Figuring the correct tire pressure.

[MadDad]

On an RV with duel tires on the rear, after getting the axle weights on scales do you take the front axle weight and divide by 2 and the rear and divide by 4 to figure the correct tire pressure?

[hoppers4]

I weigh each corner individually. The tire charts show the weights as either in a dual configuration or a single configuration. Weigh the corner and divide by two to determine the load that it's carrying and therefore appropriate inflation pressure.

Using the highest calculated pressure, inflate all four duals to the same pressure. Steer tires should also be equal. They may be different than the duals, but equal to each other.

[WaltW]

I mostly agree with hoppers4, with the following caveat...add 5 lbs to the air pressures that you get if you don't weigh 4 corners to compensate for any rt to left imbalance. Also never run your tires at less than 75% of rated capacity. Got that from a tire exec on another forum.

[MadDad]

Originally Posted by hoppers4

I weigh each corner individually. The tire charts show the weights as either in a dual configuration or a single configuration. Weigh the corner and divide by two to determine the load that it's carrying and therefore appropriate inflation pressure.

Using the highest calculated pressure, inflate all four duals to the same pressure. Steer tires should also be equal. They may be different than the duals, but equal to each other.

But when you get the weight of the rear dualies, do you divide the weight by 2 or 4 to calculate the pressure to run them at. Can only weigh front and rear. No scales around here to do each corner individually.

[hoppers4]

I ended up running max pressure (80 psi) on the rears according to the scales. These large Cs are pretty close to their weight limits anyway when they're loaded out. My guess is that you'll have to do the same. But in answer to your question, for the duals, divide your measured rear axle weight by 4.

Michelin LTX M/S 225/75R16E XPS Rib
Michelin chart says:
75 psi single is 2560 lbs per tire
80 psi dual is 2470 per tire

My actual weights were:
LF 2450 and RF 2350 for a total of 4800 on the axle (5000 allowed)
LR 4550 and RR 4450 for a total of 9000 on the axle (9600 allowed)

So in your case if you weighed the rear at 9000, divided it by 4 and came up with 2250, your safest bet would be to run at 80 psi. The inflation table calls for 75 psi at 9320 but for me that's a bit too close. I'd rather go with the 9880 figure and 80 psi.

When I weighed my rig I had half a tank of water, half gas, two people, full propane, and full travel loadout of gear. So it could conceivably get considerably heavier.

Good luck getting this mess figured out.

[norty1]

I always tried to make it simple for me. I ran the rear and front tires at the cold pressure recommendation on the door sticker of my truck.
Two trucks and no worries about how much pressure to start with. It's printed on the door sticker.

[abarkl]

Originally Posted by norty1

I always tried to make it simple for me. I ran the rear and front tires at the cold pressure recommendation on the door sticker of my truck.
Two trucks and no worries about how much pressure to start with. It's printed on the door sticker.

x2

We figure those with degrees in design and engineering have best knowledge of the required pressures assuming typical conditions and load. We don't second guess those who have the knowledge and testing to back it up.

Typically for those who full time in their RV and carry close to or at the load limit weighing all four corners and adjusting tire pressure can be more relevant.

[hoppers4]

I have to agree that plain and simple is just fine. After all the weighing and calculating my tire pressures ended up being the same as door sticker. My main reason for hitting the scales was to make sure I wasn't overweight.

[MadDad]

I'm actually trying to figure out my sis-in-laws Thor Vegas. I ran it across a Cat scale at a truck stop and got these numbers.
Front axle 4900 LB
Rear axle 7100 LB
Gross weight 12000 LB
These would put the front at 2450 LB per tire weight and 70 LB air pressure, but the rear at 1775 LB per tire weight and 45 LB air pressure which can't be right. Might try a different truck stop. Meanwhile going to run at 70 LB in all tires.

[WaltW]

For the rear duals just divide by 2, NOT 4. The table entry for duals takes that into consideration.

Setting tire pressures for cars, trucks, vs super C's/class A's is different. I've never seen tire pressure charts for cars or PU's. My tires have a max pressure rating of 120 psi. Based on the loaded weight of the rig, at 120 psi the tires would be way over inflated, the ride would be harsh and handling and tire wear would suffer.

