On 1/5/2016 9:32 PM, Tony Hwang wrote:
wrote:
On Tue, 05 Jan 2016 20:06:49 -0700, Don Y
wrote:

On 1/5/2016 6:43 PM, Ed Pawlowski wrote:
Newer cars have a TPMS and warn you when the pressure gets low. Now that cold
weather is here,check to see that your tire is at least the recommended
pressure. I forgot about it and this morning it was 7 degrees. One tire was 1
pound under and set off the warning. It would not reset after driving as it
had
to come up even more than driving did. Filled it up when I got home.

Not a major deal as I knew the pressure was adequate to drive, but it
annoys me
to have that yellow light on when driving. I understand this is a common
happening for the first really cold snap.

We're back at Costco every week or two having them add nitrogen to
the tires. On a cool day (30-ish in the AM), tire pressure (all
around) will be low. However, on a warm/normal day (80-ish in the PM),
pressures will be high -- TOO high if we'd added nitrogen on one of
those colder mornings!

[I think it's 1 psi per 10 degrees F?]

And, the Costco tire droids want to "overfill" by ~3 psi claiming
the tires are "hot" now that you've driven on them. So, instead
of 35/33 psi, they'll fill to 38/36 psi. Then, the ambient
temperature climbs 40 or 50 degrees and the tires are considerably
overinflated.

So, bleed out some nitrogen to bring them down to ~40/38 ("hot")
and hope we don't get another cold day to bring them *down*, too far.
If you are getting that kind of pressure change they are NOT using
nitrogen

Nitrogen is very thermally stable pressure-wize,The calculations for
this change are based on the Ideal Gas Law. A good rule of thumb is
this: For every 10 F degree change in temperature, the pressure will
change by 1.9%. With dry nitrogen,if a tire is filled to 32 psi at a
temperature of 75 F degrees and the temperature drops 10 degrees, the
tire pressure will drop to 31.4 psi; a difference of .6 psi. If filled
at 65 degrees to 32psi, and driven untill the tire temp is 95 degrees,
the pressure will rise to 33.8psi

A 50 degree temp rise will only add 3psi - which is no problem at all.

With air which contains moisture, you will get more pressure change.,
but not a huge amount.

My hunch is leakage. TPMS set without proper torque when installed can leak
air. I have TPMS on summer and winter tires on separate OEM rims. Never
have such problem. I usually over inflate by 5% or so. You'd feel the
ride is deffirent over inflating by ~10% or so.

A *leak* would show LOW pressures, not *high*! I am continually bleeding
pressure from the tires as the "few" cold mornings are the exceptions
yet the tire monkeys plan on them as the *norm*.