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Thread: What's the problem with tire pressure?

  1. #31
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    Quote Originally Posted by lkchris View Post
    It's always interesting to see reference to "newer, higher pressure tires."

    Correct me if I'm wrong, but in most cases this is the difference between radial and bias-ply tires.

    In most cases, the tire specified for your bike is one or the other but never both. If that's the case, there simply aren't "newer, higher pressure tires" applicable to your bike.

    (One exception may be Conti's new radials directed towards Airheads and maybe old Ks, but haven't studied that. They have just been introduced this summer.)

    And, the numbers on the sidewall are completely meaningless except that they indicate the maximum pressure safe to use in this tire. This has NOTHING to do with BMW's or the tire manufacturer's recommended pressure for your particular bike and load conditions.

    Except for the following, that is ... A recent presentation at our dealer by a Metzeler tire rep featured the notion that if your goal is to maximize tire mileage, i.e. tread life, then you should indeed run your tires at the maximum pressure indicated on the sidewall. It's printed a little less strongly in Metzeler brochures, too. This of course, is NEVER the basis for BMW's pressure recommendations for your bike and most obviously may involve some handling and/or comfort and/or performance compromises. I'm not doing it, as my goal is fun associated with riding and for sure not carefully saving every penny and diminishing the fun. I mean, if it's about only tread life, just never ride.

    If you've a current model BMW, recommended tires by brand are listed at BMW's website. These charts don't go very far back as regards discontinued models IIRC. There is, not completely coincidentally, sometimes not sizes for older bikes in the latest/greatest tire models, and the older bike guys then use the older models anyway. The notion that perhaps tires (brands) not on the recommended list somehow require completely different pressures is a bit of stretch in logic, too, I think. Look for that brand to publish something instead.

    Bottom line, I think, is that BMW's recommendations are simply fine and are good for a long time. If a tire manufacturer will publish in writing or on its web page that they indeed have different recommendations for you, then pay attention. There's not a lot of data provided by various internet contributors to back up their claims when it comes to varying from BMW specifications, and they should direct you to data rather than their opinions and unscientific observations. I know for sure that I've never seen specifically published by a tire manufacturer that pressures recommended by BMW in, say, the 1990s or 80s or 70s have been superceded by "new technology." I'd love to be directed to it.

    You can also rationalize that somehow BMW's specifications don't apply specifically to you because you must weigh something different than their average rider or something like that, but I'd instead conclude their figures can be recognized as covering normal variations. BMW's figures are quite fine and it's not really about demonstrating that you know more than they do, is it? When it comes to "voice of experience," nobody tops BMW.
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  2. #32
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    Quote Originally Posted by pffog View Post
    Remember nothing is free, yes higher pressure lowers rolling resistance and heat, and makes tires wear longer, BUT also reduce traction due to less heat in the tire and smaller footprint on the pavement.

    Pick your poison!
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  3. #33
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    Quote Originally Posted by chewbacca View Post
    There is an old timers rule of thumb and I'll probably screw it up. Set the pressure cold. Go ride for maybe half an hour. Check the pressure. If it goes up about 10% your good. More than 10%, add some air and try again. Less than that, let some air out.

    IMO, to get good, consistent readings you have to measure cold. Also, don't use the TPS readings for anything else than to determine you have leak.
    no screw up, that 10% gain is the target. That translates to a ~ 3 to 4 psi increase.
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  4. #34
    rangerreece rangerreece's Avatar
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    More importantly

    I've been running PR3's at 38 front and 42 in the rear on my R1200RT down here in lower alabama, ambient temperatures have been running in the low 90's and for the life of me I can't figure out what tire pressure is best for keeping the love bugs from hitting the bike....
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  5. #35
    Ed Kilner #176066
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    The partial pressure of nitrogen in tire air is much greater than oxygen. See http://www.chem.wisc.edu/deptfiles/g...Tx96/tx96.html for details. A bit of water will have some vapor, but I don't think it will be significant. Some chemical engineer might comment.

    Max sidewall pressure applies when the carcass and air inside are at 60F or 20C at sea level. Other temps or altitudes will require some adjustment.