If you doubt go to a reputable truck tire dealer (not NTW, Costco, etc.) and ask for advice.

[hoppers4]

According to the Michelin load and inflation tables, the number in the dual column has to be divided by 4. The largest dual number in the table for my tires says 9880 lbs. Right next to that figure is the max load and pressure for the sidewall which says 2470 lbs. 2470 X 4 is pretty close to 9880.

Perhaps the tables you're using are different but that's what the Michelin truck tire table says.

[oldmanAZ]

Keep in mind if you are lowering tire pressures with dual tires, the sidewalls of the two tires MUST NOT touch.

[jadatis]

Originally Posted by MadDad

On an RV with duel tires on the rear, after getting the axle weights on scales do you take the front axle weight and divide by 2 and the rear and divide by 4 to figure the correct tire pressure?

I made a Motorhome tirepressure calculator, an Excell spreadsheet , using my determined formula based on the official European and what I read in an article of an American IR J.C.Daws.

If you only weigh the axles , I add 10%, because weight division R/L is for motorhomes often between 45/55% and 48/52% .

Then for Dual load you have to use the maximum load for dual load of tire.
This is mostly 2 LI steps lower then for single load, and 7.5% lower then for single load . America often uses 9% reduction for dual / 4 Loadindex steps lower.

If you give the axle weights and from tires next, I will calculate and give picture of my filled in spreadsheet.
From tires: Maximum load or Loadindex for Single load and Dual load.
Loadrange to determine the pressure needed for maximum load also called reference-pressure, but will call it AT-pressure furtheron.
Thirth is speedcode of tire ( mostly not to be found on American tires but can do without).

I also made extra save pressure /loadcapacity lists ,so if you like , you can use that to do it yourselves. Given per tire/axle single/axle dual , so you dont have ot devide yourselfes.
https://onedrive.live.com/?id=A526E0...26E0EEE092E6DC

[WaltW]

hoppers4, I run XZE2 LRG 275/80 R22.5. My drive axle weight is 15000 lbs. If I half that = 7500 lbs. In the table for Dual the lowest entry is 8190 and 70 psi. Note that this would not take into account any side-to-side imbalances. But, given as the "rule" is don't run tires lower than 75% of maximum pressure, I would inflate to 82.5 psi, which also covers me for any side-to-side imbalance.

If you divided buy 4 you would have to use the single not dual part of the table. The entries are close but not quite the same. I think Michelin provided the dual weights for a reason. Perhaps for your smaller tires there is no "dual" entry? We need to ensure that the response given is applicable to rig in question.

[hoppers4]

You're apparently using different tables than I am. I couldn't find your tires in those tables.

Here's a link to what I use. Load & Inflation Tables | Michelin Truck

[rgolding]

Don't know about all this, seems like a lot of trouble. Cold pressure on our Seneca is 110 psi. on each tire. We have the guys at Love's set them at 105 psi and really don't want them below 100 psi cold.

Keep it simple guys. People spent a lot of money sending their kids to understand these things. Relax, enjoy life, be dilligent, but relax.

[jadatis]

Originally Posted by WaltW

hoppers4, I run XZE2 LRG 275/80 R22.5. My drive axle weight is 15000 lbs. If I half that = 7500 lbs. In the table for Dual the lowest entry is 8190 and 70 psi. Note that this would not take into account any side-to-side imbalances. But, given as the "rule" is don't run tires lower than 75% of maximum pressure, I would inflate to 82.5 psi, which also covers me for any side-to-side imbalance.

If you divided buy 4 you would have to use the single not dual part of the table. The entries are close but not quite the same. I think Michelin provided the dual weights for a reason. Perhaps for your smaller tires there is no "dual" entry? We need to ensure that the response given is applicable to rig in question.

That rule of not going lower then 75% of pressure needed for maximum load , is American way, and reason why not going lower then 26psi for a normal P-tire with that pressure needed for maxload of 35 psi.

In the official European formula, I once got hold of and went running with, advice was not to go lower then 150kPa ( 22psi) on a tire with maxloadpressure of 2.5 bar( 36 psi) , so 60% of maxloadpressure as lowest.

