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Old 06-11-2015, 06:18 PM   #1
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Can Anyone Help Me Figure Out These Weights & Tire PSI

OK, I am still trying to get a handle on the tire PSI vs weight thing. So picture one, is the placard from my Hurricane....showing that the tire PSI should be 95 PSI, which I presume to be the fully loaded weight of the coach, at 22000LBS. OK... fair enough...then I look at the Goodyear chart from their tire guide, which I included...and I can't figure out how the weights from the Thor placard jive with the Goodyear guidance...help? The highlighted chars is my tire size 245/19.5
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Old 06-11-2015, 06:33 PM   #2
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IMO. Looks like Thor is recommending slightly less weight per tire. 4000 lbs per tire on the front axle and 3750 for each of the rear tires at 95 psi.


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Old 06-11-2015, 06:54 PM   #3
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The tire pressure sticker is based on every axle being fully loaded. On top of that they toss in a fudge factor for the tires on each axle to NOT be balanced.

In other words, they are guesstimating how much of the full axle weight will be on an individual tire because the coach is not balanced left to right. This also includes using the same chassis for coaches of the same length but with different floor plans/slide configurations which means differences in tire weight even when unloaded.

At least that is how Newmar explained that to me.
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Old 06-11-2015, 06:59 PM   #4
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Ok thanks, but shouldn't both axel GAWR's = the coaches 22,000 LBS GVWR? in this case, the front axel 8K + rear axel 15K = 23,000? Exceeding the GVWR by 1k?
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Old 06-11-2015, 07:06 PM   #5
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Your first part is right. if you have 95 lbs in the tires, they will carry 15000 lbs on the rear axle and 8000 lbs on the front axle. That adds up to 23000, max for the chassis is 22000.
Can't read the tire pressure chart, but 95# on your size tire should carry about 4000# of weight, slightly less on a dual.
You need to get the coach weighed. Coming back from Wisconsin last fall we stopped at a place (truck stop) that just put in a new segmented scale. Drove the front wheels up to the line and each axle and the toad was on a different section of the scale. The girl punched one button and the weights for all came out on one printout, and she didn't even charge me.
If you have 6000# on the front axle, that would be 3000 on each tire, just see where 3000# is and go up to see how much pressure is needed to support that weight on that tire.
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Old 06-11-2015, 07:15 PM   #6
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I did a double take on your tire pressure charts. You have 2 lines highlighted but you should only be using the bottom set of pressures because, if you see the note, they apply to tires built after 2006.

For the front axle the 95 PSI is pretty spot on for the front tires supporting 4080# assuming you do not exceed that weight on either front tire. So, it doesn't seem account for much if any imbalance. I assume they rounded up to the next value.

For the rear axle the 95 PSI appears to also be a rounding up to the next value of the table. It seems more extreme so that might be what is throwing you off. Remember, for duals you have to divide the axle weight by 4 to use on the chart. Given that they did such a significant round up, the issue of axle imbalance will be somewhat mitigate.

If you have individual axle end weights then you use those to determine your PSI for all wheels on that axle. So...in that case for dual wheels you divide the heaviest axle end weight by 2 for the chart.

Obviously you want to get "4 corner" weights for the most accurate way to use the charts. If you can't do that then I recommend you add 5% to the full axle weight then divide by 2 for singles and 4 for duals to determine the chart weight.

Beyond that, many will also suggest adding 5 PSI to any chart for good measure. That is up to you.
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Old 06-11-2015, 07:16 PM   #7
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Quote:
Originally Posted by Tytlfamily View Post
Ok thanks, but shouldn't both axel GAWR's = the coaches 22,000 LBS GVWR? in this case, the front axel 8K + rear axel 15K = 23,000? Exceeding the GVWR by 1k?
The manufacturers set the GVWR, by a bunch of factors, if you take the time to check your car , SUV , P/U whatever , you will find the two AWRings always add up to more than GVWR.
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Old 06-11-2015, 07:17 PM   #8
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Can Anyone Help Me Figure Out These Weights & Tire PSI

