Changing tyre load ratings?
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Thread: Changing tyre load ratings?

  1. #1
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    Default Changing tyre load ratings?

    This thread:

    Tires For 14 X 6 1/2 Basket Weaves - '02 General Discussion - BMW 2002 FAQ

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    Post #13

    Pro tip #2: If you are keen to work your noggin, remember that load rating changes with tire pressure. Drive a car hard, the tire heats up, tire pressure increases, as does load rating. Many manufacturers will have charts for load ratings for a specific tire at different cold tire pressures. For high performance tires, this heat also increases grip (and thus stress on the tire), to a point.

    How true is this or is this a bunch of "hooey"????

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    Quote Originally Posted by 85Fuego View Post
    This thread:

    Tires For 14 X 6 1/2 Basket Weaves - '02 General Discussion - BMW 2002 FAQ

    Post #13

    Pro tip #2: If you are keen to work your noggin, remember that load rating changes with tire pressure. Drive a car hard, the tire heats up, tire pressure increases, as does load rating. Many manufacturers will have charts for load ratings for a specific tire at different cold tire pressures. For high performance tires, this heat also increases grip (and thus stress on the tire), to a point.

    How true is this or is this a bunch of "hooey"????
    Hi
    What a misconstruction of some facts. A pub type discussion based on a few facts obtained from a "mate" who works in the industry I would say. Basically a load of "hooey".

    The "load rating" is written down and is fixed for set conditions as specified. All tires get hot when they start to turn, we all know that as well as the manufacturers, so ? Drive hard they get hotter, so ? Perhaps truckies should just drive faster so they can overload more ? Some logic here ?

    jaahn

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    I disagree Jaahn. The following propositions are true (& thus not hooey):

    1. Load capacity of a tyre does change with tyre pressure.
    2. Tyre pressure increases with not just more gas but also gas at a higher temperature.
    3. Generally speaking, a tyre's grip under lateral load is a factor of retention of contact patch size & interlocking behaviour of the contact patch with the micro surface of the road. More pressure can help the contact patch resist adverse distortion & contact pressure changes.
    4. Moreover, a car's handling responsiveness has, as one variable, the slip angle of the tyres (see old post in technical section). The tautness of the sidewall is the major factor in the slip angle a tyre operates with. Increased pressure almost always increases sidewall tautness & lowers a tyre's slip angle & thus, ceteris paribus, increases its handling responsiveness. This fact is behind the capacity to alter a car's handling profile by playing around with tyre pressures (especially front/rear differences).
    5. Further, as a tread heats up, the compound usually becomes more flexible & interlocks better with the road surface, thus gripping better.
    6. Most structural failures are bonding failures of steel belts to the rest of the carcass. The problem is a combination of localised heating & mechanical flexion (also a cause of heat). Primary bonding failure cause is low pressure (not high) or very high speed rotation causing diameter growth by inertial force. One can also cause the carcass to rupture by over pressurisation (by either over-gassing or over-heating).

    Of course, despite all of this, one has probably already set a road car up with cold pressures in mind. So it should already be near optimal (by reference to one's desired behavioural criteria) prior to any heat induced pressure increase. Mind you, if one has simply used the manufacturer's pressure recommendations then more benefits of the tyres heating a bit should manifest themselves (not that the sort of person who just leaves pressures as the handbook advises is likely to much notice).

    The only bonus benefit from the above list even for a deliberately set up car is increased compound flexibility - with a caution that that improvement plateaus & then declines if the compound is heated past a certain point & too many of the volatile "plasticising" compounds are lost. It can also overheat & soften to the point of losing mechanical strength & "chunking". This can occur is sustained vigorous road use, track days & so on.

    cheers! Peter
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    Quote Originally Posted by 4cvg View Post
    I disagree Jaahn. The following propositions are true (& thus not hooey):

    1. Load capacity of a tyre does change with tyre pressure.

    cheers! Peter

    I agree 100%.

    Sometimes my trailer gets worked harder than it should and in those situations I go by the look of the tyre more than the actual number on the gauge.
    That might see pressures of 70 psi, but it gets rid of that flat spot and deformation of sidewall that generates the terminal heat, so in my opinion/experience increasing pressure increases the load carrying potential of the tyre.
    The little number on the sidewall is not made from structural lettering, so I dont pay it too much credence except when selecting tyres. The shape (hence performance) of the sidewall is everything.


