Measurng Tire pressure and inflating

[Rocketman43]

I have a Class A, Winnebago Journey. I have an issue/question regarding inflating tires to proper PSI. I know that we are to measure the PSI when the tires are cold (not driven on for a while). I know that the appropriate tire PSI is calculated from the tire pressure charts in combination with the weight on each tire/wheel. My issue is, if my PSI is low then I have to drive from my garage or campground to the service center to have air added and by that time I get to the service center the tire has built up pressure to be over the appropriate COLD pressure. So, do I add or not?
An exapmpe:
Appropriate PSI for the tire/wheel is 105 (when cold)
I measure the pressure before driving and it is 100, so I am 5 lbs low.
I drive to get air and the pressure has built up to 110.
The tires are rated for a max of 110 PSI.
Once at the service center do I add 5 lbs to the 110 up to 115 and go over the max PSI rating of 110 for the tire?

I could not find on this forum the answer to this specific issue. I may be on the forum, but I could not find it.

Thanks in advance for any advise,
Bob

[Wayne M]

I posted on another forum, but here is a link.

Roger Marble engineer.

[be happy]

Let me ask a question. It sure seems it would be easier to get a portable compressor to add air. I have carried a small 110v compressor for years to add those 5lb or so to get the 100/110 psi. Yes it may take a little time to do, but for me it sure makes more sense than driving cl A around trying to find a place that can deal with psi for our tires. As a side note my Meridian
(mate to journey) has a quick connect to use the on board compressor. To much math involved your way takes away from the enjoyment.

Good Luck

[creativepart]

I drive to get air and the pressure has built up to 110.

Cold tire pressure is a tire that has not been driven a mile or more.

So, that's simple.

Get a tire inflation solution that you can use on your parked motorhome before it's been driven a mile.

I carry the Vair RV kit. Available on Amazon for $180. But there are cheaper 12v compressors that are suitable for 1/4 of that price.

https://www.amazon.com/VIAIR-30033-3.../dp/B000X90YUO

[creativepart]

It's my personal opinion that it's not an exact science. If your cold tires are filled to 100 psi on 65 degree days and you leave at 9am and it's 45 degrees outside so your Tires read 95 PSI I would not stress over that little discrepancy. Perhaps others would, but I don't.

In fact, I don't change my tire pressure all that often. I do monitor my pressures before each trip and refill if they are consistently low by more than 5 psi. But other then that I just leave them alone.

You have to be reasonable. Otherwise you'll be changing tire pressures 5-times a day.

But that's for you to decide.

[Ray,IN]

The rule of thumb is, write down the cold tire pressure and the desired tire pressure and subtract. The difference is now much air pressure you add when the tire is hot.
That is not exact,as the added air is cold, but it's close enough to prevent tire damage.
FWIW over 80% of tire failures are the result of under-inflation/overloading.

The retired tire engineer that posts on these forums, says to add 10-15% to the load/inflation charts to provide a safety cushion, otherwise your tires are running at 100% capacity all the time.

[bigb]

Like Ray says, if you find yourself in a position where you need to drive somewhere to get air, just measure the tire pressure first and when you get to the air station put in the same amount you would have needed when you measured them cold.

I always wonder how Discount Tire and others get the pressure accurate at their drive thru air stations. I guess they've been doing tires long enough to get pretty close on passenger cars but I wouldn't trust that with a heavy vehicle.

[Ray,IN]

Originally Posted by bigb

Like Ray says, if you find yourself in a position where you need to drive somewhere to get air, just measure the tire pressure first and when you get to the air station put in the same amount you would have needed when you measured them cold.

I always wonder how Discount Tire and others get the pressure accurate at their drive thru air stations. I guess they've been doing tires long enough to get pretty close on passenger cars but I wouldn't trust that with a heavy vehicle.

Yes, auto tires have a 35% safety cushion between recommended air pressure and minimum required air pressure. RV tires can be less than 10%, so Discount tire has a wider range to play with with auto tires.

[cruizerEd]

There is a quick connect fitting up front intended to let a tow truck pressurize the air system.

You get an air gauge and attach it to an air chuck.
If you want to get fancy you can add a push button air blead valve up steam of the gauge.

Get a air hose long enough to reach all the tires.
Connect up everything.
Turn on the air at the fitting at the front of the coach.
Start the engine and wait for the air to come to full pressure.
Start filling the first tire keeping an eye on the gauge, if the pressure drops below your target pressure stop filling.
Bleed air out of the hose using the chuck or you fancy push button valve until you hear the compressor start.
Watch the gauge and as soon as it pass your target pressure start filling.
Repeat until you get the right pressure.
Do the next tire.

Takes slightly longer than using a quality air pump.
Uses a lot less equipment.
Fills the tires with dry filtered air.
If you do it right before you leave it doesn't add much idling time.
If you want use your cruise control switch to engage the fast idle to speed things up a bit.

