Before you over inflate watch this.

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

  1. MooingLizard

    MooingLizard Well-Known Member

    This may be common in other cars, but I know that my '05 Hyundai Sonata specifically states the mfr-recommended pressure as 30 psi under normal loads and 33 psi under heavy loads... and the tires on the car (put on by the Hyundai dealership after they got it as a trade-in, so I assume they're cheap but of the appropriate specs) are only rated at 35.:(

    Even in normal use tire pressures will sometimes be more than marginally (which I'll define as 2-3ish psi) greater than even the sidewall rating. To use Hobbit's example, if I were to put 33 psi in my cold tires in VT, load the car up and drive down to Florida, my (warm) tires could potentially be ~40 psi, 15% greater than the sidewall and over 20% more than the mfr's recommendation. Add in to that as well that my tire gauge and/or compressor might be off by a few psi. The consensus here seems to be that the lawyers and/or engineers have such scenarios and deemed that such an occurrence would be safe, or at least not deadly enough to make tort lawyers salivate (which, given lawyers, means "very safe" to me;)).

    Thanks gents. This has turned into a very productive, friendly, and helpful thread.
  2. dare2be

    dare2be Well-Known Member

    Allow me to add to that anecdotal evidence. I inflated both my cars to near max sidewall (41-42), and I notice at least as well cornering if not better than before.
  3. 2way

    2way Electromagnetic Wave [:-h

    That's where the real world comes in and observing tire wear. Center tread wear would indicate that they are over.
    I don't define it by either. However, I would consider above max sidewall to likely be overinflated.
    Above a certain point in inflation it isn't going to make any significant difference.
    Hydroplaning is a somewhat different story. However, stopping in the same conditions is also a different story.
    A good question... what exactly do we promote? No, a car with tires that are in good shape, inflated to max sidewall, is going to continue straight down the road when the driver turns the wheel sharp enough on a dry road... even at hypermiling speeds. I have a very light vehicle.... at max sidewall, I can assure you I'd continue stright down the road with a sharp enough turn. Load per tire is important. This is also why the manufacturer's placard is important. Tire manufaturer's have no idea how their products willl be applied.
    Tires still slip w/ABS. Glad you agree that the tires are going to slide.
    I don't know what planet you drive on. But, I'm here on Earth & sh1t happens... often right in front of you. When you have a 150 mile daily commute in Massachusetts, emergency maneuvers ARE a necessary part of your ordinary driving experience:rolleyes:.
    Too sharp a corner and you're gonna slide through it. Add wet pavement to the mix and you're in the woods. But that can happen underinflated, too. Where you really want to be is between under inflated and over inflated... I suppose that would be properly inflated.

    kwj, just so you know that we are actually pretty much in agreement. This was what I thought it should be: Tires should be, at a minimum, inflated to the vehicle specific pressure recommended by the automobile manufacturer. Inflating beyond that recommendation, up to the tire manufacturer's maximum rating, should only be done by those knowledgeable and familiar with the impacts of doing so.

    soD, in my case, inflating to max pressure certainly does result in a loss of grip. Light car at only 2,500lbs (about the same as a Yaris - but with much better tires & more hp). Tires will break free much easier on a launch on wet roads, even at my current 3psi above placard. Tire load is important;) I can only imagine what a Yaris would be like @ max sidewall and skinnier tires... it would probably feel like driving on stilts to me?

    I am also able to tell when I've got a tire that has dropped in pressure, by as little as 2 psi.:) You know you're really in tune w/a vehicle when you can notice such subtle differences.
    Last edited: Jul 11, 2008
  4. Right Lane Cruiser

    Right Lane Cruiser Penguin of Notagascar

    The thing that has been getting me with all this discussion of friction and traction is that they are measuring two different things that though related, don't vary in direct relationship to each other.

    The friction we are talking about is rolling resistance. Specifically, a measure of how much force it takes to maintain a state of rolling -- a (mostly) non-slipping activity.

    Traction on the other hand is a measure of the force it takes to cause slipping. Better traction requires more force to cause slippage.

    The first is a compressive force (of the structure and the surface interface), the second a shear force.

    From that standpoint, why are some people so sure that traction has to be reduced at higher pressures?

    (Honest question here -- just trying to identify holes in my reasoning... if they exist. ;))

    I might mention as an extreme case that a woman's very small area high heel will have significantly higher "traction" on asphalt than a man's dress shoe heel (positing the same weight on both and rubber soles). Deformation of the high heel sole (surface) will be significantly higher than that of the dress shoe heel due to force distribution, though the rigid structure underneath will not vary (much).

