Before you over inflate watch this.

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

  1. kwj

    kwj I hypermiled this

    I'm sorry -mr. bill, here is the link to the original, in which one can easily see that the two items of interest are in different scales. It was this one to which I referred:

    The one you've posted was photoshopped. Someone (I thought it was you since you were the one who posted it) took part of one picture, and put it with part of the other, to show a more "side-by-side" comparison. But, even in the "side-by-side" image, one can easily see that they are of two different scales. Since I thought it was you that manipulated the original, I thought it would be easy for you to decrease the scale of the upper image to match the scale of the lower image. Then we'd have a true "side-by-side" comparison. As it is, it is about worthless.

    Sorry that I thought you had attempted to better portray the two images. Again, this was because you were the first to post it in this changed fashion.
  2. -mr. bill

    -mr. bill Senior Member

    I don't think you are understanding what I'm saying.

    1. *I* created the images I posted derived from CapriRacer's post.
    2. I did not use photoshop, I used a different toolchain, but that's not important. Perhaps when you used the term "photoshopped" you were misusing it as a generic term, like "Xeroxed."
    3. Yes, it would be "easy" for me to manipulate the images.
    4. *I* won't do that. Not because I *can't* - but because I *won't.*

    Integrity of data and its visual display is important. (See Edward Tufte's wonderful books.)


    I'm just curious. If I understand you correctly, you believe the left pair are "nearly identical" because if you mentally scale one half just right, they are "nearly identical."

    You then went on to say "What this actually shows, then, is that a tire at a placard of 31 psi and the same tire at Max Sidewall, 44 psi, has roughly an identical footprint."

    So, when you were drawing this conclusion you based it on the left pair alone?

    In the conclusion you drew above, is that a narrow conclusion, or a broad conclusion? (Holding only for 44% loaded tires? Or more generally holding for tires at various loads?)

    If you look at this pair at 78% max load, do you come to the same conclusion, restated slightly, that a tire at a 31 psi (top) has roughly an identical footprint as the same tire at 44 psi (bottom)?

    -mr. bill
    Last edited: Jul 18, 2008
  3. jamesqf

    jamesqf Well-Known Member

    Sure, and now let's apply the particular law of physics which says that the footprint doesn't matter, at least until you get into the domain where you're shearing off little pieces of tire rubber.

    In any case, why the big argument? Seems to me it's all about tradeoffs. Lots of American drivers are willing to trade off significant amounts of stability & maneuverability for the perceived benefits (whatever they are) of larger, heavier, and higher vehicles, and no one (other than eccentrics like me) seems to have any great problem with that. So why should there be a problem with a tradeoff of higher tire pressure giving better fuel economy at the POSSIBLE (not certain) cost of a little reduction in handling?
  4. kwj

    kwj I hypermiled this

    -Mr. Bill, here's the deal. It is apparent that the middle and top imprints on the left are nearly identical, but that the middle one is of a larger scale. This is what I said in the beginning, and it is still clear when you look at that original image.

    All one has to do is look at the original to realize that the tread segments of the left middle imprint are larger than those of the upper left imprint, because they are of a different scale.

    Since you can't seem to see this, I suggested that the images first be put on the same scale, as they should have been from the beginning, and then when the tread segnemts are all shown to the same scale, it would be readily apparent to the most causual observer that they were for all intents and purposes, the same image.

    So, it is true that you were the one to manipulate the original (that is what I had thought).

    I don't "believe" that the two tire prints are nearly identical, I can see that one is printed larger than the other. But for those who don't possess my keen eye, a ruler will show what I'm saying is true.

    The only conclusion I drew was that the image indicated a tire at probably normal loads, has an almost identical tire print at Max Sidewall of 44 PSI as it did at a probable placard of 31 PSI.

    I never entertained anything about the tires at 78% load, as those are not normal hypermiler loads.
    Last edited: Jul 14, 2008
  5. some_other_dave

    some_other_dave Well-Known Member

    Interesting! This does go to show that tread-center wear is not completely gone. My thought is that the wear is probably caused by the higher tire pressures that you run. If that is the case, it would be due to a combination of various effects; the construction of the tire, the width, and so on.

    I will agree with CapriRacer that tire traction does not follow the "classical mechanics" model of friction--that has been obvious since the first time a drag racer accelerated at better than one G.

