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Tire pressure/weight

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echo11

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Tire pressure/weight
« on: April 18, 2012, 08:55:00 am »
I just weighed my 2011 PC 2350 w/o slide at a CAT scale.  The front steer axle was 3180 lbs and the drive axle was 7820.  Gross weight was 11000.  Looking at the Michelin tire pressure chart, I can't figure the correct pressure for the tires.  Was I supposed to get the weight done differently?  Can anyone help with this?
Thanks- Roni

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Barry-Sue

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Re: Tire pressure/weight
« Reply #1 on: April 18, 2012, 10:04:17 am »
Roni
The inflation table lists the load for each end of the axle (each tire for single axle or each pair of tires for dual axle).

If you go by what exactly the inflation table indicates then the front axle weight of 3180 pounds divided by 2 =  1590 pounds per axle end.  This would result in tire pressure being set to 40 pounds.

If you go by what exactly the inflation table indicates then the rear axle weight of 7820 pounds divided by 2 =  3910 pounds per axle end.  This would result in tire pressure being set to 60 pounds.

This assumes the weight is equally distributed side to side but this may not be the case.  It is possible that the left front tire might have 1700 pounds of load and the right front tire only have 1480 pounds of load.  This would result in tire pressure being set to 45 pounds in both front tires.  It is important to have the same tire pressure in all tires on the same axle.

The left or right side of the motorhome would need to be weighted to determine the exact required tire pressure.  This would allow you to determine the heavier axle end.


Barry
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echo11

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Re: Tire pressure/weight
« Reply #2 on: April 18, 2012, 10:32:20 am »
Barry- thanks for your response.
We are currently set at 75 in the front and 65 in the rear duals, as suggested by Phoenix.  Dropping to 60 and especially 40 in the rear sounds low.  Would it cause problems to be "over inflated"?

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Bob Mahon

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Re: Tire pressure/weight
« Reply #3 on: April 18, 2012, 11:39:37 am »
Better overinflated than underinflated.
I personally would not inflate to only 40 front and 60 rear.
When we had a 2350, I inflated 65-70 front and 70-75 rear. Tire to tire pressures on each axle were equal as near as I could get them.
Never had problems with the ride quality either.
Carry on, regardless..................
"In God we trust" to save our country and bring our troops safely home.

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bobander

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Re: Tire pressure/weight
« Reply #4 on: April 18, 2012, 12:53:35 pm »
I just weighed my 2011 PC 2350 w/o slide at a CAT scale.  The front steer axle was 3180 lbs and the drive axle was 7820.  Gross weight was 11000.  Looking at the Michelin tire pressure chart, I can't figure the correct pressure for the tires.  Was I supposed to get the weight done differently?  Can anyone help with this?
Thanks- Roni

You don't say if the RV was at travel weight, you should weigh the RV when it is fully loaded for travel including gas, water, propane, passengers, and all the other stuff you take with you.  It is probably easiest to do this when you are actually on a trip.

Bob
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echo11

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Re: Tire pressure/weight
« Reply #5 on: April 18, 2012, 01:08:40 pm »
Yes, we are fully packed and on the first day of a 3 weektrip.  We would like to  have the tires inflated properly.
Thanks-

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bobander

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Re: Tire pressure/weight
« Reply #6 on: April 18, 2012, 01:39:39 pm »
Yes, we are fully packed and on the first day of a 3 weektrip.  We would like to  have the tires inflated properly.
Thanks-

Then as Barry says, the charts specify 40 front and 60 rear, but to account for possible side to side variation you should add to those numbers, maybe 45 front and 65 rear as a minimum.  That may sound low for the front, but if that is the actual weight then the tire will be at optimal road contact and side wall bulge as per Michelin.  Personally I would probably go 50 front and 65 rear.  On my 2551, I found that being overinflated caused poor steering response, the edges of the tread were not gripping the road surface and steering was not reponsive.

