Inflation Q. on a dually pickup

[AITG]

I got the Big Dog (2003 Chev Dmax ECLB) weighed today.
Total weight: 7700 lbs.
Front axle: 4380 lbs.
Rear axle: 3320 lbs.
That means each front tire carries 2190 lbs. and each rear carries 830 lbs.
The Goodyear inflation table puts the fronts between 65 and 70 psi. The rears would be over-inflated at 35 lbs.
How soft do I dare run the rears?

 

[Boomer]

Depends on the tire. If they are LT tires, you cannot run them at a P tire pressure or they will overheat fast. Run what the door label suggests.

 

[SteveSRT8]

We run Savana vans at 9200 pounds curb weight every day. As we have single tires rear we run 80 psi rear and 65 front. Factory recommended is 80 rear 55 front.

Be very cautious about under inflating E range tires. They will die fast.

 

[bullwinkle]

I disagree-I run my 9400 pound diesel E-350's front tires at 55 PSI, and they wear just fine, plus they don't beat you to death that way. I wouldn't go below 50 PSI on your dually-and make SURE the sidewalls are not near each other between the 2 rear tires!

 

[SteveSRT8]

Originally Posted By: bullwinkle
I disagree-I run my 9400 pound diesel E-350's front tires at 55 PSI, and they wear just fine, plus they don't beat you to death that way. I wouldn't go below 50 PSI on your dually-and make SURE the sidewalls are not near each other between the 2 rear tires!


That's because in your truck the weight is over the rear axle. Our trucks are very different. I can get you a link if you want, but the facts are they are two very different vehicles in terms of WHERE they carry the poundage.

Note that there is no difference in ride quality with Michelin LTX MS2's at 55 or 65 from the drivers seat in our fleet trucks. Just longer life.

 

[AITG]

OK, I'll elaborate. Every tire has a capacity at a given inflation. This information is in a table that is tire specific (if you have LT215/85R16 LR E you use that table. Much like using a map of Toledo when you're IN Toledo.)

Here, the issue is that the truck has very little loading on the rear tires due to uneven weight distribution when empty. Tire wear is idealized when the inflation is appropriate for the weight carried.

I may be over-analyzing this, but it seems that I have been grossly over-inflating the rear tires.

 

[CapriRacer]

Originally Posted By: AITG
I got the Big Dog (2003 Chev Dmax ECLB) weighed today.
Total weight: 7700 lbs.
Front axle: 4380 lbs.
Rear axle: 3320 lbs.
That means each front tire carries 2190 lbs. and each rear carries 830 lbs.
The Goodyear inflation table puts the fronts between 65 and 70 psi. The rears would be over-inflated at 35 lbs.
How soft do I dare run the rears?


There's a multi-level problem here:

First is making the assumption that if you use the table, the tire will wear flat. Nothing could be further from the truth.

Tire wear is very complex. There are a lot of things that affect it. In particular, steer tires tend to wear the shoulds and drive tires tend to wear the center. That's why it is important to rotate your tires on RWD drive vehicle - you wear different portions of the tire and get more miles!

But tire engineers will design a tire to wear evenly at typical load/inflation combinations - and that has changed lately. Many years ago, the load table was used as is, but the Ford / Firestone situation a few years ago pointed out that this was a poor enginering practice. Overdesign/ under-utilize should be the norm for components.

So the way to do this is to find the GAWR's (Gross Axle Weight Ratings), compare them to what the tire load table says the max load is at the placard pressure. The difference would be the "Reserve Load".

Then using the actual weights, add the Reserve, then look up in the table for the corresponding pressure.

HOWEVER: Because of ther recent changes in the way this is done, older pickups were done the old way, but tires are now done the new way. So I would recommend that if the reserve isn't at least 15% of the GAWR (that is, the tire should be inflated to 115% of the GAWR), then THAT value (15%) should be added in when figuring the inflation.

But here's another part of the problem: The vehicl;e's spring rates, sway bar sizes, and shock damping were all tested by the vehicle manufacturer using the placard pressure. They tested it both fully loaded and empty. Changing the pressure split changes the handling balance front to rear - and that might have negative affects. So I urge extreme caution. This is not an area to be cavalier about!

 

[SteveSRT8]

Oh so true! Strange handling trucks are not your friend when they are that big!

I have a few Silverados as well, but none of them are big boys, all are 1500's.

We actually do set the pressure by the load. Unloaded I run my 05 with 265/70/17's at 36 front 32 rear, and I get excellent wear and surprising handling. In the rare occasion where we get some heavy cargo on board I have run the rears all the way up to 44 psi. That was with a pallet of sod.

So I'm sure a dually owner would want to run drastically different pressures loaded vs. unloaded.