BS: Proper tire pressure

Subject: BS: Proper tire pressure
From: frogprince
Date: 13 Nov 06 - 09:10 PM

Seeing the nitrogen-in-tire question recycling again reminded me that I've been meaning to bring this up:
I've heard warnings for many years against trusting the pressure gauges on filling station air pumps. So I've always kept a pen-style pressure gauge, like my Dad always did. More recently I've come across warnings against trusting the accuracy of pen gauges. I compared at least two of them I've had around, and they varied by about 5 psi. So I bought a dial gauge. Then I happened to buy a little lighter-plug compressor, apparently unused, at a church rummage sale. The first time I used the compressor, the dial on it and my other dial gauge matched just about exactly. The next time I used it,just a few days later, they varied by about 5 psi.
Every single dial gauge available in our area is the same Chinese made brand. I've been meaning to look online and see if I can find an affordable gauge with an accuracy guarantee. How in hell does anyone know what pressure they really have in their tires?

Subject: RE: BS: Proper tire pressure
From: The Fooles Troupe
Date: 13 Nov 06 - 09:14 PM

John In Kansas

Time for you to regale us with a short discourse on 'Accuracy Vs Precision', I think....

{:P

Subject: RE: BS: Proper tire pressure
From: JohnInKansas
Date: 13 Nov 06 - 10:34 PM

Robin -

I doubt if you're up for the whole lecture, and you can deliver it as well as I can.

Maybe later.

Short version:

If you can get the same "reading" three times in a row, within one psi, your gauge is repeatable enough to meet your needs. (If you let it leak while testing, you'll have to start over of course.)

If it matches another good quality gauge within about 5 psi total spread between the two I wouldn't worry too much about it, since you don't know which one is closest to actual.

If you don't use the same gauge each time you check, or don't do it with reasonable frequency, it doesn't matter much how good the gauge is.

John

Subject: RE: BS: Proper tire pressure
From: Rapparee
Date: 13 Nov 06 - 10:38 PM

I like a 20 gauge, myself.

Subject: RE: BS: Proper tire pressure
From: catspaw49
Date: 13 Nov 06 - 11:49 PM

Cheap may be okay.....and maybe not. Your best bet for overall accuracy/reliability/repeatability is to look for a BRIDGEPORT of SUPEREX pencil style gauge. Dial types often cost more and may or may not be better. The MILTON dial type gauge (mine says 901 on it) has been widely used and trusted for a long time and mine is excellent.

Spaw

Subject: RE: BS: Proper tire pressure
From: The Fooles Troupe
Date: 13 Nov 06 - 11:59 PM

Well, John,

You got me! The main trick is 'repeatability' anyway. :-)

Those really interested in learning the difference can check Wiki anyway.... :-)

If you are accurate, you can usually hit the target with the arrow - somewhere...

If you are precise, you can always group your arrows closely, somewhere on the target...


How's that?

;-)

Subject: RE: BS: Proper tire pressure
From: catspaw49
Date: 14 Nov 06 - 12:10 AM

And I suppose if you are not precise and not accurate the arrows will all wind up in your ass.

Spaw

Subject: RE: BS: Proper tire pressure
From: The Fooles Troupe
Date: 14 Nov 06 - 12:25 AM

... or in the ass of guy standing next to you, making disrepectful comments in your ear...

Subject: RE: BS: Proper tire pressure
From: JohnInKansas
Date: 14 Nov 06 - 01:33 AM

The pressures shown on the load plate on the left front door frame are the manufacturer's recommendations for average drivers with average loads. They're pretty conservative so getting "close enough" is adequate for "simple users," as long as you maintain consistent pressures all around by using the same reasonably accurate gauge every time you check.

The recommended pressures usually are for fairly light loadings, so they'll be good for anything down to an empty vehicle. They'll be safe for anything the mfr expects you to carry around in "normal use" but may not be quite what you'll want if you carry a lot of heavy stuff regularly.

If the gauge is good enough to assure you're not significantly below those pressures, and that the tires are all close to the same pressure, your ride and handling will be at least "acceptable."

From that point on, you may need to adjust for your personal driving habits and usual loads. For the adjustments, it's "how much you change" with "where you start from" being pretty much unimportant - as long as the starting point is in a safe range and has been maintained consistently.

