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Old 07-19-2015, 11:30 AM   #1
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One more tire pressure question...sorry

I know this subject has been discussed to death but I need opinions please.

Getting ready for a five-week trip. Loaded the coach including a full tank of fresh water since I wanted to achieve maximum possible weight.

Filled up with diesel and then got it weighed on a CAT scale. I have no way to get individual wheel weights at the moment. I took the axle weights and applied the 55% "fudge" factor to arrive at individual wheel weights to compensate for possible load imbalance side to side. Went to the Goodyear chart and found the correct PSI based on the individual tire load.

Now for my question. Do you add additional pressure above the recommended pressure even though by applying the 55% you have already increased pressure above the baseline? In my case it adds 5lbs more than if you didn't use the 55% trick. My goal is to run a safe pressure while still getting the best ride.

I will thank you in advance for sharing your personal preference and experience.
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Old 07-19-2015, 11:51 AM   #2
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I personally just add the extra 5 PSI only. Guess it sort of depends on where you are on the scale to begin with. If you find your running your rear tires at near the bottom of the scale like I do then adding a couple of PSI is better safe than sorry. On the other hand my front tires are much closer to the tire max so just run them just a little above what the table shows if any.
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Old 07-19-2015, 09:43 PM   #3
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You did not find the correct inflation pressure/PSI, you found the MINIMUM PSI for the load, anything below that is overloading your tires, which are now also under-inflated. The correct tire pressure is listed on the mfgrs. tire placard in your RV.
You may read this in this pdf from the RMA-Rubber Manufacturers Association addressing tire care. Chapter 4 pertains to motorhomes, page 55 states:
"Inflation pressure recommendations may also be
determined based on the tire manufacturer's
specifications, which define the amount of inflation
pressure necessary to carry a given load. These
inflation pressures may differ from those found on
the vehicle tire placard or certification label.
However, never use inflation pressure lower than
specified by the vehicle tire placard, certification
label or owner’s manual. Nor should inflation
pressure exceed the maximum pressure molded on
the tire sidewall."
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Old 07-20-2015, 05:15 AM   #4
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You can probably go higher in the pressure .
If you take care that the real weight on the side of the axle with lowest load, is more then 85% of the weight the pressure is looked back for in the list, or better calculated for with use of one of my spreadsheets, you still have acceptable comfort and gripp.

Will give general text I saved so I dont have to write it all over again, but can be little of the subject at points.

Tirepressure advice is all about load on tire and speed ( and sometimes about alighnment - camber angle).

So if you can give details of car and tires , I can calculate an advice pressure with some reserve for things like, pressure-loss in time, unequall loading R/L, incidental extra load, misreadings of pressure scales,and misyudging of weight, etc.

This is from tires next and can be read from sidewall:
Maximum load or loadindex.
Kind of tire to determine the AT-pressure/pressure needed for the maximum load up to maximum speed of tire, or if lower 160km/99m/h/reference-pressure, wich is not the maximum pressure of tire.
Maximum speed of tire, most given as letter ( Q=160km/99m/h,N=140km/86m/h fi)
If you have offroad or tires looking like that , with large profile blocs that cover a part of sidewall, also mention, they are allowed lesser deflection then a normal road tire, then the tire maker used to determine the maximum load (to my conclusion the case for the Bridgestone tires on Ford Explorer in the Ford/Firestone affaire).
If you cant find all of it give sises of tire and Loadkind, then I will google for it.

From car next and mostly can be found on same plate as the original pressure advices:
GAWR and GVWR ( Gross Axle/Vehicle Weight Rating)
But best would be to determine the real weights in your use on seperate tires or estimate it as acurate as possible, by weighing per wheel(pair) or axle.
Maximum speed , you dont go over for even a minute in your use, eventually different for different situations, for instance when towing or fully loaded.This apart from trafic regulations, if you drive faster then allowed give that speed. Nature punnisches with tire-failure, police only with a penalty.
Give all that and I will calculate and give a picture of one of my filled in spreadsheets in my answer.
If other then original tires, indead as is already answered other advice is needed, a stiffer tire ( fi C-load instead of P-tire) needs a higher pressure for the same load, or the other way around, has lower loadcapacity for the same pressure.

Greatings from a Dutch pigheaded self-declared tire-pressure-specialist.
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Old 07-20-2015, 08:08 AM   #5
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I don't want to help beat a dead horse but...

You CAN get individual weights at a CAT scale if you:

1. Get a second weigh with only one side set of wheels on the scale.
2. Subtract those weights from the original and you can figure out corner weights.

You can PM me on how that is done if you like. It isn't hard at all.

As to what Ray said...YEP! I wouldn't run less than the tire pressure placarded weights and would probably run extra since you don't have corner weights.

OTOH...here is where things can get a bit sticky. Let's say you have tires with a MAX COLD PSI of 120 PSI but after you figure out what your actual weights are (or guesstimate them ) you find you need the full 120 PSI. You now have to be more conscious of over inflation if you travel from areas of cooler temps and tires are inflated accordingly for those temps. All factors being equal, a 10* change in temps will cause a gain/loss of 2 PSI. If you inflate your tires to 120 PSI at 60* in central IL and then travel south to FL with temps in the low 80s your tires will now be about 124 PSI when "cold" per FL temps. When I trek from IL to TX I generally make tire pressure adjustments in the morning of each stop and have yet to add pressure.

