Tire Pressure -- when to measure?

[FrankO]

I measured the tire pressure of my fronts yesterday afternoon. I measured again this am and there is at least a 3 or 4 lb difference. There is a 30 degree difference in temp which affects tire pressure but I was just wondering at what temp is the standard set from for the tire pressure tables on the Michelin site.

Probably a dumb question but I had to ask since there is a strong feeling on in the industry that tire pressure is critical for ride and tire life/safety.
Frank O.

[FrankO]

I measured the tire pressure of my fronts yesterday afternoon. I measured again this am and there is at least a 3 or 4 lb difference. There is a 30 degree difference in temp which affects tire pressure but I was just wondering at what temp is the standard set from for the tire pressure tables on the Michelin site.

Probably a dumb question but I had to ask since there is a strong feeling on in the industry that tire pressure is critical for ride and tire life/safety.
Frank O.

[Clay L]

They say to set the pressure when the tires are cold but don't define cold.
I remembered that in my engineering work we used something called a "Standard Temperature" but I couldn't remember what I used. I thought it was either 20 or 25 degrees C but wasn't sure. So I did a search looking for Standard Temperature and didn't find a definitve answer. But 70 degrees seemed to be a good number based on what I found.

All that being said, since the tire people don't define cold, I suspect the few pounds difference between setting pressure at 50 degrees and 70 or 80 degrees probably isn't all that important. It's probably more important to have enough pressure rather than too little when the tires heat up when running. If set at 50 degrees they will have enough pressure going down the road when they get up to 120 degrees or so. Also if set at 70 degrees they will have more pressure but still enough when running. I suspect the tires are capable of handling the higher pressure.

[Pusherman]

You are correct that you will see a 3-4 psi variance in the tire pressures if you check them at different times during the day, specifically if you are driving the coach.

So the best procedure is to "check them when cold" -- meaning check them the first thing in the morning before driving the coach. Yes the ambient temperatures will vary, but I believe that driving the coach generates the most variance in psi.

So my process is to check the pressure each day before driving the coach.

[John_Canfield]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by FrankO:
--snip--
Probably a dumb question but I had to ask since there is a strong feeling on in the industry that tire pressure is critical for ride and tire life/safety.
Frank O. </div></BLOCKQUOTE>You are the master of understatement Under inflated tires will get hot and then blow and perhaps ruining your camping trip (and perhaps your life) in the process. You need to weigh your coach with all of your gear aboard, some water, full fuel, propane and inflate your tires in the morning to the tire manufacturer's tire pressure chart for your particular tire.

[Cruzer]

The base temperature is around 60 degrees Fahrenheit. The rule of thumb for automotive tires is 1 PSI change for every 10 degrees over or under that temperature. However, this is really based upon a percentage and an RV with 95 PSI tires isn't going to react the same way as a car with 32 PSI tires. So I'd think more along the lines of 3 PSI for every 10 degrees.

Either way, these are "cold" readings in that the vehicle should not be driven before testing. By monitoring my SmartTire system I've seen where tire temps can be as high as 140 degrees, which is 60 degrees higher than ambient temperature. In some cases the air in your tires can be as much as 25 PSI higher so don't check them after a hot run. The air inside the tires needs time to cool down before an accurate reading can be taken.

As mentioned earlier, weigh your coach in the fully loaded condition, check the manufacturer's tire pressure charts to see how much air you need, then fill them in the morning before you leave. Running a little extra doesn't hurt and gives you a safety cushion. Tires do leak down gradually over time and it's better to have them leak down "to" the required pressure than "from" it.

[Gary the Wombat]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Clay L:
I remembered that in my engineering work we used something called a "Standard Temperature" but I couldn't remember what I used. I thought it was either 20 or 25 degrees C but wasn't sure. </div></BLOCKQUOTE>

Standard Ambient Temperature and Pressure (SATP) is 25?C and 1 Bar. Ahhh--brings back memories of all those wonderful Thermodynamics lectures!