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  #1  
Old 08-13-2008, 10:30 PM
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Tire pressure question

I just bought a 99 Mitsubishi Fuso FG in North Carolina and I'm driving it home to Denver. It's a 12000lb GVWR 4wd truck with a service body on the back and some lube oil tanks (probably almost empty) so there is not much weight on the truck. Probably similar to an empty pickup truck. It was giving a pretty harsh ride and I remembered a comment on another board by someone who bought just the cab and chassis and reduced the tire pressure on the ride home to soften the ride. I checked the pressure in the rear duals and they were at 70psi. I reduced them to 60 psi and the ride improved pretty significantly. I'm wondering what the lowest safe pressure is with essentially no significant load on the back. The manual says 80psi lbs for load range E tires, which is what is on the truck. But even at 70psi it felt as if the back of the truck was bouncing up over the interstate cracks.
I've done nothing to the front tires, but I assume keeping them at spec would be appropriate, or could they even be reduced with no load on the truck? The manual calls for 65psi for load range D tires on an FE 639 with GVWR at 11,600 lbs although mine is the 12,000lb GVWR calling for Load Range E at 80psi.
Thoughts?
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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13
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  #2  
Old 08-14-2008, 01:21 AM
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Kerry, I am jealous..........later on put the 235-85x16 Yokohama rally tires on them. For now stick to 75 and look for sidewall and wear pattern by marking the outder edges with chalk, if the wipe out, you are too low.
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Last edited by Gurkha; 08-14-2008 at 04:58 AM.
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  #3  
Old 08-14-2008, 04:51 AM
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.

I agree with what he said.

Used to deal with lots of heavy equipment.

Impossible to know the weight of the equipment all the time.

Keep some chalk in the cab.
Put marks on the tires from rim to tread.
Drive a short distance, few hundred feet or so.
If the chalk has rubbed off the tire near the tread,
your pressure is too low.


RichC

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  #4  
Old 08-14-2008, 04:25 PM
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My buddy had one of those a few years ago and it always rode like, well, one of these:



You can see the resemblence... Since you're sitting out over the front wheels, you'll get an accentuated effect from every bump. My F250 has a GVW of 8800# and I can run the E rated tires at 40PSI in the rear and 50 PSI up front without it feeling too mushy.
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  #5  
Old 08-14-2008, 07:04 PM
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Thanks for the advice. I didn't have any chalk with me on the trip but I will experiment. At 60psi, the sidewalls on the duals were not deflecting at all so I don't think it's an issue on the rear.
I'll try different pressures and see what happens. The weight of the empty cab and chassis is 6000lbs.
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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13
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  #6  
Old 08-14-2008, 10:25 PM
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I think you should as Obama. He's an expert on tire pressure.
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  #7  
Old 08-15-2008, 06:12 AM
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I take it down to forthsomething on my Dodge 3/4 for comfort when there is no load. I have 87K on the ones that came on the front so far and am shooting for 100K.

tom W
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..I also have a 427 Cobra replica with an aluminum chassis.
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  #8  
Old 08-15-2008, 09:06 AM
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The first Goodyear store I worked at as an assistant manager handled road service for local municipalities as well as NJ Transit non-revenue vehicles - boom trucks, repair trucks,etc. I spent a lot of time with tires ranging from 235/75R15s up to 12R24.5 and 295/75R24.5s as well as "super singles". Just remember that it's the air pressure that handles your load capacity. Any Load Range E or higher tire will ride very stiff no matter what.
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  #9  
Old 08-15-2008, 09:14 AM
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Quote:
Originally Posted by AdvisorGuy View Post
The first Goodyear store I worked at as an assistant manager handled road service for local municipalities as well as NJ Transit non-revenue vehicles - boom trucks, repair trucks,etc. I spent a lot of time with tires ranging from 235/75R15s up to 12R24.5 and 295/75R24.5s as well as "super singles". Just remember that it's the air pressure that handles your load capacity. Any Load Range E or higher tire will ride very stiff no matter what.
I think I'm going to try 60psi in the front also when empty to see if it makes the same kind of difference. They're currently at 70psi.
I should have taken my non-contact thermometer with me so I could monitor tire temperature. I was checking with my hands and the 60psi duals did not seem any warmer than normal.
Some people are putting super singles on Fuso FG's when they turn them into campers. Any opinion as a tire guy on the advantages/disadvantages.
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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13
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  #10  
Old 08-15-2008, 09:24 AM
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You've got a couple working against you here.

1. There's a tremendous difference between the empty and GVW.

2. The huge unsprung weight of that heavy rear axle with the duals.

When you get the truck finished and the camper fitted, you might want to consider "de-rating" the suspension down to the new weight of the truck. Since the weight of the truck won't vary as widely as it would when it was a beast of burden you can tune the suspension for a much better ride. You might be able to swap springs from another model or another make of truck.

Compare the weight of the super singles to the duals. Any weight reduction will help out.

I've always thought these trucks were pretty cool. I've considered buying one to outfit as a tow vehicle with a sleeper cab. Kind of like a mini semi for towing 5th wheel campers.

Need some pictures!
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  #11  
Old 08-15-2008, 09:43 AM
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Here's one picture. Some Australians have modified the suspension on these to meet their use better.
Attached Thumbnails
Tire pressure question-99fuso.jpg  
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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13
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  #12  
Old 08-15-2008, 11:58 AM
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One thing to keep in mind is the difference between front and rear presure for the load you are carrying. You probably want to maintain close to that difference in presure. Remember, all things being equal, if you just raise the front presure only, the car will oversteer more. Raise the rear only and the car will understeer more.
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  #13  
Old 08-17-2008, 11:08 PM
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Ran across this:

http://www.goodyear.com/truck/pdf/databook/loadInflation.pdf

VERY helpful in determining proper tire pressure. I could be running lower pressures.
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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13
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  #14  
Old 08-17-2008, 11:13 PM
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Excellent links, thanks Kerry.
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