Tire pressure PSA, something I just learned

Discussion in 'Equipment' started by AZQKR, Aug 30, 2018.

  1. AZQKR

    AZQKR Long timer

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    I did not know this until the other day when I read it on the bmwmoa site.

    Tire inflation recommendations [ on Beemers anyway ] is set at 68 degrees F. For every 10 degrees above or below that in ambient temperature, add or substract 1# to the tires.

    So, I've been underinflating my two Beemers tires since the ambient temp in my garage has hovered at 95-98 degrees here in the desert. Which means for me, instead of 38/42 on the GS cold, I should have been setting them at 41/45 pressure.

    On the K bike, instead of 36/42 they should have been set at 39/45 cold before riding off for the day.

    I'm not sure how many actually know this or if I'm the only one here who was a duh on this, but thought it might have value to some members who were oblivious as I was until recently.
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  2. AZQKR

    AZQKR Long timer

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    The question I have for members who've been aware of the above-------------

    When I've ridden the tires and they have warmed up, and I want to deflate the tires for gravel road travel, do I just take out 4# or whatever from where they read at the time or do I set them to recommendations I've received here for air down for off road?

    Thanks
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  3. MarkM

    MarkM Been here awhile Supporter

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    I do not agree with adjusting the pressure based on ambient temps. If you are setting yours to 38/42, then that is the gauge pressure you should be setting them to when the tires are "cold", which means at the ambient temperature before riding.

    If you have TPM, then follow the instructions in your manual.

    Here's the instructions from Michelin.
    https://www.michelinman.com/US/en/safe-driving/tire-safety/tire-pressure.html
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  4. AZQKR

    AZQKR Long timer

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    That's for car tires, not motorcycles. I don't adjust on the vehicles as I'm told to do on the bikes. Got a link showing Michelin suggestions for their MC tires?
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  5. MarkM

    MarkM Been here awhile Supporter

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  6. MarkM

    MarkM Been here awhile Supporter

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    If you increase the pressure above the recommended setting, it causes the tire to be harder which reduces the contact patch, resulting in less traction. If you reduce the pressure below the recommended setting, you increase sidewall flex which can reduce the life of the tire.
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  7. AZQKR

    AZQKR Long timer

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    Here's the read I'm referring to. Specifically the section on Effects of Temperature when Setting Tire Pressure

    Thoughts?

    Tire Pressure and TPMS Systems: How it Works
    Qustions about tire pressure and Tire Pressure Monitoring Systems or TPMS constantly arise on motorcycle forums. I wrote this article provide answers to those questions. There are a lot of very knowledgeable folks on this forum. If you have time, please read it and let me know if you think it can be improved or needs to be corrected. Sorry its so long. If you don't want the detail, just skip to the last paragraph.

    Tire Pressure Monitoring Systems on BMW Motorcycles

    This information on tire pressure and tire pressure monitoring systems is intended to answer the many questions that constantly seem to arise on these subjects.

    Definitions (you must understand these to understand the rest of the article)

    Ambient pressure: The atmospheric pressure at a specific location and elevation where tire pressure is being checked or set. Ambient pressure decreases as elevation increases. It is also affected by weather events like low and high pressure systems, wind, etc. Weather events can cause ambient pressure to fluctuate by usually no more than + or - .25 psi.

    Relative pressure: The amount of pressure by which the pressure in a tire exceeds ambient pressure. Manufacturer recommended tire inflation pressures are always specified in relative pressure.

    Absolute pressure: The total pressure inside a tire, which includes the ambient pressure plus the relative pressure.

    Ambient temperature: The temperature at a specific location where a tire pressure is being checked or set. This could be an outdoor temperature or an indoor temperature.

    Cold tire: A tire containing air that is at the same temperature as the ambient temperature. This usually means the bike has not been ridden for long enough for the tire to cool down to the ambient temperature.

    Tire gauge: A typical hand held tire gauge directly used on a wheel valve stem to check tire pressure. Be sure to check your tire gauge against a known good gauge to be sure it is accurate.

    Tire pressure: The same as Relative Pressure.

