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Old 09-15-2015, 08:41 PM   #1
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Need help determining Tire Pressure

I've had a small travel trailer for years, but my Class C is new to me. I just had new Firestone Transforce HT's installed, LT225/75/16 and had the Motorhome weighted with a full tank of gas and gear. The Front weighs 3,940 and rear weighs 9,560 for a total of 13,500. I Could not locate a scale to weigh each tire.

Firestone load charts says the following:
For Dual Axle:
1765 - 50 psi
1875 - 55 psi
1995 - 60 psi
2150 - 65 psi
2220 - 70 psi
2330 - 75 psi
2440 - 80 psi

For Single Axle:
1790 - 45 psi
1940 - 50 psi
2060 - 55 psi
2190 - 60 psi
2335 - 65 psi
2440 - 70 psi
2560 - 75 psi

How do I determine the psi to go with for each axle and what should I set the front tires and the rear dual tires at?

In advance thank you for the help

Greg
2009 Winnebago Access 31N
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Old 09-15-2015, 09:53 PM   #2
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ok how on ur weight allowed 5000 frt rear 9500 so 80 in rear 65 frt both are max. total for rig is 14500 mine is 14.050. also u weighed with no people. don't carry to much fresh water just enough to use potty ect on road wtr is 8 lbs a gal gas is 6
take rear weight divide by 4 frt div by 2 alway use the high # all across axle
forgot to ask it's a ford right?
you need to add to your profile as far as what u have, really helps when u ask a ?
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Old 09-16-2015, 06:24 AM   #3
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2390# is average wt on rear tires. Go with the 80# of air.

Front. Divide by 2, then go next highest. 55# to 65#.

Because you do not know weight on each tire exactly, go a little over the chart pressures.

Personally, I would go 80# rear, 65# front.
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Old 09-16-2015, 07:59 AM   #4
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I weighted it with a full tank gas, gear and people. The gray, black and water tanks were empty as I go to state parks with full hookups.

My tires are actually rated to go to 80 psi, but I did not include it as my fronts are at 75 psi now and I feel every little rut or bump in the road. I just did not know how to determine the psi to go with. I will try 65 in the front.

Thank you

Greg
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Old 09-16-2015, 02:35 PM   #5
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Used my made motorhome calculator and came to Front 65 psi and rear 87 psi .
The list you give comes to higher loadcapacity's for the pressure then European lists would give for the same tire specifications.

My spreadsheet even uses a saver formula then european and ads 10% to the given loads to cover , pressure loss in time, unequall loading R/L etc.

Also gives weight division F/R and here front 29,2 % and rear 70.8% of vehicle weight, once read advice to keep at least 30 % on front for best driving.

Determined your maximum loads Single and Dual to be AT 80 psi so E-load/10pr. Single calculated from dual 2440 lbs at 80 psi so Single 2638 lbs AT 80 psi.
Check this all on sidewall and also search for maximum speed of tire, then I can refine the advice to probably higher pressure without bumping and so more reserve. Most likely N speedrated for 140km/86m/h max speed.
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Old 09-16-2015, 04:19 PM   #6
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It seem that after all of that calculating you came up with the same pressures that are printed on the door jam of my Ford E450 Class C . Front 65, Rear 80.
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Old 09-16-2015, 04:24 PM   #7
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Quote:
Originally Posted by jadatis View Post
Used my made motorhome calculator and came to Front 65 psi and rear 87 psi .
The list you give comes to higher loadcapacity's for the pressure then European lists would give for the same tire specifications.

My spreadsheet even uses a saver formula then european and ads 10% to the given loads to cover , pressure loss in time, unequall loading R/L etc.

Also gives weight division F/R and here front 29,2 % and rear 70.8% of vehicle weight, once read advice to keep at least 30 % on front for best driving.

Determined your maximum loads Single and Dual to be AT 80 psi so E-load/10pr. Single calculated from dual 2440 lbs at 80 psi so Single 2638 lbs AT 80 psi.
Check this all on sidewall and also search for maximum speed of tire, then I can refine the advice to probably higher pressure without bumping and so more reserve. Most likely N speedrated for 140km/86m/h max speed.
sidewall says 80 MAX,so thats that frt axle is max weight 5000
R is 9500
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Old 09-29-2015, 09:21 PM   #8
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First look at the tire pressure rating on each of your tires. It is never advisable to use less than the recommended pressure.
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Old 09-29-2015, 09:31 PM   #9
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Poeman, I hate to disagree but the pressure on the sidewall of a tire is not always what should be used.

The pressure stamped on the side wall of every tire is the maximum pressure you would use IF (and only IF) the tire is carrying it's maximum weight as determined by weighing the coach. If you use a higher pressure than required for the weight it is carrying it causes a harsher sometimes tooth rattling bone jarring ride.

As twinboat mentioned: After all that figuring you ended up putting in the pressures recommended by the manufacturer on the sticker inside the coach. That's always a good place to start. Then after you've loaded the coach and gotten a 4-corner weigh you can adjust as needed.

TeJay
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Old 10-01-2015, 10:00 AM   #10
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Where can you get a 4 corner weigh?
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Old 10-01-2015, 10:17 AM   #11
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Crsltman,

Not an easy thing to do. Most USA CAT scales won't do it because of the traffic, guard rails around scale so you can't straddle the scale. We had our truck and travel trailer weighed at a CAT Scale once.

Many times an RV Rally will have the state police come in and they have the scales to do it. If you are over weight they promise you won't get a ticket. There may also be some who have the equipment and specifically come to the rally for that purpose. I have not been to one that had them so others may chime in on that. I've just heard that they are sometimes there.

If you live in farming country and see a grain elevator you might talk them into doing it. I'm sure they'd be happy to accommodate an RV. When we picked up our coach in Forest City Iowa I went to a grain scale and had it done. The coach was new so I didn't get a 4-corner weigh at that time. Because of the larger grain trucks there are no guard rails so they can easily do it. Also because of the BIG WBGO Grand Rally every summer they probably stay very busy with RV weighing.

TeJay
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