Ideal air pressure to maintain in my tires

[Richard Cooper]

A couple or more months ago I had 6 new tires installed on my coach.  I specifically asked the manager of the company that installed them what air pressure to maintain and he did not tell me.  I forgot about it.  So, last week I called him and talked with him --- asked him again.  He said he would get back to me.  Never heard anything.

Last week I had Camping World in Chattanooga check pressures and even they wouldn't tell me whether they added or took away.  What's with people these days?  The service tech said to keep 100 psi all the way around, but I don't quite think that is correct.

You all know more about this anyway.  I have the 2001 Marquis 40JSP --- non-tag axle model = 6 tires.

My tires are:  Michelin 295/80R/22.5 XZA2 ENER

How much air pressure to maintain in front 2 tires?

How much air pressure in back 4 tires?

[Larry and Heidi Lee]

Hi Richard, congratulations on the new Michelins. Your tire pressures can only be determined based on the weight of each corner of your fully loaded rig. Find a tire center that can weigh each corner of your rig on a scale. Until you have done this nobody can tell you what tire pressure to run for optimum performance and safety.  

[Jerry Carr]

Hi Richard,

Both the tire dealer and CW really can't give you an ideal tire pressure, you need to weigh the coach to know what the actual weight is on each axle  then you can look at the published tire pressures in your Beaver manual if you guess 100 lbs on all tires this is just that a guess (FYI not a bad guess but not correct).

It is very important to know your weights you may very well be under or over inflated. On our rig based on the weights we run 105lbs on the fronts and 90lbs on the driver and tags but again this will be different for every coach.

I recommend that you fill your coach to the average way you travel, then fill your tanks water and fuel then go to a scale and get your weights its the only way that you will every know what your tire pressure should be. If you choose the best guess method that your tire dealer offered you will have wear your tires or worse have a failure. The number one reason for RV accidents is tire failure.


If you can visit a FMCA rally they have folks that will provide weights for each tire with the use of portable scales this is really a must have. If you can even get to a public scale you can at least get the weight on each axle this is not a good as getting each tire position. Once you have the actual weights you can look up your tire in your Beaver owners manual then you have the system correct.

[Richard Cooper]

Assuming one fills up the diesel tank, fills up the water tank, and is loaded up for a trip.  You air your tires for that.  What about when your diesel tank is half full and your water tank is lower?  Won't that affect your tire pressure?  I mean you can't always have a full tank of diesel or water.....well, water is easier to keep full than diesel.

I'm going to a rally next week and will ask --- it's a chapter of FMCA, but not a FMCA rally per se.

[Joel Weiss]

Assuming one fills up the diesel tank, fills up the water tank, and is loaded up for a trip.  You air your tires for that.  What about when your diesel tank is half full and your water tank is lower?  Won't that affect your tire pressure?  I mean you can't always have a full tank of diesel or water.....well, water is easier to keep full than diesel.

Richard--

If your tires are inflated for the load with full tanks then they will be completely fine if you only have half-filled tanks.  Nothing prevents you from running slightly higher pressure than is needed; for the kind of weight differences you are talking about you won't even notice it.  Remember 100 gal of water is only ~800 lbs and diesel fuel is slightly less.  Compared to the coach weight this is pretty trivial.

Joel

[Gerald Farris]

Richard,
You should inflate all your tires to 120 PSI until you weigh your coach and know the proper inflation that is needed for the weight of your specific coach.

Gerald

[Richard Cooper]

Where do you find scales to weigh each tire side?  Besides a FMCA rally that is.

[Joel Ashley]

Richard,

Go to the RVSEF site here   http://rvsafety.com/rvsef-authorized-weigh-sites/
and click on the tab for "Weighing Program".  That will give you some options, one of which may help you.  There may be a link to State scales sites as well, but you can check individual State department of transportation websites for locations and availability also.  Most may not be capable of weighing individual wheels though, like the portable scales of RVSEF, and you need individual wheel weights to know if you have an overloaded quarterside and have to redistribute some load.

After they weigh your coach, they analyze manufacturer info along with your actual weight distribution, and mail you a report that recommends appropriate tire pressures.  It includes other data and safety ideas as well, and is well worth the minimal cost.

