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Old 04-06-2016, 06:15 PM   #1
TJ
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Another tire pressure question

I've used the search function and read many of the resulting threads, but am left as confused as when I started.

My coach is a 2014 Newmar Ventana LE 3436; one of the smaller coaches Newmar makes.

The OEM tires on it are Goodyear G670RVs in size 275/70R 22.5. The sticker Newmar puts on the wall behind the driver seat says all tires (front and rear duals) should be inflated to 120 PSI.

I have been unable to find a scale outfit around here that is willing or knows how to weigh each corner, so right now I don't have that information.

Beside a harsher/rougher ride, is there a problem with just inflating all tires to the Newmar spec (120 PSI)...at least until I can get corner weights established? That's what they were when we picked up the coach from the dealer and we've traveled about 5K miles that way. I'm not seeing any unusual wear pattern and the ride does not seem objectionably rough.

Looking for some wisdom here.

Thanks.

TJ
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Old 04-06-2016, 06:25 PM   #2
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The sticker gives the minimum pressure the tires require to support the maximum rating of the vehicle. That is what people should use till they get all "corners" weighed
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Old 04-06-2016, 09:08 PM   #3
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As others have said, go with the sticker pressures until you get it weighed.

If you're near the Love's Country Store I-5 Exit 136 Tacoma, WA, it looks like you can pull the coach up to just before the gantry with one set of wheels off the scales. You don't have a tag axle so it should be pretty easy to get each axle on a separate pad (they have three or four separate pads so different sets of axles can be weighed). I virtually scouted the location out using CAT's scale locator with it's satellite view.

To weigh you pull through once with all wheels on the scales, then a second time with either the left half or right half of the wheels off the scales. Subtracting the one side weight from the total weight will get you the other side weight. Just go into the station and tell the attendant what you're doing and they should be very helpful, they won't do the math but will provide you with two different weight slips.

Using this method I have weighed our coach on a CAT scale a couple of times. You have to find a CAT scale that does not force you to keep all your wheels on the scales using guardrails, fences, curbs or something similar.
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Old 04-07-2016, 09:14 AM   #4
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Jim, When I picked up our 3434, I also thought that the ride was harsh and actually questioned if my Comfort Drive was functioning correctly. I was being pushed around by the semi's and having difficulty keeping a straight line.

Now, since we both have the same axel weight ratings, (12K front & 20K rear) here's what I can recommend until you get the 4 corner weight completed.

Front: 12K divided by 2 wheels = 6K load....from the Goodyear tire chart, you scroll the the nearest number and get 6070 lbs. which = 105 psi for the front. I run mine at 110 psi.

Rear: 20K divided by 4 wheels = 5K load.... again from the same chart, you get 4980 as the closed weight which = 90 lbs. tire pressure. I set mine at 100 psi.

These pressure adjustments have completely change my ride and handling characteristic and also maintain an additional margin of safety beyond the Goodyear chart. Remember. the worst thing is to be under inflated.

Good luck.
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Old 04-07-2016, 01:03 PM   #5
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Thanks for the input, folks.

In looking at the Goodyear tire website, I found the following statement for RV tires:

"Tire pressure should never be reduced below the vehicle manufacturer's recommended levels to support load conditions in order to improve the ride quality of a vehicle. The difference in ride quality is not significant. When minimum inflation pressure requirements are not met, tire durability and optimum operation can be affected."

Now, if I am reading this correctly, Goodyear recommends sticking with the RV manufacturer's recommendation on the sticker. I know, this statement is probably there because of the corporate lawyers. But, it makes you wonder how much input the engineering side might have had in setting those inflation numbers.

As I understand the argument by RVers for lower inflation, it is based on actual weight loading per wheel and uses the tire manufacturer's weight vs inflation chart. And, I can see that argument...if you frequently do individual corner weights and adjust tire pressure.

Loading can change significantly after a fuel stop, additional passenger coming aboard, etc. So, it would appear that you would have to adjust tire pressures fairly frequently to accommodate those changes. If using the RV manufacturer's sticker inflation numbers, you are covered for the maximum weight for which the coach is designed...right?

For the record, my coach seems to handle and ride just fine at the sticker inflation numbers. That's why I am confused as to what I might gain by running lower pressure. At least Goodyear seems to be on that page as well. What am I missing here?

TJ
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Old 04-07-2016, 05:30 PM   #6
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Jim....You wouldn't change tire pressure due to varying loads, such as fuel. You typically load the coach with full water and fuel and then weigh it. You use those numbers and not worry about pressure as you empty a tank or add a passenger.

I can tell you I can feel the difference when I raise my tires 5 psi. You should get your weights and adjust your tires to match. If you're concerned, add an extra 5 psi. As stated, over inflated tires can cause you to wander more than you should. Theoretically, they can wear the center of the tires more quickly and even cause you to loose some traction. Even though you don't think the coach is riding harshly, you're unnecessarily pounding such things as cabinets and fasteners.
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