Psi++ Wtf?

i filled my tires to 38/36 a couple weeks ago and when i checked them tonight they were about 41/40. i haven't really changed elevation much. what would cause this? air temp?

Did you check them while they were cold? If you check them after you drive, the pressure will be a few PSI higher than when they are cold.

that is probably it. today i checked them after driving home. should i be setting my psi based on cold or hot?

Cold.

Recommend you check them while they're cold - good Saturday morning habit to get into, provided of course that it hasn't been ridden hard a few hours before and put away wet. FWIW - I notice a 1-3 lb drop in tire pressure every week on at least 2 of 4 wheels from the previous week's pressure sets. Perhaps "spirited" driving through turns and on ramps extracts it's toll on AP....:D SBT

Recommend you check them while they're cold - good Saturday morning habit to get into, provided of course that it hasn't been ridden hard a few hours before and put away wet.

 

FWIW - I notice a 1-3 lb drop in tire pressure every week on at least 2 of 4 wheels from the previous week's pressure sets. Perhaps "spirited" driving through turns and on ramps extracts it's toll on AP....

 

SBT

 

I check em and set em cold - and after driving, the tires are up another 2-3 psi as you would expect.

 

And yes, I too have noticed a drop of 2-3 psi every week or two.

i have the bfg kdw tires. after a little over 2 weeks, i was expecting to put more air in the tires but the result was this thread. are you guys running the re92's?

Bridgestone RE750's on two of the Leggy's (L & GT), RE92 on the other GT. Bridgestone G900 on the Forester and Pirelli P400 on the OBS. All lose some air per week and never from the same wheel positions car-to-car.

 

SBT

Tires are have a miniscule amount of porosity themselves anyway, and hopefully they don't lose much air.

 

One thing that I didn't realize, but heard recently, that if the tire's bead, and the wheel's bead seat are even the slightest bit dirty, can create open pockets at the bead seal, which can lose pressure over time.

 

Some schrader valves seal less effectively than others, as well. All sorts of reasons for tires to lose pressure.

 

As for gaining pressure, it would almost have to be a temperature issue. for every 10 degrees of ambient temp change, the tires change about 1 psi. Inside the tire, the air is insulated a bit by being seperated from the outside air, so tire temp changes might actually magnify that effect a little bit by heating the air in the tires more than ambient temp changes, I am not sure.

 

BTW, subietonic, how do you like the Potenza RE750s? I am trying to decide between them and Toyo T1-S for the miata's new summer shoes this spring.

BTW, subietonic, how do you like the Potenza RE750s? I am trying to decide between them and Toyo T1-S for the miata's new summer shoes this spring.

IWSS - Here's a post with some of mine and other's thoughts on RE750's http://www.legacygt.com/forums/showthread.php?t=2730&highlight=RE750. Bottom line, I've begun the migration of all of the Subes to the RE750s and will only load out a winter set when I head to the mountains for snow fun.

 

SBT

Air temp. is the biggest beast. I had lost almost 10 psi in my wives car from Aug. to Dec just from the air temp. Second, is the that the psi will go up after a drive because of friction.

Don't forget that good old sunshine pumps a lot of thems into a tire so that you can start out with equally balanced tires in the morning, but if you don't move your car and the sun shines on it all morning, you can have a +2 psi variance from one side to the other.

 

before sunrise or driving Saturday morning early, is when I check the tire pressures so that I'm starting from a cold tire baseline side to side. Makes it much easier to verify that you're setting them at the same pressures.

 

SBT

Interesting. Lost about 3-5 psi over 2 to 2 1/2 weeks, wasn't sure if this was normal or not. My last car didn't seem to lose this much tire pressure. However since it seems I uniformly lost this amount on all my tires on my Legacy... *shrug*

 

Currently only got about 1150 miles on this baby.

You might check out this thread, too:

 

http://legacygt.com/forums/showthread.php?t=16030

I check me pressures when they are hot when it's the summer time, and cold when it's the winter time.

 

The reason being is that you can lose traction when your tires get too hot and the tire can balloon and lose contact patch.

 

In the winter, you dont want your tire to drop too low of a pressure, causing the tire sidewalls of not being able to hold up the car's weight.

 

In autocrossing, I can gain as much as 6psi per corner under 2 runs (about 1 to 2 miles worth of driving in a summer day) on a set of Falken Azenis RT-215s.

 

In the past, I have gained much as 10 psi on each tire on a set of Yokohama Avid H4s (H-rated) on a 5 min run of 120 mph. You can make up a lot of heat from the friction created in the air of the tire...

 

 

 

Keefe