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Old 04-05-2020, 02:38 PM   #1
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Can you guys help with some new guy questions on weight/tires/inflation please

My coach is a 2005 Newmar Mountain Aire 4304. It's a 43 foot RV Class A diesel with tag axle. According to the sticker inside the RV, it has a Max GVWR of 44,200. The front GAWR is 14,200 lb and the rear GAWR is 30,000 lb.


The tires are 2 year old Toyo M144A in size 295/80R 22.5 They're marked as Load Range H. They're all in excellent shape.



I've located the Toyo Tire Commercial Load and Inflation Tables. I believe it's the correct one. I'll try to attach a link to the online tables that I'm using. I'm hoping that will work.



https://www.toyotires.com/media/2155...ion_tables.pdf



Anyway, the table gives various pressures and associated loads the tire will carry at that pressure, I presume. Assuming that's correct, based upon differing pressures, the tire will support between 4920 lb and 7830 lb each in a single configuration. If I understand this correctly, I would double that figure for an axle with two tires on it. That makes the front easy - my axle can't have any more than between 9840 lb and the max gross GAWR of the actual axle which is 14200. At 14,200 lb, the tires would need a max pressure of between 105 - 100 PSI. Of course all this depends on having the coach weighed fully loaded up as it would travel and seeing what the actual front axle weight is. Assuming for example the front axle actual weight came in at 13,900, I would inflate the front tires to 110 PSI, giving them a load capability of 7065 lb each, or 14,130 for the axle. Assuming I'm pretty close on my understanding of this (I hope) then the front end of things is fairly straightforward.



Now we get to the back, and the dual (tag) axle setup. Now, I'm clueless. Do I weigh each of the axles separately? How do I do that? The weights on the Toyo Tire tables are shown as both single and double. Do I count the tag as single and the drive axle as double? Do I use the single figure and multiply by 3 on each side? I'm just clueless here as to how to properly do this.



The GAWR of the rear is given as 30,000. I assume that's a total for both tag and drive axles combined? Do I just treat them as one with 6 total tires? Do I use the single or dual numbers? This is very confusing!


Also, why are the load bearing numbers at a given pressure lower for dual vs. single at the same pressure?



If I have to weigh the drive vs. tag axle separately, how do I do that?


Is it safe to say that for quality of ride you want to run the minimum pressure you can that will support the actual weighed load on that particular axle?



I'd appreciate any help you more experienced guys can give me on this. Thanks!
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Old 04-05-2020, 04:40 PM   #2
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Ok , I'll try and help where I can.

Your understanding of the front axle is correct.

Personally I don't got to the minimum inflation/ per weight , because of the possibility of side to side differences in load ( that would require a six corner weight , in your case ) I always go 5 psi over the minimum. that allows for pressure drop for lower ambient temps too.

Rear with tag , your 30,000 is probably , 20,000 on the drive and 10,000 on the tag. Could be 22/8 but maybe some one with better info will post to that.

A Cat scale at a truck stop should be able to get you individual axle weights , depending on your location another member could provide info on availability of 6 corner weights.

The lower pressure , dual vs single , is a balance by the manufacturer between carrying the load and an acceptable ride.

Remember when weighing , all your axle weights can vary with the pressure you apply to the tag suspension , does your owners manual give any guidance as to a minimum pressure to apply there.

Perhaps " Dutch Star Don " will notice your post and provide more info.

I'll see if he's online this afternoon and have him chime in.
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Old 04-05-2020, 05:39 PM   #3
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You will find many posts on this and they get really particular. So to keep it simple, you have the front correct. Apply the same logic to the other two axles and yes, you have to weigh them separately. Most CAT scales like at Loves will do that. Some RV places will do it such as NIRVC and I believe Escapees in the Houston area does weights. If you want to really get picky, weigh all six "corners" if you will to match your chart.
On the drive axle, use the dual line. On the tag axle, use the single line like you did on the steers. This should be when you have the coach loaded. Until you get the weights you can use whats on the chassis info that came in your Newmar. Mine is on a sticker near the Drivers seat.
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Old 04-05-2020, 09:16 PM   #4
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Tireman9 is the resident tire expert, he says if individual wheel corner weights are unavailable, to use the 60/40 rule. That is assume one corner/side is supporting 60% of that axle weight, then inflate accordingly, with a 10psi cushion above the chart listing.
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