[Morich]

Can we assume you are not yet using any form of tire pressure monitoring system?

I had to develop a totally different approach , once we added a TPMS that let me know what the pressure does on our tires!
Once I had a full time readout of each tire's pressure setting right in front of me with alarms to tell me when pressure changed, I had to change my thinking on how to best maintain that pressure!

This was not on a large class A but only a 30 foot and our recommended pressure was closer to 85 but once I found how often small things changed the tire pressure, I stopped worry about getting it right because it never was going to be right over a few hours!
With a readout on all six tires, I set it up to be "exactly right", only to find it would not stay there but changed constantly!

We parked the RV next to a board fence which shades different tires at different times of day and the pressure changed depending on which tire was in the sun.
When we went to a campground and the sun went down, I had to open the low pressure alarm setting to avoid it letting me know the tires were low at six AM!
I found the inside rear dual where the exhaust came out, tended to heat more after a couple hours of driving!

My final thoughts came around to not chasing that bouncing ball as it was never going to be right. Over time, we got the tire pressure down to a level where the ride was much better, opened the alarm setting to the levels we actually found meant something was wrong and stopped worry about something I could not change!

When we started at 6 AM, the tires were slightly lower than recommended but they soon warmed up and if we drove until mid afternoon, the pressure might be slightly higher than recommended but I'm certainly not going to stop and adjust it every 25-30 miles or every time the road changed directions to put different wheels in the sun!

I recommend getting a TPMS to let you watch what is happening and it will soon relieve the stress of not knowing and chasing a setting that you can't reach!

Too many things change the temperature and that changes the pressure! Sun/shade/ ambient temperature, The crown on the road that makes the inside dual carry more load, how the tailpipe fits and even whether you are driving in water will change the pressure.

You'll likely find you can never get it exactly right, once you get something to show you what it is doing, you stop the worry about something you can't control!

[ldfeat]

Originally Posted by Rocketman43

I have a Class A, Winnebago Journey. I have an issue/question regarding inflating tires to proper PSI. I know that we are to measure the PSI when the tires are cold (not driven on for a while). I know that the appropriate tire PSI is calculated from the tire pressure charts in combination with the weight on each tire/wheel. My issue is, if my PSI is low then I have to drive from my garage or campground to the service center to have air added and by that time I get to the service center the tire has built up pressure to be over the appropriate COLD pressure. So, do I add or not?
An exapmpe:
Appropriate PSI for the tire/wheel is 105 (when cold)
I measure the pressure before driving and it is 100, so I am 5 lbs low.
I drive to get air and the pressure has built up to 110.
The tires are rated for a max of 110 PSI.
Once at the service center do I add 5 lbs to the 110 up to 115 and go over the max PSI rating of 110 for the tire?

I could not find on this forum the answer to this specific issue. I may be on the forum, but I could not find it.

Thanks in advance for any advise,
Bob

That's how I've done it. The 110 max pressure is when it's cold. So you're correct when adding what ever lbs of pressure is needed to reach the 110 PSI measured before driving.
Larry 2014 Reyo P

[jadatis]

100 psi rising to 110 psi is easy calculating, rises 10%.
The gascompound in tire reaches such a temperature that pressure rises 10%

Not important what ambiënt temperature was, as long as its practically the same when you measured cold at home 100 psi and in the garage you measured 110 psi.

Then to get 105 cold, you have to fil also 10% more, so 115.5 psi, as far as you can measure it that acurate. That little air you fill up tires with, of ambiënt temperature, only marginaly cools down the gascompound in tire at that moment.
You can check it the next morning, if ambiënt temperature is about the same as when you measured 100 psi cold. Then you will see its 105 psi.

The tiremakers give cold 110 max, and know it will rise when driving..

Dont worry about the high pressure warm, only worry about tge temperature in tire it represents, and gives a certain temperature of tire-material, when no external factors. Tire material may not go above a certain temperature.

If you are happy enaugh wih this explannation, you can stop reading, but if you to know more, read on.

I now am busy with pressure rising, and will give list for 110 psi cold at 68 degr F( 20 degr C.)

Here a list, after that I will explain the temperatures in tire.
Are if dry gascompound in tire, at the end extra by enaugh water in tire behind W

68 F/ 20 C/ 110 psi
113F/ 45C/ 120.6 psi / rise 9.7%/ W 1psi
140F/ 60C/ 127 psi / rise 15.5% / W 2.6 psi
167 F/ 75C/ 133.4 psi/ rise 21.3% / W 5.2 psi


68 degr F / 20 degr C
The assumed ambiënt temperature when the 110 psi was filled.