    I draw a direct analogy to high psi tires with different contact patches bearing the same weight (vehicle).

    Poke away at the above, please. If my reasoning is faulty I want to know what I'm missing so I don't do it again.
  5. kwj

    kwj I hypermiled this

    2-way, you say "I don't know what planet you drive on. But, I'm here on Earth & sh1t happens... often right in front of you. When you have a 150 mile daily commute in Massachusetts, emergency maneuvers ARE a necessary part of your ordinary driving experience."

    I agree that driving outside of Massachusetts might seem like being on a different planet to some. If evasive maneuvers are part of your everyday commute, you are both driving too fast and too close. Slow down, leave a buffer and live for tomorrow.

    Your responses were generally evasive, but that's okay, people here are pretty smart.

    We know that an escape driving maneuver involves cranking the wheel full in one direction or the other, to force a power spin and a pull out at 180 degrees. Practice on a skid pad until you feel confident of your ability to do it on a dry road. Travelling at 55 and slamming on your brakes can create a skid. Both maneuvers can be promoted by bald tires. The second one can be avoided by proper attention to the task at hand, by driving slower, by keeping a proper buffer for conditions, and by practiceing good defensive driving techniques, such as always having a safe out.

    In the worst case, following none of the above good driving techniques, you go dumb and slam on your brakes because there is something bad immediately in front of you, what is the delta in stopping distance between tires at Placard and tires at Max Sidewall? No guessing please.

    You also say "Tires will break free much easier on a launch on wet roads, even at my current 3psi above placard." You definitely drive differently than I.
    Last edited: Jul 10, 2008
  6. kwj

    kwj I hypermiled this

    Oops, another leading question came to mind; if a drag racing slick gets more traction than a tire with treads, why don't we all drive on tires that have no treads? Or are our tires more of a compromise for less than ideal conditions?
  7. Shrek

    Shrek Kaizen Driver

    One word. Hydroplaning.
  8. Shrek

    Shrek Kaizen Driver

    Your observation also correspond with the nice graphic posted above. Higher pressure gives more 'penetration' of the rubber, and thus should get your more grip by that theory

    (at least when you start to slide, because by that theory the rubber is mechanically torn by the pavement - if you agree that that is a benefit. Other posts note that the torn off rubber will act as balls that will reduce your grip anyway.... I'm a bit confused now.)
  9. CapriRacer

    CapriRacer Well-Known Member

    Welcome to my world, guys. I am really glad of the direction this thread is taking - in particular that change in tire pressure affects my than just a single property.

    First, let's talk about tire wear.

    Most tire wear occurs in cornering. In order to turn the vehicle, tires have to develop a slip angle - the tire is pointed at a higher angle. This causes the tire to slip and that generates a grinbding action in the tread.

    Put another way, cars that operated in the city - with frequent turns - won't get as good of tire wear compared to tires that travel strictly on the interstate - where there is very little turning. I've seen the cheapest of the cheap tires get 100,000 miles simply because the vehicle spent mall most at of its traveling between cities - hardly any turns at all. I've seen the same tire deliver less than 10,000 miles when used in a city delivery situation.

    OK, next:

    Steer tires tend to wear in the shoulders. The reason for this is than the tire tends to tuck under in cornering.

    Drive tires tend to wear in the center. I don't have good explanation for this but much observation and others who have noticed that same thing tend to confirm this.

    In a RWD, the steer tires and the drive tires are separate - this is where you can observe this.

    On FWD cars, the front tires are both steer and drive and they tend to waer evenly, while the rear tires just sort of follow along.

    I was hoping someone would point this out - but that didn't happen - but if you look at the graphic with the 6 color footprints, notice that with the same load, pressure increases cause the pressure distribtution to shift. The center of the tread becomes more loaded and the shoulders becomes less loaded. If the hypothesis is that more pressure causes the center to the tread to bulge more - and that causes the center of the tread to wear more, then the graphic supports the first part of this hypothesis.

    BTW be very careful with the graphic. I have looked at many of these foorprints and this set is among the best footprints I have every seen. Many tires don't have footprints that look this good - and change a lot more with pressure. (I suspect this tire has a cap ply - maybe 2).

