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Old 04-13-2021, 10:46 AM   #1
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Tire pressures for heavier rigs

I have always followed the load curve vs tire pressure with success. My current rig (expedition) runs about 25k pounds. I’m about to take delivery of a new-to-me mountain Aire (it has a tag )which runs something like 45k pounds. I have the weights and tire load charts... and those charts indicate I am WAY below minimum load on the charts, indicating something like 80 psi in the rear and 90 psi on the steering tires ( there are different tires front and back).

However, the dealer swears inflation should be more like 110 psi all around. He says the handling is “squishy “, or “not tight”. Those of you with heavier rigs, do you find this to be the case ... or should I give the lower inflations a try and see how handling and ride is?
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Old 04-13-2021, 11:02 AM   #2
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Quote:
Originally Posted by MN_Traveler View Post
I have always followed the load curve vs tire pressure with success. My current rig (expedition) runs about 25k pounds. I’m about to take delivery of a new-to-me mountain Aire (it has a tag )which runs something like 45k pounds. I have the weights and tire load charts... and those charts indicate I am WAY below minimum load on the charts, indicating something like 80 psi in the rear and 90 psi on the steering tires ( there are different tires front and back).

However, the dealer swears inflation should be more like 110 psi all around. He says the handling is “squishy “, or “not tight”. Those of you with heavier rigs, do you find this to be the case ... or should I give the lower inflations a try and see how handling and ride is?
Well,
The answer is simple. It's NOT THE DEALERS RIG, IT'S YOURS! The tire folks kind-a know what they're doing when building tires. Most of them have been at it since Christ was a pup. Sooooo, that means if they put out a tire pressure chart, for a given load on THEIR TIRES, it means THEY know just EXACTLY what kind of pressure is best FOR THAT LOAD, PERIOD!!!

Regardless of what the manufacturer states or stated what the coach weighed when it rolled out of the factory, load it as you would for your trips etc. and GO WEIGH IT. And you can goof around with the four corner stuff it you like but, a front GAWR, rear GAWR, tag GAWR and total GCWR will tell you ALL you need to know, in regards to the proper air pressure on YOUR tires, ON YOUR RIG, not the dealers.

I have never, ever did a 4 corner weight. I can't move anything around anyways. It all fits EXACTLY where I need to put all my stuff. I've weighed this coach at least three different times with just a front, rear and total of weights. I went to the tire pressure chart of the make of tires I had on it at the time and, went with what they recommended. Your choice on this one.
Scott
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Old 04-13-2021, 11:10 AM   #3
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Weigh it then use the tire manufacturers recommendation. Too much causes the center of the tire to wear faster and, in my experience, the handling and ride to suffer. I do add five pounds as a safety measure and as insurance in case I am heavier on one side. The scales at truck stops generally won't give side to side or 4 corner info.
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Old 04-13-2021, 11:20 AM   #4
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Not my first rodeo. I do plan on getting weights for myself, but like I said, the mfgs weights are *so* far below minimum it will make no difference. My question is specifically for owners of heavier rigs and whether they have seen reason to deviate upwards from the charts?
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Old 04-13-2021, 12:22 PM   #5
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Forget about what the dealer says and what the charts that came with the rig say! Take it to a scale that can weigh all four corners, being sure it is loaded the way you would normally travel, and with a full freshwater tank and full fuel tank! Then go to the tire manufacturer's website and get the correct information for your series tires. I always add 5 PSI as a "fudge factor"!

Good luck, happy trails, and God bless!
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Old 04-13-2021, 02:25 PM   #6
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Well,
The answer is simple. It's NOT THE DEALERS RIG, IT'S YOURS! The tire folks kind-a know what they're doing when building tires. Most of them have been at it since Christ was a pup. Sooooo, that means if they put out a tire pressure chart, for a given load on THEIR TIRES, it means THEY know just EXACTLY what kind of pressure is best FOR THAT LOAD, PERIOD!!!

Regardless of what the manufacturer states or stated what the coach weighed when it rolled out of the factory, load it as you would for your trips etc. and GO WEIGH IT. And you can goof around with the four corner stuff it you like but, a front GAWR, rear GAWR, tag GAWR and total GCWR will tell you ALL you need to know, in regards to the proper air pressure on YOUR tires, ON YOUR RIG, not the dealers.

I have never, ever did a 4 corner weight. I can't move anything around anyways. It all fits EXACTLY where I need to put all my stuff. I've weighed this coach at least three different times with just a front, rear and total of weights. I went to the tire pressure chart of the make of tires I had on it at the time and, went with what they recommended. Your choice on this one.
Scott
You can do that but I would suggest if you have axle wts only dont ASS U ME the wt is equal side to side.
I have seen side to side difference of close to 10% of rated capacity. So I would use avg wheel wt + 10% when you go to the chart. Most also round up or add 5 PSI from calculated as a safety factor.
Results when you do the above will be higher than just assuming avg wts.
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Old 04-13-2021, 02:27 PM   #7
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My last coach came in at about 37k lbs. All the tires were somewhat above inflation table minimums, but not at max either. With the tires about 5 psi above the minimum psi for the axle loads, the fronts felt a bit squishy on really curvy roads, so I added another 5 psi up front to firm them up a bit. Rears seemed fine at the table value, perhaps because they weren't steering anyway.


