mneblett
#32806
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).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.
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.
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.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.
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?