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Thread: dare I ask about tyre pressures?

  1. #1
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    dare I ask about tyre pressures?

    Hi all,

    Yes, the good old tyre pressure discussion. I have been reading up on it over the past years, experimenting myself as I go along and somehow I still feel that there is a lot of non-scientific stuff floating around this subject. Mind you, I am NOT against these methods but I would like to try to really get my head around this subject once and for all, where possible.

    I have seen plenty of tyre size / pressure opinions out there. Heavy laden vehicles with 40 psi on tar, and less in sand. Helpful people post their tyre pressures online but they are all related to their vehicle setup and tyre size. I am not sure how one get's to the basis of these pressure though. Some use the 4psi rule (higher with tougher tyres it seems) and others simply follow someone else and find that it works for them.

    Having modified my P38 to a point where nothing is standard anymore (read: weight due to all the additions, tyre width, diameter and even offset and LT instead of automotive.) I have found it pretty hard to determine a starting point. After much googling I came across this a while back: Optimum Tyre Pressure - On or Off Road 4x4 Driving Land Rovers

    I thought this calculation makes sense so I tried it on my vehicle. After having put the car on a scale I topped out at 2500kg's (empty) and the weight distribution was almost perfectly 50/50 by chance. Putting the numbers for my tyres into formula I ended up with 2.5 bar (or 36 psi roughly).

    With my tyres inflated accordingly I observed that the sidewalls still bulge more then I would expect them to and I have quite a few of my MT thread blocks in contact with the road. A good thing since I do not want to skid of the road but I feel like I am not running as economically as I could and the flex in the sidewall makes me worry about long high speed runs in high ambient temperatures.

    so, in short, my question is: Given any type of car and any size/type of tyre, how does one determine the optimum tyre pressure? Starting with just the on road pressure and we'll take it from there

    Cheers!
    -P

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    G’day Prelude

    Nobody bit so I wondered if an idea out of left field might add to the discussion. Clearly tyre temperature is involved here and I would expect tyre manufacturers to be able to quote an operating temperature. Some I looked up on the web do. Too hot and they break, too cold and they are likely overinflated.

    The heat comes from the road temperature (racing circuits monitor track temperature) and tyre flex.

    The 4psi rule gives a hint but is a bit awkward to carry out in practice. I note that ebay offers lots of aftermarket tyre pressure and temperature sensors. Some of these can be fitted through the valve stem, others replace the valve stem. Not too many dollars.

    Even with only pressure it should be possible to estimate temperature changes.

    So I wonder if a bit more instrumentation might help to understand what is going on?

    Cheers

    Graeme

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    Hi Graeme,

    Yes, it seems people are sick and tired of talking about tyre pressures

    I have in the mean time found these two documents, the first is a method that I think is more commonly known; comparing load ratings and pressures and they have been put neatly into a table here

    The second method I found is a lot more complex but seems to be actually quite a decent scientific study of the matter here I have had a quick go in calculating my tyre pressures and they seem to correspond to the information in those two documents.

    When and if I find more info, I'll get back to you guys In the mean time, feel free to chip in!

    Cheers,
    -P

  4. #4
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    The basic problem, and why this subject attracts so much attention is that there is no such thing as "optimum" tyre pressure.

    Tyre pressure is a compromise between ride, adhesion, handling, fuel consumption, and tyre life. You can work out the optimum pressure for any one of these, but it is fair to say that all these "optimums" will be different, so you will have to compromise. And this is where the arguments start. And keep going, since there is no such thing as a single answer!
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    Quote Originally Posted by prelude View Post
    so, in short, my question is: Given any type of car and any size/type of tyre, how does one determine the optimum tyre pressure? Starting with just the on road pressure and we'll take it from there

    Cheers!
    -P
    There is No single answer to this question as different people want different things from their tyres.
    Traction, fuel economy, tire longevity and comfort are what you have to consider when choosing what pressure to run in your tyres and everyone has different expectations.
    All you can do is to run the tyres at whatever pressure that you are comfortable with But the general rule is higher pressures for bitumen and lower pressures for dirt roads and sand work.
    There is NO "Definitive" answer to your question.
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  6. #6
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    Thanks for you replies

    I do agree that there is no single answer and optimum means different things for different tyres or people. I have already accepted that What I am looking to do though is find the science of a road (tarmac) tyre as a basis for the rest. Under and over inflation results in incorrect wear of the tyres as we all know but a 0.1bar or 1psi difference won't do much harm so there will never be a pressure set in stone. When changing tyres from the factory standard and having modified your car to weigh quite a bit more, one needs a basis I guess to start from. The rest can be figured out later.

    Thus far I have come up with presures that SEEM low to me. I run around 2.6 bar in the back and 2.4 in the front for tarmac and I weigh in at around 2700kg fully loaded. The temperature of the tyres is good, handling is fine and I guess this is just all as good as it can get, but I am a bit puzzled by the relatively large contact patch of the tyres. Also, I do show a bit of a bulge. I would expect highway pressure to be higher but I can't be sure, that is why I thought to start looking for the science behind tyre pressures

    Thanks for you input!

    Cheers,
    -P

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    I suspect there is an maximum operating temperature though, and I interpreted this to be in the back of prelude’s mind when he mentioned side wall flex. Too much of that at higher speeds may well influence economy but it can surely make the tyres go bang at higher speeds.

    How influential are tyre working temps, when below the maximum, on comfort, economy and adhesion?

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    Quote Originally Posted by grh View Post
    I suspect there is an maximum operating temperature though, and I interpreted this to be in the back of prelude’s mind when he mentioned side wall flex. Too much of that at higher speeds may well influence economy but it can surely make the tyres go bang at higher speeds.

    How influential are tyre working temps, when below the maximum, on comfort, economy and adhesion?
    I would think that extreme temps cold/hot would have quite an influence on a tyres performance and pressure But ambient temps above 0C and under 45C wouldn't have too dramatic effect on a tyre, The pressure will likely fluctuate a few degrees as it does when the tyres are cold and then heat up But nothing to be super concerned about.
    One thing that I do is to put my hand on the tyre after I have travelled 50-100km to see how warm it is and if it is too hot I adjust the pressure and check again in another 50-100km.
    An excessively warm tyre is telling you that there is a problem
    You only get one shot at life, Aim well

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    Another perspective... I adjust tyre pressures without science, physics or calculation. I do it by feel.

    IME driving a Defender is all about feel. I know this can apply to all vehicles, but there’s something particular about the handbuilt quality of Defenders that rewards this approach. Over many years Ive tuned in to my Defenders to the point of being able to sense, feel, hear and diagnose slight changes, issues, shifts in how the Defender is feeling. Defenders often feel more like organisms than machines. Afterall they are handbuilt entirely out of modified raw materials from nature. Their steel, aluminium, rubber bodies flex, change and adapt to prevailing conditions, temperatures and driving style. They are carapace full of fluids, flexing joints and dare I say it, individual character.

    ...all of this comes into play when it comes to intuitively adjusting tyre pressures. The impossible optimum or balance to measure, between ride, handling, traction and durability can best be calculated intuitively, by feel and experience. My 235/85R16 AT’s invariable end up being inflated between 36-45 psi depending on feel, load and prevailing conditions. Ride quality, visual sidewall bulge, number of lugs on the road, traction in certain conditions and variable temperatures all contribute to the correct psi outcome, not the other way around.

    Just like everything else about a Defender It’s not something that can be solely measured IMO.
    This can apply to all living machinery of course, but in a Defender it’s even more akin to being alive.

    Cheers, Z

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