The attached chart is in section 22 of the Owners Manual. The heaviest side on the rear is 12,660 for a total weight of 25,320. Using this chart my rear mains should be 65 psi and my tags should be 50. The lowest pressure shown on the Michelin chart is 75 psi. Would you run the tires at 75 or what the chart indicates.
I have had the same dilemma looking at my weights/charts and talking to Wayne at FOT. I can't run my tires that low. At less than 1/2 of the max pressure (120 psi) I would consider that tire flat. Google "when is a truck tire considered flat" lots of info out there.
I would send an e-mail to Michelin to get their input. Until then, I wouldn't run at a lower inflation pressure than the one shown in their load/inflation table.
At least in the past, Michelin has always recommended carrying the minimum PSI on the chart. If that is 75 PSI, that is their recommended PSI.
It would be interesting to measure the rear tire foot print (when set at the chart minimum pressure) and compare it to the front tire foot print when set at the correct chart pressure. If the rear tire foot print is the same or smaller than the front I would be comfortable with it.
Could running at these lower pressures have something to do with side wall cracking?
What I deal with when you talk about tag PSI is simply this --75 PSI allows the front to porpoise a bit too much . 80 PSI reduced it some , 85 made it stop porpoising all together , So , what to do ? My chart says run 75 PSI . So ---I run 80 PSI and deal with what ( porpoising ) I get , which is not all that bad . I see this as a see saw kind of thing . Probably all wet , but that is what I do . On the 40 ft Phenix I had , didn't seam to matter much , but this coach makes it show up . 2 cents worth . Brad Metzger
As a long time dirt bike builder and suspension tuner at a quick thought the increased tag tire pressure might be an excellent adjustment of the suspension externally to compensate for too low rebound dampening on the front shocks.
Good fix
I think if you look at the charts it would be indicating much higher pressures than 70 pounds. on your drive axle you are carrying 0ver 12,000 lbs per side which is over 6,000 pounds per tire which would have your pressures over 105 lbs, I can't see the chart as I am writing this, but don't know how you came up with such low pressures. On our 320 after weighing found need 120 in the front and 105 in the drive axle and I run 80 in the tag
Second chart shows combined rear axle weight at 25,500 needing 60 on the main and 50 on the tag. I agree this seems to be low but there are three tires on the rear on each side.
I would have the coach weighed as a 25,000 combined rear axle weight seems very low with a 42 ft coach I would think maybe 24,000 on drive axle and maybe 8,000 on the tag
We are 16,600 drive and 7300 tag as loaded for travel in a 40 foot 2-slide.... Total of 23,900 combined rear. Steve D's weight seems reasonable to me.
Plus the drive axle is rated 21,000 lbs so I'd be surprised to see a tag axle coach at 24,000 (3K over) on just the drive....
I dropped of coach today at FOT and while there I researched the history of the charts in the original post. That data was from Michelin and only applies to Michelin tires. The pressures are so low because the weights are low on that model coach. It confirms my experience with the coach when I checked temperatures across the tire width after operating at high speed. The temperature was consistent across the tread width and did not show a deviation that would show up with under inflated tires.
I run 75 based upon weight from truck scale.