    TPMS: A system comprised of wireless sensors located within the tire cavities plus a remote receiver that receives signals from those sensors. The system also includes a computer that processes those signals and produces an adjusted tire pressure result that is displayed on the vehicle dash board. In this article TPMS only refers to systems manufactured by BMW and used on BMW motorcycles.

    Tire Pressure Basics

    BMW provides recommended inflation tire pressures for their motorcycles. Always use the tire pressure recommendation of the vehicle manufacturer, not the maximum tire pressure rating typically embossed on the sidewall of tires.

    Always use a tire gauge to check and set the pressure of your tires. Do not attempt to use the TPMS system for this purpose. Always check and set your tire pressures with the tires “cold”. See Definition of cold tire. BMW also specifies the temperature at which the recommended tire pressure is correct. This temperature is 68 degrees F (20 degrees C). If the ambient temperature is not 68 degrees F, you will need to set your tire pressures to an adjusted pressure. See below.

    Effects of Temperature when Setting Tire Pressure

    The ambient temperature may be higher or lower than 68 degrees F when you are setting tire pressure. You need to adjust the factory recommended tire pressure to account for this difference in temperature. This is easy to do. For every 10 degrees F that the ambient temperature exceeds 68 degrees F, increase the factory pressure by 1 psi. For every 10 degrees F that ambient is below 68 degrees F, decrease the factory pressure by 1 psi. This will result in the correct tire inflation pressure when the ambient temperature is at 68 degrees F.


    How Tire Pressure Gauges and TPMS Sensors Work

    A tire that measures 0 psi with a tire gauge will still have ambient pressure within the tire. If this is done at sea level, the ambient pressure inside the tire will nominally be 14.7 psi. If you add 42 psi of additional pressure to the tire as measured by a tire gauge, the absolute pressure within the tire will be 56.7 psi (42 + 14.7).

    Tire gauges sense both the absolute pressure and the ambient pressure. The gauge displays the difference between these 2 pressures which is called the Tire Pressure or Relative Pressure. When setting tire pressure, we always use relative pressure. Tire gauges are reliable and simple to use. The ability of these gauges to measure accurately is not affected by temperature, elevation or barometric pressure.

    Elevation Change and TPMS Sensors

    TPMS sensors are located inside the tires and have no way to sense the ambient pressure. These systems typically use an assumed ambient pressure of 14.7 psi (sea level pressure) when calculating relative tire pressure. But atmospheric pressure decreases as elevation increases. The ambient pressure at sea level is 14.7 psi. The ambient pressure at 6000’ in elevation is 11.8 psi. For this reason, TPMS systems do not calculate relative pressure accurately at elevations above sea level. The TPMS calculated tire pressure will be low when compared to a tire gauge pressure by .5 psi for each 1000’ of elevation above sea level.

    Example: Let’s assume you are located at sea level and have set your tire pressure at 42 psi. If you ride your bike up to an elevation of 6000’, the pressure on the TPMS will remain constant at 42 psi. If you check your tires with a tire gauge, you will find the tire pressure to now be 45 psi. If you then reduce your tire pressure by 3 psi to 42 psi using a tire gauge, this will lower the pressure displayed by the TPMS to 39 psi, a 3 psi error.

    Effects of Temperature Change and “Corrected Tire Pressure”

    The purpose of the TPMS system is to detect an air leak occurring from a tire. The pressure inside of a tire increases as a bike is ridden and the tires heat up. This pressure increase can camouflage a leak. The TPMS sensors constantly measure the temperature inside the tire and send this information to the TPMS computer. The TPMS computer is constantly calculating how much of the increased or decreased pressure in a tire is due to the tire temperature being below or above 68 degrees F. It then adjusts the actual pressure received from the sensor by this amount of pressure difference and displays the corrected result on the dash. This adjusted pressure is referred to as the “temperature corrected tire pressure”.

