Before you over inflate watch this.

Discussion in 'Fuel Economy' started by ALS, Jul 2, 2008.

  1. guroove

    guroove self-proclaimed expert

    The max pressure is intended to be used as the maximum pressure that the tire can safely be used at, which would imply hot. As we have found though, tires can safely be used at much higher pressures than recommended by either the placard or the sidewall on the tire. I'd say it doesn't really matter. Try 44psi and see where they end up after the car sits, and after you drive on the highway. If you set them at 44 psi cold, they will go up much higher after some highway driving. If you aren't comfortable with this, you can let some air out. Many people go much higher than this. Definitely don't run any lower than what the placard says cold.
     
  2. some_other_dave

    some_other_dave Well-Known Member

    I certainly hope not--more advertising is not welcome here.


    Back onto topic, I will try to contact some of the street-tire drag race crowd, and see if there is a consensus about tire pressures for longitudinal grip. As I said earlier, and was pointed out again more recently, my own tests (and other autoXers tests) have concentrated primarily on lateral (cornering) grip, as that is what is most important for us.

    -soD
     
  3. worthywads

    worthywads Don't Feel Like Satan, I am to AAA

    44 psi Max Pressure is cold.
     
  4. diamondlarry

    diamondlarry Super MPG Man/god :D

    This looks suspiciously like advertising. If you are going to post it in the same way again it could have consequences.;)
     
  5. guroove

    guroove self-proclaimed expert

    I stand corrected:embarassed:
     
  6. MaxxMPG

    MaxxMPG Hasta Lavista AAA-Vee Von't Be Bach

    Wow, that Auto-Rx solves so many problems. So I went to the bank and poured 6 ounces into the ATM's deposit envelope slot and my bank balance went up by 20%! And then I doused my CocoPuffs with another 6 ounces and my biceps grew 3 inches! I put a little dab on each collar bone, and the scent is bewitching to all the supermodels I hang out with. And all so very unrelated to the current topic of setting tire pressure and the resulting effect on handling/reliability/safety. :D
     
  7. diamondlarry

    diamondlarry Super MPG Man/god :D

    I suspect Mr Baines is a spammer and may not be around for long.;)
     
  8. -mr. bill

    -mr. bill Senior Member

    I apologize for beating a dead horse, but I'll have to correct you twice. One, it almost certainly is a Bridgestone - specifically a Bridgestone Turanza EL41. Two, it isn't a possible explanation - it is almost certainly *the* explanation. See Tire Rack. Particularly, click on "Click here for a 1000x1000 SUPERVIEW."

    So, I'm going to use a cut from The Tire Rack picture without permission (but I believe this is fair use) but I'm going to highlight two sets of tread elements.

    [​IMG]

    Clearly the element sizes *do* vary.

    Bottom line - for a given tire with a given load, tire contact area is a function of tire pressure.

    -mr. bill
     
  9. dare2be

    dare2be Well-Known Member

    And I said earlier:

    Now I'm no longer on the fence. This explains precisely why the images do not look the same scale, but only in the horizontal direction. Thank you mr. bill.
     
  10. kwj

    kwj I hypermiled this

    CapriRacer and -mr bill, you may have found the answer to the disparity, but realize that both the horizontal and vertical directions are suspect if you were to actually measure them.

    So, -mr. bill, if one strictly adheres to the placard, where there is rarely any adjustment for load, and another fills to Max Sidewall, but then loads to 75%, could the contact patch equal the unloaded car at placard?

    Why do most manufacturers not specify a pressure increase for a load increase, if the idea is to have the unloaded Placard pressure tire print on the road. Or, more likely, is the tire contact patch actually within Manufacturer's specifications over a wide range of pressures and loads. Additional questions, if you load a vehicle, does the pressure increase? If I fill my tires "cold" at -15 degrees in Springfield, MA, and drive to Tampa, FL, and like most people, don't check my tire pressures for a month, have I placed myself at great risk of a loss of traction, or has this been considered in the design of the car and the tire?