    Gotta be a nerd (me?) to do the math ... My RT will do temp calculations, but altitude, must be negligible or BMW would have done that too?
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  6. #36
    Cam Killer marchyman's Avatar
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    Quote Originally Posted by BMW Triumphant View Post
    at 60F or 20C at sea level.
    68F, not 60F. 68F = 20C

  7. #37
    not so retired henzilla's Avatar
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    Quote Originally Posted by rangerreece View Post
    I've been running PR3's at 38 front and 42 in the rear on my R1200RT down here in lower alabama, ambient temperatures have been running in the low 90's and for the life of me I can't figure out what tire pressure is best for keeping the love bugs from hitting the bike....
    well played
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  8. #38
    Registered User chewbacca's Avatar
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    Quote Originally Posted by kantuckid View Post
    Not that I'm taking potshots at the "theory" above but obviously much is missing with the process as "go take a ride" for a rider in the desert, in the summer would far different than the result for "go take a ride" in the winter on say, the US/CA border.
    As for Osbornk & his N source, I just googled the subject & the geologist whatever website says while N hasn't changed appreciably, oxygen levels have changed. Now , do we have a "tire pressure expert" to step forward & debate the effects of more or less oxygen in a MC tire?
    Take your potshots, but also think a bit. Why it works has to do with operating temp of tire. Operating temp is what you are shooting for. In the desert of AZ you will get hot quicker and have to let air out. In the cold you will have to add air to get it up to operating temp. As the man said you already have 78% nitrogen in air, but that universal gas law really doesn't tell you a whole lot. It is under perfect conditions for a perfect gas.
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  9. #39
    Ed Kilner #176066
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    Quote Originally Posted by marchyman View Post
    68F, not 60F. 68F = 20C
    Yes, thanks for the correction.
    Ed
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  10. #40
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    Quote Originally Posted by PGlaves View Post
    Yes, moisture in a tire can theoretically add to pressure fluctuations. If the interior tire temperatures reach the point that water droplets are vaporized - something in excess of 212F - the steam can increase pressures more than simple heating of air. At the extreme, triple digit ambient temperatures and sustained Interstate speeds my rear tire gets to somewhere between 150 and 160F. No steam.

    If concerned, put a drier on your compressor.
    This is from the nitrogen institute it is a direct quote

    Fundamentally; air, oxygen and nitrogen will all behave exactly the same in terms of pressure change for each 10 degrees of temperature change. However temperature alone is not the whole story.

    Ambient air contains moisture, nitrogen does not. If moisture is present it contributes to a greater change in pressure simply because at lower temperatures water condenses to become a liquid. The liquid form of water occupies very little volume and contributes only a negligible pressure to the tire. But at higher temperatures, such as those in a running tire, water evaporates inside the tire and becomes a gas which increases pressure in the tire.

    Ambient air contains about 21% oxygen. Oxygen?s smaller molecular size allows it to permeate through the rubber of the tire. By inflating with nitrogen, which is much less permeable than oxygen, the pressure changes due to oxygen loss are greatly reduced.

    The racing industry is correct; nitrogen is more predictable. Because nitrogen is dry it has no moisture to contribute extra pressure changes with temperature. Because nitrogen permeates out much slower than oxygen pressure changes due to that leakage are almost eliminated compared with ambient air.

  11. #41
    Registered User chewbacca's Avatar
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    Quote Originally Posted by Gonzo49 View Post
    This is from the nitrogen institute it is a direct quote

    Ambient air contains about 21% oxygen. Oxygen?s smaller molecular size allows it to permeate through the rubber of the tire. By inflating with nitrogen, which is much less permeable than oxygen, the pressure changes due to oxygen loss are greatly reduced.
    I love it when motorcyclists talk science or at least what they believe is science. Molecular size of molecules in a gas is pretty damn small. The difference in size between O2 and N2 means practically nothing. Actually Nitrogen's mass is less than Oxygen so it is probably smaller, depends a bit on the size of electron orbits and nucleus components. Both are made up of a whole lot of space. The only thing in this entire discussion that has any merit is IMO the amount of a water vapor in the tire. For the average rider, you are better off just measuring the "air" pressure in your tire and adjusting it in whichever brand of voodoo you subscribe to.
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  12. #42
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    Quote Originally Posted by chewbacca View Post
    I love it when motorcyclists talk science or at least what they believe is science. Molecular size of molecules in a gas is pretty damn small. The difference in size between O2 and N2 means practically nothing. Actually Nitrogen's mass is less than Oxygen so it is probably smaller, depends a bit on the size of electron orbits and nucleus components. Both are made up of a whole lot of space. The only thing in this entire discussion that has any merit is IMO the amount of a water vapor in the tire. For the average rider, you are better off just measuring the "air" pressure in your tire and adjusting it in whichever brand of voodoo you subscribe to.
    Please see the following link to see the entire explanation

    http://www.getnitrogen.org/pdf/graham.pdf

    I'll give you the basic here.

