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#2999586 - 05/11/13 11:40 PM Learning about 19.5 tires, loading and inflation.
mrsilv04 Offline


Registered: 12/08/06
Posts: 6146
Loc: Illinois
I've entered the new world of 19.5" tires. I purchased a 2003 Coachmen 31' motorhome with 225/70R-19.5 LR-G tires.

The previous owner had all 6 inflated to the maximum pressure listed on the sidewalls, and not surprisingly, the motorhome rode and handled like the tires were full of concrete.

These tires aren't OEM, the last owner replaced them in 2010, with tires with 2010 date codes.

So, after a visit to the local grain elevator, I was shown how to turn on the exterior scale display during non-business hours.

I weighted it empty (3 people inside), and it was front axle 5200#, rear axle 8400#. This was with about 25 gallons of gas (75 gallon capacity), and about 20 gallons of water (100 gallon capacity) on board.

So...I unloaded the old travel trailer, and carried everything into the motorhome, and headed for the elevator for a "loaded" weight.

With the same three people inside, it weighed 5380# on the front axle, and 8800# on the rear axle.

I don't expect to ever carry over 50 gallons (1/2 tank) of water while travelling, so that would add roughly 250 pounds to the rear axle weight, and a full tank of gas would add another 300 pounds of rear axle weight.

So, to give an additional margin, I think we could reasonably bump the rear axle weight up to a max of 9800#, and increase the front axle to a max of 5700#.

Looking at the BFGoodrich spec sheet for the tires (model ST230), it shows:

65psi = 5510# single, 10400# dual
70psi = 5790# single, 10880# dual
75psi = 6080# single, 11440# dual

As an alternative to the previous owner's strategy, I've already tried 70psi in all six, and the difference in the ride quality and handling is like night and day.

According to the BFG specs (if I'm reading them correctly), I'm fine at 70psi. If anything, I may go up to 75psi on the fronts for some wiggle room.

A question: Both the water tank and the gas tank are located *behind* (aft of) the rear axle. In theory, shouldn't those, since they are located behind the rear axle, help to effectively reduce some of the weight load on the front axle, as I add water and gas to them?
_________________________

President of the Illinois chapter of 'Motorcyclists for Global Warming'.

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#2999609 - 05/12/13 12:39 AM Re: Learning about 19.5 tires, loading and inflation. [Re: mrsilv04]
gaijinnv Offline


Registered: 02/14/13
Posts: 231
Loc: Nevada, USA
I think the only part of the equation that's missing is the GAWR (Gross Axle Weight Rating) numbers for your rig.

The GAWR should not exceed 94% of the Load Limit of the tires on that axle.

If that looks good, then you should be OK.

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#2999681 - 05/12/13 06:23 AM Re: Learning about 19.5 tires, loading and inflation. [Re: mrsilv04]
CapriRacer Offline


Registered: 02/22/03
Posts: 2439
Loc: Somewhere in the US
You should find the vehicle tire placard for your vehicle. It should be located on a doorpost or in the glove box, but I've heard that some RV's have it on the curbside door.

You might also look in the oowners manual.

A common misconception about RV's is that the tire load tables published are a recommendation. Look carefully at the table and you will see they are maximum loads for a given pressure. That is NOT a recommendation.

I would recommend you use MORE pressure than what the table calls for. Without further info, I'd recommend a 15% load reserve (difference between the actual load on the tire and the table's rated load). Don't forget that the actual load on a tire is different side to side. So if you only weighed the vehicle by axle, then you need to adjust for that.
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#2999736 - 05/12/13 08:39 AM Re: Learning about 19.5 tires, loading and inflation. [Re: mrsilv04]
bullwinkle Offline


Registered: 10/09/04
Posts: 3889
Loc: Cincinnati, OH, USA
Out of curiosity, what is the max pressure on the tire sidewall for your tires-120 PSI? In my (admittedly a little old) RV experience, higher pressure seemed to help a little with the flat spotting that my tires would get from sitting on concrete for an extended period-but my old '73 25 footer always had 17" bias ply tires.
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#2999754 - 05/12/13 09:12 AM Re: Learning about 19.5 tires, loading and inflation. [Re: bullwinkle]
Corvette Owner Offline


Registered: 01/22/03
Posts: 1555
Loc: Waldorf, Maryland
Originally Posted By: bullwinkle
Out of curiosity, what is the max pressure on the tire sidewall for your tires-120 PSI? In my (admittedly a little old) RV experience, higher pressure seemed to help a little with the flat spotting that my tires would get from sitting on concrete for an extended period-but my old '73 25 footer always had 17" bias ply tires.


Usually 110-115 psi for these tires.
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#2999800 - 05/12/13 10:19 AM Re: Learning about 19.5 tires, loading and inflation. [Re: mrsilv04]
bullwinkle Offline


Registered: 10/09/04
Posts: 3889
Loc: Cincinnati, OH, USA
I would say, try 80 PSI cold (which would mean 12000 lbs. rear axle load) & see if you could live with it. I would rather bounce a little than blow a tire with unsecured passengers & load and try to prevent a deadly rollover accident.
_________________________
06 Ram 3500 CTD 4X4 48RE SRW, 93 GMC C3500 6.2 diesel, 89 F-450 7.3 IDI, 98 Cherokee 4.0, 05 Scion xB, 82 Mercedes 300D, company van 12 Ford E-250 4.6

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#2999803 - 05/12/13 10:23 AM Re: Learning about 19.5 tires, loading and inflation. [Re: mrsilv04]
mrsilv04 Offline


Registered: 12/08/06
Posts: 6146
Loc: Illinois
Capri-

I've seen that info. Coachmen included it all on a sheet that was included with the 4" thick, 3 ring binder that came with the motorhome.

I'll have my hands on it later today and will be able to report back with some numbers.

Thanks!
_________________________

President of the Illinois chapter of 'Motorcyclists for Global Warming'.

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#3000174 - 05/12/13 06:31 PM Re: Learning about 19.5 tires, loading and inflation. [Re: mrsilv04]
Ken2 Offline


Registered: 12/02/02
Posts: 6059
Loc: Washington St.
There are scales that weigh each corner. These are often at RV get togethers and maybe a local RV dealer or RV tire specialist shop would know where get to one. As Capri mentions, don't assume that your rig has symmetrical weights side-to-side.

Yes, filling the tanks more behind the rear axle will take weight off the front axle, but it might not be worth bothering with. Use the heaviest weights you'll see plus the safety margin for your calculations.

Can you feel a reaction in the rig as if the water in the tank is sloshing to one side when your corner, or sloshing forward when you brake hard? You might want to try to run with the tank full for that added stability.
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