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Old 06-30-2012, 07:00 PM   #1
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Confused about tire pressures

On my Thor ACE, the Ford and Thor stickers both say tire pressure 85psi all tires. Here are all the numbers I know:
GAWR: Front: 6,500 Rear 11,000
GVWR: 16,000

Recorded weights on a CAT scale, typically loaded and with 3/4 fuel and water:
Steer Axle: 6,340
Drive Axle: 9,060

Tires are 225/70R19.5 Continental HSR.

The Continental Tire Inflation chart shows 3315 pounds at 85 psi single tire so I'm close on the front (6340/2 = 3160)

The rear tire values have me confused. The chart shows 3115 pounds at 85 psi Dual tires. If I divide the rear axle weight by 2 I get 4530 and if I divide by 4 I get 2265, neither of which weights are even in the range of the chart for duals. It goes from 2720 at 70psi to 3750 at 110 psi.

So my question is, how do I calculate the rear tire pressure?
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Old 06-30-2012, 08:20 PM   #2
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The 3115 pounds is the amount of weight each of your 4 drive axle tires will support at 85 psi. So you can carry more than your drive axle GAWR. Most truck tire experts consider 85 psi to be the minimum for 22.5" wheels. If it were me, I would continue with the 85 psi. I might up the steer axle to 90 just in case one side is much heavier than the other.
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Old 06-30-2012, 09:21 PM   #3
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Please remember that those charts from the tire companies are not the recommended pressure for a given load - it is the Minimum pressure for that load. Or looking at it the other way, it's the Maximum load for a given pressure.
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Old 06-30-2012, 10:20 PM   #4
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Thanks Steve and jmckinley. Both answers make sense. I think I'll go with 90 front 85 rear.
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Old 07-01-2012, 03:14 PM   #5
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Now go take a look at the "Cheap Handling Fix" thread. Costs nothing, big improvement.
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Old 07-01-2012, 09:28 PM   #6
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85 psi on the rears is ultra-conservative with those tires and I would not be afraid to run 70-75 as recommended by Continental. If you can't trust the tire manufacturer's load tables, who are you going to trust? Something you see on the internet?

85 psi is spot on for the fronts. You have to make allowance for side-to-side imbalance, so 80 psi is cutting it too fine.

On the other hand, running at a higher pressure just gives you a somewhat rougher ride. At worst you wear the center tread on the tire a bit more than the edges.
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Old 07-02-2012, 11:24 AM   #7
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Just a general comment...I was running my tires at the recommended pressures listed in the RV, and I was getting uneven tire wear. After asking the "experts", they all agreed that I should run the tires at 5 lbs below the max recommended pressure (125) for the TIRES, not the recommended COACH pressures. Yes, the ride is a bit harder, but the tires are wearing evenly now. At $600 per tire, I'll live with the rougher ride.

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Old 07-03-2012, 03:19 PM   #8
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There is always the possibility of individual reasons for adjusting pressure on a particular coach or driving situation, but I would hate to have anyone assume that they should always run at 5 under max or always run at any particular pressure.

The coach builder gives a general psi recommendation for those who don't want/need to fine tune. It may not be optimum, but it works safely for everybody.

The tire manufacturer gives inflation tables in case you need/want to fine tune for your coach.

Individual tire experts may suggest methods to correct for various observed conditions in a specific coach or specific operating environment. It is not generalized information.
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Old 07-03-2012, 03:35 PM   #9
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The lowest pressure on a chart is the lowest the tire should ever have when running it.
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Old 07-29-2012, 05:40 PM   #10
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Quote:
Originally Posted by Gary RVRoamer View Post
The coach builder gives a general psi recommendation for those who don't want/need to fine tune. It may not be optimum, but it works safely for everybody.

The tire manufacturer gives inflation tables in case you need/want to fine tune for your coach.

