Before you over inflate watch this.

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

  1. CapriRacer

    CapriRacer Well-Known Member


    Perhaps this will help:

    [​IMG]

    The rubber penetrates the road macrotexture and more force is generated before the tire starts to slip.

    Once the tire starts to slip, the peaks in the road surface macrotexture tear off bits of rubber, which generates more force than could be generated by friction alone.

    You can see this when you watch a NASCAR race and they talk about the "marbles" that accumulate just outside the racing groove. Those "marbles" are the bits of tread rubber that has been torn off.
     
  2. worthywads

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

    I'd have to agree, seems pretty obvious.
     
  3. CapriRacer

    CapriRacer Well-Known Member

    This may be way unscientific, but I counted the "notches" in the center grooves. I got 7 on one and 5 on the other. So I think the footprints are not the same size.

    Just as a reality check - does it make sense for the footprints to be the same size? None of the others are.
     
  4. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    No, it doesn't make sense for them to be the same. But for a valid comparison, they need to be the same scale. The size of the tread blocks can't change. The number of blocks touching the road is what changes.

    I count 6 on top and 5.5 on the bottom.

    [​IMG]
     
  5. Shrek

    Shrek Kaizen Driver

    So what does this have to do with tire pressure then? this is more about rubber hardness and its effect on grip. Tire pressure changes the area and tire deflection. Smaller area would give higher force and more penetration of the rubber, giving a hard tire more more grip.
     
  6. CapriRacer

    CapriRacer Well-Known Member

    I was attempting to explain that the following was incorrect:


    Area is important and is involved and it is because the tread rubber penetrates the road surface macrostructure.
     
  7. dare2be

    dare2be Well-Known Member

    Precisely, which to me explains why higher pressures create better handling. Higher pressure = smaller footprint = greater downard force per square inch = better tread penetration with road surface = better grip / higher coefficient of friction.
     
  8. dare2be

    dare2be Well-Known Member

    I'm on the fence on the scale issue...notice the vertical scale is roughly identical but the horizontal scale appears to be compressed. Playing devil's advocate, perhaps the added pressure and thus less "malleableness" of the tire is causing less flattening out of the tread, thus explaining why the individual treads look wider. Or the higher force per area is compressing the treads together. I guess either thought is a "stretch". Sorry, couldn't resist.
     
  9. Shrek

    Shrek Kaizen Driver

    OK. I agree that at the boundary between sliding and not sliding the relationship between friction and downforce is not so simple ('nonlinear') and can be area-dependent.

    I would say it is the other way around. The road penetrates the rubber, not opposite around.
     
  10. 2way

    2way Electromagnetic Wave [:-h

    Wow... what a lively discussion and one that, I for one, am thankful has been digressed into this. BTW, thank you to the OP for bringing such a safety concern to our attention.
    I don't think it could've been much better said.
    Absolutely! It's all about a balance.

    For me, I use a pressure that is between OEM Vehicle spec and OEM Tire spec. My choice. I find that even then, I run into situations where I have reduced traction when forced to accelerate. Going to max sidewall for me is just unsafe. For a die hard hypermiler, you probably won't encounter such situations very often;) In my mind, there is no doubt that maximizing tire pressure will result in improved fuel economy. So will using a skinnier tire. Coefficient of friction;)

    For the discussion on "high speed"... I won't get into the definition debate. But, there used to be a rule of thumb for increasing tire pressure (2-4psi I think) for sustained speeds of 70-75 MPH (I forget which) or higher.

    The other factor that has received little mention is load. Obviously, you need to increase pressure with a heavier load. IMHO, a lightly loaded, one occupant vehicle shouldn't be run at max sidewall pressure other than for purely fuel economy reasons. Without the necessary traction (even at 55MPH) if a driver executed an emergency hard turn, he/she would find themselves continuing in a straight line. That nasty coefficient of friction thing... which reducing it improves economy... but, also reduces traction. If you want cornering/handling improvements go to a wider tire or larger rim/low profile combination. Turning ease does not = handling improvement... it just means you've reduced that darn coefficient of friction thing;)

    Along those same lines - the oft quoted officer.com article - some things seem to be being overlooked when viewing that article. You must consider how "loaded" a fully equipped law enforcement vehicle is and the suspension tuning the factory provides them with (just take a peek at their sway bars). Radios, roll cage, supplies, firearms, cameras/recorders, speed detection devices, auxiliary lighting, etc. all add vehicle weight that are not necessarily accounted for in the manufacturer's inflation recommendation. You also have to consider that "Pursuits and Code 3 responses are not normal driving conditions." Additionally, many Law Enforcement vehicles utilize "pursuit rated" performance tires, not the common consumer fare. In Goodyear's case, Eagle RS-A's with higher load/speed ratings than their consumer counterparts.

