spider_mun

I know this may be a stupid question, but I noticed when I set my tire pressure on my 2003 CR-V in the winter (-12 Celcius) to the recommended setting of 26psi, I find the ride is not that good, unitl the tires heat up. Now I am not a rocket scientist, but I am sure the colder the air, would it not give an improper reading, I know 26 psi is 26 psi, but at -12? or colder outside temp? or I am just being stupid? If the tire pressure increases after driving by about 4psi, in the winter would a setting of 26psi be too low to start with? just wondering when manufacturers put the recommended tire pressure, normally at sea level, and outside air temp?

sanimalp

you are all messed up because of boyle's gas law.. you are dealing with 6 variables, 3 of which are in a closed system that is measured relative to another "open" system.

as long as the temp inside and outside the tire is the same, the pressure you are reading should be the pressure above or below outside(outside the tire) pressure. So in your case, if your car has been sitting, we can assume that the inside and outside air is the same temp, which means that the pressure reading you are getting is the positive pressure inside the tire, above what the atmospheric pressure is at the time.

I believe tire pressure recommendations are made at STP(Standard temperature and pressure), which is, i believe 70 degrees F and 1 atmosphere or pressure(sea level).

now here is the trick.. if you fill your tire to 30psi of internal pressure, and change the "outside the tire" temperature from -12c to +25c, the internal pressure of the tire will go up significantly because the density of the outside air most likely would have changed quite a bit(it got a lot lower), making the reading inside your tire appear much greater.

the increase in tire pressure you are seeing by driving is the heating up of the tire, causing the gas to expand in your tire, which causes the higher pressure reading if outside conditions have otherwise stayed generally the same. Nitrogen can help with this issue a little bit, but given that atmospheric air is already 70% nitrogen, it doesnt make a whole lot of sense to pay extra for pure nitrogen for your tires.

The moral of the story is that you need to adjust your tire pressure with the seasons, by setting them to the recommended pressure at all times, regardless of the outside conditions. The more you set it, the better off you will be. Boyle's gas law explains the math so you can maybe understand the problem a little better.

Crx Jimmy

26 psi is pretty low, it feels better when warmed up because the pressure goes up, I would run at least 32 psi, try up to 40 and see what feels better. (just dont exceed maximum psi on tire sidewall) You will also get better gas mileage with more pressure just don't over do it.