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Old 01-16-2015, 02:06 PM   #1
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Proper Tire Pressure and Weight Distribution?

Hello Everyone,
I am very new to motorhomes and am the recent owner of a 24ft Cobra Monterey. I am about to transport my family cross country to our new home in North Carolina and am needing to know more about setting the proper tire pressure. The repairman from a local motorhome repair center suggested I just fill each tire to the max air pressure (80 psi) and if the ride is too uncomfortable then I should let out 10 psi at the most from that maximum fill. He also suggested that I put the majority of cargo weight as close to or in front of the front tires to increase traction in the front for better steering stability.

What say ye to these suggestions? Thanks in advance for your feedback and suggestions.
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Old 01-16-2015, 02:31 PM   #2
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You need to get the corner weights as described in this thread

How/Where to weigh an RV

Load the coach as you would be driving it. That means water tank full and half tanks in the gray and black tanks, with all the stuff you'll be traveling with in the cupboards and fridge.

Then, go to your tire manufacturers web site and look up that weight for fronts and rear duals. Put that pressure in and maintain it +/- 5psi. Each corner will be different, for instance the grey and black tanks are probably not centered between the rails. That's why you need to do corner weights.

The method given by the repairman is good for not knowing the weight. Using that on our 28' Monterey (14,000 gvw) we started at 110 psi and ended up at 100 psi. That drop down to 100 make a big difference in the ride.
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Old 01-16-2015, 02:45 PM   #3
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Hello jplante4, thanks for the advice. Since we live in a very remote location and the nearest possibility of corner weight scales would be hours away, I think I will have to start with the max tire pressure method. I will however carry the tire manufacturer weight/tire pressure chart with me in case I have the opportunity to do a detailed weighing of the rv whilst on the road.
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Old 01-16-2015, 03:27 PM   #4
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If not 4 corners then front and rear will do. Better than estimating. Lots of places available to do this. Check locally for public scales. Feed stores and such will have them.
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Old 01-17-2015, 01:40 PM   #5
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And if you are not even able to manage that weighing, you will have to read from the car the GAWR's and GVWR ( Gross axle/vehicle weight rating)
Mostly on the same plate as the pressure advices.

Then from tire the maximum load , pressure needed for that ( yours 80 psi) wich is not the maximum pressure of the tire, and the speedcode of tire.

Give that here and I will calculate it with my multilingual motorhome-tire-pressure-calculator.

The I calculate with a saver formula then the American tire makers use and even slightly better then the European use.
Also I add a reserve percentage to the load .

Can also make an extra save pressure/loadcapacity list for your tires.

Greatings from a Pigheaded Dutch self declared tire pressure specialist.
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Old 01-17-2015, 02:12 PM   #6
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I did some research on public weigh stations in California and will be going to a truck stop in Ukiah which will be about 3 hours into our cross-country journey Monday morning. The truck stop can weigh front and back, but not the four corners. As some of the tires are as low as 55psi at present, I think I will inflate the tires to 75 psi at the nearest town which is an hour from where I live, then refine the inflation at the truck stop.

jadatis, the tires are Firestone LT 215/85R16. Here is the info I could glean from the tire while kneeling in the rain:
Max Load Single: 2680 lbs
Max Load Dual: 2470 lbs
Followed by the number 550 for both single and dual, then
Max Pressure 80psi.

From the Firestone website at http://www.firestonetire.com/tires/t...transforce-ht:
Service Desc: 115R
Load Range: E
Speed Rating: R

I cannot find info in the motorhome on the GAWR and GVWR - where would that be listed?

Many thanks!
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Old 01-17-2015, 04:13 PM   #7
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Those load/inflation charts reflect the minimum acceptable pressure for the corresponding load. Without knowing what your RV weighs, they are immaterial.
The RMA-ch 5 covers RV tires, says tires should never be ran at less than the RV placard stated air pressure, and load/inflation charts may be used if your load requires pressure in excess of the RV placard stated pressure.
You cannot go wrong by inflating to sidewall stated air pressure, but you will get a harder ride.

As to what the repairman said about putting as much weight as possible near the front tires, IMO he is wrong. Weight should be balanced so neither axle is overloaded.
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Old 01-18-2015, 01:09 PM   #8
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Then I asume you have a single axle rear .
then the maximum GAWR is 2 times 2680= 5360 lbs
Front is mostly about 10% lower so say about 4800 lbs and so total GAWR's.
10160 lbs . GVWR is mostly about 2 times GAWR front so 2x4800 = 9600 lbs.
That is if its not dual load rear so 4 tires on the(imaginary) axle.

Its also possible that the OEM tires where D-load with 65 psi pressure needed for maximum load. Googled that sise in D-load and found 2340 maximum load single.
This would give because mostly OEM tires yust enaugh for the rear axle.
Rear GAWR 4680 and front about 10% less 4220 lbs so GVWR 8440 lbs.
Then you would expect for rear 65 psi needed because maximum load of tire .
But for E-load wich is a stiffer tire you would need higher pressure for the same load.
Calculated it with my motorhome calculator and with 5% adding for reserve gave 73 psi for E-load, same system for D-load gave 69 rear so a bit above the AT pressure but is allowed, this is not the maximum pressure of tire.

So before I go on speculating and educated gambling , I will wait for more information, this all is to give you an idea of what info is needed.
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Old 01-18-2015, 02:00 PM   #9
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Thanks jadatis.

There are dual tires on the rear.

