Tire pressure

On Mon, 23 Oct 2006 11:57:46 -0700, the following appeared in rec.autos.makers.jeep+willys, posted by "Dennis Croll" :

That sounds backwards; the tire pressure should be lower when the air is colder. That said, you should check the air pressure at least once/month and set it to whatever's recommended.

The owners manual says to check them cold. No matter the time of year....

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail >

If you are going season to season without ever having to put air in your tires, you are one lucky guy! ;) Air molecules expand in the heat and contract in the cold. You should notice more air leaking out in the cold than in the heat of summer. Always check your tire pressure when cold as that's what most tire manfacturers require. That way, the ambient air temperature will dictate how much air you put into the tire.

tw

Dennis,

Air pressure in a tire is directly proportional to temperature. In the absence of other forces, you will gain or lose about 1 psi for every 10 degrees (F) temperature change.

You should always check the tire pressure when the tire is 'cold' with cold meaning that it had been driven less than 1 mile or sat parked for more than

3 hours.

Generally speaking your tires will gain about 4-5 psi during normal suburban driving and more at highway speeds.

Everyone should own an accurate tire pressure gauge and you won't be likely to find one in the 99 cent bin at K-Mart.

If you haven't a compressor or bicycle pump you can stop by the gasoline station or tire dealership and add your preferred tire pressure PLUS 5 psi and before you go to bed tonight you can bleed the pressure down to the exact pressure you want. If PLUS 5 psi doesn't make it when the tire is cold add a bit more the next day until you get your required pressure after a bleed down.

You will find that tire pressure variance and loss is less if you have 100% Nitrogen in your tires instead of just air and yes, I am aware that air is

80% Nitrogen.

As you are using oversized tires the pressure you need is probably less than shown on the door placard. Other users here can probably give you approximate pressures and there is an easy way to double check in your driveway. I'm sure someone else will chime in with the procedure and the best was to appropriate the supplies from your sproutlettes.....

Pressure also varies with altitude so you don't want to be filling the tires in the valley and bleeding them off at home in your mountaintop cabin..

In a Real Jeep, the tires need to be filled for each day ...

Only if you are getting that metric air, it doesn't quite fit correctly in American tires.

No... I think what Jeff is saying is that the 35's on my CJ (AKA "Real Jeep") won't hold air for more than a day or 2 because of mud in the bead or the sidewall rock damage.

good one.

You don't have to set the tire pressure for each season, but you DO have to use the right air, summer air in the summer, fall air in the fall, winter air in the winter, and spring air in the spring.

Using the correct seasons air helps your tire pressure stay stable with that seasons weather conditions.

Of course as Billy Ray pointed out, you also must use American air in a Jeep, not that funky Metric air.

Jeff DeWitt

Dennis Croll wrote:

And up here at 7000' Above Sea Level we have to use Mountain Air. :-)

I've heard of that. Tastes great, less filling. :)

Yes but mountain air contains less pressure than regular air and the lack of heavy metals makes it particularly unsuitable for American cars..

On Tue, 24 Oct 2006 20:38:59 -0500, the following appeared in rec.autos.makers.jeep+willys, posted by "DougW" :

And in Ireland, be sure to use Londonderry Air.

No, that's Lite air. Mountain air is non-union ...

And "cold does not mean cold daytime temperatures. "Cold" means not having been driven for at least a few hours. For example: you could check your tires when "cold" on a 95 degree Florida day.

Tom

I was a little surprised by that effect. Aired up to 28psi after a day's wheelin', temps in the high eighties. Drove home (same elevation) and the next morning (60s) they were all @ 23psi.

Yeah, but I always wondered about places like the high desert where daytime can be 100 and nights can be 60. It would seem that the differential between these extremes could cause a tire that was inflated to proper pressure at 5 AM to be seriously overinflated by 4 PM.

Any thoughts?

DAve

mabar wrote:

Lookup Boyle's Law... Just remember though that the temperature is referenced in degrees Kelvin, not Celsius or Fahrenheit... 100F = 38C =

311K... 60F = 16C = 289K... Thus you will have a (311-289)/289 percent difference (i.e. 7.6%)... If your pressure is at 30psi at 60F, it will be at 32.3psi at 100F... Of course, in this example, I'm assuming that volume remains constant, but since we're talking about a rubber tire, that's not exactly true, but it would only add a small change to the result and it would be a reduction in the percentage anyway...

Yeah, I know that Boyle's Law refers to *ideal* gases and that air is not one, but it's close enough for what we're talking about here...