    How could manufacturers around the world subject themselves to such a precise placard pressure, when every one of their cars around the world are likely to be at different pressures?

    It is my understanding that the Placard is a good suggestion for most people for the tradeoffs of gas mileage and comfort, all within a range of pressures that are entirely safe.
     
  11. kngkeith

    kngkeith Well-Known Member

    kwj- you have illustrated why this thread really can't accomplish much more. There are too many variables, and the recommended tire pressure covers a wide range of possibilities. For instance- placard on my Highlander is 30psi, with max payload including passengers of 860 lbs. Curb weight is about 3900 lbs. There is a 15% difference in gvw with only one passenger and no cargo. Should psi be adjusted for that? Additionally, if the vehicle is to be driven at speeds above 100 mph (or is it high speeds for over 100 miles, the manual is a bit unclear) the pressure should be raised to 35psi.

    Consider my semi for a more dramatic example. I keep my tires inflated at 105 psi to safely carry max gross (80,000lbs). However, I'm at max gross about 10% of the time. 60% of my miles at less than half that. At those times I can be at 70 psi according to many manufacturers. Note that I said can be, not should be. The charts use the phrase minimum recommended. Does the truck handle differently- absolutely- due more to suspension geometery than tire geometery. Do my palms sweat when empty or lightly loaded- no.

    Keith
     
  12. jamesqf

    jamesqf Well-Known Member

    You know, you may be on to the solution to America's gasoline problems here. The price of gas goes up because of demand - that's basic economics. The demand is high because lots of Americans choose to drive oversized pickups & SUVs. The most common reason they do this is to compensate for a (perceived) lack of size in a certain portion of their anatomy. So if rubbing 6 ounces of Auto-Rx on it cures the deficiency, they'll all happily swap their FSPs for Fits and Yarises, gasoline demand will drop, and you'll probably win a Nobel Prize :)
     
  13. 2way

    2way Electromagnetic Wave [:-h

    Possibly. Depends on whether you mean 75% tire load or 75% vehicle load;)
    It isn't the unloaded pressure on the Placard.
    Manufacturer's spec is for xxPSI cold @ xxxlbs additional load max.
    Not until you move it.
    Better over than under. But, yes.
    It also has to do with meeting EPA MPG specs.
    Ah yup. It is the starting point from which one can deviate from.
     
  14. -mr. bill

    -mr. bill Senior Member

    Neither the horizontal nor the vertical directions are suspect. Look at the data that is actually there.

    I believe that you have a dramatic misunderstanding of yet another part of the data presented. The data presented in the JPEG is for % of MAXIMUM LOAD CAPACITY of the TIRE! For the tire under test, that is 1356 pounds. The left column the tire has been loaded to 44% of 1356 pounds, or 597 pounds. The right column the tire has been loaded 78% of 1356 pounds, or 1058 pounds.

    I invite you to look up the MAXIMUM LOAD CAPACITY of *YOUR* Hankook's, look up the curb weight of your Aveo, see if you can find the weight distribution for your Aveo, add in 150 pounds for a "standard" driver (you can SWAG that that 150 pounds will be distributed 50%/50% between the front/rear wheels) and calculate the % of MAXIMUM LOAD CAPACITY *your* tires are loaded to. And report back what you find.

    (I will tell you that doing the calculation with my Michelin Pilot MXM4 on my Honda Civic Si Coupe, the tires are loaded 76% front and 50% rear with only the driver. And with four on board with luggage, it goes to 82% front and 63% rear.)

    -mr. bill
     
  15. kwj

    kwj I hypermiled this

    2-Way, you respond: "Manufacturer's spec is for xxPSI cold @ xxxlbs additional load max." So how precise is "cold"? and I've seen very few placards with anything about load.

    The question was: "...have I placed myself at great risk of a loss of traction, or has this been considered in the design of the car and the tire?" and your answer was "Yes."

