Before you over inflate watch this.

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

  1. 2way

    2way Electromagnetic Wave [:-h

    Put your car on a skid pad to obtain a quantifiable risk from overinflation.

    I do, definitely, agree that underinflation and overinflation tend to be rather nebulus terms. For a normal driver, without the scientific means to easily quantify over/underinflation (other than the crude chalk method I mentioned), he/she must rely on vehicle feel and observations or go strictly by the placard. A pure hypermiler that is usually under speed limits and takes corners at low speeds may have a tougher time making slip/grip observations. Arbitrarily raising your pressure to max sidewall just doesn't make sense w/o considering other factors. The placard is a "standard" or defines the point from which to deviate from. Typically, the optimal tire pressures for normal driving seem to be somewhere within ±10-15% of placard.

    For me, I've observed less "grip" or "stiction" on my daily driven vehicle/tires in the summer with pressures any much above 4-5psi above placard and better snow grip in winter at or slightly below placard and adjust them accordingly. I also observed a very noticeable "grip" change when I went from summer performance tires to performance all seasons. You might as well have skis on in the winter than summer/rain tires:(.

    BTW, "cold", as technically applied to tires and placard settings, usually means an ambient temperature of 70 degrees and not driven on for 3-4 hours minumum, preferably longer. For ambient temperatures above and below 70 degrees, pressures are decreased/increased by approximately 1psi for every 10 degrees of change. It is more commonly taken to mean first thing in the morning temp on undriven tires.
     
  2. some_other_dave

    some_other_dave Well-Known Member

    Very interesting--you must be driving on drastically different tires than the ones I have tested on. (I think it was 5 sets?) Or my friends. (At least 4 sets.) We noticed no fall-off of lateral grip until well above placard pressures--usually more than 15 PSI above placard.

    -soD
     
  3. kwj

    kwj I hypermiled this

    Capri Racer, I measured all dimensions, top to bottom, tread segments width and height, vertical distance between tread segments, and horizontal distance separating tread rows. In all instances, the measurements for the center left tire print were greater than identical measurements for the top left tire print.

    Some difference I could understand and explain away, but not the consistent measurement differences in all aspects.

    The center left tire print seems undeniably of a larger scale than the top left foot print. It has been my contention that if they were of the same scale, then they would almost be identical.

    One use I do have for this image, is how much difference loading makes, and that makes me wonder why auto manufacturers typically don't include a range of pressures to include different loading percentages.
     
  4. kwj

    kwj I hypermiled this

    I think it would be better to say "I'm very lightly loaded, and I think that MIGHT reduce my contact patch, which would be too much for my FEELINGS of safety."

    What I meant to convey is that almost all placards I've seen, only give one pressure, the "cold" pressure. Very few give a range of pressures based on load, and yet it is evident that load greatly affects the tire patch (and as Exploder drivers found, the sidewall flex problems).

    I believe this was a decent "tool" when tires were bias ply. I'm not sure this translates well to radials. For radials, I think chalkmarks on the outer tread onto the sidewalls would be a more of a "teaching moment" for underinflation, as underinflated tires tend to roll onto the sidewall during cornering.
     
  5. kwj

    kwj I hypermiled this

    I don't drive on a skid pad, and that would not quantify my risks.

    We've been trying to consider other factors, and finding it is difficult for people to bring proven good reasons why not. Since hypermiling will also mean slowing down and maintaining a buffer, I feel we've more than made up for any perceived loss of "grip." We would also consider the typical understanding that as tread wears low, so does our stopping distance. My old Honda Accord (at placard) could skid in rain while slowing and making a downhill, left turn, before I reached the normal indications that I'd entered the realm of tire replacement.


    And this comes from where?

    I've not noticed anything similar to your experience with my cars and their various tires, even at 15 psi above placard (much less at 35 psi above placard).

    Where does this come from? Having a well understood "cold" temperature would certainly be a help. Why don't they just say what you said?
     
  6. 2way

    2way Electromagnetic Wave [:-h

    Know doubt that higher pressures reduce contact patch. I'll give ya the 2nd half.
    True. That one pressure is "supposed" to be good for the placard load. But, you're right. I seem to recall older vehicles carrying much more info related to load and pressures.

