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Old 07-21-2003, 11:02 PM   #1
JDMTEG98
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Default Proper Tire pressure for stock tires

I checked my pressure in my tires all out of wack

Driver Side - Passenger
Front: 38.00 PSi 39.00 PSi

Rear: 38.50 PSi 39.00 PSi

Owners manual says 36 psi in front and 30 in rear

*** mine is way off

I just took the pressure after not driving my car for several hours

Weird
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Old 07-21-2003, 11:26 PM   #2
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Mine were all in the 40's when I picked it up.

I have them at 36 front and 32 rear. It works for me.
{I love the banana SMILIE}.
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Old 07-21-2003, 11:36 PM   #3
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one thing to remember is that air guages are sometimes way off.

for example, in my truck i have super duper gauge that cost $$$. then i bought this $15 for my then wrx. the 2 differed by like 5 PSI...

my truck has big ass tires so while 5 PSI is a difference, on a smaller volume tires like that on the wrx, now my STi, 5 PSI is HUGE....
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Old 07-21-2003, 11:39 PM   #4
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For some reason some of the dealers seem to think inflating the tires to max cold pressure is the right thing to do.
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Old 07-22-2003, 01:06 AM   #5
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I know everybody will have their personal preference. But in approximate terms, what is the right thing to do for the street, auto-x and track days? What should the cold pressure be in the 070? About where shuold it be hot? Will this vary greatly over the different makes of performance tires? Any insight is greatly appreciated.
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Old 07-22-2003, 02:40 AM   #6
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run the pressures on the card. just about perfect for autoX.
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Old 07-22-2003, 09:10 AM   #7
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Default High Pressures are from Japan?

I wonder if the higher pressures are being put there in Japan?

Before I store my tires for the winter/summer, I always over-inflate the pressure to the max indicated on the sidewall. That way, when I'm ready to install them 6 months later, I don't have to top them up before I put them on... I actually let the pressure out a bit normally.

Maybe, after the problems with underinflated Firestones, the safe way to play it is to over-inflate them for the shipment over so that the tires have at least the bare minimum air pressure.

Shipping cars to North America is a year-round thing, and they don't know if these cars are going to be shipped to Northern Alberta or New Mexico eventually. (I would guess that they ship by boat to Alaska.) These cars have to be jockied around after arrival. If the car leaves Japan with 30 psi in the rears, that could be 22 or lower when it arrives in Canada due to temperature differential and shipping delays.

Safe way to play it as a manufacturer? Overinflate it and let the dealer adjust it for the specific climate at the PDI.

Now, look at your PDI sheet to see if the dealer documented the actual pressures.

(P.S.: My pressures at all four tires were 1.5psi above spec. That's just a one day to the next difference in temperature. No big deal. They also re-balanced all of my tires and did an alignment before delivering it to me.)
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Old 07-22-2003, 09:21 AM   #8
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Default Manufacturing Reality

Another issue is the production line. The tires are probably mounted to the wheels in a different location than the wheel/tire combo is put on the car. Thus, they put in enough air for the tire to be mounted to the front or the rear. Another production reality is that they won't have a guy with a tire pressure gauge like you have checking each individual tire. They'll have one air line regulated to ~38 psi with dual analog pressure indicators confirming the pressure on the line.

(For you young guys, every neighbourhood gas station used to have a pressure regulated air pump. Used to DING as it pumped air, too. You set the pressure you wanted, and it automatically went until it hit that pressure, then stopped.)
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Old 07-22-2003, 11:33 AM   #9
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Ok so 36 and 30 is correct?
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Old 07-22-2003, 12:23 PM   #10
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Default Correct tire pressure

Quote:
Originally posted by JDMTEG98
Ok so 36 and 30 is correct?
Well... that depends. Yes, 36/30 is fine for daily driving. What type of driving do you intend to do? What loads do you intend to carry?

For daily driving, start with the manufacturer's recommendation and increase/decrease pressure 1-2psi. See if you prefer it for the type of driving that you do.

If you plan to do high speed driving, (90mph/150km/h and up) consider the adjustments that Michelin suggests in it's tire guides. You can find it on the second page, under the fitment chart and stipulations #1 & #2. Generally, you should increase tire pressure 1psi for each 10km/h additional speed you wish to do, if the manufacturer doesn't provide other guidelines.

http://www.michelin.ca/ca/eng/local/...lotsportas.pdf

To give you an example, my front wheel drive GM's (Regal GS, Bonneville) had equal Font/Rear tire pressure recommendations. (I think it was 32 psi on the Regal, 30 psi on the Bonneville.) However, the weight loading on the front axle was ~70% where the rear was ~30%.

Does this make sense? I didn't think so. Especially after I noticed that unless I fully load the rear of the car to it's limits, the edges of the rear tires were so over-inflated that they never even touched the ground. (Contact marks were different across the tread... visually the sides appeared to touch the ground.)

I figured that this couldn't be good for panic breaking when most of the weight would dive forward anyway, so I generally reduced the pressure 2 to 4psi in the rear, and increased it 2psi in the front. +2psi in the front resulted in stiffer road-crack jolt, but better turn in without the road-imperfection tendency that +4psi had.

Start with the manufacturer's recommendations and work from there. And remember, the maximum sustained speed and weight ratings on your tires only apply at the maximum infation pressure indicated on the side of the tire.
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Old 07-22-2003, 12:45 PM   #11
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I would run the car for a while so the brakes heat up the rims causing the air to expand.

