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Old 01-31-2017, 10:59 AM   #1
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Duals Tire Pressure / Load Question

I'm sure this has been asked a thousand times, but I've researched until I'm blue in the face, but cannot find an answer. It may be so obvious that nobody ever asks, but since this is a safety issue, I'm going to ask anyway.

My RV has been weighed at all 4 corners. I know the heavy side of my rear axle is 11,100 lbs. So when I look at my tire chart, and use the dual load rating based on air pressure, do I use 1/2 of 11,100 to determine the proper pressure?

If I have this right, my tire is rated at 5570 lbs at 95 PSI, which would be the correct pressure for the duals to handle 11,100. Then, possibly add 5 psi to bring it to 100 as a buffer.

I found documentation in the RV where the previous owner attended a class on tires, and his notes seem to agree with this. Strangely, they gave him a different pressure for inside/outside tires, but everything I have from Goodyear both in my paperwork and within their internet documentation states that paired duals should always be run at the same pressure.

His documentation also says that tire pressure should be adjusted per corner, so on my lighter side it being 10,000 lbs, 85 PSI is the recommended pressure. Do you all run different pressures at each corner based on load?
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Old 01-31-2017, 11:08 AM   #2
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Tire pressure should be the same across all tires on that axle. If you have four "corner" weights you take the highest weight on a "corner" to get the pressure, then all tires on that axle are run at that pressure. You can add 5 psi for safety.
My owners manual is different:
Quote:
Tire Industry Changes
The tire industry, as a whole, has changed its traditional stance on adjusting cold tire inflation pressure for RV tires installed on recreational vehicles and busses. Previously, tire manufacturers supported a policy where tire inflation could be adjusted according to the actual loaded weight of the vehicle. Now, the major tire manufacturers recommend that medium duty truck tires be maintained at the pressure that corresponds to the Gross Axle Weight Rating for the axle to which they are mounted. To make this recommendation uniform across the industry, tire manufacturers strongly urge the consumer to keep all tires inflated to the pressures recorded on the Federal Tire Label.

Tire Inflation
Country Coach recommends that the cold tire inflation pressures should at all times be maintained at the inflation pressure(s) recorded on the Federal Tire Label. There are no acceptable circumstances where tire inflation pressure(s) should be reduced below that pressure recorded on the Federal Tire Label.
Every tire manufacturer says to run the same pressure across all tires on an axle using the heaviest side.
Many manufacturers publish a tire guide, they are available online. They answer all of the questions, even down to correctly weighing your rig and applying the results to get the correct pressure.
As per my owners manual I run the pressure on the Federal Tire Label at all times and my TPMS is set for that.
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Old 01-31-2017, 11:23 AM   #3
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Thank you. I was not going by the owner's manual, but reading through some documentation he had from a class he attended. This isn't my first dually vehicle, but given the weight and height, I want to get this one right.

I'm using the tire guide for my tires to get them set correctly.

I had a Pressure Pro that the PO installed, but I hated it. I'm currently in process of installing a new EezRV TPMS. So far I like it, it seems to be a much better product than the Pressure Pro I removed.
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Old 01-31-2017, 11:39 AM   #4
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Quote:
Originally Posted by RobbieH View Post
Thank you. I was not going by the owner's manual, but reading through some documentation he had from a class he attended. This isn't my first dually vehicle, but given the weight and height, I want to get this one right.

I'm using the tire guide for my tires to get them set correctly.

I had a Pressure Pro that the PO installed, but I hated it. I'm currently in process of installing a new EezRV TPMS. So far I like it, it seems to be a much better product than the Pressure Pro I removed.
Years ago I was a Pressure Pro distributor/dealer but their product fell behind the curve and were passed by newer products. I went to a TST 507 system for my Dutch Star and the Magna came with a SmarTire internal system. I'm thinking about setting the TST system up for my towed.
I looked into the EeZ system and becoming a dealer but the cost and late wife's poor health kept me from it. Seems like a good system.
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Old 01-31-2017, 12:15 PM   #5
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The batteries were dead in most of my sensors, and the cost of replacing them (no user-replaceable batteries) was going to be $300, plus another $200 for toad monitors. The monitor was ugly, and huge. The Eez had all the features I wanted - high and low warning, rapid loss warning, temp monitor, and it has a lithium ion battery which allows me to use it while walking around the RV and toad.