I have not been able to find the placard anywhere but will continue to look.
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Old 01-18-2015, 02:16 PM   #10
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Pressure for RV tires shown on the sidewall is NOT the maximum pressure for the tire, it is the MINIMUM pressure for the MAXIMUM load the tire can carry.
And the pressure shown in the charts is the MINIMUM pressure for the weight stated.
This is in the various tire brochures published by the tire companies.
The pressure on the sidewall of a Michelin RV tire and many others is not the "Maximum" the tire should ever have (unlike car tires) it is the minimum to support the maximum rated carrying capacity of the tire.

From page 2 of the 06/07 Michelin RV Tire Guide:
Quote:
"If you look at the tire's sidewall, you'll see the maximum load capacity allowed for the size tire and load rating, and the minimum cold air inflation needed to carry the maximum load."
From page 6 of the GoodYear RV Tire and Care Guide:
Quote:
"How much air is enough?
The proper air inflation for your tires depends on how much your fully loaded RV or trailer weighs. Look at the sidewall of your RV tire and you’ll see the maximum load capacity for the tire size and load rating, as well as the minimum cold air inflation, needed to carry that maximum load."
From Cooper:
Quote:
Tire Service Life

Cooper recommends that all tires, including full-size spares, that are 10 or more years from their date of manufacture, be replaced with new tires.

Tire service life is not determined by chronological age. The useful life of a tire is a function of service and storage conditions. For each individual tire, this service life is determined by many elements such as temperature, storage conditions, and conditions of use (e.g., load, speed, inflation pressure, impacts and road hazard damage) to which a tire is subjected throughout its life. Since service and storage conditions vary widely, accurately predicting the service life of any specific tire based on calendar age is not possible.

Cooper Tire is not aware of scientific or technical data that establishes or identifies a specific minimum or maximum service life for passenger and light truck tires. However, Cooper recognizes a consumer benefit from a more uniform, global industry-wide approach to the tire service life issue. Accordingly, Cooper recommends that all tires, including full-size spares, that are 10 or more years from their date of manufacture, be replaced with new tires. Tires 10 or more years old should be replaced even if the tires appear to be undamaged and have not reached their tread wear limits. Most tires will need replacement before 10 years due to service conditions. This may be necessary even if the tire has not yet reached its tread wear limits.

Under no circumstances should a "maximum" service life recommendation for a tire be considered as an "expected" service life. Tires must be removed from service for several reasons, including tread worn down to minimum depth, signs of damage (cuts, cracks, bulges, impact damage, vibration, etc.) or signs of abuse (underinflation, overloading, improper repair, etc.).
From TOYO:
Quote:
Q: What are the consequences of inflating the tires to accommodate the actual loads?
A: If the inflation pressure corresponds to the actual tire load according to the tire manufacturer’s load and pressure table, the tire will be running at 100% of its rated load at that pressure. This practice may not provide sufficient safety margin. Any air pressure loss below the minimum required to carry the load can result in eventual tire failure.
But then they go ahead and publish a weight/pressure chart allowing lower pressure for RV's!!

From the August 2010 Motorhome Magazine "Tread Carefully" tire article:
Quote:
The maximum load capacity allowed for the size tire and load rating and the minimum cold air inflation needed to carry that maximum load are located on the tire’s sidewall.
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Old 01-18-2015, 02:19 PM   #11
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Quote:
Originally Posted by jplante4 View Post
You need to get the corner weights as described in this thread

How/Where to weigh an RV

Load the coach as you would be driving it. That means water tank full and half tanks in the gray and black tanks, with all the stuff you'll be traveling with in the cupboards and fridge.

Then, go to your tire manufacturers web site and look up that weight for fronts and rear duals. Put that pressure in and maintain it +/- 5psi. Each corner will be different, for instance the grey and black tanks are probably not centered between the rails. That's why you need to do corner weights.

The method given by the repairman is good for not knowing the weight. Using that on our 28' Monterey (14,000 gvw) we started at 110 psi and ended up at 100 psi. That drop down to 100 make a big difference in the ride.
DO NOT run different pressures on the same axle. You take the highest weight and use that on all tires across that axle.
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Old 01-18-2015, 02:22 PM   #12
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Great information everyone, thanks so much.

At this point however I am confused about how much tire pressure I should begin with for the first three hours of my drive until I get to the nearest weigh station. Any ideas?
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Old 01-18-2015, 02:48 PM   #13
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80 PSI, Unless you find the manufactures plackard that shows the recomended tire pressure.
An hour driving at freeway speed with 55psi in 'some' of the tires, could be a recipe for disaster !
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Old 01-18-2015, 02:51 PM   #14
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There is very little to virtually no information regarding 24 Ft. Cobra Monterrey on the Internet for your GVW and CCC. Without those numbers if it were me in your shoes I would inflate the tires to 80 psi and be done with it. You don't need corner weights, no need to stop at a weigh station.

Corner weights from a weigh station are more useful for the larger RV's with 19.5 and 22.5 tires that can carry many thousands of pounds.

I am over 36,000 lbs when fully loaded and close to 47,000 lb's or more while traveling with my rolling garage behind me.

BTW, I carry 80 psi in my trailer tires which is the max for those 16 inch rims. My axles are 6,000 lb axles with about 11-12,000 lb's after it's loaded.

Just make sure you are NOT overloading the coach above the CCC (Cargo Carrying Capacity) maximum for your specific RV. Since you don't have the CCC then just use common sense.

Do as you please but in your case KISS is better.

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