Joel

[Edward Buker]

The advice given about getting the coach weighed by Gerald and others is spot on. It is the only way to really know what the pressure should be. The fronts being set at 120lbs is probably a minimum safe value. Those Michelins in duals are rated at 11,500lbs at 100psi, 12,000 at 105psi, 12,550 at 110lbs, 13055 at 115psi, 13900 at 120psi. Coaches like ours have come in at 11,000 and 11,800 on the rears depending on the side. This is not to say that your coach could not be somewhat higher weight depending on options etc but 110psi at 12,550lbs may be a safe estimate before getting the actual weights but it is a guess without knowing for sure.

 As a query has anyone ever measured more than 12,550lbs on a rear single axle Marquis from 2000 to 2002? I ask this because going to 120lbs will stiffen the ride considerably in the rear and add some suspension/coach wear and tear that may not be necessary.

Later Ed

[Steve Huber]

Once you know the weight on each wheel, go to the Michelin web site and select RVs. You'll find a load/PSI table that will define air pressure for a specific weight on each wheel (duals in back). Site link is  http://www.michelinrvtires.com/michelinrv/tires-retreads/load-inflation-tables.jsp

You can get your coach weighed at any truck stop that has the CAT scales logo. As noted earlier, be sure to have the coach fully loaded; water, fuels, etc. I filled my fresh  water tank and had my black/grey tanks 50% full.
Steve

[LarryNCarolynShirk]

Richard,

Do not use the tire chart in the Beaver Owner's Guide.  Your coach probably originally came with Toyo tires, and their specifications are different than Michelin's.  The Michelin web site is up to date.

Ed,

My rear axle weighs 23,000 lbs.  You must have one of those light weight specials at 13,000 lbs.  

Larry

[Edward Buker]

Larry,

Only when I fill the coach with helium...I should have said 12,500lbs per side.

Later Ed

[LarryNCarolynShirk]

Another consideration for filling tires is the ambient air temperature.  Most charts are established for 70 degrees at sea level.  Check the chart to be sure.  I have seen a chart listing corrections for temperature.  If it is cold when you check the pressure will read lower in the tire.  Of course, altitude could be considered also.  These big tires cam vary 5-6 pounds either way.

In Oregon, many of the state run truck weigh stations are left operating even when closed.  We can weigh for each axle free and then weigh one side front and back.  With a little math, we can figure the weight of each corner.

Larry

[Richard Cooper]

You can get your coach weighed at any truck stop that has the CAT scales logo. As noted earlier, be sure to have the coach fully loaded; water, fuels, etc. I filled my fresh  water tank and had my black/grey tanks 50% full.
Steve

A friend at my rally this weekend suggested the CAT scales, but said that due to the design of the scales which you drive through --- you can only get the weight of the front axle and then the rear axle --- not the 4 tire corners.

Will that be adequate?

[Joel Ashley]

It's not entirely adequate, Richard, because it won't tell you how well-balanced your coach is.  You need to have the same pressure in your tires on one side as on the other;  so if, as an example only, you have 110psi in your two left rears, you need 110psi in the right rears too.  

To reiterate what's been said above, if like most rigs, your coach is heavier on one side than the other, just weighing the front and back won't tell you that.  You need to know just how heavy the heavier corner is, because the tires on that side need to be at a pressure to handle that side's load.  And then the other side will need to be pumped up to the same pressure.

For example, the right rear of my coach is a full 1500 lbs heavier than the left rear, when the rig is fully loaded for a trip (which is how it should be whenever you weigh it).  The recommended pressure for the load on the right is 110 psi.  I then put 110 psi in the left rear tires as well, even though 100 psi might be adequate for the lighter loaded left rear.  The pressure must be the same across an axle or across the front.

Weighing just the front and rear won't help you come up with the optimum pressures.  The precision RVSEF weighing program is best, as they send you a report with appropriate recommendations.  But if you can at least find a State scales that isn't full of trucks, you may be able to get a ball-park four corner weighing, and then using tire manufacturer charts or coach builder certificates, you should be able to zero in on the right numbers.  In my state of Oregon, the State truck scales are left on 24/7, and since many are on State highways rather than just Interstate freeways, it's not hard to find one in the evening or at night that is virtually deserted.  You simply drive up and weigh the front total, then the rear total, then maneuver to check individual corners.  The scales' digital readout board is easily visible so you can jot down each weight.

In the meantime, I'd consider taking Gerald's advice;  120 psi won't hurt until you have more precise numbers.

Joel