    Example: We begin our ride with a tire pressure of 42 psi and a tire temperature of 68 degrees. During the ride, the tire temperature goes up to 98 degrees. According to the ideal gas law, PV=nRT, this will raise the pressure within the tire by 3.22 psi to a pressure of 45.22 psi. The TPMS system recognizes that the temperature inside the tire has increased to 98 degrees F and deducts 3.22 psi from the measured pressure inside the tire to correct for this temperature increase. If the tire has not lost any air, the TPMS will display the corrected tire pressure on the dash as 42 psi. The TPMS system does this automatically, so the driver sees a constant tire pressure reading on the dash no matter how much the actual tire pressure changes due to temperature increase or decrease. This lets the driver know the tire is not losing any air.

    The Use of Pure Dry Nitrogen Gas and Condensation Inside of a Tire

    Air compressors use ambient air, so that’s what goes into most tires. There is a belief that the use of pure dry nitrogen gas to inflate tires has many advantages. I do not want to support or refute that idea in this article. The reality is that for all intents and purposes, tire gauges and TPMS systems perform identically using either type of gas.

    It is recommended that you allow your air compressor to come up to full pressure, usually about 125 psi or greater, before you use the compressed air to fill your tires. This will wring high levels of humidity out of the air before the air then passes into your tires and will prevent any condensation from forming inside the tire. Condensation is undesirable since it can potentially cause corrosion inside the tire. Condensation can also have a very minor effect on tire pressure, probably less than .1 psi.

    Tire Dimensional Change

    The volume of a tire will change as temperature and pressure inside the tire change. This could vary from tire to tire depending on tire manufacturer, tire design, tire loading, etc. Since tire volume change cannot be accurately predicted, my opinion is that Manufacturers of TPMS disregard tire volume change when calculating temperature corrected tire pressure. This will cause the TPMS to understate tire pressure when a tire gets very hot. I estimate that the amount of this error is probably less than 1 psi most of the time.

    What does this all mean?

    1. Always set your tire pressure with the tire “cold”.
    2. When setting your tire pressure you must adjust for the ambient temperature. For every 10 degrees F that the ambient is below 68 degrees F, deduct 1 psi from the factory spec pressure. For every 10 degrees F that the ambient is above 68 degrees F, add 1 psi to the factory the spec pressure.
    3. Use an accurate tire pressure gauge to set your tire pressure, not the TPMS system.
    4. Recognize that the TPMS system will always have an elevation error. For every 1000’ of elevation that you are above sea level, it will read .5 psi low.
    5. Recognize that due to the many errors created by inaccurate temperature measurement, elevation change, barometric pressure change, tire volume change and rounding errors, the pressure displayed by the TPMS will not be accurate and will vary a little during your ride. The purpose of the TPMS is not to accurately display tire pressure. The system is there to immediately alert you to a sudden unexplained drop in air pressure of more than a few psi, which it will do very well.
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  8. MarkM

    MarkM Been here awhile Supporter

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    It's a good article and I agree with it except for the part you highlighted. That part may or may not be reasonable advice for bikes with TPMS, but it makes no sense otherwise. The performance and safety of a tire is determined not by the pressure corrected to an arbitrary temperature, but by the relative pressure at ambient conditions.
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  9. AZQKR

    AZQKR Long timer

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    Relative pressure is the key IMO. At 98 degrees, the tire set at 3#'s over recommended will be correct at 68 degrees having lost 3#'s. BMW makes their recommendations for tire pressures at 68 degrees.

    Does that make sense?
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  10. MarkM

    MarkM Been here awhile Supporter

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    By relative pressure I meant the tire pressure shown on a tire gauge. That's how the article defined it.
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  11. canoeguy

    canoeguy Long timer

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    Lets be honest...Who the hell does this. Do I need to adjust my tires in the Spring and Fall multiple times a day, because there is a 30-40 degree swing. Actually I have never even heard of this, not once in decades of riding. I set my tires to my preferred PSI regardless of ambient just like, statistically speaking, everyone else. We all know after a few miles under your belt you can get a feeling for what your tires are doing by feel anyway. But regardless put me in the camp of just filling them regardless of ambient temperature.
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  12. Jmunk

    Jmunk Adventurer

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    Is few pounds enough to worry about? I think that there are gauges out there that vary more then that. If it has been working out up until now is it worth worrying about ?
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  13. flei

    flei cycletherapist

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    I wouldn't sweat it about the air temp; it's likely your tire gauge is off by more % than what the difference in ambient temperature would cause. Unless you have a very expensive "racecar type" gauge that has been calibrated, setting your PSI in the ballpark is close enough.
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  14. AZQKR

    AZQKR Long timer

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    When I lived in the northeast, I always had to air up my tires once it got cold/winter time from where they had been set during the summer. It's known that tires lose air when the weather turns colder, and gain some when the weather gets hot by about everyone that lives in such a climate between summer and winters.