I would be very skeptical of going 20-30 psi over the recommended inflation as the dealer suggests. Try the inflation table settings first - you can always add more later. Just make sure you have the right table for the size & model.


Do you have the huge 365/70's on the front? They can carry 8000 lbs apiece at 90 psi, so that isn't exactly soft.
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Old 04-13-2021, 02:31 PM   #8
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I have found that dealerships often think that they know more than they actually do and this is especially true if you are talking to someone from the sales department.
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Old 04-13-2021, 02:38 PM   #9
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Yup-- believe the tire engineers who designed the tire, oversee QC and develop the Inflation Tables...


OR


Someone at the dealership??? And, even if he is clairvoyant, how is he going to know how you will load your coach. Full timer doing lots of boondocking (so full tanks and lots of stuff) or occasional overnight to watch a football game? Tire PSI by ouija board???
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Old 04-13-2021, 03:52 PM   #10
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With the tires about 5 psi above the minimum psi for the axle loads, the fronts felt a bit squishy on really curvy roads, so I added another 5 psi up front


Do you have the huge 365/70's on the front? They can carry 8000 lbs apiece at 90 psi, so that isn't exactly soft.
fronts are the Michelin XZA, 365/70R22.5. Table says they can carry 14700 lbs at 80psi (yes - that is *single* tire load, not double). Factory weight on the front axel is 16480 lbs, split 8145 and 8335 left and front respectively. So ... on the fronts, *minimum* load on the tires is almost double factory weight.

The rears are Michelin XZA1 315/80R22.5. Table says minimum load is 23360 (double) lbs at 85psi. Factory weights on the duals are 17495 lbs total, split 8845 and 8650 lbs left and right (again, double weights). These factory weights are *less* than half the *minimum* load at 85 psi.

I will definitely weigh the rig, but I *strongly* suspect even fully loaded I am going to come anywhere near adding something like 7000-8000 lbs per corner to lift things above minimum inflations - hence my question (btw - I have looked repeatedly at these tables, and I dont *think* I am getting the numbers wrong - if someone wants to double check me based on the information above, by all means please do).

I am intrigued by your comment about feeling like the front end felt "squishy" and so you upped the pressure. This does echo what the dealer says, he is indicating a greater pressure bump than you used.....
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Old 04-13-2021, 03:59 PM   #11
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Yup-- believe the tire engineers who designed the tire, oversee QC and develop the Inflation Tables...

OR

Someone at the dealership??? And, even if he is clairvoyant, how is he going to know how you will load your coach. Full timer doing lots of boondocking (so full tanks and lots of stuff) or occasional overnight to watch a football game? Tire PSI by ouija board???
I fully understand that dealers can have a bad reputation for saying "anything it takes" .... and in this case there *are* a few items where the dealer (and I was working with the owner) did not know something .... BUT they admitted they did not know the answer. I think you folks here are being a little too fast to demonize what this dealer said: they are a nationally known dealer, family owned, and have been dealing in these units for many years. I believe them when they say they want to protect their reputation, AND they have had ample time to hear feedback from customers (many of whom are repeat). I am not perfect at this .... but I have gotten absolutely zero "bad vibes" about them .... in fact, quite the opposite. Will I take what they say about tire pressures as gospel? No. But will I listen, and at least consider their experience. Yes. Again - hence my question. Please note that at least one person responding here said they ended up deviating upwards from the table values based on experience with handling.....
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Old 04-13-2021, 04:06 PM   #12
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I played with the tire pressures on my 52,000# rig and found the handling to be better with the weight sticker pressures. And "squishy" is a good description at lower pressures!
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Old 04-13-2021, 04:12 PM   #13
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The reason for most of the "less than positive" comments is that each coach and each owner is different in terms of needed tire pressure. Again, a huge range of weights for the same coach based on how it is to be used.


Best answer from a dealership is always:


As soon as you have it loaded as you will use it, have it weighed. Individual wheel position weights best (use heavier wheel position on each axle to determine the minimum PSI for all tires on that axle). Add 10% or so to that minimum.


If only axle weights are available (most truck stops), you have to add a little more to account for left/right imbalance.


Until weighed, use the PSI on the GVWR sticker in the coach. That PSI is based on each axle being loaded to its GAWR (Gross Axle Weight Rating).


Sorry, but any other answers DO concern me.


Yes, I completely understand that dealerships do not want to burden new owners with "homework", but (OK large BUT)....
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Old 04-13-2021, 05:11 PM   #14
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I noticed something that I should have noticed in the beginning. You already understand well how to use weight and pressure tables. If I read it correctly you are below the minimum, not sure how it effects the needed pressure and also got BS from a salesman.

Ignore the salesman. You have 2 ways two proceed. Best would be to weigh the vehicle. Then contact the tire manufacturer and speak with someone, possibly in R&D or technical services. If they will they can take your information and then give you a definitive pressure for your situation. They have that information already from development and testing. If they won't do that then the second option is to run the minimum from the table. Less may void your warranty unless specifically instructed to do so by the company. You can play with more in different places without risk if it helps but you will be safe.
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