    Once the heavy load is shifted, the pressures get put back to 30 something or the empty trailer bounces all over the road. Thats about the only time I get to see it when driving the scenic rx4.

    Jo

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    Default Hmmmm

    Hi
    I thought I was answering a statement or question "Changing tyre load ratings?"
    They are set down by the manufacturer for car tires and are not AFAIK changeable by the operator Perhaps I was remiss taking it as a bald statement only.

    Indeed I agree the load carrying capacity of a tire is dependant on the air pressure. The maximum air pressure is set by the manufacturer in relation to the tire construction. There are no normal car tires with a cold pressure rating of more than 36PSI AFAIK. Commercial rated tires or extra load etc can be much higher.

    Jaahn
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    Way overinflated tyres have catastrophic failure, sidewall bursting, if they hit a bump.

    The instantaneous pressure when hitting a bump will increase the pressure P1V1 = P2V2.
    Throw in Temperature as well from cold 70psi to hot at ??? psi then becomes 0 psi when the tyre fails

  7. #7
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    Quote Originally Posted by jaahn View Post
    There are no normal car tires with a cold pressure rating of more than 36PSI AFAIK.
    Nonsense…
    51psi is what my tyres have written and they are bog standard.

    jo

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    Default Pressures !

    Quote Originally Posted by jo proffi View Post
    Nonsense…
    51psi is what my tyres have written and they are bog standard.

    jo
    Hi Jo
    For the sake of advancing my education I would be obliged if you would put up the exact size description of the tire with load and pressure rating. They sound like light commercial tires to me not bog standard car tyres. However I am always willing to learn.
    Jaahn

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    Ok, in a minute,

    Mums i30 has a number of 44 on the Oe tyres.

    Jo

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    Nothing odd….
    High Performance Sports Tyres | Kumho ECSTA SPT KU31 | Kumho Tyres Australia

    kumho ku31, 225/60/r16 98v. load 760 kg max pressure 51 psi

    Jo

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    Quote Originally Posted by jaahn View Post
    Hi
    I thought I was answering a statement or question "Changing tyre load ratings?"
    They are set down by the manufacturer for car tires and are not AFAIK changeable by the operator Perhaps I was remiss taking it as a bald statement only.

    Indeed I agree the load carrying capacity of a tire is dependant on the air pressure. The maximum air pressure is set by the manufacturer in relation to the tire construction. There are no normal car tires with a cold pressure rating of more than 36PSI AFAIK. Commercial rated tires or extra load etc can be much higher.

    Jaahn
    Jaahn, I rather think that you were indeed remiss in how you interpreted 85 Fuego.

    A usual analytical technique for ascertaining what is mean't by someone's turn of phrase is not to look at that turn of phrase alone but at the context of its use. In this case, it was surely obvious that he would not be entertaining the proposition that a formal load rating as set by some official agency could be changed by individual fiat by driving in a manner which heats one's tyres.

    So, what else could reasonably be taken to be what's meant so that the question being asked (perhaps by deployment of ill-chosen terminology) can be addressed?

    How to tell? Look at the whole utterance. In context, it is pellucid that 'load rating' is being used to denote load carrying capacity. Thus my answer, which I hope was of some use to 85 Fuego. I could, I suppose, have added a final observation: 'Mind you, I think that your source could be more careful with language & would have been better advised to speak of load capacity, not load rating.' I chose not to divert from the substance of the question.

    On Jo's over-pressurising practice:

    1. The manufacturer's "x kg at y max. pressure" is indeed a cold inflation figure &, obviously, if one inflates to that & then drives on a hot day then both pressure & load capacity will rise.

    2. Also obviously, a tyre can be inflated to such a pressure (by gas quantity or heat) that it ruptures. Just where this point would be is at least a function of two variables. First, as correctly observed, the stresses inflicted on the tyre by road imperfections would affect that point. But, second, another influence is tyre quality & type. On these latter, tyre, features, I have in mind such things as: low, low, price budget tyres versus tyres of better structure design & implementation; &: the number of sidewall plies respectively. An extra ply is the major feature of so-called 'XL' variants of normal car tyres. (For what it's worth, I always recommend these unless comfort & fuel economy dominate one's priorities.)

    3. Finally, bear in mind that, whatever the figures on the sidewall, tyre manufacturers have built in a safety buffer. How big that would be, would vary across manufacturers.

    cheers! Peter

  12. #12
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    Unfortunately another person was killed yesterday by a burst tyre

    https://au.news.yahoo.com/qld/a/2630...s-at-qld-mine/

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