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Old 03-10-2010, 09:04 PM   #1
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Tire inflation pressure

I have always run 100 psi in front and rear (dual) tires as this is what the data sticker on the wall at the drivers seat said. It's a 38' Pace Arrow with 22.5" tires. This pressure seemed odd to me but it is what it is. Today I found another data sticker in the rear closet and guess what it said, front 95 psi and rear (dual) 85. Now this makes a lot more sense and matches what I have been hearing here. This miss information sux, this is not rocket since.
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Old 03-10-2010, 09:46 PM   #2
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Why don't you take your fully loaded MH with all the tanks full into a truck stop and weigh it on some CAT scales. Make sure your front and rear axles are on the separate platforms so you get separate axle weights. Then divide the front by two and the rear by four and look up those weights on the tire manufactures load and inflation charts. Add 10% of what ever air pressure they say for safety.

If they are Goodyears then go here for the PDF tables
On The Wings of Goodyear | RV Tires - Tire Care: Proper Tire Inflation

If Michelins go here
Michelin North America RV Load & Inflation Tables

If some other brand and you can't find it then I bet somebody on this forum can direct you.
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Old 03-10-2010, 09:57 PM   #3
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Plan that next trip. I have only gone on two trips so far. They are Michelin.
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Old 03-11-2010, 12:08 AM   #4
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you will be amazed at how much better your motorhome will handle and ride if you

a) have it aligned

b) weigh each axle

c) inflate according to the table for your tires ...
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Old 03-11-2010, 07:11 AM   #5
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The bottom line is you can't properly determine the proper inflation pressures without knowing how much your rig weighs when it is loaded for travel. The pressures on the stickers as determined by the manufacturer are usually based on the maximum weight rating for the axle, not for the actual weight.

Even better than weighing each axle is weighing each corner. Side-to-side weights can vary, too. Set the pressures on each axle according to what is required for the heavier side on that axle.
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Old 03-11-2010, 08:48 AM   #6
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The sticker by the driver seat is for the max axle capacity stated thereon, per Federal regs. The sticker in the back is the manufacturers recommended inflation, but that is based on "typical" loads. It is unlikely you need either one, but the only way to know is to weigh it, as others have already said. It is well worth the $8-10 it costs to us a public scale.
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Old 03-11-2010, 10:21 AM   #7
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For the maiden voyage in our brand new coach I hadn't yet gotten it weighed so inflated all tires to the 110lbs on the sidewalls. It rode so rough and loud that I was devastated... thinking I had bought a bucket of bolts.

After getting axel weights, referencing the mfg web site and inflating properly... what a difference. good luck!
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Old 03-14-2010, 10:51 AM   #8
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The number on the sidewalls is for max tire carrying capacity. The number on the wall by the driver seat is for max axle capacity. Both are likley more than is needed for the actual load (but not always).
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Old 03-14-2010, 11:17 AM   #9
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Thanks everyone, I will get it weighed on my next trip. We are going the end of the month for about a month so it will be loaded. Will go to a truck stop so will be front and back not each corner. I have a high frequency vibration most noticeable at 60 and above, do you think this could be the problem, running 100 PSI? That would be nice.
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Old 03-14-2010, 11:30 AM   #10
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My guess in the high frequency vibration is a tire balance issue. If they are steel wheels then they should be checked for tire runout the aluminum wheels are trued on a lathe and are normally right on. On my Dolphin I had the front tires rotated on the rims until a had minimum runeout then had them trued and spin balanced. They are as smooth as glass after many years of high frequency vibrations.

The CAT scales are normally divided up into several platforms so you can put the front axles on one and the rear axles on another platform. It is hard to do just one side then the other because the scales are usually a little above ground so you put the MH on a tilt which will make the wheels being weighed slightly lighter. I think just weighing the axles as a whole is close enough especially if you increase the tire pressure by 10%. Remember air temps are going to change the tire pressures. If you are in the cold north and drive to warmer Florida then you tire pressures are going to increase as you drive south and should be adjusted and it will happen going the other way also. The tire pressure will drop when it gets colded.

One thing a forum member taught me was if you have an air bag suspension that you should line up with the scales and dump the air and let them refill before you weigh the MH to ensure the MH is level. This especially applies if you have to make a tight turn before you get on the scales.
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Old 03-14-2010, 11:57 AM   #11
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Proper Pressure

Pete & Linda
Most of the info posted here is pointing you in the right direction. If you can't get individual sides weighed at least get each axle weighed. Until you get data on actual side to side it might be safe to assume one side is 5% heavier than the other so multiply the total load on each axle by .575 then with that number look up the inflation needed to carry at least that load (go up to the next higher inflation in the table) This would be the Minimum Cold inflation for every tire on that axle. An extra margin would be to add another 5 psi.
If you go to a major rally like The Rally or a big FMCA Convention you can get each corner weighed accurately by RVSEF and they will do all the math for you. You can check their schedule at RV Safety, Merritt Island, Florida.

FYI Cold inflation means not driven on for about 3-4 hours and not in full sunlight. Also be sure you know your pressure gauge is accurate. A quick check would be to borrow another pressure gauge and compare. If they agree +/- 2 or 3 psi they are both probably OK. If they disagree then one or both need to be checked against a third gauge.
At a recent rally I did a gauge verification check against a certified digital gauge and most were OK but 12% of the gauges were off by 10 psi or more with a couple off by more than 15 psi.
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Old 03-14-2010, 11:58 AM   #12
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What does cold tire pressure mean?

I understand that tire pressure increases with temp. but how do you determine the correct pressure when the ambnient temp. is really cold (below freezing for instance). The cold temp. pressure would vary with the ambient temp. Should the tires be inflated to the recommended cold pressure regardless of the air temp. If I inflate the tires to the recommended cold pressure at 20F for example and drive into a warmer climate where the air temp is 70F will the tires be overinflated?
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Old 03-14-2010, 01:05 PM   #13
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Good question. I doubt that you will wake up one morning and find it 20F and then later in the afternoon be driving where it is 70F. Tire pressure only changes about 1 to 3 psi per 10 degrees change in air temp so even in your extreem example if you set your pressure when it was 20 then the cold temp might increase by about 10psi when it was 70 is not a big deal. In general tires warm up by 30 to 60 degrees above ambient when run hard (fast & high load) so a pressure rise of 5 to 20 is normal.
I know I am giving broad ranges but there are a lot of variables.
In general the main thing to remember is not to blead down a hot tire to the cold inflation spec. If you see more than say 3 to 5 psi change from morning to morning then you might want to adjust. I check my TPMS and only take action when the cold goes down by 5. I can do thin because I have a bit more than 10 psi safety factor from the minimum inflation based on individual corner weights.
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