Generally, you should not go below the load plate inflations, since handling and safety even at low loads can be adversely affected; and low inflation pressures can be extremely dangerous with "higher than average" loads.

The sidewall of your tire should indicate the pressure at which the tire has its maximum rated load capacity, and you ordinarily shouldn't exceed them. All 4-ply rated tires should show max capacity at 32 psi, although I've seen a few "mavericks" that indicate 35. Adding 5 psi when you'll be driving long distances at sustained high (public highway) speeds in hot weather is sometimes recommended and should be permissible when appropriate. (If you do this frequently, you may want to look at the speed range ratings for your tires.)

All 4-ply rated tires have a burst pressure (when new) of at least 250 psi, so you're not likely to make significant errors on the high side.

Tire pressure can be increased enormously if you crash into a curb or hit another obstruction at even moderate speed, so the 250 psi burst isn't as much margin as it looks like, even when the tires are new. The real-world burst can decrease significantly over the useful life of a tire. Don't over due the "add a little" thinking.

With modern "high mileage" tires, tread wear is so low that it may be difficult to detect uneven wear before it progresses quite a bit, but of course the old rule still works, that edge wear (both outside edges) indicates underinflation and center wear indicates overinflation. The proper inflation is the pressure that gives flat wear across the width of the tread. This is the condition for minimum tread flexure, and is the condition that will give the best useful fuel economy and longest tread life.

Wear on one edge may indicate chassis misalignment that should be corrected by your favorite shop. It may also indicate "excessively aggressive driving" which ultimately may be corrected by your local police, although your own adjustments before that happens would be preferable.

What's important is that inflation pressures be near, perhaps preferably a little above rather than below, the manufacturer's recommendations as shown on the load plate, without exceeding the tire max rated pressure (+ the 5 psi "speed allowance if applicable), and that the tires be maintained so that all four are kept at very nearly the same pressure (for safe handling) and all consistently maintained at the same uniform cold pressure all the time (so you don't have variable handling to contend with, so that you don't fail to catch one that goes down a little).

With most modern tires you can't tell by looking whether a tire is low. Sitting at the curb where you can look, 15 psi looks the same as 35. More often in fact, you'll think the tire looks low when it isn't. You must use a gauge regularly. The gauge should repeat the same indication when plugged into the same pressure within one psi, but it doesn't have to be exactly on the absolute value as long as you know what pressure - in that tire and with that gauge - you want to see.

Since loading has a lot of effect, finding the right "indicated pressure" for your vehicle, your normal loads, and your driving habits, and your gauge is the tricky part. Until you find your spot - use something that might be within a couple of psi of the maker's recommendations - consistently.

John

Subject: RE: BS: Proper tire pressure
From: The Fooles Troupe
Date: 14 Nov 06 - 07:22 AM

I like to keep the spare a few psi above the 'normal' of the tyres on the road - every few months when I excavate it to check - it's always dropped a bit anyway.

Subject: RE: BS: Proper tire pressure
From: Jeremiah McCaw
Date: 14 Nov 06 - 08:42 AM

Very knowledgeable, JohnInKansas; thanks.

I've always run my tires at the maximum indicated sidewall pressure (usually 32psi). A soft barca-lounger ride has never appealed to me as much as handling and squeezing every last mpg out of the vehicle, both of which the max pressure thing achieve.

Depending on the vehicle, I've sometimes run from 1 to 3 lbs less pressure in the rear tires to fine-tune the handling.

Subject: RE: BS: Proper tire pressure
From: Bunnahabhain
Date: 14 Nov 06 - 09:18 AM

I'm not too fussy on tyre pressure. It's about 100-120 psi, but you can get away with alot on 25mm race slicks...

Subject: RE: BS: Proper tire pressure
From: GUEST,Russ
Date: 14 Nov 06 - 12:38 PM

A suggestion

If you are really concerned,

Go to the place where you buy your tires.

Ask one of the guys/gals there to measure your tire pressures with his/her instrument of choice.

Measure the pressures with your gauge.

Make a note of the difference, if any, and use it in the future when you use your own gauge.

The assumption, of course, is that it a reading is good enough for a professional, it should be good engough for you.