Will the R1300GS have temperature adjusted TPMS?

[mneblett]

At 28F, the external air (what's pressing on the outside of the tire) is also of a lower density, thereby requiring a lower internal pressure to maintain the tire shape.

Not correct. To the contrary, colder atmospheric air is more dense than warmer air. Ask a pilot -- aircraft need more runway during takeoff at higher temps due to the lower density of warm air (less available lift due to less air mass flow at the wings).

Moreover, in this application differential pressure is negligible. The differential pressure inside/outside of 4 psi (32 psi vs. 36 psi) will have essentially no effect across a robustly-designed tire carcass which has essentially no measurable expansion/contraction at such low dp's.

The fact remains that you need a certain amount of pressure inside the tire to generate sufficient force at the contact patch to counteract the load being carried by the wheel, and that to maintain the same wheel-supporting force at lower pressure, the tire must deform to increase the area over which the lower pressure acts.

However, while riding in 28F weather, the internal air temperature of the tire is still going to be higher than that external 28F, due to road friction and flexing of the tire carcass.

That will be the case at any temperature, i.e., typically a 2-3 psi increase with heating by tire flexing, whether at 28F or 68F.

The key is that an increase in the range of only 2-3 psi at road speed from desirably-low amounts of carcass flex heating is what you get at any particular temperature when the tire profile is maintained at the design profile. This is not what you get with the BMW approach, which results in lower (i.e., under-) pressure at 28F -- instead you get more flexing and heat, and often sloppier handling from the extra flexing.

I'm not saying that the differences are so huge to cause life-threatening tire carcass heating. I am saying that, as is well known, under-inflated tires wear prematurely and usually provide and sub-optimal handling.

What I don't like is that BMW's approach leads people to unknowingly believe their tires are at the manufacturer's recommended pressures at all temperatures, when physics/thermodynamics instead shows them to be under-inflated at colder temperatures and over-inflated at higher temperatures.

Again, is the amount of under- or over-inflation critical? No. But why not just give actually pressures so the rider instantly knows whether they need to make any pressure adjustments, sans pressure math gymnastics?

 

[tsconver]

With respect, I believe you are wrong.

The manufacturers set tire pressures to, among other things, establish an optimum tire cross-section profile to obtain a desired contact patch. Per thermodynamics (PV=nRT), tires with 36/42 psi at 68F will have pressures at ~32/38 psi at 28F -- in other words, at pressure levels which do not hold the tire sidewall as "tall" as at 68F, thereby distorting the profile and increasing sidewall flexing due to the reduced air pressure support of the tire carcass.

If I'm at 28F at the start of a ride, I'll inflate the tires to 36/42 psi to maintain the as-designed tire profile. If I later find myself at in an environment which will stay around 68F (e.g., shifting from winter to spring, or going from mountains down into Death Valley), I'll bleed off some air to reset the pressures to 36/42 psi at the then-current temperature.

Think back to the days before TPMS systems: We'd all set the pressure to the recommended level (e.g., 36/42 psi) at whatever temperature we were currently standing in -- nobody stood there calculating how much they should reduce or increase pressure to reach the 68F pressures.

The old way always resulted in the manufacturer's desired tire profiles, without, for example, the potential for a heavily-loaded rear tire that is under-inflated at 38 psi to be subject to detrimental excess heat from increased sidewall flexing.

I still maintain that the BMW approach to TPMS has created unnecessary confusion in what was a simple subject (witness the amount of keyboard warrior energy that has been generated by BMW's choice). If the system reported actual pressures none of this "what is the actual pressure" calculation foolishness would be necessary.

Has nothing to do with tire profile. Just because you think you are right does not mean anything. Here is a Toyota document from NHTSA site showing tire pressure is set based on ambient temperature using temperature compensation.

In their examples they tires pressure is set based on the expected ambient temp not simply set to the placard on the door.

https://static.nhtsa.gov/odi/tsbs/2014/MC-10133726-9999.pdf

 

[brownie0486]

I was of the impression recommended tire pressures [ like 36/42 ] were max tire pressures for that tire. That always suggested to me that unless I'm loaded up with gear and full panniers/top box I can set the tires to something less than max.

In regards to what pressure I'll use based on ambient temps. If it's 40 in the garage, setting the tires to 33/39 doesn't automatically mean the tires are underinflated. If the garage is 90, setting the tires to 38/44 isn't over inflating beyond the tires ability to keep that pressure.

I've had the tires set from the house at 70F which I'd used 36/42 fully loaded up and leaving on a trip. That afternoon, crossing the Navajo rez at 3pm, it was 115. I checked the tire pressure with an accurate gauge and found them to be a little alarming at 45/48. So those tires can certainly hold that much without an issue on the road.

My master bmw tech sets the pressures on a new set of tires for same motor at 38/40 whether it's over 100 or 50 here.

 

[mneblett]

LOL! The Toyota bulletin you linked proves my point.

The bulletin states that to avoid getting too many low pressure warnings (thereby annoying customers) if the vehicle is expected to be used at low temperatures, ADD pressure when the vehicle is at shop temperature!

See for example, Example 1 -- they say ADD an extra 4.9 psi to the tires if the temperature is expected to go from 68F (shop temperature) to 14F - in other words, Toyota recognizes that when temperature goes down pressures go down, and that the cold tire pressures should be adjusted accordingly.

At this point I believe we'll have to agree to disagree. You've posted a couple incorrect statements now, and have offered nothing to directly rebut the physics/thermodynamics points I've made, other than unsupported assertions that I'm wrong and you're right. So, slightly re-worded from your last post, just because you make an unsupported assertion does not make you right.

I may be old, but I haven't forgotten everything I learned when I earned my engineering degree. I'd rather rely on facts I can find in my engineering textbooks and >40 years of experience than assertions which are inconsistent with the science.