    Now I live in the Az desert. Presently it's 100+ and come 5 months from now it can in the 40's. Every year since here, I've had to air the tires up in the winter just like back east when it turns colder by 50-60 degrees. Same when back east when it turned from 90 in the summer to 30 in the winter months.

    It's fairly understood that tires gain and lose air pressure based on ambient temperature. If tire pressure recommendations are based on 68 degrees ambient by bmw and are set at that temperature, when it's 100+ ambient the tires will have gained pressure and when set the same at 68, if the tire sees 30 degrees they have lost air.

    Here, I can see 80 in the valley and drive north and see 40 degrees at the Grand Canyon in 4 hours. The tires will be lower than recommended after a time at the canyon. I'd never seen the advice to air up or down 1 degree based on ambient due to the recommendations being for 68 degrees either until that was reported. But it seems to make sense.
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  15. AZQKR

    AZQKR Long timer

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    A few pounds in the front can make the difference between cupping and not cupping of the tire. Therefore I think it makes a difference. When I called Heidenau on the K60 Scout recommended pressure settings for weight of the bike, me and the gear heading to Ak. the guy said to air the tire every morning to 39/41 [ front/rear ]. That most people that experienced problems with their tires underinflated them, knowingly or inadvertently. I followed his recommendations and had plenty of tread left after 8K on both tires upon returning with no cupping on the front tire.
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  16. AZQKR

    AZQKR Long timer

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    My gauge has been tested side by side with a calibrated gauge and is off .5#'s.
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  17. Hammerdown77

    Hammerdown77 Long timer

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    This isn't that hard. The recommended pressures are at 68F. If it's 98F when you set your tire pressure, you need to set it to a higher indicated gauge pressure, unless your gauge is temperature compensated.

    The TPMS system on, say a BMW, calculates for this compensation. It will show you the corrected-for-temp pressure. If it's warmer than 68 degrees, your gauge will show a higher temperature. However, that is still the right pressure as it matches the factory recommended pressure which was specified at 68 degrees.

    I don't understand why you think it needs to be that way with TPMS, but without it (just your handheld gauge) it doesn't. The equation is still the same, it's just one compensates for the temperature change over what the spec was set to, and the other doesn't.
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  18. AZQKR

    AZQKR Long timer

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    Just an fyi, Many BMW's don't come with TPMS, my GS doesn't have it, the K1200GT does. BMW's recommendations for tire pressure have nothing to do with TPMS readings.

    Seems you agree with the theory of adjusting pressures based on temperature at the wheel.
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  19. spoon

    spoon Rubber's gone!

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    Here is food for thought. My work installed a tire pressure gage calibration test stand. This is run by the calibration folks. I gathered an assortment of TPG's for a comparison. These ranged from the $2.00 type found at the auto parts store checkout register to some higher dollar "professional" models. The cheap pencil type were dead nuts on. Now granted the resulation was at best + - .5 psi, but the fact that they were that consistent and accurate amazed everyone. Out of the 7 that I tested one was 4 psi off at a 40 psi test pressure. So your chance of getting a good reading from an off the shelf pencil type pressure gage is fairly high. What does this have to do with what your pressure should be? Not much except you can expect it to be close to what you think it should be when you do check it. Check often and prosper, check occasionaly and pray for luck.
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  20. CharlieInStLouis

    CharlieInStLouis Been here awhile

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    You're kidding, right?
    I didn't know they made them. https://pitstopusa.com/i-18722901-l...EFknyfw-KVvO30bswnDdxeMGrrWnvH4BoC7YUQAvD_BwE

    I can't believe it.
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