    I'll throw another little wrinkle in... I haven't seen anything that says Max sidewall is "cold"... only max allowable pressure. So, by setting to max sidewall cold you will easliy exceed max once you start rolling. If I'm incorrect in this observation, someone please correct me. Granted, I'm not going to make any safety conclusions from that.
    In honesty, I haven't tried it. But, I am interested (for educational purposes only) in trying it out as an exercise.
    Guess I didn't make it clear that it was from personal experience.
    Honestly, I don't recall where I saw it and I'm not 100% positive I quoted it correctly. But, it made sense to me, where 70 is a common temp standard I've had to use, above and below which compensations need to be made. Oddly, thinking about it... it tends to track w/what I used to set my pressures for summer/winter with my 32psi placarded tires.... I used to do 34 summer/28-29 winter (I've gone higher summer now). Max sidewall would definitely not be a good thing around here in the winter. It would be as bad as having the summer tires on... no grip.

    Ya know, thinking back (memory ain't what it used to be), I think I noticed it more on takeoffs than lateral grip... other than one particular event involving a wet sharp turn.... that definitely told me my pressures were off. Although, I had the same issues w/a fwd Taurus work wagon where the pressures were quite fussy. I don't seem to notice it as much w/the Avalon. Then again, the Avalon has never been a daily driver for me. I'm going to be taking it on a nice long trip shortly and plan on elevating pressure a bit above my norm for load/economy and will see how it does:)

    SCCA auto-X or race? I know auto-xer's run 35-45psi (maybe more)... and do some pressure adjustments to balance the steering/suspension. I'm going to inquire w/my SCCA M3 racer buddy & see what he does. I would certainly think that you'd notice things in the 914.... the 914 is a bit heavier, yes? I do know they handle VERY nicely:)

    Ran across this: MECHANICS OF PNEUMATIC TIRES... looks like I've got some vacation reading:)
    ^^ Tons of info

    "In general, the cornering stiffness of tires increases with an increase
    of the inflation pressure." Nothing new there. But, the graph on cornering coefficient shows very minor gain from 38psi to 48psi.

    I've been out of school WAY to long...lol. A little birdie told me CapriRacer can help.
     
    Last edited: Jul 21, 2008
  7. some_other_dave

    some_other_dave Well-Known Member

    Nope, in stock form the 914 is just a tiny bit lighter than the CRX (which is, granted, the heaviest CRX made--not counting the Del Sol). It's just a touch over 2000 lbs, less when I autoX it.

    I've done the Time Trial thing (running on the Big Track) once, it was a blast. I've autoXed for quite a long time, only once on "soft-compound" tires.

    I am starting to see a little bit of consensus that the drag racers like lower pressures than the autoXers do, though I haven't heard back from too many of the straight-line guys so far. So it sounds like most tires change longitudinal grip differently than lateral grip with pressure changes. Makes sense...

    AutoXing, I have used tire pressures to change the oversteer/understeer balance of the car. Generally, by letting air out of the end opposite of the one that isn't sticking as well--because it's easier to let air out than to pump it in, especially when you don't carry an air tank with you. ;) That's how I did my testing, though--add air to one end of the car and see if that end sticks better or worse. I find it's a lot easier to feel a change in the balance of the car (which is generally quite well balanced) than to feel a change in overall grip. I borrowed an air tank for that...

    -soD
     
  8. 2way

    2way Electromagnetic Wave [:-h

    Yeah... the more I read, the more confusing it gets... lol. But, I think you're correct. I may be somewhat confusing lateral w/longitudinal "grip".

    For kwj's Aveo, (185-60-14?)it has a smaller tread width and larger sidewall than what is on my vehicle. Increasing pressure on such a tire wouldn't have the same effects as it would on mine. An increase in pressure on the Aveo will stiffen the sidewall more, not really reduce the lateral portion of the patch (unless you overinflate to the point of "crowning"), and only slightly reduce the longitudinal portion of the patch.

    Whereas, with my larger wheels, wider tires, and shorter sidewalls (205-50-16), an increase in pressure doesn't improve the sidewall stiffness as much, doesn't reduce the lateral portion of the patch (again, unless overinflated to the point of "crowning), and greater reduces the longitudinal portion of the patch compared to the Aveo. Makes a little more sense to me now... if I've got it right. Part of the "factors" to consider, kwj. That could also explain why the draggers w/"fatties" go with lower pressures to get better longitudinal "grip" as SoD was mentioning.

    Pardon my education, I'm going to hazard a guess that the closer to a circle you get (contact patch generally being oval either laterally or longitudinally) the better the balance between longitudinal and lateral "grip"?

    However, I would think this still points to what I was saying that it is best to strike a balance. While my tire/rim combo is no where near those of most sportcompact "tuners", I can see where an elevation in pressures can benefit "normal" passenger car tires more. Course, this doesn't explain my experience w/the Taurus wagon... unfortunately, I don't recall what it had for rubber on it. Another "factor" that comes into play in all this is the transfer of weight onto or off of tires. All this crap is damn complex:( There's also pavement penetration.

