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Old 08-18-2015, 05:14 PM   #1
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(Yet Another) Post on Tire Pressure - Help Me Stop Worrying!

I just bought a used, but awesome, class A motor home. One of the things I remember reading all throughout the buying process is to 'make sure the tires are inflated properly'. I had the unit weighed, and it comes out at 7760 lb steer axle and 14380 lb drive axle. It is a dually on the rear, with Michelin 235/80R22.5 G all around.

Using the table at Michelin RV Tires | View Michelin RV Tires load and inflation tables for motorhomes and recreational vehicles. | Michelin RV Tires I'm pretty sure this means I need ~82 lb on the front and ~84 lb on the back. Checking the tires, they are currently 86 lb on all, apart from the passenger rears which is 82 lb in one and 84 lb in the other. Two questions then;
  • Assuming my math is correct and 86 lb is in the ballpark, why do the tires seem flat when at rest? I.e. when I'm parked, the tires look worryingly flat on the bottom.
  • Should I worry about the difference between the two passenger rear tires and then the passenger rears and the driver rears? I.e. should I rush out to my nearest truck stop and add 2 and 4 lbs to them respectively, or should I just do it whenever I happen to be somewhere with air?

I will note I'm about to take a ~200 mile journey on the highway, so I'd like to be as safe as possible.
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Old 08-18-2015, 05:28 PM   #2
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Quote:
Originally Posted by Ropetin View Post
I just bought a used, but awesome, class A motor home. One of the things I remember reading all throughout the buying process is to 'make sure the tires are inflated properly'. I had the unit weighed, and it comes out at 7760 lb steer axle and 14380 lb drive axle. It is a dually on the rear, with Michelin 235/80R22.5 G all around.



Using the table at Michelin RV Tires | View Michelin RV Tires load and inflation tables for motorhomes and recreational vehicles. | Michelin RV Tires I'm pretty sure this means I need ~82 lb on the front and ~84 lb on the back. Checking the tires, they are currently 86 lb on all, apart from the passenger rears which is 82 lb in one and 84 lb in the other. Two questions then;


  • Assuming my math is correct and 86 lb is in the ballpark, why do the tires seem flat when at rest? I.e. when I'm parked, the tires look worryingly flat on the bottom.
  • Should I worry about the difference between the two passenger rear tires and then the passenger rears and the driver rears? I.e. should I rush out to my nearest truck stop and add 2 and 4 lbs to them respectively, or should I just do it whenever I happen to be somewhere with air?



I will note I'm about to take a ~200 mile journey on the highway, so I'd like to be as safe as possible.

If your weights are correct then the psi of 86 all around is adequate. Please note, the pressure in the table is the lowest pressure required to carry that particular weight. It will be perfectly fine if you add 5-10 psi all around. This will protect you if one end or the other of each axle is significantly heavier than the other end. Many RV owners routinely add 5-10 psi as a safely buffer.

Try to keep the dual pairs on the drive axle within 2-3 psi of each other. The psi will rise as you drive and the inner tire usually rises more than the outer tire. Down worry about these pressures as you drive.


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Old 08-18-2015, 05:32 PM   #3
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First, right now, quit worrying. Second, go buy a portable air compressor to have on hand either at home, or stored in your motorhome. Third, at your convenience set the pressures to what the charts say.
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Old 08-18-2015, 05:54 PM   #4
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Thank you both for your responses, make me feel much better that I'm not doing something totally stupid.

Wildtoad; do you have any suggestions on compressor? We live in our coach now, so size and weight is important. I've seen the Dewalt D55140 has good reviews, but it looks pretty chunky.
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Old 08-18-2015, 06:38 PM   #5
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First off, those load/inflation charts reflect the minimum air pressure for the corresponding load, not the optimum. Air to the mfgrs. tire placard in the MH, never less according to the RMA = Rubber Manufacturers Association, of which, Michelin, Goodyear and virtually all major tire mfgrs are members. CH. 4, pg 51 pertains to inflation of MH tires.
"
However, never use inflation pressure lower than
specified by the vehicle tire placard, certification"
label or owner’s manual.Many MH owners ignore this information because they prefer a soft spongy ride, at the expense of tire safety.
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Old 08-18-2015, 07:52 PM   #6
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My Class A has a compressed air coupling where I can attach an air hose. That is how I added air to all my tires last week.

Check to see if you have one? You could add one via an empty port on the compressed air manifold.
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Old 08-18-2015, 08:25 PM   #7
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Radial tires always look a touch underinflated when cold and at the right pressure. In fact, I was just going through my Spartan manual and it had that very comment right there in print.

I can't tell you how many times I have been sure I had an issue, grabbed a gauge, and found the tire spot on for pressure.

Personally, if 86psi was the right number, I'd likely round up to 90psi. That addresses any side-to-side imbalance, and isn't so much over the top that the ride or tire wear is compromised.

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Old 08-18-2015, 10:14 PM   #8
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One note about air pressure is the pressure affects ride and handling. While observing the minimum and also the maximum you can play with pressure between. Sometimes a little higher pressure in the rear can improve sway stability but as the pressure goes up there is some harshness to ride as the tire is stiffer. The only observation with over inflating is watching wear patterns but at the weights you have I think you could do anything you wanted.
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Old 08-19-2015, 07:49 AM   #9
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Originally Posted by SteveLevin View Post
Radial tires always look a touch underinflated when cold and at the right pressure. In fact, I was just going through my Spartan manual and it had that very comment right there in print.

I can't tell you how many times I have been sure I had an issue, grabbed a gauge, and found the tire spot on for pressure.

Personally, if 86psi was the right number, I'd likely round up to 90psi. That addresses any side-to-side imbalance, and isn't so much over the top that the ride or tire wear is compromised.

Steve
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Old 08-19-2015, 06:55 PM   #10
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Thanks for the advice everyone! 90 lbs is where it's at. I had a nice smooth, comfortable, safe trip today. Now the roof leak I found when I got here is another story for another thread :/
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