    What Sgt. Storton leaves out in his performance, hydroplaning, and tire wear paragraphs (but, mentions in his opening paragraph) is including the caveat that this largely applies to tires that are under inflated &/or carrying increased loads. Nothing we haven't heard before. The more probable reasons for 50psi on their trainers are that it does help reduce their tire wear (tons of cornering) and also helps them out inducing a skid on the pad.

    You might want to take a look at: Police Tires Proper Tire Inflation
    Note Goodyear's emphasis on "NEVER EXCEED THE MAXIMUM INFLATION PRESSURE LISTED ON THE SIDEWALL OF THE TIRE." and how the 44psi sidewall rating came about.
     
    Last edited: Jul 9, 2008
  11. lightfoot

    lightfoot Reformed speeder

    I was wondering the same thing. As I recall, cars used to have a higher recommended pressure rating for greater loads, but neither of my cars does. Anybody know why this changed? Seems especially strange for the Subaru which could carry a lot of additional load.

    Also, the Goodyear article mentions 35psi and 44psi but the Nokians on my Subaru are rated for 51psi?
     
  12. kwj

    kwj I hypermiled this

    "Without the necessary traction (even at 55MPH) if a driver executed an emergency hard turn, he/she would find themselves continuing in a straight line. That nasty coefficient of friction thing... "

    Which sounds legit, until you consider the dynamics of a radial tire. The higher pressure keeps the sidewall from flexing as much in a turn, which keeps more tread in contact with the road, and less sidewall. Actually this will increase traction in an emergency turn. Of course, assuming decent tread to begin with.
     
  13. some_other_dave

    some_other_dave Well-Known Member

    That was true in the past, but it is almost entirely a thing of the past. However, it got entrenched in the "conventional wisdom", which very few people seem to question.


    That may be a bit of an overly-strong statement. It might better be said that "This community has observed many cases where treadwear is not an issue, and has so far observed none [or few??] where it is an issue." Also, note that in many of those observations, the tires have been run at somewhat more than the max sidewall pressure, which would (presumably) exaggerate uneven wear if high pressure was a cause of such.

    You can view that as proof, or merely anecdotal evidence.

    I know that I feel just fine running my tires at the max sidewall pressure, and also recommending that other people run their tires at max sidewall.

    -soD
     
  14. kwj

    kwj I hypermiled this

    Lightfoot, I thought I mentioned the problems the Exploder had with such a low recommented tire pressure (that most people rarely check) and then the drivers put in the kiddies and all the vacation gear and go out and drive 80 MPH on a hot road, and wonder why their tires separated and blew out.

    The Volkswagon Beetle does have a placard that increases tire pressure with load. Their placard is in the fuel filler cap, meaning the people see it each time they fill up. Now, they may not know what that means.

    Shouldtn't people know something about the cars they drive? Who's responsibility is it to make sure people understand these things? The person who is driving.
     
  15. lightfoot

    lightfoot Reformed speeder

    I agree with you 100%.

    My thoughts were more along this line: if the auto mfrs recommend a higher pressure if the vehicle is loaded to its maximum load rating, that would mean that the higher pressure could not in itself be considered "dangerous". That's all, a bit peripheral to the current line of this thread.
     
  16. kwj

    kwj I hypermiled this

    Lightfoot, great minds think alike, which makes me wonder why I always think differently?

    Actually, your train of thought is very interesting. I was wondering along those lines. It would seem to me that if you pumped your tires up to placard, then proceeded to load up the car (SUV, etc.), what physically happens to the tire? More tread in contact with the road, better traction, more rubber pushed into all those little knooks and crannys? Maybe even an increase it tire pressure - yes?

    So, why, at that point, is it better (safer) to increase the tire pressure prior to loading it up? Then comes the question as to why some truck tires placard at 70 PSI or more? Wouldn't that just mean less traction - according to what some are telling us?
     
  17. 2way

    2way Electromagnetic Wave [:-h

    It is legit... from personal experience (among other things). If you've reduced the friction between the tire and the road by over inflating, it doesn't matter whether the sidewall flexs or not.... you've got less rubber on the road & less traction caused by the over inflation. The same forces that cause improved fuel economy with overly inflated tires work as a detriment to traction. Turning the wheel too sharply or at too high a speed and the car's inertia overcomes the friction between the tires and the road. You wind up taking the path of least resistance, sliding in your original direction, until that inertia is no longer greater than the friction between the tires and the road. If you had ever hit a patch of black ice, you'd totally understand. Overinflation has the same effect. It will also negatively impact your stopping distances in a panic braking situation. You need to balance FE vs. safety.

