GXV Patagonia on the Kenworth K370 chassis

gregmchugh

Observer
With that weight rating you need a CDL licence to drive your truck?
It is titled and registered as an RV/Motorhome in Florida so anybody can drive it with a normal license, pretty much the same for RVs of most sizes in most states. Now you know why you should drive defensively around all the old folks who are thundering down the highway in their RVs. And I surely fit into the old folks category... :)
 

gregmchugh

Observer
Greg, will you share where you purchased the totes and size, I’m thinking of following your lead on storage.
I sent you a message with my email. Might be easier to discuss the details via email.

I assume you are referring to the underbed storage cabinet that is accessed from inside the camper. We use 4 different sizes of IRIS Weathertight totes on three shelves, the bottom shelf for larger totes and the top two shelves for smaller totes. The cabinet was specifically designed for the sizes of these totes. I will look up the specific sizes of the totes and let you know what they are and how they are arranged. It was worked fine but the cabinet latch prevents using a tote in one spot so we store other stuff in that one spot which is accessible with the cabinet doors open.
 

Britboaters

Observer
Greg,
If I understand correctly you are using 425/65R22.5 tires.
The tire pressures you quoted from the GY military brochure are for max rated load - 11,400 lbs at 120 psi (according to GY tech data).
If you look at the load / inflation tables, page 18, you will see minimum cold inflation pressure for various loads, at highway speeds.
http://fifthwheelst.com/documents/Goodyear_Tire_Inflation___Load_Charts.pdf

You quoted earlier that you think your max weight is around 26,000 lbs. So if we assume 50/50 front rear, then 6,500 lbs per tire.
From the tables, the minimum quoted pressure is 80 psi, with a load capacity of 8,270 lbs, at highway speeds.
I would suggest you weigh the axles to be sure, but you are probably less than 8,270 lbs on all the tires.
So run them at 80 psi on the highway. This will give you a better ride, better tire wear, etc to run them at their design pressure. Air down per the military brochure if necessary.
Bob
Retired Michelin tire engineer.
 
Bob, you are making a lot of assumptions. Expedition campers are almost always heavier in the rear. Fully loaded mine is 5000 lb/tire in the front but 9000 in the rear.
Second people almost always underestimate their weight, sometimes grossly.
Third, all the very large tires have low speed ratings due to heat buildup issues which can lead to catastrophic blowouts. How is this ameliorated in addition to speed discipline? A modest amount of overinflation, not exceeding max rated cold pressure. American drivers are not noted for driving RVs slowly on Interstate highways. A slightly rougher ride and a bit of central tread wear is a small price for safety.
You of all people as an engineer should know the danger of assumptions in place of facts.
At least you did suggest Greg weigh his truck. He should do it maximally loaded and round up a bit, each axle separately. If he has TPMS he can use a fairly simple formula I have posted previously to calculate tire temperature.
 
Last edited:
Tire temp formula:
T(hot) = T(cold) x (P(hot) + 1 atm)/(P(cold) + 1 atm)
T must be on absolute scale, F + 453.6 or C + 273. Use 1 atm = 14.5 psi. T and P cold are what you observe before starting travel. T hot will be estimated tire temp. I have been told by ANOTHER Michelin engineer at Eurosatory 2010 that 167F is the “danger point “
 

waveslider

Outdoorsman
Following this discussion intently as I was trying to understand the relationship of loading, pressure and speeds from the brochure GREG posted.

Recognizing that some assumptions must be made in lieu of Greg actually weighing his rig, just using some sample weights in order to have the discussion is still useful for those of us trying to wrap their head around that speed/heat/load relationship.

Charile, is it useful to perform some testing of pressures and speeds by using a infrared thermometer to get actual tire temps or is it more of a “core temp” vs surface temp kind of thing?
 

gregmchugh

Observer
I am using a cold tire pressure of 95 psi front and rear which was the pressure recommended by GXV for highway driving. The ride is very smooth on the highway at this pressure and I limit the speed to the 65 mph limit for the tires. When I get a chance to weigh the truck I will look at the options for lowering the pressure.
 

waveslider

Outdoorsman
using a cold tire pressure of 95 psi front and rear which was the pressure recommended by GXV for highway driving. The ride is very smooth on the highway at this pressure and I limit the speed to the 65 mph limit for the tires.
So, is that max speed independent of loading or is it relative to loading rates?
 

gregmchugh

Observer
So, is that max speed independent of loading or is it relative to loading rates?
Max speed on highway is 65 mph with tire pressure no lower than tire inflation load charts above but max speed would be lower if you reduce tire pressure off highway as shown in the military brochure above, tires still handle the same load at the lower max speeds.
 

Britboaters

Observer
Just to summarize:
- weigh each axle fully loaded
- Refer to the Goodyear published document I posted - note these are industry standard load / pressure tables for 65 mph.
- Use those recommended pressures for best tire performance, if you like.
There are also instructions in the document on the relationship between speed and pressure requirement.
Bob
 
Last edited:
Top