The first thing to do is determine when your tires were manufactured so you are using the correct numbers. Secondly you must weigh your unit when it is fully loaded, fluids, water, fuel, etc to get the loads for each axle. It is best to get the weights at all four tires as the total axle weight for the front or rear may be within the vehicle limits but most of the time the weights are not evenly distributed from side to side which could cause an overloading condition. Once the weights are obtained, you then can compare them to the tire manufacturer's chart to determine what the tire pressures should be for each tire. Note, if you do weight the four corners use the largest weight for the tire pressure on each side. Front and back tire pressures can be different but side to side should be the same. Then add 5 psi to the pressure to accommodate most conditions including elevation changes. If you send me a PM I have a worksheet that can be used for the calculation including determining what the real towing capacity is when the rig is fully loaded.
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Old 06-11-2015, 07:17 PM   #9
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Quote:
Originally Posted by Tytlfamily View Post
Ok thanks, but shouldn't both axel GAWR's = the coaches 22,000 LBS GVWR? in this case, the front axel 8K + rear axel 15K = 23,000? Exceeding the GVWR by 1k?
AHHH...you will see that for gasser chassis it is VERY NORMAL for the combined axle weights to exceed the GVWR. Trust me on that or you can go through almost any brand's brochures and see that to be normal. LOL
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Old 06-11-2015, 07:18 PM   #10
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...if you take the time to check your car , SUV , P/U whatever , you will find the two AWRings always add up to more than GVWR.
Not true for DPs...at least I haven't seen that yet. Very common for gassers.
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Old 06-11-2015, 07:21 PM   #11
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The first thing to do is determine when your tires were manufactured so you are using the correct numbers. ...
I think it is safe to ASSUME his tires were built way after 2006. LOL He only needs to use the lower of his highlighted figures and since his tires are G rated he can use the whole range of tire pressures. The good news it that he will never have to use max tire pressure.
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Old 06-12-2015, 06:43 AM   #12
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Thanks everyone for the responses....makes sense to me now. The ultimate knowledge base here on irv2 ������������

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Old 06-12-2015, 08:32 AM   #13
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One last thought...

I would go ahead and use 100 PSI for all tires until you get 4 corner weights. This will cover you for any reasonable side to side imbalance. The only issue with running more PSI than needed (within reason) is that it might make your ride a bit harder and you could feel bumpy roads more through the seat of your pants. Many RVers routinely add 5 PSI to chart weights.

HOWEVER running all tires at 100 PSI will not cover an axle that is totally overloaded. If you do the math you can see that your tires will handle more weight in combination than the axle is rated for. So, in interim you can easily get full axle weights at most any available scales such as CAT scales found at truck stops. That weight will at least ensure you are not overloading either axle and/or exceeding GVWR. As long as you ensure you don't exceed GVWR and individual axle weight ratings I think the 100 PSI in all tires will keep you safe.

A final word about gasser chassis. It is far more common to max out the rear axle weight before you ever exceed the GVWR. By design the rear axles carry most of the variable weight. By that I mean fuel, fresh water and holding tank contents along with all the other stuff. Other stuff is all the things you carry to enjoy your RV like food, clothes, lawn chairs, family/kids not in the front 2 seats and what every you want to make it fun. Because of the relatively short wheel base it can be difficult to shift much basement weight to the front, assuming your front axle has room to take the weight.

If you have a CAT scale close by, use it a lot at first as you load your coach. It could take 3-4 weighings to get a handle on weight and balance.

Getting 4 corners is a bit more difficult. Many folks suggest contacting something like the state's branch that monitors trucks. As an example, in Illinois the Secretary of State police carry portable scales that can be used on individual wheels. They will often be glad to come to you and help you with this issue.

Good Luck!
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Old 06-12-2015, 09:23 AM   #14
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The GVWR is set according to several parameters, with axle GAWRs being very important but not the only factors. Transmission capability, braking and driveline gearing are some others. In all cases, whichever parameter has the lowest rating becomes the limiting factor (they weakest link in the chain).
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