This shouldn't be taken to be me screaming with my hair on fire proclaiming your sky is falling. LOL It is something to be aware of.

BTW...when folks add 5 PSI "just because", they are protecting themselves from the opposite when traveling from warmer to cooler temps.

The only "problem" with running the added 5 PSI is both for what I said about over inflation if close to max PSI rating and increasing the harshness of the ride.
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Old 07-20-2015, 09:19 AM   #6
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Gentlemen...thanks much for your wisdom.

Unfortunately I can't get individual wheel weights on my local Cat scale. They have installed concrete ballads that won't allow you to be anywhere except the center of the scale.

I'm truly sorry for bringing this tire inflation mess up for the 1,000 time but I want to get the best ride and still be safe. Here are the details:

Motorhome placard:
GVWR - 32,400
Front GAWR - 12,400 Recommended PSI - 115
Rear GAVR - 20,000 Recommended PSI - 100

After loaded weigh:
Front axle - 9,340
Rear axle - 19,980
Total weight - 29,360

Tires - Goodyear - 275/70R 22.5 Load Range H
Max load single - 6,940
Max load dual - 6,395
Max PSI - 125

Wheels:
Max load - 7,390
Max PSI - 120

The rear tires are a no brainer, it's the front I"m trying to get right. There is a 3,000 lb. difference between actual weight and GAWR. If I run the placard recommended pressure of 115 lbs. it's like ridding on solid rubber tires.

According to the Goodyear chart the baseline pressure, even with compensating for load balance left-to-right (55% fudge factor), is 90 PSI. I want to be safe but is eels to me 115 lbs. is too much.

Thanks for your patience. You guys are the best.
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Old 07-20-2015, 09:33 AM   #7
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Quote:
Originally Posted by Jim28730 View Post
Gentlemen...thanks much for your wisdom.

Unfortunately I can't get individual wheel weights on my local Cat scale. They have installed concrete ballads that won't allow you to be anywhere except the center of the scale....
Looking at the sat photo of the one located at Waynesville, NC I don't see a barrier except on the arch. On the second weigh you drop back to pads 2 & 3 and do the math.

Now, if there is another barrier at the beginning of the scale that is not on the sat photo...yep...you can't do it there. I haven't seen anything like that at any of at any of the CAT scales I have used or observed.
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Old 07-20-2015, 09:43 AM   #8
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Thanks Don, i will ride up there later and check it out. I hadn't thought about using the second and third scales. I am going to get individual weights by Excapees when i head south this winter.
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Old 07-22-2015, 03:54 AM   #9
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Quote:
Originally Posted by Jim28730 View Post
Gentlemen...thanks much for your wisdom.

Unfortunately I can't get individual wheel weights on my local Cat scale. They have installed concrete ballads that won't allow you to be anywhere except the center of the scale.

I'm truly sorry for bringing this tire inflation mess up for the 1,000 time but I want to get the best ride and still be safe. Here are the details:

Motorhome placard:
GVWR - 32,400
Front GAWR - 12,400 Recommended PSI - 115
Rear GAVR - 20,000 Recommended PSI - 100

After loaded weigh:
Front axle - 9,340
Rear axle - 19,980
Total weight - 29,360

Tires - Goodyear - 275/70R 22.5 Load Range H
Max load single - 6,940
Max load dual - 6,395
Max PSI - 125

Wheels:
Max load - 7,390
Max PSI - 120

The rear tires are a no brainer, it's the front I"m trying to get right. There is a 3,000 lb. difference between actual weight and GAWR. If I run the placard recommended pressure of 115 lbs. it's like ridding on solid rubber tires.

According to the Goodyear chart the baseline pressure, even with compensating for load balance left-to-right (55% fudge factor), is 90 PSI. I want to be safe but is eels to me 115 lbs. is too much.

Thanks for your patience. You guys are the best.
Then I filled this all in in my calculator and made picture of it.
What I did now know I asumed to be M speedrated = max speed of tire 81m/130km/h.
The picture uses as given maximum load , but if I lower maxload to give the tire the same deflection a Q speedrated tire would have, so highest presssure without bumping and most reserve it would give for GAWR's
Front 128 psi ( wich is above AT pressure of 125psi and above maximum allowed cold pressure of rimms) . For your given weighed axle loads with 47/53 division it gave 101 psi , so also allowed for the rimms.

Rear I did not lower because its already done for dual load 3 LI steps.

I used part 2 for GAWR's and part 3 I assumed a 47/53% loaddivision on the axle-ends to give advice pressure for your given weighed axle loads.
This to get the both calculations in 1 picture.

For GAWR's the spreadsheet gives Front 117 psi and Rear 108 psi wich are both below the maximum allowed cold pressure of Rimms of 120 psi.

For your weighed loads it gave, Front 92 psi and Rear 108 psi ( because almost GAWR)

So my advice is F 101 psi and rear 108 psi , to give maximum reserve with acceptable comfort and gripp.

Now here the picture and also mind the things below the advices , also handy to know.

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