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Old 07-19-2014, 10:45 AM   #1
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Isuzu turbo diesel owners..question?

I assume that most all classic AS motorhomes with the Isuzu turbo diesel have similar weight load on the front axle. I'm curious as to what tire pressure you all are running in your front steer tires. Ive read through tons of threads about tire pressure and weight distribution for both motorhomes and trailers. Since the front end of the classic motorhome seem to be about the same in design and dimension, and the main variable being the difference in weight between the gas and diesel motor, i think all the classic diesels weigh about the same on the front axle. Im sure there are some differences. I just put new tires all the way around on my 310 and am curious what pressure the classic diesel owners are running.
Ive read posts that say any where from 65psi to 100psi. The tire guys ive talked to have all said to inflate to 100psi, it seems the classic motorhome owners disagree. So........if you would be so kind as to state what you are running on yours, i would much appreciate.
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Old 07-19-2014, 10:53 AM   #2
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I run 95 front and rear. Seems to ride well and they don't get hot. If you take it to a scale and get a weight by axel you can inflate them to the manufacturers specifications. I think some people tend to overthink it a bit.


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Old 07-19-2014, 11:42 AM   #3
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Mayco,


The book that came with the motor home states front 70 psi and rear duals and tag 60 psi. I assume they lowered the inflation to the minimum amount that would carry the rated load to provide a smoother ride. The fronts always look low. I am planning to go up another 10 lbs on each which is still well below max rated inflation. Would would increase the hardness of the ride a bit but decrease the heat build up and add a better buffer for GVWR vice just enough pressure to meet minimum allowed for that weight.
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Old 07-19-2014, 01:38 PM   #4
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Wxbouy,
Ive seen those same psi numbers and thats what makes me wonder. Also, since nobody stores gear of any sort up in the cab area (that ive heard of) our weights should all be about the same, for the diesel that is. Air Dog i have a tendency to agree that this gets overthought a bit and im not trying to contribute to that for sure.
I had a flat last fall before i had put the new rubber on over in central Indiana. When the fella looked at the tire to repair it he didnt find any evidence of damage. He inflated the tire to 100psi and its held ever since. I told him that i was running 75psi and he immediately said, "well thats probably what caused the flat, must have broken the seal". So thats why im asking what folks are running.
Talked with the installer of the new tires and he said to run at 100psi. Now i know through reading here on the forum that most folks run their tires under 100psi but i was wondering if there is some consensus, at least on the steer tires. The rear are a different story i realize. Model, tag/no tag, actual load, etc.
Thanks for your patience.

I will say that new tires made a heck of a difference in the ride up front. Even at 100psi its much softer on new rubber.

Hopefully a few more classic diesel owners will chime in with what they are running up front.
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Old 07-19-2014, 04:10 PM   #5
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I'm sure that you are all aware the weight difference between the 7.4L 454 ci and the 5.8L Isuzu is about 400+ lbs.

Good inflated front airbags 55 PSI make more of a difference than tire inflation on ride performance but I have found that 80 PSI seems to give the best performance on fuel economy and ride comfort. I would think 70 would be too low for the diesel.

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Old 07-19-2014, 04:30 PM   #6
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We were wearing tires at 80 psi, when we had fresh ones put on, the Goodyear guys had it weighed and we have run 105psi ever since. We do have fresh heavy-duty coil springs, and they have been a big help without the air bags. We are running the most expensive Goodyears, by the way.
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Old 07-19-2014, 06:50 PM   #7
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Quote:
Originally Posted by mayco View Post
Wxbouy,
Ive seen those same psi numbers and thats what makes me wonder. Also, since nobody stores gear of any sort up in the cab area (that ive heard of) our weights should all be about the same, for the diesel that is. Air Dog i have a tendency to agree that this gets overthought a bit and im not trying to contribute to that for sure.
I had a flat last fall before i had put the new rubber on over in central Indiana. When the fella looked at the tire to repair it he didnt find any evidence of damage. He inflated the tire to 100psi and its held ever since. I told him that i was running 75psi and he immediately said, "well thats probably what caused the flat, must have broken the seal". So thats why im asking what folks are running.
Talked with the installer of the new tires and he said to run at 100psi. Now i know through reading here on the forum that most folks run their tires under 100psi but i was wondering if there is some consensus, at least on the steer tires. The rear are a different story i realize. Model, tag/no tag, actual load, etc.
Thanks for your patience.

I will say that new tires made a heck of a difference in the ride up front. Even at 100psi its much softer on new rubber.

Hopefully a few more classic diesel owners will chime in with what they are running up front.


We put about 12,000 miles on the mix of 12 year old yakatories. It had a mix of Asian tires. The PO said he ran 95 psi. We've run the Goodyears for about 6000 miles so far. What a difference in ride and road noise. 95 psi was recommended by the shop that installed them and I believe that is what is stamped on them. No signs what so ever on wear and they still look like new. I always check them at stops with a calibrated temperature sensor. (My hand). They barely warm up and are all uniform. I plan on getting individual axel weights some day. The Goodyear specs say 95 is a good number for what I'm guessing the weights are. However, my engine is in the opposite end.


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Old 07-19-2014, 07:46 PM   #8
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I may be a little controversial but I go pretty much buy what the book says. My Airstream manual says 55 psi. rear and 65 psi. front. There is a note in the 79 to 81 manual that says to add 5 psi for radials but I don't see that note in the 82 manual. I run 70 psi. all around and according to Goodyear that puts me where I should be according to actual scale weights.

The Goodyear inflation chart at 95 psi. gives a total capacity for 6 wheels of 18,740 lbs. which is a little more than our classics should weigh, no tag axle here.

Also for those of you with Alcoa aluminum wheels, the wheel is stamped with a max inflation of 95 psi. Those running more might rethink.

My TPMS shows both pressure and temp. The running pressure increases by about 10 to 11 psi. and the temp gains around 10 degrees over ambient. Sitting in the hot sun the temp may go up 30 to 40 degrees over ambient but comes down as soon as you start rolling. I find it interesting watching the numbers in real time going down the highway.

All things concerned I think my pressures are pretty close to where they should be and the tire wear looks very even. Mine is a 280 diesel and rated at 14,500 GVW.
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Old 07-19-2014, 08:23 PM   #9
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Well......great information from all. I appreciate all the input.
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Old 07-23-2014, 04:01 PM   #10
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Got over to the barn today so I checked some numbers.

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Here's a copy of the table in my Spartan manual.

My tires are stamped 95 psi for the max load.

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The Alcoa rims are stamped for 110 psi max. Cold.

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The numbers for your wheels and tires would be different.




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Old 07-23-2014, 04:23 PM   #11
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Nice chart, good to look at even if its not for the p30
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