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Rear tire pressures with tag

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.
Steve DeLange
2005 U320T 40'
Pearland, Texas

Re: Rear tire pressures with tag

Reply #1 –
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.
2002 U320 40Ft  MC # 15759
Coach # 5995
Previous coaches
2002 U320 coach  #5941
1996 U270 coach #4808

Re: Rear tire pressures with tag

Reply #2 –
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.
David and Carolyn Osborn
1995 U320C SE 40' Build 4726 Feb 1995
FMCA 147762
Motorcade 17186

Re: Rear tire pressures with tag

Reply #3 –
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.
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: Rear tire pressures with tag

Reply #4 –
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.
John Duld
1995 U320C SE 40'

Re: Rear tire pressures with tag

Reply #5 –
Could running at these lower pressures have something to do with side wall cracking?
2002 U320 40Ft  MC # 15759
Coach # 5995
Previous coaches
2002 U320 coach  #5941
1996 U270 coach #4808

Re: Rear tire pressures with tag

Reply #6 –
             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
Brad Metzger
2010 Phenix 45'

Re: Rear tire pressures with tag

Reply #7 –
Quote from: Brad Metzger –
              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
"Riding and rejoicing"
Bob
1997 U320 40' Mid entry, build 5132,  wtbi ce27, 4th owner
2007 Solara convertible
2 prodeco tech outlaw ss electric bikes

1095 watts solar
08 Ls 460 and a sc430
2000 Ford F-250 superduty 4x4

Re: Rear tire pressures with tag

Reply #8 –
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
previous 1984 35 ft ORED 250 HP 3208 Cat       
previous 1998 40 ft U295 CAI 325 hp Cummins
previous 2003 40 Ft u320 build #6140 450 Cummins M11.                                                         
1999 Mazda Miata
Ron, Nancy, Tipper the cat, Max The dog
1997 U 270 36 ft build number 5174 8.3 Cummins

Re: Rear tire pressures with tag

Reply #9 –
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.
1999 40 ft. U-320 wtfe build 5563 Chuck & Lynda's "Rollin' Inn"  2030 watts solar
prev. mh's 71 GMC 5 yrs. 73 Pace Setter 1 yr. 78 Vogue 5 yrs 81 FTX 40ft all electric 18 yrs. 1996 Monaco Signature 3 yrs.
2014 Grand Cherokee Overland
Dream as if you will live forever. Live as if you will die today.  James Dean

Re: Rear tire pressures with tag

Reply #10 –
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
previous 1984 35 ft ORED 250 HP 3208 Cat       
previous 1998 40 ft U295 CAI 325 hp Cummins
previous 2003 40 Ft u320 build #6140 450 Cummins M11.                                                         
1999 Mazda Miata
Ron, Nancy, Tipper the cat, Max The dog
1997 U 270 36 ft build number 5174 8.3 Cummins

Re: Rear tire pressures with tag

Reply #11 –
Quote from: red tractor –
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....
Learn every day, but especially from the experiences of others. It's cheaper!  - John C. Bogle

2000 U320 36' non-slide / WildEBeest Rescue
2003 U320

Re: Rear tire pressures with tag

Reply #12 –
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.
Steve DeLange
2005 U320T 40'
Pearland, Texas
 

Re: Rear tire pressures with tag

Reply #13 –
I run 75 based upon weight from truck scale.
'07 Phenix Double slide, Bunk beds, 42'
'14 Wrangler Unlimited Rubicon