Filling Tires

[Ned]

Keep in mind that the pressures shown on the chassis label apply to the original equipment tires.  If they have been replaced with a different, often larger, size, or sometimes the same size in a different manufacturer, you should use the inflation tables for the new tires.

 

[jlazar]

Ned.  The load and inflation table on the Toyo site shows my tire size with an asterisk.  The asterisk states that it is compatible/comparable to the original Michelin tires put on by Workhorse as standard and retained by Tiffin as standard.  Based on that I thought the inflation specs from the Workhorse label would also apply to the Toyo tires.  I really appreciate everyones input on this.  I am quickly learning how more important attention to tire wear and care is on an RV compared to what people give to their auto tires.

 

[Ned]

If Toyo says the tires are compatible, then you should be safe using the placard pressures.  Most of us will inflate to 5 or 10 psi over the specified pressure, whether from the placard or a chart, to allow for leakage over time and for those cold mornings when the pressures drop.  The most important item is to never under inflate, as others have stated.

 

[jlazar]

Thanks.  Will do.

 

[SargeW]

I've never found a statement about any tires that would allow using less tire pressure than the vehicle manufacturer's recommended pressures.

Not true. Checking the tire inflation pressures on the manufactures certification label lists the max inflation pressure of the installed tire.  Which may apply if the vehicle was being operated at the max GVWR as listed on the certification label.  All one needs to do is look in the owners manual for the vehicle, as well as the Michellin tire manual included with the rig.  Both specify to obtain a 4 corner weight and adjust the tire pressure accordingly. 

 

[jstrubberg]

Just to muddy the waters more, if you've never chalk-tested your tires, you really should.  Particularly if you have a tire size different from OEM.

You use sidewalk chalk to mark a heavy line across the tire, then drive straight forward for 100 feet or so, then check the wear of the chalk. If the chalk is wearing in the center and not towards the edges of the tread block, you are overinflated.  I can just about guarantee you that you will have to drop air pressure to get a good, square contact patch.  When we did your rig, we had to drop 15lbs off of the manufacturer "reccomendation" to get things right.

The rig rides one heck of a lot better.  We haven't had our RV long enough to see what this does for tire life, BUT I did the same thing to my Jeep three years ago and I am at 54,000 miles on a 40,000 mile tire!

 

[sluggermike]

A couple weeks ago I bought a 3 gallon pancake type compressor from Harbor Freight for about $60.  It is rated at 100psi which is good for me because I run about 85 to 90 psi in the front and the rear tires.  The compressor is light weight and easy to store.

 

[jlazar]

The past few mornings when I checked the air pressure in the tires, the side facing the sun was 2-3 PSI higher than the side in the shade.  How do you adjust pressure in this type of situation?  Do I fill the low side to match the side in the sun?  I would think that side would then be higher than the sun side once I am running and the shade side gets in the sun.  Or do you just make sure you start with the same pressure in the tires regardless of whether they are in the sun or shade?  I want to make sure I am monitoring and filling my tires properly.  I do not yet have a TPMS but am leaning towards the TST.

 

[mygreyhound]

jlazar

My Workhorse W24 chassis states similar limits for my coach (37 footer) 110 front, 100 rear. Not sure exactly where on your coach the specs would be, but on my 2005 Suncruiser I believe it is on the gross weight info chart on the back wall of the one clothes closet in the bedroom. I've heard that's a common place to find it.

My take on it is there are 3 separate entities giving their choices of the correct tire pressure. The chassis mfg. (in this case Workhorse), the coach builder (mine is Winnie & is slightly lower pressure than the chassis mfg.), and then the tire mfg. (Michelin in my case) that allows even lower pressure according to specific, four corner weights. I went with a slightly higher (2 lbs.) than the Michelin chart. It rides smooth and after traveling over 50,000 miles on the Michelins they still looked new. The only reason I took them off was they were timed out. They wore so evenly, you couldn't identify the drive tires from the steer tires.