    I still do not agree with the contention that placard pressures will grip better than max sidewall pressures. That goes against everything that I have seen in terms of tire testing for hard-compound street tires. That includes on my CRX, on my 914 (four or five different brands/models/sizes of tires on that!), and at least three other friends' race cars. Not to mention the "convential wisdom" in the street tire classes of the SCCA.

  6. 2way

    2way Electromagnetic Wave [:-h

    Think about the low pressures used for dragging.... to get better "grip". Higher pressures will give you additional stability in cornering.... up to the point where you have reduced the contact patch beyond the stability gains. Low pressured tires increase the contact patch... high pressured tires decrease it (part of the reason it is good for MPG) ... it's all about a balance for me (what anyone sets their's at is their choice). Additional weight will also increase contact patch (the point in the article). It is all about getting the right contact patch based on tire load. Auto-X & dragging are specific purpose driving under semi-controlled conditions. While hypermiling is a somewhat specific purpose, it is pretty much in an uncontrolled environment. Auto-X & draggin are also done on a fairly level surface and you also pick up time in Auto-X from the reduced steering resistance.

    Overall, it is better to be over placard than under.
    Last edited: Jul 15, 2008
  7. CapriRacer

    CapriRacer Well-Known Member

    I can’t believe we are still discussing the footprint sizes. Nevertheless, here’s what I know about the chart. (Please excuse the long post, but I think it is worth it if we can resolve this issue.)

    The chart was lifted directly from “The Pneumatic Tire”, a book published in August, 2005 by the National Highway and Traffic Safety Administration (NHTSA) and edited by A. N. Gent and J. D. Walter of the University of Akron. It is a book that summarizes what is known about tires and ways of analyzing tire performance. The book is over 700 pages.

    The chart in question is on page 620 of Chapter 15: Introduction to Tire Safety, Durability, and Failure Analysis, co-authored by J. D. Gardner and B. J. Queiser of Bridgestone Americas. This is interesting since I recognize the tread pattern and it is NOT a Bridgestone or a Firestone tire. They must NOT have gotten the chart from an internal Bridgestone source. In other words, it must have come from somewhere else, possibly another published paper. However, the chart is uncredited – as are all the charts, diagrams, and graphs in the entire 700 pages of the book – so I can’t quickly go back and look at the source document.

    But let’s see if we can establish if the size of the footprints are reasonable:

    Here are 2 charts that appear a page earlier:

    (Editorial note: I've tried repeatedly to get the image to a smaller size without success and rather than waste more time, I'm posting a link.)

    Figure 15.2 is merely a diagram of a loaded and unloaded tire. In particular, notice dimension “d” – the difference between the loaded and unloaded tire’s radius. We tire engineers refer to this dimension as “deflection”. Unfortunately, the diagram doesn’t label the footprint – but it’s the flat portion of the tire in contact with the reference surface (near the spring symbol, labeled “K”.)

    Also, notice that everything in Figure 15.3 (the tire, loads, inflation pressures, etc.) are identical to what is in the chart in question (Figure 15.4) except that the different data is charted.

    I should point out, that the loads and pressures are the same as needed to do a rolling resistance test according to SAE J1269 – so I suspect the source document was about rolling resistance testing.

    I think it is easy to see that the footprint length would be some function of the deflection - the more the deflection, the longer the footprint . Looking at the relative ranking of the deflections, it is clear that the relative length of the footprints are in proportion – and the 2 footprints in question (31psi / 44% load and 45 psi / 44% load) appear to be correctly sized relative to the others.

    I think this demonstrates that the footprints were not altered.

    More to the point, the chapter that these figures appear in is not about rolling resistance or footprint size and shape. These figures are being used to demonstrate that load and pressure directly contribute to heat generation – and ultimately, tire failure. Put another way, there is no reason for the co-authors to manipulate the footprints to make their point.

    So why does it appear that one the footprint is out of proportion lengthwise?

    Possible explanation:

    Tread patterns are built up of a series of individual elements that are repeated around the tire. It is common practice to vary the size of the elements to prevent the tread pattern from inducing a vibration – and it is done by carefully selecting the pattern – commonly called the pitch sequence – such that there is a discontinuity around the tire.

    I’ll go so far as to say that virtually all tread patterns have varying element lengths. As confirmation of this, I invite you to go out and look at the tires on your vehicle and see if you can see the differences in element length. You may see 5 or 7 different element sizes, but most common is 3 different sizes.

    Needless to say, the element width has to remain the same, so only the length of the element varies. I think I see differing element lengths in the longest footprint and the difference is element length is fairly small.