    In particular be careful about the evenness of the pressure distribution at a particular pressure. The evenness is going to be a function of what the molded shape of the tire is - not to mention the angle of the belts and the presence of cap plies. In other words, it would be wrong to conclude that ALL tires get even pressure distributions at elevated pressures. This one does, but I assure you others don't.
  10. worthywads

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

    13 pages and we aren't any closer to evidence showing if or by how much max sidewall is any worse than placard from a safety standpoint.
  11. Shiba3420

    Shiba3420 Well-Known Member

    As Shrek said hydroplaning, however some people do use "near slicks" on the road where they shave down the tire tread so its just barely legal. Sometimes the tread is thicker as you get near the core, but most of the advantage comes from the tread not being able to move latteraly; Or, in short, it makes the tire stiffer. That's the same thing that is happening when we add air, although we only stiffen the main part of the tire and not the tread.

    If you ever get a chance, feel the warmed up tires of a top fuel dragster. They are gummy and actually stick to you. Becasue of that fact, they experience traction beyond that of ordinary friction. We could have street tires like that, but they wont last very long. Either the sticky surface would wear out and we would be left with ordinary tires, or the sticky could extend through the tread, but then the entire tread would quickly wear out. Besides that, for hypermiles, that much traction would be bad. While we want to be be able to stop/start/turn in a reasonable manor, we really don't want to have to overcome a flypaper effect with every inch we move forward. Very poor mileage would result, but people running 1000hp engines for less than 10seconds aren't exactly concerned about economy.
  12. lightfoot

    lightfoot Reformed speeder

    I have been wondering if the slip angle changes as a function of inflation pressure? Would higher pressure stiffen the sidewalls of a radial tire and thus reduce the slip angle? Would this reduction in slip angle make the car turn quicker and thus make the driver feel that the tires have more "grip"?

    Just questions, no agenda attached.
  13. hendu

    hendu Well-Known Member

    woops wrong thread
  14. 2way

    2way Electromagnetic Wave [:-h

    Believe me... I'm much more cautious than I was in my youth;) You also have to consider that it is very difficult to maintain a buffer to your side(s). I swear people have no clue these days on how to merge onto a highway. The more dedicated right lane cruisers probably see even more of this than do I.
    Beats me. Feel free to supply it. But, it will vary from vehicle to vehicle and tire to tire.
    It was just the easiest way to explain a 1st gear clutch engagement take off. But, you are probably correct. I don't extreme hypermile (yet - lol).
    I like it. It's got you thinking:) As is mentioned later, you slide when you turn the wheel (even a little). Traction/grip is what is going to reduce that slippage and get you turning in the right direction.
    And they run low psi for better traction.
    I would certainly think that the angle changes. What most feel is that the vehicle is easier to turn (the steering wheel doesn't fight you as much). This is because the tires are slipping more.
    By all means:)
    More like sidewall flex or lateral tire roll. Also, OEM tire sidewall aspect ratios have decreased over the years and sidewall flex is less of a factor than it used to be. Twenty+ yrs ago you would commonly see OEM aspect ratios of 78, 75, & 70. RLC's Elantra has 60's and his Insight has 65's.

    Max sidewall is just what the manufacturer has safely rated the tire for. One size does not fit all. It is too arbitrary to recommend using it no matter what the vehicle. The ONE difference would be if fuel economy is your only goal and you want to reduce rolling resistance as much as possible. Don't think that I don't get it. I just don't think max sidewall is something one should advocate for the inexperienced and using the article to justify doing so is seriously flawed.

    Last edited: Jul 11, 2008
  15. Ophbalance

    Ophbalance Administrator Staff Member

    Well, but the thing about recommending those pressures is that we also say "Slow down, drive with more attentiveness, look ahead on the road" to go along with those pressures. It's a culmination of all of these things that helps lead to better gas mileage. We don't ever say that any one thing is a magic bullet, and stress that slowing down will garner the most fuel savings out of any of the techniques. So, hopefully, no one comes on and thinks "Hmmm... well, I don't want to slow down, that's too much of a bother. Looking ahead?? Who has the time. Not racing to lights?? Not for me. This tire thing, though... I can take time out of my day every week for that!" I'd think that if someone cannot be bothered to slow down, they're not going to be bothered about their tire pressure either. And above all, we also stress that you must try this techniques in little to no traffic to get a feel for how your vehicle handles first.
    Last edited: Jul 11, 2008
  16. hobbit