What I concluded of the administrative research I did on tirepressure-advice, is that the old american formula gave to much loadcapacity in the lower pressures , so to much deflection and heatproduction, and so stopped at 75%.

America stepped over as late as 2006 to the European formula , but only for P-tires in Standard load and XL/reinforced/Extraload, and left LT tires to the old less worse calculation.
This European formula , that is used still for any kind of tires ( from P-tires to trucktires)gives lower loadcapacity in the lower pressures , so more the same deflection as when using maxloadpressure when maxload on tire. So then more the same heatproduction, what the goal is of all the calculations.

The european calculation , so is the conclusion of an article of an American IR J.C. Daws, is adequate for P-tires , but still gives more deflection for LT- and truck-tires in the lower pressures, then when maxloadpressure and maxload on tire.

So the European calculation is also not that holy as I declared it in the beginning, but comes close.

If you would use the to natures law ideal calculation , you could even go as low as zero pressure , if the load on tire is low enaugh, and still the same deflection so heatproduction of tire. Zero pressure the average tire can bare only on its construction about 100 lbs/45kg, is my estimation, so that low you shal never calculate on RV's

Mayby this is also the reason why in this topic the difference between pressure/loadcapacity lists of Michelin occurs.

I found the Michelin lists also different calculated on American site of them , then on European site.

Mind that if you devide by 4 on a dual axle , because P/LC list gives it per tire, to use the Loadindex or maximum load given for Dual load, and not that for single load.

European lists give it mostly per axle, American lists mostly per tire given , so then you have to devide the axleload by 2 ( single load) or 4( dual load).

[BrentJ]

My biggest concern is with fluctuations between cold settings based on different climates. I set the tires at 100 in front and 105 in rear in the Texas summer heat. Drove the rig to Michigan and was surprised at the loss of Cold pressure just due to ambient temp drop. Had to add air to restore original settings. Pressures were fine for a trip to Colorado. However upon leaving the cooler temps I had to let air out again as soon as entered Texas as now they were on the high side. The biggest lesson learned was the value of Tire Pressure Monitoring System. Without the TPMS I wouldn't have caught the Cold pressure drop when going to cool climate or the increase when back to hot climate. Either of these could have caused problems. TPMS provides tremendous confidence that you know what is going on with tire pressures.


Sent from my iPhone using Tapatalk

[jadatis]

Dont worry about the pressure-chanche when ambiënt temperature chanches.
Yes, when colder the tires get lower pressure and by that more deflection so more heatproduction then where the advice pressure is given for.
Pressure advice is going from 18to20degr C / 65to 68 degr F.
If colder more heatproduction , but also more cooling down of the tire by the larger temperature differences.

And the other way around, when hotter outside, the pressure rises , and by that lesser deflection so lesser heatproduction. But then also the cooling down is lesser because lesser temperature differences( between rubber and inside air, and between inside air and outside air). The heated rubber is cooled down to the outside air on one side, and the inside air on the other side.

so only for savety of the tires , you dont need to highen up the pressure to adviced when colder outside, for fuel saving it may be done .
But lowering the pressure to adviced when hotter outside is dangerous.
When it gets that 65 degr F again outside, the pressure is to low , and more heatproduction.

The tire can stand a 2 to 3 times as high pressure as given AT-pressure on sidewall ( maximum load xxxx lbs AT yyy psi( cold))
So dont be affraid of higher pressure , even cold , then this AT-pressure.
Once calculated that if filled with 1.4 times AT-pressure at freesing point of water ( 0 degr C/ 32 degr F) and the temp in tire rises to boilingpoint of water ( 100 degr C/ 212 degr F ) that you would measure a pressure of 2 times the AT-pressure, and this is still below that 2 to 3 times .

Made a list of what the advice pressure filled at that 65degr F , will get when inside tire temp rises or lowers. Will give picture of it here.

[WaltW]

hoppers4,for my rig I use the tables at
michelinrvtires.com

Different chassis, different tires.

rgolding, proper tire pressures improves ride, handling, and tire wear. Setting proper inflation pressures is simple. Pump works the same either way.