    Perhaps that is what is driving the visual difference between individual people who view the footprints.

    In the meantime, I am going to try to find the source document. I’ve met Jim Gardner (I haven’t had the pleasure of meeting Mr. Queiser) and hopefully, he will remember where he got the figures in question. But since Jim’s area of expertise is failure analysis, it’s possible that he wasn’t responsible for the portion of the chapter that contains these figures. However, he has a reputation of keeping a huge file of supporting documentation which he brings with him when he gives legal depositions, so perhaps he has those figures in that file.
  8. kwj

    kwj I hypermiled this

    Thanks CapriRacer. Your background and experience are very important to this thread. I am not implying that the originator of the image was being deceitful, nor that anyone on this tread is, just pointing out what I saw.
  9. vtec-e

    vtec-e Celtic MPG Warrior

    I stuck a steel ruler into the tread and looked across the tire. Then repeated further out towards the shoulder of the tire. Not as accurate as a tread guage but not bad all the same.
    I must also apologise to CapriRacer for being a little cheeky with my comments. It's just that this subject is dragging on and on with no actual "hard" data on braking distance versus tire pressure. Which is part of the core issue. I'm a little p***ed off about it actually!

    Last edited: Jul 15, 2008
  10. kwj

    kwj I hypermiled this

    My experience, which brings all my driving experience baggage with me, is that inflating my tires to Max Sidewall provided better cornering, and better MPG. I experienced nothing to suggest I had less traction in braking, even in the rain.

    So, I've gone to higher pressures.

    Last night, during a quick rain, I pressed it in a stop (but not an all out wheel dragger) and threw it into a corner at a speed above my usual. Either I have exceptional tires in these Hankooks, or the increased pressure has just not yet reached the point where "breakaway" is pronounced.

    Now, this is just my experience. I've slid, purposely and by accident, in my cars over the years, with placard pressures. I've suffered a front flat and a rear blowout (with the rear being a catastrophic 6 inch loss of tread) on normally pressurized tires and didn't find them to be too much to handle.

    I've always replaced tires because of wear on the edges, never for wear in the center.

    It would be nice if someone could assign absolute risk numbers to what we are discussing. For instance, since I never go above 55, how much have I reduced my risk of tire failure, and how much have I increased my benefit by having more time to react and less speed to bleed off if a tire failure occurred.

    So far, I have no evidence that having my tires at the pressure they are, has decreased my traction or my ability to stop. By maintaining a good buffer, I also think I have realized an offsetting benefit of reaction time.

    I think the additive benefits far outweigh any perceived negatives for me, with my car, and my experience. I think Max Sidewall is a very positive and safe suggestion, when taken with all the other Hypermililng benefits, specifically with slowing down and leaving a buffer.
  11. CapriRacer

    CapriRacer Well-Known Member


    I share your frustration with the lack of data. I want folks to work off of good solid information and not having ANY is not only a bit discouraging, but also leaves open an opportunity for antecdotal information, which in my experience is subject to misinterpretation.

    But, the engineer in me wants to know why there is no data. I've come up with 3 answers:

    1) The normal operating range for passenger car tires is 26 to 35 psi (with the exception of Extra Load tires, which go up to 41 psi.) Placard pressures are always within this range (with a few interesting exceptions). Since tire manufacturers are in business to make a profit, any activity that doesn't contribute to this goal is frowned upon. Needless to say, testing tires outside this range makes little sense - and it becomes difficult to get company support for testing at elevated pressures.

    Note: I am not going to take this opportunity - as good a lead-in as it is - to discuss where the pressure listed on the sidewall comes from and what implications that may have. It would be a long post, so I'll leave that for a later discussion.

    2) The post above where is I discussed the footprint sizing issue (it's about 4 posts back) took me 2 hours to prepare and publish. Writing a technical paper is obviously much more involved and also takes a lot of effort and time - not to mention the necessary steps of anticpating all the questions that might be asked. I've noticed that some researchers present a new paper at every opportunity - and while each paper is slightly different - some are merely rehashing the same data, and some are merely reporting on the next step taken in the research. There is certainly an economy of scale if all you have to do is add the latest data or repackage the old data.

    I don't think I have to mention how difficult it is to actually structure a test and arrange to have equipment available to get reliable and repeatable data on the subject under study.