    "O2 "permeates" approximately 3-4 times faster than does N2 through a typical rubber, as is used in tires, primarily because O2 has a slightly smaller effective molecular size than does N2."

    I try to take advantage of every technological breakthrough. I'm putting my life on two tires with very small contact areas. I'll make use of anything that will increase my safety and the safety of my passenger. It's also why I've chosen to ride a BMW, safety, desgn,technology etc rule.

  13. #43
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    Quote Originally Posted by chewbacca View Post
    I love it when motorcyclists talk science or at least what they believe is science. Molecular size of molecules in a gas is pretty damn small. The difference in size between O2 and N2 means practically nothing. Actually Nitrogen's mass is less than Oxygen so it is probably smaller, depends a bit on the size of electron orbits and nucleus components. Both are made up of a whole lot of space. The only thing in this entire discussion that has any merit is IMO the amount of a water vapor in the tire. For the average rider, you are better off just measuring the "air" pressure in your tire and adjusting it in whichever brand of voodoo you subscribe to.
    Please see the following link to see the entire explanation

    http://www.getnitrogen.org/pdf/graham.pdf

    I'll give you the basic here.

    "O2 "permeates" approximately 3-4 times faster than does N2 through a typical rubber, as is used in tires, primarily because O2 has a slightly smaller effective molecular size than does N2."

    I try to take advantage of every technological breakthrough. I'm putting my life on two tires with very small contact areas. I'll make use of anything that will increase my safety and the safety of my passenger. It's also why I've chosen to ride a BMW, safety, desgn,technology etc rule.

  14. #44
    rangerreece rangerreece's Avatar
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    Define science and who has exclusive membership to that club.

    "I love it when motorcyclists talk science or at least what they believe is science. Molecular size "
    It struck me as odd, this quote as I am aware of a couple of scientist who ride bmw's or did. On Montesano mountain in Huntsville AL, I'm told that Verner Von Braun's widow still lives in the house that her late husband owned when he was "liberated" from Nazi Germany to work on the rocket program that put a man on the moon. I've been told that in the house's garage is the BMW with sidecar that they road all over Huntsville and Tennessee in the 40's 50's and 60's.
    I remember going into Northrop Grumann's headquarters in California when I worked on the RAH 66 Comanche project and walking down the many long hallways with doors leading to project offices, I saw a poster on the wall near the door of one of these offices that read, "yes we are rocket scientists!" I guess that takes all excuses off the table.
    Moreover, I suspect the real question would be, If a scientist did ride BMW motorcycles and was a BMW MOA member, what would possibly motivate him to pursue the litany of diatribe that flows down the threads of this forum like the 'salmon of Capastrano'?
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  15. #45
    Registered User chewbacca's Avatar
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    Quote Originally Posted by Gonzo49 View Post
    Please see the following link to see the entire explanation

    http://www.getnitrogen.org/pdf/graham.pdf

    I'll give you the basic here.

    "O2 "permeates" approximately 3-4 times faster than does N2 through a typical rubber, as is used in tires, primarily because O2 has a slightly smaller effective molecular size than does N2."

    I try to take advantage of every technological breakthrough. I'm putting my life on two tires with very small contact areas. I'll make use of anything that will increase my safety and the safety of my passenger. It's also why I've chosen to ride a BMW, safety, desgn,technology etc rule.
    This guy says that Grahams Law of "Diffusion" which is what it was taught as in my HS and college chem classes is wrong. If it wasn't for that law Oak Ridge wouldn't have pumped out weapons grade uranium for nukes back in the last disagreement we had with the Rising Sun crowd. Grahams Law depends on molecular weight. Now size is in the eye of the beholder. Most and I mean by a bunch of the molecule, any molecule is just space. Why O2 goes through rubber faster than N2, I would bet has more to do with the electron configuration of O2 and N2 as compared to the long chain molecules of rubber than it does with pushing a molecule through gaps in other molecules. The term effective size was thrown out. Now there is a totally nebulous statement. The effective size of any molecule would depend on what was around it, that is what kind of van der Waals forces could be generated between the molecules, what kind of loose bonding could be formed between electron pairs and empty shells, etc, etc.

    Yes there very well may be a couple "scientist" who ride motorcycles. Maybe Einstein tried to jump the Grand Canyon before Evil. Bottom line is your average motorcycle rider isn't up for a Nobel Prize. Scientists probably rank in the statistically insignificant category along with most other specific classifications.
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