Community Forums

wa8yxm · 08-02-2014, 06:13 PM

I use a 150 PSI to push my tires to 115 PSI. no tribble at all.

Jburklo · 08-02-2014, 07:32 PM

Quote:

Originally Posted by wa8yxm

I use a 150 PSI to push my tires to 115 PSI. no tribble at all.

I'm just curious why doesn't everyone use the compressor on the coach? I have a 2012 Aspire and am able to use the supplied air to fill up my tires. Yes I need to have the coach running to keep up the pressure but it works just fine.

Is there something wrong with this process? I have done the same with two other coaches and did not realize a separate compressor was needed.

Just curious.

Jeff & Terri Burklo
2012 Aspire / 2012 Jeep Liberty
New Braunfels, Tx

Sky_Boss · 08-02-2014, 07:41 PM

Quote:

Originally Posted by Fasteddie08

With respect, how does the full weight of the front end not have an effect. If you had an unmounted tire an aired it up to 120 psi (lets say) and then mounted it and put the full front end weight of the coach on it are you saying that the psi would not be higher? If his compressor is not strong enough to get the desired PSI that he's looking for if you take some weight off the front and the air will go in the tire.

I am saying that if you put 120 PSI in the tire off of the coach and mount the tire, it will still read 120 PSI.

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It's because the volume of the tire is constant regardless of the car's weight. If the volume of the tire changed due to the vehicle's weight then the pressure would change. The tire's shape will become more squashed from a heavier car, but the volume remains the same. This formula says it all: pV=nrT, which means pressure times volume is equal to the amount of gas times a constant times temperature. No matter the amount of weight, the volume will always be the same, as will the amount of air in the tire, r is constant, and the temperature is the same, therefore pressure remains the same.

ruffian · 08-02-2014, 07:54 PM

If you put 120 psi into a container, the only ways to change the pressure, exempting a leak, is by either changing the temperature of the inside air or by decreasing the volume of the container.

While a mounted tire does deform some it doesn't change volume. It only changes shape by pooching outward at the bottom. Jacking up the MH won't help air up the tires.

I also use my onboard compressor for the few times I needed air while on the road. Works well but does take a little time.

Sky_Boss · 08-02-2014, 07:54 PM

Quote:

Originally Posted by Jburklo

I'm just curious why doesn't everyone use the compressor on the coach? I have a 2012 Aspire and am able to use the supplied air to fill up my tires. Yes I need to have the coach running to keep up the pressure but it works just fine.

Is there something wrong with this process? I have done the same with two other coaches and did not realize a separate compressor was needed.

Just curious.

Jeff & Terri Burklo
2012 Aspire / 2012 Jeep Liberty
New Braunfels, Tx

I don't use the on board compressor for a couple reasons.

The first is that for the front tires the working around the cut in and out pressures is not my cup of tea.

The second reason is that I would rather move the compressor around with a short air hose and the light power cord. In good weather I can plug the compressor into the power post on the driver side and reach all tires including my toad and only need to move the compressor a couple times. I can then unplug from there, use a power outlet in the basement on the passenger side and not move it around much either. I also use a good quality curly hose to make reaching places easier. With the onboard compressor I would have to have at least 60' of hose to reach my toad if it is at the back of the coach.

Its just my preference.

Jonnyboy 316 · 08-02-2014, 08:08 PM

My 150 PC 3 gal. compressor fills my tires to115 to 120 easily. Make sure your compressor tank comes to 150 and shuts off.... then you can top off your tires. Mine are typically at 110 or so...then I fill to 115 or 120...depending on load...temperatures....etc.
Good luck in your travels. Jonnyboy 316

Fasteddie08 · 08-02-2014, 08:11 PM

Ok, so when your tire blows because it gets hot was the pressure pressure constant ? Answer No. You are using a formula that you don't understand fully. No disrespect. I will make this simple.

Say you have 4 tires that read 25 pounds pounds per square inch while off of the car. (The tires have a maximum pressure of 50 psi, so there is room for more pressure.)

Once you put them on the car would the weight of the car increase the psi reading?
The equation you are looking for is the ideal gas law:

PV = nRT

P = Pressure
V = Volume of the container
n = The amount of gas
R = 8.314472
T = Temperature of the gas in the container

Solving for P you get:

P = (nRT)/V

In your scenario, you fill the tires and get a reading. You then put them on the car and read another reading. What has changed?

The amount of gas, R, and the temperature remain constant. (The temperature actually increases a little but that's not important right now.) However, the car slightly decreases the volume of the tire by deforming it.

In terms of the equation, if V is smaller, P must be larger. Ergo, the pressure goes up.

So back to the original statement, if he is getting close to his required PSI with his air compressor but just can't quite reach it, then taking the weight off the tire will help reduce the opposing pressure and allow more air to go into the tire. The inlet force has to be greater then the force trying to come out.

By your theory, pressure is constant regardless of any "outside forces" Go get your on open container of orange juice out of the refrigerator. It has a constant pressure, now stand on it and tell me what happens.

Sky_Boss · 08-02-2014, 08:26 PM

Quote:

Originally Posted by Fasteddie08

Ok, so when your tire blows because it gets hot was the pressure pressure constant ? Answer No. You are using a formula that you don't understand fully. No disrespect. I will make this simple.

...

WOW...I'm kinda stupid so you didn't make it any simpler. LOL Besides, the scenario we are discussion was whether or not tire pressure changed after mounting without any other factors affecting the tire.

I'm just gonna say that I have done the exact thing with checking tire pressure before and after mounting and saw no significant difference. That is about as simple as can understand.

BTW...regarding tire blow out due to hot temps, I'm totally in agreement that rising temps will change pressure.

Mychesh · 08-02-2014, 10:16 PM

Been reading this compressor issue with interest I have been using power tank ever since I have been RVing and I would not leave home without it!!!

There is so much info on their site that it best you read and make your own

http://powertank.com

Be interesting to hear your thoughts on it

Steve

johnnyreno · 08-02-2014, 11:27 PM

the porter cable just wont push mine may be a bad unit I also tried it while up on jacks would not cut the mustard

oneal · 08-03-2014, 06:42 AM

Quote:

Originally Posted by johnnyreno

the porter cable just wont push mine may be a bad unit I also tried it while up on jacks would not cut the mustard

I agree, I think you got a bad one Johnny

Sky_Boss · 08-03-2014, 07:44 AM

Quote:

Originally Posted by Mychesh

Been reading this compressor issue with interest I have been using power tank ever since I have been RVing and I would not leave home without it!!!

There is so much info on their site that it best you read and make your own

POWERTANK - CO2 Air Systems

Be interesting to hear your thoughts on it

Steve

This Powertank HD Pressure Gauge is great! You get over 5' of stand off distance and the ability to add or release air from the handle.

I'm not convinced that the space and weight of their tanks is worth it. I'm happy with my portable compressor for versatility.

Sky_Boss · 08-03-2014, 07:54 AM

Somewhat related...

I would like to say that there is perhaps one advantage to the onboard compressor that might be worth considering. That is because it uses dried air. I'm not so certain it is a huge factor but perhaps over a cumulative period of time it might be a factor.

With any portable compressor I would do my best to get any moisture out of the system that is possible.

Dogpatch · 08-03-2014, 08:15 AM

I use the same Porter Cable 150psi. It has to cycle a few times, but I can get my tire pressure to where I want it. I thought about the coach compressor, but out of respect for my neighboring campers, don't like running the Diesel engine for 20 minutes to use that method. I have room for the compressor.