    3) Sometimes testing results in knowledge that might hold a competitive advantage. Needless to sat, all us technical folks have signed a non-disclosure agreement and need permission to publicly publish data - and data that would give a company a competitive edge would never be publicly published. (Do you see the irony of needing to do research that generates a profit, then not being able to publish it?) I suspect that was the situation with the SAE paper I summarized - it seemed like the testing was done much earlier and the only reason it was published was that it lost its value to the company.

    There is a 4th reason, but I don't think it applies here. Sometimes research turns up something that the company (and its lawyers) would rather not have available to plaintiff's attorneys. Even a warning that might come out as a result is twisted to the plaintiff's attorney's advantage. A good example is the warnings coming out about the age of tires (which is what the beginning of this thread is all about - Whew, we've gone far off the original track!!).

    At any rate - my intent is all these postings is to try to steer folks towards understanding how things really work - and not how we would like them to work. It's a difficult and never-ending task!
  12. 2way

    2way Electromagnetic Wave [:-h

  13. Somewhat confused with this issue, I have a 2003 Olds Alero and th tire specifies 32 psi and the car door sticker recommends 30 psi. Which is correct?
  14. lightfoot

    lightfoot Reformed speeder


    I think you have put your finger on the issue. We are exploring an area where there is little or no data, and many of the preconceived notions may prove incorrect. For example, from an intuitive viewpoint it would seem that driving at a constant speed would be the most efficient mode. We now know that this is not true, that a modest variation (P&G) improves FE.

    Don't forget to include yourself in the group seeking greater understanding. Even people with many years of experience in a particular area need to step back and take a fresh look when entering uncharted territory.
  15. 2way

    2way Electromagnetic Wave [:-h

    The Germans knew this w/the V1 Buzz bomb, which the US developed the cruise missile from.

    Even the government has trouble with it: Tires And Passenger Vehicle Fuel Economy
  16. CapriRacer

    CapriRacer Well-Known Member

    Something is wrong.

    Nothing on the tire should say anything about 32 psi.

    So let's start with the basics:

    1) What does your car door sticker - commonly called the placard - say the tire size is supposed to be? You already told us the pressure written there says 30 psi.

    2) What tire size do you have on your car now?

    3) Are you sure the sidewall says anything about 32 psi? If it does, please tell us the brand name of the tire and the tire line name - Like "Goodstone FireEagle ST3".

    It might also be helpful if you recorded the DOT number. It's 10 to 12 digits long, and will have both letters and numbers. It's located near the letters "DOT". From that I can tell you where the tire was made, what the tire size is, and when it was manufactured.
  17. psyshack

    psyshack He who posts articles

    Ive read all of this thread. Its quite funny.

    I will keep the tires on our Accord at max side wall. With 73k miles on them and still good rubber to roll on I expect better than 80k miles on them before replacement.

    I will keep running the tires on my Mazda3 at 47 to 50 psi. They are wearing out faster. But they were also 2 tenths short on tread depth when the car was brand new compared to Goodyears published tread depths. So right off the bat I got screwed with what Ive always called OEM shorts. A complaint was filed with Mazda and Goodyear. Im sure it fell on deaf ears.

    I have been running max sidewall or above for 25 years. As as everbody in my family has for years and many friends. This has been very common place for steel belted tires among us.

    This discussion will rage on Im sure. And it will remain unconclusive. There are so many varibles at work over all.

    Let the thinkers have at it. But the doers know the truth.
  18. Daryl Baines

    Daryl Baines New Member

    I ran across this post on one of the web sites and thought it was interesting.

    My A/C in my 99 Buick Regal LS was worked on by someone who did not know what they were doing. As soon as a few days after I had lots of water from unit all in passenger floor board. They fixed that than when A/C was on,temp guage would go to half way mark and transmission would buck felt like it was seperating from the engine. They just could not fix it and i had to much money in the car to junk it. Added 6 ounces of Auto-Rx and shudder & bucking is
    gone MPG is back ( had dropped 6 miles per gallon from normal 22 MPG) I am putting 1000 miles on and doing a T-Tec. Damage could include a pin hole in the transmission line going to radiator, if so this transmission should be fried. Will post again.
  19. worthywads

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

  20. Thanks Capri,

    DOH! I am not sure what I was thinking there.
    Tire size is the same as on the placard is the same as on the car, 215/60R15
    The are BF Goodrich and the tire reads Max 44 psi not 32 as I suspected as that is what the garage fills them to. Is this number cold or hot?

    I hope this helps. It seems like a huge discrepancy between GM and BF Goodrich.
    Last edited: Jul 16, 2008

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