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Author Topic: Yet another tire pressure question  (Read 291 times)

fijidad

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Yet another tire pressure question
« on: August 28, 2017, 12:38:08 PM »
Have our 36-foot DP in for servicing and the tech said they were going to set the tire pressure at 120, per the manufacturing tag behind the driver's seat. I said "isn't that the MAX. tire pressure?"...he said no, it's what the TP should be set to.

I'd read much before this experience and had set my TP at 80 based on front and rear weight and tire size.

Any thoughts on this?
2001 Monarch Windsor 36' DP

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kdbgoat

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Re: Yet another tire pressure question
« Reply #1 on: August 28, 2017, 12:55:18 PM »
Go back to what has worked for you in the past as long as it isn't below the tire manufacturers recommendations.
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SeilerBird

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Re: Yet another tire pressure question
« Reply #2 on: August 28, 2017, 12:57:01 PM »
The 120 is wrong, that is the maximum pressure the tires are good for. Weight the rig and consult the tire manufacturers chart for the proper pressure.
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Larry N.

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Re: Yet another tire pressure question
« Reply #3 on: August 28, 2017, 01:02:02 PM »
Tell the tech not to mess with your tire pressure, just use what you've been doing, based on the mfr's charts.
Larry and Mary Ann N.
2016 Newmar Ventana 3709 -ISB6.7 XT 360HP
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TonyDtorch

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Re: Yet another tire pressure question
« Reply #4 on: August 28, 2017, 08:20:42 PM »
Some people over on the FMCA forum are recommending +5% of the recommended max pressure.

Harvard

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Re: Yet another tire pressure question
« Reply #5 on: August 28, 2017, 09:07:13 PM »
From the stand point of handling, the tire pressure on the fronts should be set to the minimum pressure required for the actual load. To do otherwise would reduce the pneumatic trail which will reduce the "Self Aligning Torque" (SAT) which has an effect similar to reducing the positive caster.

https://en.wikipedia.org/wiki/Self_aligning_torque

Gary RV_Wizard

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Re: Yet another tire pressure question
« Reply #6 on: August 29, 2017, 07:27:56 AM »
Quote
tech said they were going to set the tire pressure at 120, per the manufacturing tag behind the driver's seat. I said "isn't that the MAX. tire pressure?"...he said no, it's what the TP should be set to.

The psi shown on the placard by the driver seat will be for the coach when it is loaded to its max capacity (GVWR). That is NOT the tire's max load pressure, which is embossed on the sidewall. [I think the latter is what Seilerbird referred to.]

If you have scaled weights that show your coach is loaded to less than its GVWR and each axle's GAWR, then you can safely use a lower pressure, which you would obtain from the tire manufacturers load & inflation tables. However, I am skeptical that the difference between the actual load and GVWR load would allow a 40 lb difference in psi.  That's an unusually large variation, though not inconceivable. I suggest that you re-visit the inflation table and make sure your 80 psi is correct for the load.

Your tire psi should be set at least 5 psi above the minimum for the loaded weight as shown in the inflation table. That allows for variations in load and ambient temperature, so that you need not be concerned on a daily or even hourly basis.
« Last Edit: August 29, 2017, 07:29:48 AM by Gary RV_Wizard »
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Rodney Davidson

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Re: Yet another tire pressure question
« Reply #7 on: August 30, 2017, 08:45:50 PM »
Be sure to remember that you will gain 10 to 15% in pressure once your on the highway for some time.
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A Traveler

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Re: Yet another tire pressure question
« Reply #8 on: August 30, 2017, 09:57:39 PM »
You need to find a new tech. He's clueless about this. That is NOT how you determine the correct tire pressure.

Increasing air pressure as the tire heats up is of no concern in this process. You set the tires to the correct pressure (according to a load chart) when the tires are cold, and you're done. The pressure increase as the tire warms up is figured into the cold set pressure by the tire manufacturer.

I set my tires to the correct pressures 3 years ago and have not touched them since. For my coach that's 110 psi in the steer tires and 95 psi in the rears. I've run them over 50,000 miles since then. Drives and rides like a dream.

Larry N.

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Re: Yet another tire pressure question
« Reply #9 on: August 31, 2017, 07:52:06 AM »
Be sure to remember that you will gain 10 to 15% in pressure once your on the highway for some time.

That isn't necessary to determine cold pressure desired, but with a TPMS also having a temperature readout, it certainly is a needed understanding to avoid changing tire pressure because of those pressure gains. Of course the tires are designed with that in mind.
Larry and Mary Ann N.
2016 Newmar Ventana 3709 -ISB6.7 XT 360HP
2015 Wrangler Sahara Unlimited toad
Formerly: Trailmanor 2720SL
  de N8GGG

 

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