Tire Pressure

[Yankee802]

Dumb questions I know, but....when getting our tire pressure reading, it's supposed to be a 'cold' reading, correct? If my fronts are supposed to be 110lbs, it's before I go down the road, right?

Now it's hot where I am, and one day before a run I took my pressure readings, one side was a little higher than the other, could the outside temprature and/or sun play that big of a roll in tire pressure? Should I take the readings at first light so daylight doesn't effect them?

Sometimes I realy feel stupid, this is one of those times.

[JIM CHALOUPKA]

You have the right idea. The sun can affect your readings.

JIM

[Jon Wehrenberg]

The ideal is to take the readings when there is no sun on the tires, and before they have been heated up by driving.

Do not take the pressure readings after driving unless the tires have had ample time to cool down. The heat will raise the pressures and provide false readings.

The ambient temperatures will have an impact on your readings so just recognize if you are going to travel from one temperature extreme to another and back again you will see the pressures follow the temperatures. If you are checking pressures on a sub zero morning, and inflate the tires to the recommended pressures, then drive to a warmer climate, when you check pressures again they will be in excess of what you started with. The reverse will occur when you go from warm temps to cold. I don't bother to bleed off air if the pressures are higher because the difference is usually 5 PSI or less. I do add air if I am below the specified pressures however.

FWIW if you ever suspect a loss of tire pressure while traveling you can check pressures, but recognize they are going to be higher than specified. That is OK. The key is to recognize if one tire is significantly lower than its mate. You need to also recognize that the tires on the sunny side of the coach are likely to be a few PSI higher than those not in the sun. To stretch my legs and get a sense of how my tires are doing I aim my infra-red temperature gun at the center of the tread on each tire immediately after stopping. As long as they are all approximately the same I'm OK. I also measure the temps at the wheel near the hub cap. If any shows up hotter than the others I suspect a dragging brake or possible issue with lube on the bearings.

At the tire tread on a warm day it is not unusual to see all readings around 135 degrees. I can't remember how high the pressures get relative to their initial pressure, nor do I remember the temps at the wheels.

[george frudakis]

Jon
You mention that you have a infra-red temperature gun, I want one, what brand and where can i get one. Thanks George

[travelite]

Ahh, Tires, one of my favorite topics.

Your tire's gauge air pressure (as measured with a tire pressure gauge) will increase approximately 2% for every 10 deg F increase in tire temperature. As an example: if you check your tire pressure when the ambient (and the tire) is at 75 deg F and the pressure is 100 psi, then if the tire heats up to 135 deg F the tire's air pressure will rise to approximately 112 psi.

Also, your tires gauge air pressure will increase as you ascend above sea level by approximately 0.5 psi for every 1000 feet of altitude. As an example, if you leave NJ with 100 psi and make a climb to Aspen Co, you'll ascend approximately 8000 feet and your tire pressure will increase to 104 psi.

The dynamic changes in air pressures are well understood by tire engineers. Turns out that a tire's stiffness (spring rate: lb/in) and dampening (lb-s/ft) both decrease with speed and are offset, to a degree, by increases in air pressure due to increases in temperature.

[Ray Davis]

The manual I received with my SmarTire system indicated that the specified tire pressures are defined for 65 degrees. They have a chart which tells you what the pressures should be for higher or lower temperatures. If I remember correctly, David's numbers are just about what SmarTire indicated, i.e. about 2 pounds for 10 degrees.

Ray

[Jon Wehrenberg]

Jon
You mention that you have a infra-red temperature gun, I want one, what brand and where can i get one. Thanks George

I have a Fluke, but there are many brands and prices for the most part are under $100.

[Jon Wehrenberg]

One thing worth mentioning. Tire pressure gauges may not be accurate. Ironically the Milton gauges (a good respected name in the indusrty) are not as accurate as a cheap one with a dial I got at Wal-mart.

I took all my gauges to a large tire place and had them checked against their master gauge. I also exceed the Michelin recommendations on pressure by 5 PSI for each of my tires. I do that because I theorize it give me a little additional margin if I develop a slow leak. Both Michelin and Goodyear agree if you run a tire 20% (or more) below the recommended pressure you have effectively ruined the tire and it should be removed from service. That's a strong reason for being very careful about maintaining tire pressures.

[michaeldterry]

I have a Fluke, but there are many brands and prices for the most part are under $100.

I just ordered a Raytek MT6 for around $50. It appears to be substantially the same as the Fluke 62 mini.

[Yankee802]

Jon, thank you for the tire temprature info, I'll take temp readings of my tires from now on to use as a guide, and expect them to average around 135 degrees.

So just to summarize and clarrify, if my tires recommended pressure is supposed to be 110, that is 'cold' and at an ambient temprature of 65? So giving for 2lbs per 10 degrees, If it's 85 when I check (before getting on the road), I should make sure my tires have 114lbs?

And thank you guys for the information and presenting in such a way so I don't feel dumb.