    The question was: "How could manufacturer's around the world subject themselves to such a precise placard pressure, when every one of their cars around the world are likely to be at different pressures?" Your answer was: "It also has to do with meeting EPA MPG specs" but placard tire pressures predated EPA MPG specs.

    And Mr. Bill, you answer: "Neither the horizontal nor the vertical directions are suspect. Look at the data that is actually there." Of course they are not suspect to you. But I did look at the data, and I actually measured it. It is suspect. Why don't you go look at the data.

    I don't think I've seen anything, or read anything brought to our attention that begins to show any problem with exceeding placard pressures, but only things that would support doing so, and I've personally not experienced any slipping or sliding, just increased MPGs.

    So, bottom line Mr. Bill, is it really dangerous to exceed the placard pressures? This is how this all started and we've been going down every rabbit hole anyone can think of.
     
  16. CapriRacer

    CapriRacer Well-Known Member

    I'm truncating a lot of the following so it flows better:

    kwj:

    I am confused.

    Are you saying all of the chart is suspect or just part of it?

    What is it that seems suspect to you? (It would help me understand what the issue is if you posted the measurements you took that show it is suspect.)
     
  17. 2way

    2way Electromagnetic Wave [:-h

    kwj, I've never said it is bad to exceed placard?? I've only said that max sidewall is just too arbitrary. Personally, except in winter, I do go above placard. But, not max sidewall. I'm very lightly loaded and that would reduce my contact patch too much.

    Vehicle manufacturers must identify vehicle load capacity on the placard with the following sentence: "The combined weight of occupants and cargo should never exceed XXXX pounds." So, I don't understand how you could not have seen anything about load on a placard.


    "Cold" is as precise as you make it.
    Yes, you could've placed yourself at risk, not that it was a design consideration.
    The "around the world" question.... well, in some cases they are mandated to indicate it. Actually, you need to view placard as a recommendation, not a rule. A starting point from which you can deviate from. Simply changing to tires other than OEM will likely change what pressure should be.

    I think this is one of the better articles I've seen: http://www.sportcompactcarweb.com/tech/0208scc_tire_pressure_guide/index.html

    However, a very simplified way to find out approximately what pressure may be optimal for your wheel combination is to draw a chalkline across the width of the tire, drive for a bit, and look at the wear pattern of the chalkmark. Wearing more quickly in the center indicates pressure that is too high, and wear on the edges indicates too low a pressure.
     
  18. kwj

    kwj I hypermiled this

    With all that has been said, this is a poor statement ""Cold" is as precise as you make it." I don't "make" cold. "Cold" is an imprecise word. Because of this and other imprecise "measures," there is very little leg to stand on as to not going to Max Sidewall. You "feel" safe at a few pounds over Placard. I "feel" safe at Max Sidewall. I can't say you are placing yourself at any quantifiable risk and so far, you've been unable to quantify any risk I might assume with the offsets of my driving experience and my low speeds. Sounds like we are about even in this respect.
     
  19. 2way

    2way Electromagnetic Wave [:-h

    I'm guessing you didn't read the article in that link:
    &
    So, as long as you're comfortable with what you run at kwj, s'all good.
     
  20. kwj

    kwj I hypermiled this

    Very comfortable, for many, many months.

    This quote does nothing: "A change of 2-3 psi can completely change a vehicle. An underinflated tire will ride smoothly but feel sluggish, have mushy turn-in and go around corners with high slip angles or just not much grip at all. An overinflated tire will feel harsh on impact and over smaller bumps, and while responding crisply, will lack grip when pushed hard. To an experienced person, these characteristics are obvious even from the passenger seat." because it uses nebulus terms like "overinflated."

    We've gone 'round and 'round, and so far, no one has quantified any risk to pumping tires up to Max Sidewall. That really is a problem. People don't want to hear that in "overinflating" a tire will lack grip when pushed hard. Heck, any tire will lose grip when pushed hard, so what real information is there in that? None.
     

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