I have this set up to monitor my toad also. I got the system with 10 sensors. I don't have a tag axle so 6 is all I needed for the RV.
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Old 01-31-2017, 12:54 PM   #6
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There are a view points you have to be ware of.
I think you already used the Pressure/loadcapacity-list for dual load, with lower maximum load a tire then for Single-load.
Some lists give it per side so weight on 2 tires, and then your 11000 lbs is what you have to read from that list. Sometimes also for dual load the P/LC list is give per tire so you have to devide by 2 .

In Europe the P/LC lists are given per axle so if you do 4 point weighing you even have to multiplie by 2 to get the weight on 4 tires.

Second point of interest is that American P/LC lists for LT and TRuck-tires are made with a formula that leads to higher loadcapacity for the pressure then the EUR calculation . So beter use an EUR made list, wich would lead to higher pressure advice. To laws of nature all tires of same specifications should be calculated the same way.

Then you can even use higher pressure then the list gives, you already add 5 psi for reserve, but my idea is that if you keep weighed weight above 85% of weight the pressure is determined for for 160km/99m/h , that then comfort and gripp is still acceptable , and you have maximum reserve .

Your tires I guess are speedrated for max 75m/120km/h wich is L speedrated, so to give maximum reserve you pretend to be max speed of tire to be that 99m/h. Only for calculating the pressure to give a deflection of tire with still acceptable comfort and gripp and maximum reserve.

Point is that you can never give or measure load and pressure 100% acurate.
Weighing is a way to get load above 99% acuracy,but still your pressuredevice can easyly be 5 % off , and worst case schenario is that pressure device gives to high pressure.

If you give me front and rear weighing R and L, and from tires read from sidewall the "maximum load Single/Dual AT psi( cold) " and if you can find it the speedcode , I will put it in my made calculator part 3 for seperate wheelweights and give you that lowest and highest advice.
In my calculator I use formula that leads to even higher pressure advice then the EUR calc, maybe even pessimistic, but never unsaver then that EUR calc. Also I standard add a reserve, or in case of wheelweighing, make the percentage right and left between 85 and 95 % , yes also I give for the axle same pressure on all tires, so R/L and inner/outer tire.

But how did you weigh, was it with all the load and persons you normally have in vehicle , otherwise the load in real use is higher then you weighed, and you need higher pressure.
If weighed empty or with load missing, give what you add and I can make an
estimation of what comes to the weighed loads.
Estimation is dangerous, so acuracy gets lower then d99% then.
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Old 01-31-2017, 01:44 PM   #7
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Earlier I said Goodyear. I have Michelins. I was also looking at the Goodyear chart since it was in here and I got mixed up.

The Goodyear chart is per tire on duals. The Michelin chart is for both duals combined. I do know the dual rating is lower than the single rating, I noted I was using the dual rating.

I have the Michelin XZA2 Energy in 295/80/22.5. It has a maximum speed rating of 75 MPH.

Based on the chart, all 6 tires on my RV should be at 95 PSI. I have them set to 100.

The sensors on the EezRV system are accurate within +- 1.5 PSI. They mention that they are likely more accurate than most gauges. I do have a good gauge that I believe to be accurate and I'm getting similar readings with both devices.

Weight was done during a trip with RV loaded. It was done once at all four corners. On another trip, weighed at a CAT Scale which only does axle weights, not each corner. Weights were similar with each other.

Additional 5 PSI raises capacity significantly, so I should be good.
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Old 02-01-2017, 04:06 AM   #8
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Putted it in my calculator and gave advice 101 psi for 75m/h.
So 100 psi is the minimum , then heavy side is using 95% of the loadcapacity belonging to 101 psi, and lightest side is using 85% of the loadcapacity belonging to 101 psi for 75m/h.
To my opinion that is still poor reserve ,so I lowered the loadindex single of 152 by 4 steps (for every 10km/6.5m/h 1 LI step, to go from 120km/75 m to 160km/99m/h),still a rule of tumb system, to give the tire a deflection with still acceptable comfort and gripp.
And came to 148 LI wich is in this case the Loadindex for Dual load.