113 degr F / 45 degr C
A temperature of gascompound in tire, you best aim at by using high enaugh pressure, it will be when driving in ambiënt temperature of 68 degr F, a speed of about 55 mph.

140 degrF / 60 degr C
When driving maximum speed of tire constant ( your 110 psi most likely 75 mph) in ambiënt temperatire of 68 degr F / 20 degr C. Tire inside gascompound will reach this temperature, and with that the pressure rising. This if cold pressure is determined using best formula to laws of nature, with no reserve build in for that tire.

167 degr F/ 75 degr C.
Driving maximum speed constantl of tire in ambiënt temperature of 104 degr F/ 40 degrC, the temperature of gascompound in tire reaches this temperature same conditions futher as for 140 degr F.

The temperatures are my estimations, so mayby in time I have to adyust them a little.
Used the strange Fahrenheit temps, because exact conversion of degr C in inrements of 5 degr C / 9 degr F.

Have made list for psi to use for US, and in Bar for almost the rest of the world, that uses ISO ( intergalactical standardisation organisation).

[ashaffstall]

FYI: The tire pressure on the Journey data plate shows pressure to support the max gross vehicle weight (GVWR). My 2004 Journey has GVWR of about 27500 lbs and tire pressure of 105. I don’t run at 105 because the way I run fully loaded is around 23,000 lbs split 8,000 on front axle and 15,000 on the back. Michelin tire ( my tire type) puts out a pressure guide for motorhomes that I use to set the PSI for my load which is about 90% of max PSI and this makes for a more comfortable ride.

[jadatis]

In my post #12 I forgot to calculate with absolute pressure instead of gauge-pressure.
Luckyli the difference is small at these high pressures.
I calculated with my made calculator using 65 and 75 degr F as ambiënt temperature, and both practically 5,5 psi higher filled needed warm.

For 35 psi the diffefence is more, so my first given system is to inacurate.

Edit: also calculated for 35 psi rising to 39 psi and gave same
Only influence of water in tire is larger.

[Morich]

The change from 65 to 75 is not enough to think about.
When we start with 40 in the morning and reach 80 in the afternoon plus add any changes from tire flexing and sunshine, then it does give us different pressure which will show.
Under ideal conditions, we could set an ideal pressure for all times but that would also require an ideal world!

[Wayne M]

At 65 to 75˚ there will be about a 2% change. Not enough to worry about.

[jadatis]

I calculated for 65 and 75 degr F ambiënt temperature at home of TS and the service centre, because TS did not give what temperature outside.
And so concluded that the first easy rule of tumb system works acurate enaugh.

In post #12 I gave that what ambiënt temperature is not important, only that its the same at home and service centre..

And now realise that its not even important, only at home temperature in tire, gives cold 100 psi, and at the service centre warm gives 110 psi,

So easy rule of tumb system, acurate enaugh for the goal. And can be checked by measuring it again next morning if right, but then ambiënt temperature must be practically the same.

Or you have to calculate back to first morning ambiënt temperature ( so temperature of gascompound in tire), only if more then 10 degr F difference between the 2 mornings.

100 psi at 70 degr F going to 110 psi, gives with rule of tumb 2%/10 degr F , 50 degr warmer in tire so 120 degr F.

I made more exact list, and between 97 psi and 109 psi it gives 4.5 degr F / 1 psi, so 45 degr F warmer so 115 degr F.

Exact calculated 100 psi at 70 degr F going to 110 psi gives if completely dry gascomppound in tire, like Nitrogen says to be, 116.2 degr F. , and if enaugh water in tire, like normal air filled with all the water as humidity in it, 112 degr F.

[Kim Petersen]

I don't know if everybody is aware of a battery operated tire inflator but I bought a Milwaukee Tool cordless inflator tool that will automatically air my steer tires to 105 psi and stop there. then i can do the drivers at 100 psi each, when pressure is reached it shuts down. easiest inflator ever, even quicker than my 5hp shop compressor. I bought mine on Amazon but I see them at most Milwaukee dealers. only drawback is it requires a rechargable M18 battery and charger. I still haven't had to recharge the battery after airing up several tires.

[bigb]

Originally Posted by Kim Petersen

I don't know if everybody is aware of a battery operated tire inflator but I bought a Milwaukee Tool cordless inflator tool that will automatically air my steer tires to 105 psi and stop there. then i can do the drivers at 100 psi each, when pressure is reached it shuts down. easiest inflator ever, even quicker than my 5hp shop compressor. I bought mine on Amazon but I see them at most Milwaukee dealers. only drawback is it requires a rechargable M18 battery and charger. I still haven't had to recharge the battery after airing up several tires.

I have the M-12 version and it works great although a bit slow on big tires. I generally use it for the bikes. I wish I would have had M-18 stuff but all my Milwaukee is M-12, I went with Makita on my 18 volt stuff.