I'll then use a digital and analog gauge to make sure the pressure is ok. 36 front - 30 rear.
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Old 07-22-2003, 12:51 PM   #12
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All mfgrs. that I know of ship cars with upwards of 40#/tire. They do this to prevent flat-spotting on tires. As part of the prep for the car, the dealer is supposed to reduce the air pressure to the recommended values. (I had a test drive in a BMW that seemed to drive way too hard and steered a little wacky too. Asked the sales guy to check the tires and they were at 45-48#! With the pressure corrected it once again drove like a BMW.)

I agree that you have to use some common sense and experiment with tire pressures to get the best combinations. On most tires (I doubt this is true, however, on the STi's performance tires) the recommended tire pressure is set for "comfort" not for the best mileage or for optimum handling. (THis soft setting is part of what got Ford/Firestone into trouble.)

My own practice is to start several pounds above the recommended then adjust according to how they handle.

Of course you don't have to be this anal about it, but .... an easy way to find out if a performance tire is inflated properly is to chalk (like you would in autoX) the edge of the tire. For those not familiar with this, use carpenter's chalk or anything that washes off easily and draw a mark about an inch up the sidewall and an inch across the tread. Drive in a figure eight in a way that will be how you expect to be cornering (hard for autoX, smooth for street, or whatever) then check the chalk markings.

If the tire is too soft, the mark on the sidewall will be partially gone. If the tire is too hard, you will still be able to see a bit of it on the tread. Just right will show with a sharp edge at the edge of the tire .....

Goldilocks. J
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Old 07-22-2003, 01:04 PM   #13
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Default Pressure measurement

Quote:
Originally posted by LESLIEx317537
I would run the car for a while so the brakes heat up the rims causing the air to expand.

I'll then use a digital and analog gauge to make sure the pressure is ok. 36 front - 30 rear.
I believe that you'll find that every tire manual, and even the stickers on some door sills state that manufacturers recommendations are for COLD pressure, not pressure under hot conditions.

Actually, it's often not the residual heat from rotors that heats up the tires. Sidewall flex causes the most significant amount of tire heat. That's why tires that come off of the highway in the middle of winter can still feel hot/warm. (No braking on the highway.)
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Old 07-22-2003, 01:08 PM   #14
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Quote:
Sidewall flex causes the most significant amount of tire heat.
Tread squirm causes it too.
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Old 07-22-2003, 01:53 PM   #15
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Default Heat Increase

Quote:
Originally posted by Glenn Wallace
Tread squirm causes it too.
I suppose it would! Though it's not something that I've personally experimented with. Have you checked tire temperatures after before and shaving the tread? How much of a difference is it?
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Old 07-22-2003, 03:10 PM   #16
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Using for daily driver. No off road or rallying (Right now atleast)

I am going to try recommend 36 for front and 30 for rear

Guess I just need to let some air out (LOL) no need to visit the air pump
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Old 07-22-2003, 08:16 PM   #17
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Might want to try slightly lower pressures if you are having fun in the gravel

Like it's been said, just follow the recommended for daily driving. For auto-x, I bump mine up substantially. On my RE92s, I run 33f/30r daily, but 42f/39r for Auto-x. Then again, RE92s have much more sidewall to worry about than the RE070s.
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Old 07-22-2003, 08:36 PM   #18
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I checked my tire pressure today and the following were my findings;
FL = 39
FR = 41
RL = 40
RR = 45

The max PSI as per Bridgestone is 51 PSI.
My numbers might explain why my car was pushing left on straights. I'll check it out later tonight to see how she rides now.

I reset the tire pressures as per the Subaru tire placard to 36 Front & 30 Rear.

- Janq
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Old 07-22-2003, 08:57 PM   #19
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I run 36/32 COLD when I rember to get the tires filled. usualy this means 38/33 hot.
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Old 07-22-2003, 09:22 PM   #20
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Won't altitude changes affect preasures as well? Like if its toped off at sea-level in a Japanese port, won't the psi be even greater once the car makes it to a higher altitued in the states?
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Old 07-23-2003, 04:54 PM   #21
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Default Pressure Differential

Quote:
Originally posted by Pre-diddy
Won't altitude changes affect preasures as well? Like if its toped off at sea-level in a Japanese port, won't the psi be even greater once the car makes it to a higher altitued in the states?
I haven't personally experimented with changing tire pressures due to changing altitudes, but it's the same amount of air molecules in the tire when it leaves Japan to when it arrives in (say) Colorado. (Assuming zero air loss for this logic.)

Scuba class teaches us that if you fill a balloon filled with air below the surface of the water, that it will expand when brought up to the surface due to a differential in exterior pressure. As the volume expands, the pressure inside the balloon drops.

Now, Bridgestone RE070's don't have the same expansion potential as a balloon. Thus, if any pressure change WOULD occur, it should occur as a loss due to volumetric expansion, not an increase. However, I'd guess that any volumetric change that would occur would not be measureable on any gauge system that you could use. You'd see far more tire pressure change between morning temperature and sitting in the sun in the afternoon than from an altitude change like you mention.

That's my theory. If you have a mountain nearby... check it out.

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Old 07-23-2003, 05:14 PM   #22
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I was thinking the same thing that you said. But you put it more eloquently. I was actually even thinking of the balloon example from SCUBA as well.
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