Bob
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Barry-Sue

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Re: Tire pressure/weight
« Reply #7 on: April 18, 2012, 01:50:25 pm »
Roni

If you are satisfied with the ride of your PC with the tires inflated to 75 lbs front and 65 lbs rear I would be cautious about making any drastic changes until I got home, did some more research and had the left or right axles weighed.  I think if you had the weights done at a CAT scale within the last 24 hours you can have it weighed again for $1.00 (I know it used to be like that).

The reason Phoenix suggest the 75/65 ratio is because the 2350 PC has a 5,000 pound rated front axle and a 8,500 pound rated rear axle.  It looks like the tires are inflated for the maximum axle loading.  Doing the calculations from the inflation table 75 LBS is for a load of 5,000 pounds and 65 LBS is for a load of 8,500 pounds.  Remember these are the maximum ratings for the axles.  Since yours clearly are not loaded to their maximum you will beable to lower them but to what degree depends on the left or right weighing.

Barry
« Last Edit: April 18, 2012, 02:49:58 pm by Barry-Sue »
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DownBy TheRiver

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Re: Tire pressure/weight
« Reply #8 on: April 19, 2012, 12:49:26 pm »
So, here’s what I take away from this - you have 2 guidelines to land between:


1) The Michelin Guide (generally lower pressures) allowed for an actually weighed vehicle. To this, add Barry-Sue’s maxim, (important) that if you have the individual loaded corner weights, the pressure for front axle tires should be equal, and rear axle tires should be also be equal; and dependent on the heaviest weighted corner for that axle.  Don’t have that data?  Add a few extra pounds pressure to all and be done.

2) The Phoenix Guide (generally highest pressures) accounting for a fully maxed out GVWR.


Any pressure between these two recommendations is entirely acceptable, at least in terms of tire safety alone.

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DownBy TheRiver

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Re: Tire pressure/weight
« Reply #9 on: April 19, 2012, 12:50:37 pm »
Then there’s personal experience.  And that’s where your, eh, umm, er.. *ss - or “seat of the pants” - comes into play.  Don’t at all discount this feedback.  My “seat” has often proven far more correct far earlier than any other set of data that “something’s wrong”.

Bobander is correct in regards to the Michelin Guide data, “the tire will be at optimal road contact and side wall bulge as per Michelin”.  Optimal is optimal, in terms of safety, wear. handling and gas mileage.

I personally find a few extra PSI enhances all the above and gives me a margin for error.  And for those travelling north to south and back, especially during winter; a good rule of thumb is you lose/gain about 1 lb in pressure for each 10 degrees in temperature difference.  Driving up and down between Chicago and Tampa in winter (a blessed 60-degree shift), I usually air-down in Tennessee, then again in Tampa.  Airing-up again on departure, and once again in Tennessee.

Being too over-inflated causes more problems than it solves.  It decreases the contact patch of the tire, causing it to “squirm” along the road surface, which can result in negative handling safety, tire wear and gas mileage.

Only your “seat” knows for sure.   ;)

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echo11

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Re: Tire pressure/weight
« Reply #10 on: April 19, 2012, 07:18:37 pm »
Thanks for all the replies. It sounds like it is not an exact science.  At this point, I am leaning towards putting 70 in the front and rear, or possibly going a little higher in the rear axle.  Since our trip started in MA and we will be traveling to FL with the temps fluctuating,  we will keep a watch on the pressure and adjust as needed.  We will also see if changing the pressure effects the drive.
Roni

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gradygal

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Re: Tire pressure/weight
« Reply #11 on: April 20, 2012, 07:24:47 am »
Roni,
If you are anywhere near Punta Gorda (between Sarasota and Ft. Myers), give us a shout. We will be here til the beginning of June.

I tried to send you a PM but for some reason, it didn't go through.

Judi
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ron.dittmer

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Re: Tire pressure/weight
« Reply #12 on: April 20, 2012, 11:48:35 am »
I have yet to weigh our 2007 2350 without slide out.  I put 65psi in the rear tires and 70psi in the fronts which is 5 psi more than the door sticker states to have.

I really need to weigh our rig when on a trip.
Ron (& Irene) Dittmer