    hobbit He who posts articles

    As another Boston driver, I can confidently report that leaving a buffer
    works fine, and the answer to worries about people jumping into it is
    to simply make it BIGGER. And of course this goes for locales other
    than Boston, since it's just a good idea. As I've told many people and
    emphasized in countless threads, more space allows someone to get in,
    do what they need to do, and get out again -- like get to their exit,
    come in and move over a lane, whatever. The more room you give them
    the less likely they're going to need to brake-check YOU in the process
    because they'll already be at a better distance from the car ahead.
    I can't count the number of times I've been in a nice warp-stealth
    glide on 93 and watched all kinds of stupidity and mis-arbitration
    happen at an on-ramp some distance ahead of me, but they've usually got
    it sorted out and they're on their way by the time I get anywhere near
    it without me having to change anything. It works, and it's incredible
    to me how many people continue to disbelieve that.
    But to get back to tires and pressure, yes, after now *14* pages of
    this I'm reaching the conclusion that NOBODY REALLY KNOWS, even the
    supposed experts. The butt-dyno that says "cornering feels firmer"
    is only one piece of input and will likely be different for different
    types of tire, the interaction between different grades of rubber and
    different roughnesses of road surface will vary all over the place,
    and let's face it, there is no ONE single conclusion anyone is ever
    going to reach here. But the discussions of the micro-aspects of
    what goes on under a tire are interesting, and maybe having more of
    that information will let people make more accurate decisions about
    their own individual needs and subjective preferences.
    Example: ever since I rotated my Hydroedges front-to-back on the Prius
    after about 15K, there was apparently a very slight toe-in on the
    right rear wheel that caused just a wee bit of feathering on the
    tread blocks. Once that wheel was on the front with visibly different
    camber and now a drive/steer tire, it's been a bit on the noisy side.
    Recently I brought the pressure DOWN from high fifties to more like
    44 in all of them, to see if that would mitigate the problem any and
    possibly let that tire re-bed in a little better and maybe feather
    back to something more normal. It hasn't been long enough to really
    tell yet, but overall the tires are definitely a bit quieter but the
    butt-dyno does tell me the car is slogging a bit more heavily on the
    glides. I also think there's a bit of imbalance by now, i.e. weights
    fell off in the meantime and I definitely notice some symptoms at
    higher speeds, so I'm going Monday to have all four rebalanced before
    the big haul to Hybridfest. I'll probably split the difference and
    put 'em back up to low/mid fifties PSI before then. Heck, once you
    get out on a hot road your "cold" 44 can wind up that high anyways.
    I would suggest that for any given combination of car, driver, tires,
    road condition, and loading, that people experiment around with a wide
    range of pressures high and low and determine what they like best for
    the type of driving they do. 80 PSI for a fuel economy challenge?
    Go for it, you aren't going to be slammin' rocks and chuckholes during
    that. 28 PSI for climbing that mountain sand road? Knock yourself
    out and have fun. 43.6021 PSI at 22.39 deg C for an autocross?
    Clearly you get off on significant digits and nobody should deny your
    enjoyment of same. Carry a hand pump in the car and get a mild upper
    body workout in the process, and see how many people stop and ask
    "are you okay?" while you're pulled off in a parking lot huffin' away.
    Tell them you're a gas/wheaties hybrid.
  17. MaxxMPG

    MaxxMPG Hasta Lavista AAA-Vee Von't Be Bach

    This link seems to have our answer about slip angle changing with pressure and tire load -
    It says...
    "Tire slip angles are determined by the load carrying abilities of the tire compared with the load they are seeing at a given moment and job they're being asked to do. Bigger tires or more air pressure reduce slip angles and smaller tires or lower air pressure increase slip angles. The more weight or cornering, acceleration and braking loads you're expecting a tire to carry, the greater its slip angles. In a car with a lot of weight at one end you have some options for reducing the slip angle of the tires on the 'heavy' end: larger tires and/or higher tire pressure and/or reducing roll stiffness."

    When talking "tire pressure", I sometimes mention the Ford Explorer and Chevy Corvair because they are two perfect examples of automakers setting an inappropriate pressure on the placard on the doorpost. In the case of the Corvair, they specified a rear tire pressure that was 75% higher than the front to narrow the slip angle to tame oversteer, and specified very low pressures to keep the ride cushy. The Explorer had very low pressures to provide a passenger-car ride in a truck.