    The whole point of my observation on the officer.com article was that the reason higher inflation is beneficial to those particular vehicles is that they are operating under a laden condition and a higher pressure is warranted to offset the load squish already on the tires. They are already in a condition where the sidewall is flexed. Throw 250-500lbs additional weight in your trunk at vehicle spec pressures and see what happens to your sidewalls. (Note, I'm not saying the vehicle manufacturer's always get the spec right either. Sometimes personal real world experimentation goes a long way;))

    As for center tread wear being a thing of the past... the OEM Yokohamas I took off from my car's previous owner certainly said otherwise. The idea that tires won't "dome" is ridiculous... steel belts flex. It isn't like riding on something completely solid. They're woven and designed to flex.
     
  18. 2way

    2way Electromagnetic Wave [:-h

    The sidewalls flex. More rubber in contact w/the road. More friction, causing more heat to be generated.
    Among other things, trucks carry a higher load per tire.
     
  19. kwj

    kwj I hypermiled this

    What about trucks that are just used to satisfy the I'm-bigger-than-you syndrome, that never see a load greater than a coat of wax. There is nothing in their manual to decrease the tire pressure.

    "If you've reduced the friction between the tire and the road by over inflating, it doesn't matter whether the sidewall flexs or not.... you've got less rubber on the road & less traction caused by the over inflation."

    Of course that statement assumes that overinflation causes a reduction in friction. First, define "overinflation." Does your definition include anything over placard up to Max Sidewall?

    Second, we just pointed out how having increased tire pressure minimizes sidewall flex which keeps more tread on the road in a turn.

    Third, as the Police video points out, a car with tires inflated beyond placard, hydroplanes at a speed well above any national speed limit (except for two roads in Texas). A car with tires only inflated to placard can hydroplane at only 45 MPH passing through a puddle of less than 1/8th inch deep.

    Where is that better than what we are promoting? A car with tires that are otherwise in good shape, inflated to Max Sidewall is not going to continue straight down the road when the driver turns the steering wheel left or right on a dry road, at hypermiling speeds. Only a car at high speed or with underinflated tires could do that.

    As we all know, if you are driving down a dry road with any car, tires underinflated, overinflated (by my definition) or at placard, if you've got to fully slam on the brakes, panic style, the tires are going to slide. This is why defensive drivers learn to steer out of trouble rather than simply slide into another. ABS is fantastic in that it stops the slide in split second increments, allowing the tires to better maintain traction. Without ABS, the tires slip, generating more friction as they slide over the pavement, which heats up the rubber at the contact surface, forming many tiny balls of rubber which pretty much causes a loss of control.

    Safety point - regardless of the pressure in your tires, don't drive in such a way that emergency manuvers are a necessary part of your ordinary driving experience. Drive in total communication with your environment, such that regardless of the situation presented by surprise, that you have time to react safely. This means slowing down at night, or in the rain, and leaving even more of a buffer between you and the cars in front. Then you won't have to ever worry about tiny rubber balls under your tires, or whether you have more or less of a tire print on the highway. Your only concern will be to achieve increased fuel economy by ensuring your tires are at Max Sidewall pressure. You will know that hydroplaning is no longer your concern, and that if you have to take a turn, more of your tire tread is kept on the pavement to ensure you don't slide while turning.

    And finally, how many accidents have you heard of, where the driver lost control in a corner and slid off the road? Tons. How many of those cars had their tires above placard, and up to Max Sidewall. Unfortunately, you can't get an answer out of most of them.
     
  20. some_other_dave

    some_other_dave Well-Known Member

    Very interesting, and (the implication is) directly counter to my experience. As far as I can tell, pressing up my tires to their MAX rating has not decreased the friction between them and the road. To the contrary, it has increased the lateral grip very noticeably over what the car had at the placard pressures. And I can generally tell when a tire is down near the placard pressures, because that corner of the car starts to feel "greasy" or "floppy" when pushed hard.

    I am not as confident in the increases in straight-line traction, though, as I am more attuned to feeling lateral G-forces than longitudinal ones. (I'm talking about autocrossing, here--not driving like a sane human being on the street.)

    The above has been my experience with quite a few sets of street tires, though in some of those cases I did not go all the way up to the MAX pressure. However, my traction was improved in all of those cases over running the placard pressures.

    Now, if you were saying that you inflated your tires to double the MAX rating (for example) and the traction was significantly reduced, I can easily see that. But you appeared to me to be implying that inflating to the MAX pressure would cause a reduction in grip, which I doubt. I have yet to see (on my cars) any case where a street tire grips better at placard pressures than at the MAX pressure.

    -soD
     

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