Individual tire experts may suggest methods to correct for various observed conditions in a specific coach or specific operating environment. It is not generalized information.
Gary:

Today I weighted our new coach (2013 Thor Palazzo) for the first time in preparation of our first long trip to upper Michigan.
Steer Axle: 6960 (GAWR 10,500)
Drive Axle: 15,520 (GAWR 17,500)
Gross Weight 22,480 (GVWR 26,000)
GCWR 30,000

The chart behind the driverís seat says to inflate front and rear to 120 PSI (the dealership delivered the MH to me with all tires inflated to 120 PSI per this chart).

I have Goodyear G670 RV 255/70R 22.5 16 ply installed. The Goodyear booklet provided has an inflation range of 80 PSI (S 4190 lbs/D 3970 lbs) to 120 PSI (S 5510 lbs/D 5070 lbs) and like others; I am confused as to what pressure to inflate my tires?

Referencing the steer axle as an example 6960 divided by 2 = 3480 and as you can see this is well below the 4190 @80 PSI minimum on the Goodyear chart.

If I understand it correctly I should divide the drive axle weight by 4 or 3880 per tire. Again nowhere near anything on the chart. I do not think I should be running 120 PSI all around like the way it was delivered but I do not think I should be running 80 PSI (the minimum) either?

Your opinion as to what pressure I should run would be appreciated! Other than a harder ride what would be the problem running the 120 PSI the way it was delivered and what is listed on the chart behind the driverís seat?

Thanks in advance for you opinion!
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Old 07-29-2012, 06:40 PM   #11
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I mechaniced on semis for years .For a huge company,when one went out of the shop we always went with ten pounds lower then the max pressure on front and five on the rear below the max pressure but what you also have to think about is weather situations and the weight of the unit ours were oversized load semis,to low will cause sidewall failure and to high will take a chance on overexpansion and cause a blowout . Have to think about heat example, i live in AZ we run all 10 pounds lower for air expansion when traveling the ride was fine, yes tires can be very expensive so you want to keep a close eye on them. Thats using the manufacturer of the tire suggested inflation rating.For example our Alfa SeeYa tires say max psi 110 pounds im running them at 100psi the coach runs the road fine.On our Newmar 40ft 5th wheel had 2 dually axles it weighed 18,500ibs empty weight we also ran 10 pounds under never ever had a problem. Just giving my personal experience if that may help. : )
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Old 07-30-2012, 05:51 AM   #12
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Does anyone have experience with the Doran 360RV Tire Pressure Monitoring system? They state overheated tire waring at 248 degrees F. That seems pretty high to me.
Thanks in Advance.
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Old 07-30-2012, 03:33 PM   #13
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Do a four-corner weighing and use the higher weight from each axle with the table from the manufacturer.

See my notes on tire replacement at

http://home.roadrunner.com/~kwildman...cing tires.doc tires.doc

I don't claim to be an expert but I did a lot of research on the subject when I needed to replace tires.
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Old 07-30-2012, 05:06 PM   #14
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Do a four-corner weighing and use the higher weight from each axle with the table from the manufacturer.

See my notes on tire replacement at

http://home.roadrunner.com/~kwildman...cing tires.doc tires.doc

I don't claim to be an expert but I did a lot of research on the subject when I needed to replace tires.
UltraKen:

Referencing post #1 and post #10 (my post). We know how to weight and this is not the question. In both cases the weight of the axle/point is below the minimum weight on the tire manufactures inflation chart.

In my case the dealer delivered my coach with the max inflation per the Goodyear chart (120psi). Dealer inflated per Thorís inflation chart behind the driverís seat (120psi). If I follow the Goodyear chart, based on my weight, my pressure is below the minimum inflation listed on their chart (80psi).

If I am unable to inflate per the manufacturers (Goodyear) chart and Thor recommends to inflate to the max inflation (120psi) which leaves no room for heat expansion.

Minus any understandable suggestions/objections I am going to reduce the front to 110 psi and each duel rear tire to 105 psi. I am too light to be running 120psi (max cold inflation) in all tires.
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