    As we learn about tire pressure, we find out that higher pressures narrow the slip angle and therefore reduce wear during cornering. The narrower angle also explains why cornering is more direct and there is less perceived "mush". The only downside to the narrower slip angle is a more abrupt breakaway when tire adhesion limits are exceeded. You can also feel this as a sort of "skittering" at the rear when cornering on a rough surface, as the tires break away and then grab again in a more sudden motion than the mushy slide-n-grip that occurs at low pressure. This is not an issue for any good driver, as all of us have at some point in our driving history have felt their car "break loose" when cornering in rain/snow/icy surfaces, sometimes even doing so deliberately to test handling. Driving as hypermilers do, with attention to traffic and road conditions, and with knowledge of the capability of the vehicle, the narrower slip angle is a non-issue for handling and a great benefit for longer tire life.
  18. 2way

    2way Electromagnetic Wave [:-h

    I do believe!;)
    LOL. No argument from me on that!
  19. some_other_dave

    some_other_dave Well-Known Member

    Interesting... In the most recent issue of Grassroots Motorsports magazine, there is a discussion about, among other thing, tire pressures. It says things to support both sides of this argument. The author is "Woody Rogers of The Tire Rack". The article is written specifically about autocrossing, so it is likely that the grip that is primarily being discussed is lateral grip.

    "You must increase inflation pressure and reduce the slip angle of the front tires."

    "This ideal pressure changes with the tire, car, alignment, course, driver, ambient conditions and a host of other variables... If you plotted the relationship bdetween pressure and grip you'd see traction increasingly arcing upward as inflation pressure climbs. Eventuall a peak would be reached, and any further increase in inflatikno pressure will result in a subsequent loss of grip. It's the classic bell curve."

    "Hot front-tire inflation pressures on a stock front-wheel-drive car will often be... as high as 50-plus psi [sic] with typical street tires."

    "Going either higher or lower than ideal will reduce grip, right? In general, overinflating the ... tires to reduce traction will result in a slightly quicker initial breakaway characteristic. There can also be slightly less residual grip as the tire begins to slide... The underinflated... tire usually breaks away more slowly and with a slightly slower slide."

    And the last line in the article:
    "And whether you choose to over- or underinflate your... tires... be sure to reset your pressures back to the door placard settings for the trip home."

    Particularly worth noting, to me:
    - Increasing tire pressures does indeed reduce the slip angle. (Implied by the first quote.)
    - Best grip for "typical street tires" is in the high-40s low-50s PSI for hot pressures.
    - They still recommend placard pressures for street driving.

    My own interpretation of that last point is that they don't want the legal hassles of someone wrecking their car and then suing "because you told me to overinflate my tires"; autocrossing is (legally) a very much an at-your-own-risk affair, unlike street driving.

    Further, the best traction for typical street tires is significantly above most placard pressures, and is somewhere near the MAX sidewall rating for many passenger car tires. (Mine say MAX 44 cold pressure, which won't be above low-50s pretty much at any temperature my tires will ever see!) That very strongly implies that the MAX pressure will usually have better traction than the placard pressure. And that is what I have been contending all along--that MAX not only reduces rolling resistance, it also improves cornering grip over the placard pressures.

    Now, I know that you can get a car to spin out or slide straight on any given set of tires if you try hard enough or do something stupid enough. (BTDT, many many times--fortunately almost all on a track of some type!) But running on tires that have more grip than they "have to have" seems to give me some extra safety margin if I need it.

    As I said, though--that's my interpretation of the info in the article. The article is written by someone who works for a tire seller, not a tire maker. (I don't know if he used to work for a manufacturer or not, but Tire Rack has done a lot of tire testing over the years.) I feel that it supports my decision to run MAX pressure, but if you think your circumstance is different or it makes you worry to run them that high, set them to whatever you want.

  20. CapriRacer

    CapriRacer Well-Known Member

    I'm going to shorten the quote for clarity:

    It really would be nice if there was some documentation to back this up. The problem is there is a lot of words out there, but very little in the way of studies and graphs and published results.

    If I were trying to set up a simple test, I would take a group of tires - all the same size - and mount them on a vehicle and then record the time it takes to traverse a circular skid pad. If they all behaved the same way, then this would be a convincing study.

    Then if someone were to follow that up with a different vehicle (the more different the better) and a different set of tires (the more different the better), then this would help in clarifying whether this is true - and is it true everywhere / all the time, and if it isn't, where it might be true, and where it might not be true.

    Too many gaps!!

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