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Tire Tech! Very cool way to figure out what to run.

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Grim-Reaper, Apr 15, 2003.

  1. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    Thought I would pass this on. This came off a Travel Trailer Forum I am on. It was brought up because the new tire you put on may not be same ratings as what GM put on and that door sticker is of no use. So I don't bomb this other samll Forum I'm just pasting it here.

    [ QUOTE ]
    AccessMaster

    The Real Answer to Tire Pressure
    I worked for Michelin Tire Corporation for 7 years and Yokohama Tire Corporation for 11 years. I have given numerous tire seminars on tire maintenance and especially how to determine the correct tire pressures. So here goes.
    The pressure on the sidewall of the tire is the maximum pressure at the published load at approximately 55 mph. (The speed can vary somewhat but it is not important for our discussion).
    The air pressure is required to support the load that the tire must carry in such a manner that the tire flexes at the designed place on the sidewall of the tire.
    If the load on the tire changes then the air pressure should change accordingly to keep the tire flexing at the proper place.
    The reason for correct air pressure is to prevent the tire from overheating. It was put together with heat and it will come apart the same way. An under inflated tire will eventually self destruct due to excessive heat build up. An over inflated tire will ride harshly and be more likely to burst upon impact. Sorry for the long explanation but here is the bottom line.
    To determine the correct air pressure, check the pressure when the tire is cold. Run the tire for several miles at highway speed. Stop and immediately check the air pressue in the tire. It should be higher than we cold but no more than 10% higher.
    Now here is the hard to believe part. If the pressure is more than 10% higher you must ADD AIR and test again. For example if you start with 50 psi cold. If the pressure is 60 when hot, you have exceeded the (10%) in this case, 55 psi maximum safe heat build up pressure. You must ADD AIR. In this case I would add 5 psi which would take the tire to 65 psi when hot. After you run the tire again you will find the pressure to actually drop because the tire will run cooler. The heat build up causes the tire pressure to increase when under inflated.
    On the other hand, if the 50 psi cold pressure does not change when hot. You have more air than needed. You can remove 5 psi or so and test again when they return to cold. Like the next trip you take.
    So a fully loaded rig will require more air in the tires than one with empty tanks and a light load on board. Always error on the side of over inflation. Thus the maximum sidewall pressure indicated on the tire is usually used. It usually is more than needed. Each axle has its own requirement based upon the load on that axle.
    So how do I get a 2 Rivet rating? Does this long explanation help or hurt?



    [/ QUOTE ]

    I have been playing with my tire pressure using the method AccessMaster Outlined.

    Very neat. It really works.
    Last night I checked the pressure before heading home. Ambient temp was about 70deg. Ira, The PO of the Burb, Had told me he was running 55psi in these Michelin tires. This was right in line with the recomendations by GM on the door frame. Problem is these are not the same tires that the truck came off the line with so they probably have different ratings for PSI. The Max cold air pressure on the side wall is 80PSI. These are 10ply Heavy duty truck tires so hense the high pressures

    I checked the pressure and according to my gage they were a 56 at the current air temp. Drove the truck 19 miles down the freeway at 65mph. Checked the air pressure and it was reading 64 after heating. SO I went over the 10%.
    This morning it was 57deg. I checked the tire pressure. Showed just shy of 56 on my gage. I added air till they showed 60psi. Ran 19 miles up the road to work and checked them again. 64psi after heating. Try it again on my way home when it should be about 72 degrees outside. Tomorrow I'll add 2 PSI and see what happens.

    Neat trick and did learned something from this. Thanks for passing this on. What I really like is even if the gage is a couple degrees off it doesn't matter. You just need the pressure change after heating.

    Now accroding to Access master this will work on ANY tire. The reason is thetread temp is not really whats causing the heating it's the sidewall temp and the internal belt temps. I'll give it a try on my 75 later this week and see how it works on a 35 inch BFG.

    Let me hear from a few others after they give it a try.
     
  2. Goober

    Goober 1/2 ton status

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    Sounds easy enough.

    I'll have to give it a try.

    /forums/images/graemlins/thumb.gif /forums/images/graemlins/thumb.gif
     
  3. Blue85

    Blue85 Troll Premium Member

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    Good tip. I have heard of that before: that you have to add air to an overheating tire. It seems counter-intuitive to a lot of people.

    I have generally used the linear interpolation method between (0psi, 0lbs) and (max pressure, max load rating). The problem is that it requires you to weigh each corner (realistically, each end divided by two) or guess the weights on the tires. This cold/hot pressure method is a good way to double-check and be sure that the pressure is right.
     
  4. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    [ QUOTE ]
    Good tip. I have heard of that before: that you have to add air to an overheating tire. It seems counter-intuitive to a lot of people.

    I have generally used the linear interpolation method between (0psi, 0lbs) and (max pressure, max load rating). The problem is that it requires you to weigh each corner (realistically, each end divided by two) or guess the weights on the tires. This cold/hot pressure method is a good way to double-check and be sure that the pressure is right.

    [/ QUOTE ]
    Exactly! If you want to get real nitpicky and get a really good air pressure gage then you would be able to tailor the pressure to each wheel.
     
  5. Langosta39

    Langosta39 1/2 ton status

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    it doesn't seem to me that this method would optimized tread wear, and those of us running agressive meats need that.

    any comments on this?

    andy
     
  6. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    [ QUOTE ]
    it doesn't seem to me that this method would optimized tread wear, and those of us running agressive meats need that.

    any comments on this?

    andy

    [/ QUOTE ]

    Read what AccessMaster said again and tell my why not? (hint: the part about the tire not flexing were it was designed to)
     
  7. FRIZZLEFRY

    FRIZZLEFRY 1/2 ton status

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    Im gonna give this a shot with my 37s

    Thanks Grim /forums/images/graemlins/thumb.gif
     
  8. Chris_T

    Chris_T 1/2 ton status

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    So, did this work on everyone's off-road tires?
     
  9. shane74

    shane74 1/2 ton status

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    Yep, when I worked for a tire store, that's what we were taught to do. (hot tire..add air) It really does work! I used that procedure with my 38" swampers and found that for my truck, 32 psi in the front and 28 in the rear puts it about as perfect as it can get for tire wear and ride (for swampers anyway! /forums/images/graemlins/rotfl.gif). /forums/images/graemlins/thumb.gif
     
  10. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    24 front and 23 rear for my truck and the new TSL's wore and rode nicely to Moab and back.

    rene
     

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