    BTW, kwj, I do better see the benefit>risk for a "pure" hypermiler in all this. But, I hope you see that setting to max sidewall is as arbitrary as me posting up that everyone should inflate to 35psi or 60psi. It really should be gradually increased by a driver and he/she needs to make a determination as to what is appropriate. Many vehicles have offset pressures ft->rear for suspension/balance/load reasons and that differential should probably also be maintained.
     
    Last edited: Jul 22, 2008
  9. CapriRacer

    CapriRacer Well-Known Member


    OK, so you think the upper left image is magnified. If that is so, then for that image, the area of the footprint would be smaller than it appears. But isn't that consistent with what is being protrayed? (That the area of the footprint is smaller as you add pressure or decrease load?) So I'm still confused as everything seems to line up.

    BTW, when I measure the upper left footprint, the groove widths and the rib widths appear to be the same as the others. Word of caution: It looks like the 2 16 psi footprints have the center ribs lifted off the surface (the edges of the rib are not crisp) and the groove widths appear wider and the rib widths appears narrower than they really are.

    And to answer this question:


    My experience is that consumers can be incredibly simple minded. Anything more that a single, straight forward answer confuses them.

    It used to be common for some German vehicles to have different pressure settings for different conditions. I can't begin to tell you how many times I've had to explain that the pressures were tied to a condition and the condition was delineated on the placard. The obvious conclusion from this is that if you want folks to remember what pressure to use, you have to give them a single, straight forward, always available answer.

    So the vehicle manufacturers test at both curb weight and fully loaded. They will have experienced what the vehicle does at both conditions - which is why there is so much confidence in that pressure.

    However, the folks that look at this web site are OK with complex answers - hence our discussion.



    And one last thought before I close this post:

    We have been discussing this footprint chart:

    http://www.geocities.com/barrystiretech/footprint.jpg

    I think this chart says that for radial tires, using the chalk method to determine the proper pressure doesn't work - the entire width of the tread seems to be engaged in all the the footprints.
     
    Last edited: Jul 22, 2008
  10. kwj

    kwj I hypermiled this

    I'm not sure what you think I wrote so I'll repeat it: In all instances, the measurements for the CENTER LEFT tire print WERE GREATER than identical measurements for the top left tire print.

    Yet, you come up with "OK, so you think the upper left image is magnified." No. As I've continually stated, the center left tire print appears to have been enlarged. It measures larger on all aspects, overall, top to bottom, distance between tread segments, and distance between each row. It is of a different scale.
     
  11. CapriRacer

    CapriRacer Well-Known Member

    Opps, my mistake. Now that is clear, and I see what you are saying.

    At the same time, I also see that the center left has more tread segments engaged than the upper left - 3 1/4 or so vs about 3. So while I see your point about magnification, it appears to be slight.

    Thanks for pointing that out. That gives me confidence to use the chart again. I think it illustrates some very valuable points about load, inflation and footprint size.
     
  12. kwj

    kwj I hypermiled this

    CapriRacer, I thank you and all who have offered opinion and fact to this discussion. It is very important to understand, as those tiny four patches are all that connects your ton of vehicle to the road.

    I think we are in good shape. Never again will my tires be underinflated (unless I'm stuck in a sand pit and the tide is coming in).

    I'm still running "overinflated," being above Max Sidewall, and plan to continue, to provide actual experience feedback. I also understand the trepidation that a rational driver has about increasing tire pressure. It's a healthy thing to be skeptical.
     
  13. MooingLizard

    MooingLizard Well-Known Member

    Hyundai's definition of "Cold"

    At one point this thread was debating the definition of "cold" tires, and whether the tires should be pressurized at cold or warm temps.

    I came across some fine print in my Sonata Owner's Manual last night that basically said the following:

    1) Hyundai's door placard temps are for "cold" tires
    2) Hyundai defines a "cold" tire as one that has been driven for less than a mile in the last 3 hours.
     
  14. 2way

    2way Electromagnetic Wave [:-h

    Does that make you irrational? ;)

    The longitudinal changes make much better sense to me now. It explains my forward traction loss and why Wayne says it will impact braking.

    My thanks also, CapriRacer.
     
  15. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    I don't think I've ever heard Wayne say that.
     
  16. Ophbalance

    Ophbalance Administrator Staff Member

    I think it's in the Hows and Whys article as a con... or may have been at one point?
     
  17. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    The article actually says the opposite:
     
  18. Ophbalance

    Ophbalance Administrator Staff Member

    I could have sworn seeing something about it further down? Perhaps not, I've been reading a lot of material here an elsewhere lately.
     
  19. 2way

    2way Electromagnetic Wave [:-h

     
    Last edited: Aug 6, 2008

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