Tire Experts

Discussion in 'Archived Threads 2001-2004' started by Russell B, Dec 2, 2002.

  1. Russell B

    Russell B Stunt Coordinator

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    I just bought some new wheels and tires. The tires have a max psi of 44. I have been given many opinions on inflating raging anywhere from 5-15 psi under the max rating. What should i inflate them to?
     
  2. Philip_G

    Philip_G Producer

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    stock tire size? or did you plus size the rims?
    if it's stock I'd run the reccomended pressure from the car mfgr.
    if you plus sized I dunno [​IMG]
    I think I'm running about 8-10 under the max pressure for the tire.
     
  3. DonnyD

    DonnyD Screenwriter

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    Max pressure is usually recommended only for higher than normal loads you may be carrying. The tire manufacturer usually has a normal inflation pressure recommendation somewhere on the tire. I would imagine it will be 32-35 lbs.

    I run oversized custom wheels and tires and they still run the same pressure that my standard size had... 35lb. Much off that may cause the tire to wear unevenly.
     
  4. Denward

    Denward Supporting Actor

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    The standard answer is that if the tires are stock size, you should inflate according the car manufacturer's recommendation (it's almost always less than the max pressure printed on the tire). This can be found in your owner's manual or near the bottom of the driver's side door pillar.

    Most auto manufacturers aim to give you a good compromise betwen handling (less air) and mileage (more air). I suppose if you've done serious suspension modifications, this would change.
     
  5. Russell B

    Russell B Stunt Coordinator

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    Ok, i found the sticker near the door. It says using a 195/75/14 tire inflate front to 29 cold and rear to 35 cold.
    My question about that is my stock tires were 35 max psi tires and they want me to run the back at max, and the pressure goes up another 2-3 psi when hot so that puts it over the max rating.

    My new tires are 225/55/16 and the max rating is 44 psi.
    I just filled each tire to 40 psi but i did it hot so cold is 2-3 psi below that.
     
  6. AviTevet

    AviTevet Stunt Coordinator

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    There's only three reasons to run below max pressure:
    1. extremely hot weather and lots of hard driving (fast, hard cornering, hard braking) may cause some tires to overheat and blow. In the winter it's not a factor.
    2. Ride quality diminishes with more air, ie you will feel more bumps in the road and the ride will be more harsh.
    3. Tire life diminishes somewhat.

    With more air in the tires you will see better mileage (less rolling resistance), better dry handling (less prone to roll onto the sidewalls when cornering hard), better road feel (see ride quality), and less prone to breaking the breaking the bead on the tire (wheel & tire separating, though this is kind of splitting hairs). Don't worry about the hot pressure when you are inflating cold, but always inflate when cold and not hot. The manufacturer rates the max pressure when cold and overengineers the tire to accomodate the pressure when hot.

    The most important thing about changing your tire pressures from stock is keeping the pressures balanced... make sure the FR is at the same pressure as FL and the RR is the same as RL. You should also be sure to keep the differential between the front and rears approximately the same as stock (-6 psi). Significantly changing this (0 psi or -12 psi) will dramatically affect the balance of your car at it's limits (ie emergency braking or hard cornering). What kind of car do you have anyway? Those are some mighty odd stock pressures.
     
  7. Todd Hochard

    Todd Hochard Cinematographer

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    Generally speaking, running tires at max pressures under normal loads is a bad idea, because it tends to push the center of the tire out too much. This:
    1. decreases the width of the contact patch, reducing handling, even in the dry. You'll lose handling here, not gain.
    2. increases center tread wear, reducing tire life.
    3. increases the peak tire load during a road hazard condition (pothole, 2x4, speedbump), leading to easier tire damage (sidewall and/or belt damage).
    This can be illustrated by this simple experiment- inflate the front tires to max, then turn the wheel lock-to-lock. Now, reduce to normal pressure (probably 10psi down, at least), and try this again. Note the heavier steering feel, since more of the tire is contacting the road.
    Unless the tire is built to accomodate full pressures (i.e. certain autocross tires, etc.), or you are putting road racing loads on them (high speed and hard cornering), I'd keep the pressure down.
    Simply put- if tires were meant to be hard-as-a-rock, they'd be solid, not inflatable.[​IMG]
    Those are weird recommended numbers. What type of vehicle?
    Todd
     
  8. Denward

    Denward Supporting Actor

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    On a related note, how far can you drive to the gas station and still have "cold" pressure in your tires? Once you've heated up your tires, how long does it take to go back to cold? Does either answer change much from summer to winter?
     
  9. Ted Lee

    Ted Lee Lead Actor

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  10. Russell B

    Russell B Stunt Coordinator

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    The vehicle is a 1999 toyota tacoma 2x4. It says it on the sticker near the door, 29 front 35 rear cold with 195/75/14 tires.
     
  11. Ron-P

    Ron-P Producer

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    Here's a great test.
    Use white chalk. Run a line across your tires tread, drive a few feet, check to see where the chalk line wore off, adjust the tires air pressure so the chalk line wear's off evenly...done.
    Peace Out~[​IMG]
     
  12. Ted Lee

    Ted Lee Lead Actor

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    if you go with ron's idea better take into account tire expansion. [​IMG]
     
  13. Ron-P

    Ron-P Producer

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    Yes, make sure you do it when the tire is warmed up and the air is hot, that goes without saying, or at least I thought.
    Peace Out~[​IMG]
     
  14. Russell B

    Russell B Stunt Coordinator

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    That chalk thing sounds like it might work but you would need a road that is perfectly flat or else the dips can make the chalk rub off differently.
     
  15. Todd Hochard

    Todd Hochard Cinematographer

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    The 29/35 almost sounds like a "loaded bed" setup. If you always drive empty, you could probably run them even. I'd start at 32-34psi, and go with the chalk test.
     
  16. Russell B

    Russell B Stunt Coordinator

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    I called the dealer and he said to fill them 32-34 cold.
     
  17. Eric Kahn

    Eric Kahn Guest

    More and more tires ar being built to withstand Higher pressures for what ever reason, this does not mean you put max sidewall pressure in your tires unless you are addicted to harsh ride and low tire life, when I bought new tires for my 69 caddy cuope deville, the tires I bought, general R235-75-p15's had a max pressure listed on them of 45 PSI where the tires that came off had a max pressure of 35 psi and caddy recommended 32 psi in the tires ( car came from factory in 69 with radials by the way)
    as an aside, overfilling some modern radials over the recomended pressure will now give you a tread wear pattern like you used to get running around with low pressure, it has to do with the way the tread belts are put in the carcass of the tire
     

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