I check them early enough so the sun hasn't warmed up one side or the other. BTW Milton is a quality, American mfg of tire air chucks and gauges. I went thru 3 crappy Chinese ones in 6 months before I could find an American made one.  Good luck. GR 

 

[jlazar]

I know what range I want.  100-105 in front and 95-100 in duals.  My question is if cold temps for front (sun) reads 101 and front (shade) reads 97, if I take both up to cold 105 will they both read the same once they are both in the sun and heated up.

 

[SargeW]

Doubt it. If the sun side is higher by a few pounds and you add that much to the shade side, when the shade side warms up it will be a few pounds higher.  For what it's worth, a few pounds in a tire that is inflated to 100 psi is not that much difference. 

If you want to try to match them though, if your sun side is 102, and the shade side is 100 psi, they will be pretty close once you start driving down the highway.

 

[Gary RV_Wizard]

As Sarge says, you don't need to worry about a couple psi as long as it is at or above the minimum for your weight. If you want to be more perfect, inflate the tire on the sunny side a little extra, maintaining the same amount more than the shade (cool) tire. That way, if you turned the coach so the sunny tire was in the shade, it would be at the "right" pressure once it cooled. Of course, if you turn the coach some other tire is probably in the sun, so its get s few psi higher.

Most of us inflate about 5 psi above the prescribed minimum for the actual weight, thus insuring that the pressure remains ok across a range of temperatures and load conditions. A little extra never hurts, but a little low is a Bad Thing.

If you are using the RV manufacturers weight sticker (found on the wall by the driver seat), it is usually based on an assumed max load, i.e. each axle loaded to its max capacity. That typically makes the front tire pressures quite high, since most rigs have extra capacity on the front axle (rears tend to be loaded closer to their max). Get an actual scaled weight before you make any adjustments, though, because more than a few rigs will be exceptions to what is "typical"..

 

[jlazar]

Thanks.  I have had it weighed and Gary is right concerning back having more weight than front.  9K capacity on front weighed in at 7400.  15K on duals weighed in right at 14K.  My target for front is 100-105 and rear is 95-100.  These figures are from the Workhorse labels, the Tiffin labels, and the Toyo load/inflation tables.  All are within 5 lb. of each other.  I plan to go with about 103 in the front and 98 on the duals and see how the ride is.  I will go 105 and 100 if I ever start nearing load capacity.  Based on your feedback, I will just inflate to these values and not be too concerned about the sun/shade.  I am usually filling before 9am and I'm not sure if the difference in PSI on the same axle (sun and shade) is due to the sun or are the tires actually off by that amount.  Look forward to when I can afford a TPMS to have another tool to use.

 

[Gary RV_Wizard]

Check the pressure on each axle after driving 75 or so miles - the heat from road contact should have pretty much overwhelmed any sun effect on one side. Make the tires read the same psi then. Don't drop them down to the cold pressures - just make them read the same psi (within a pound or so of each other) across each axle.

 

[jlazar]

Will do.  Thanks again for everyone's advice.

 

[BernieD]

Use the one that was in the shade as the base reference.

 

[mygreyhound]

Don't know but good question! Any physicists out there?

 

[Gary RV_Wizard]

The physics of a gas is actually pretty simple and expressed in Charles Law. The Ideal Gas Law is an extrapolation of that for a theoretical gas that perfectly obeys Charles Law and responds uniformly to changes in temperature and pressure.

Where it sometimes gets tricky is that most gases, especially blends like common air, are not quite "perfect" and are affected by moisture content as well as the slightly different characteristics of the individual gases. That can lead to endless debates over minute factors that mostly are not relevant to the temperatures and pressures used in highway use tires. Makes for really interesting discussions on rv blogs, though!

 

[Ned]

You don't need any complicated physics, just simple arithmetic.  Those that try to apply physics to some topics don't understand physics

 

[Gary RV_Wizard]

Agree, Ned. Charles Law is just arithmetic. And we have calculators to do that for us these days.