So your 101 psi is also the upper border of pressure.
Now this comfort border is my conclusion , and some say you can even go higher,and its a subjective thing, so if you want to be save , use 105 psi ,then most likely stil no bumping and more reserve for misreadings and pressure-loss in time.
Always savety first!
And also you notice bumping less from the rear tires ,because driver and co-driver are seated close to the front axle.
For trailers with no persons in it , I use in my spreadsheets , a Loadpercentage of 80% , lower would shutter the screws of the woodwork, again to my discussable conclusion.
Most European tires are given 2 LI steps lower for Dual ,or substact 7.5% of the maximum load for single. American system often substracts that 4 LI steps, or substact 9% of the maximum load.
2 LI steps lower is 5% lower maxload, and 4 steps about 10% lower maxload ( also rule of tumb).
The difference between the systems is probably because the roads are more curved sideward in America then Europe. wich gives the inner tires more deflection. Once read about agricultural dual tires that substracted 12% even ,so that would be 5 LI steps lower.
So here also the rules of nature are the same all over the world,but the roads are different probably. Once heard that in Holland only 1% sideward slope is done for watering of ,but in America probably about 1.5 to 2% what the tire-and car makers take into acount .

If you give front seperate wheelweights , because there single load, I can calculate for that,and then there will be a lowest and highest pressure advice , then fill to highest ,and if in time it drops to the lower , you are still save. But can then also be that you calculate that highest pressure , but because your pressure-device ( wich you say is pretty acurate) is still giving to high pressure ,and you yust have enaugh pressure to prefent damage to tire by overheating when realy driving 75m/h.
For lower speed , wich you wont go over for even a minute,the pressure can dropp a bit more. 62m/100km/h for instance 2 LI steps higher then given single would give rear advice of 94 psi,so if in time your 101 drops to 94 psi it would still be save for that 62m/h.

In Holland and the most of Europe , the maximum speed for your kind of vehicle ( above 3500kg/7700lbs and not registed as personscar) is 80km/50m/h and sometimes 90km/56m/h. The speeds the truckdrivers wont go over for even a minute thoug can be that 90km/56m/h to sometimes even 100km/62m/h.

What is the maximum speed you drive ( and wont go over for even a minute)?
And what are the speedlimits in America, and are they different for persons-cars an d for instance trucks?
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Old 02-01-2017, 10:08 AM   #9
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In Texas and New Mexico, we have highways where the speed limit is 85 MPH. The fastest I will go even for a minute is 70, and although I've been doing this, we are going to try slowing down to 65 as a max on our next trip. I know the other drivers are not going to like this, as this trip will have long stretches of 2 lane (one each direction) where the speed limit is 75 MPH.
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Old 02-01-2017, 02:14 PM   #10
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Does that mean that a truck , with in Holland maximum allowed weight of 50000 kg/110000 lbs, is allowed to drive that 85m/h on the highway, and 75m/h on that 2 lane road .
In Holland on "autowegen" wich translates as car-roads, there are crossings and traficlichts possible and maximum speed is standard ( if not lower given) , 100km/62m/h for normal cars, and for trucks still 80km/50m/h.
On Highways ( no crossings and traficlight and minimum 2 lanes a direction) normal cars since a few years 130km/81m/h, was 120km/75m/h, if not given lower.
But still for trucks 80km/50m/h.
so we are used on such roads that trucks drive slower, anoying but we have to do with it.
A lot of trucks in Holland, but at sertain times of the day they are not allowed to pass . and on Highways not allowed on the 3th lane of the direction (also not 4th, 5th, etc).

So you can understand that I think your speeds are way to high.

But is it as I heard that you are allowed to drive any vehicle with one driverslicence? Most in Holland have only B, for driving up to 3500kg/7700lbs car with maximum 8 persons and driver in car.then C driverslicense for above 3500kg and 8 persons and driver. And also D licence for more then 8 passengers like busses.
For motor you need A driverslicence.
For every driverslicence you need to do a seperate exame.

