours where it isn't plowed is covered in compact snow that somehow managed to form little hills. It's like riding on a poorly maintained cobble road
     
    #14 Tideland Prius, Dec 14, 2009
  15. Airbalancer

    Airbalancer Active Member

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    So should we go with 42 front 40 rear?
     
    #15 Airbalancer, Nov 7, 2010
  16. JimN

    JimN Let the games begin!

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    I found the handling improves on wet payment with higher pressure so IMO, yes.
     
    #16 JimN, Nov 8, 2010
  17. xs650

    xs650 Senior Member

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    Unless they left the cords out of your tires when they were manufactured, the volume change is not very significant.:D
     
    #17 xs650, Nov 8, 2010
  18. Tideland Prius

    Tideland Prius Moderator of the North
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    Depends on your tyres. I'm running Nokian Hakkapeliita R winter tyres on 42/40. It's currently above zero during the day and it's fairly stiff for my liking (the bumps are transmitted right into my seat).

    I would hesitate to go higher in the winter because you need more traction (either on compact snow or just glazed asphalt). If possible, I'd try two different pressures (say factory 35/33 and 40/38 or 42/40 and do a braking test on an icy or frosty road and see how braking feel and distance compares.)
     
    #18 Tideland Prius, Nov 8, 2010
  19. Airbalancer

    Airbalancer Active Member

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    I am at 40/38, when I pick up the car the tres were at 29
     
    #19 Airbalancer, Nov 9, 2010
  20. Tideland Prius

    Tideland Prius Moderator of the North
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    Alrighty.

    Yeesh. I've had the dealer reset the pressures to 32 all around and I quote "because that's what the Corolla uses." The last time I checked, I bought a Prius :confused:
     
    #20 Tideland Prius, Nov 10, 2010
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