And give your front weights , so I can calculate, by what you wrote about your speeds , my advice is to go from 75m/h for the speed you wont go over for even a minute. Your tires dont alow faster too.
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Old 02-01-2017, 02:36 PM   #11
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Yes. I can load down my Chevy 2500 HD with a very heavy 5er and hit the highway at 85. Or a gooseneck with two big rock crawlers on it (Think very heavy, highly modified Jeeps).

My last RV would go 85 with the trailer behind it (flatbed with one Jeep). It wasn't an intelligent thing to do, but it would do it.

I'll have to go back out to the RV to get the front weights. They were 6,300 something. I know when I looked at Michelin's tire chart it said 95 PSI for those too.

For a while in Texas, we had separate day/night speeds. And we had separate car/semi speeds. But, that's all gone now as far as I know.
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Old 02-02-2017, 08:48 AM   #12
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Mr D gave sound advice. Do NOT adjust each corner to a different pressure. Use the heaviest "corner" on each axle to determine the proper inflation for that axle. Front & rear axles will likely be different, but all tires on the same axle must be the same psi.

Also be very careful to read the inflation chart properly. Michelin's RV Guide shows dual tires as weight per pair, while most other manufacturers show dual inflation weights on a per tire basis. Tires always are rated to carry less load per tire when used in a dual pair, but the "dual" line in an inflation chart does not necessarily show the weight as a pair.
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Old 02-02-2017, 08:56 AM   #13
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After doing more reading in the Michelin guide, I noted they said to make all tires on the same axle all the same pressure. So, I'm good here. I have all 6 tires at 100 psi now.

I noted the difference in GY vs Michelin above. It's pretty obvious when you look at it, sine Goodyear says 5k something and the Michelin says 11k something.

EDIT: Correction, I was reading through the RV's manual and it said to put all tires on the same axle at the same pressure.
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Old 02-02-2017, 09:11 AM   #14
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Then your loads on front where heavyest side(because you determined pressure for heavyest side between the values of 90 and 95 psi .Looked it back in the list of Michelin the 295/80R22.5, ad read between 6425 and 6755 lbs .
Then i assume you looked for front in the single load list?!
The lighest side we still dont know ,but will go from 6700 lbs on heavyest and make that 95% of the weight, the pressure is calculated for.
Gave 108 psi advice for 75m/h, and 122 psi with max reserve and still acceptable comfort and gripp.

So Front advice 122psi ( so 120 psi read on your device),and if in time or by inacuracy it gives 108 psi , you still are save with some reserve for 75m/h.
So a range of 14 psi in wich your tires are save and still acceptable comfort and gripp.

If I would make a Pressure/loadcapacitylist for your tires with my made extra save formula, it would give for 75m/h at 95 psi 6250 lbs loadcapacity.
European calculation would give at 95 psi would give 6495 lbs loadcapacity, And so I now see that the list given on this site from Michelin Michelin RV Tires | View Michelin RV Tires load and inflation tables for motorhomes and recreational vehicles. | Michelin RV Tires
for your tire is calculated with the European formula, the goodyear list I also will search for and it will give higher loadcapacity.
Found the goodyear list here
http://www.goodyearrvtires.com/pdfs/rv_inflation.pdf
and against my expection it gives AT 95 psi ,6285 lbs loadcapacity, but they go from an AT-pressure from 125 psi.
Do I calculate with EUR formula it also comes to 6286 lbs, so these larger tires are also in America calculated with the saver European Formula.

But I bett that if I calculate it for for instance an LT tire with 16 inch ( did once for Goodyear and Michelin RV here given link of) it will prove to be calculated with the American formula. My calculation would give at 95 psi going from AT-pressure 125 psi a loadcapacity of 6050 lbs.

This all to give an idea of the differences.
My calculation to my conclusions takes care that at every pressure/load combination ,the deflection of tire is the same,so never unsaver then the official calculations.

Thanks for the speed info ,now about the driverslicence .
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