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Old 05-01-2010, 08:50 PM   #1
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Tire psi

The weight of my motor home fully loaded is
Steering axle = 7220 lb
Drive axle = 12520
Acording to my calculation ea. Front wheel can carry 3610 lb
Rear wheel can carry 3130 lb
Acording Michelin chart I shoul run 80 PSI After inflating the tires to 80 PSI and checking them with two different guage It seam to me that they were UNDER INFLATED ?
Would 85 PSI be OK for all tires ?
Thank you.
Mario
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Old 05-01-2010, 10:33 PM   #2
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Your tires will heat up possibly 10 to 15 lbs while driving adding 5lb's will not cause a problem.
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Old 05-02-2010, 12:37 AM   #3
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msmorto;
Just a question: Are the weights listed actual weights or GAWR? If actual, it looks like you are on the edge and need to be careful how much more you add. Not trying to join the weight police, just curious.
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Old 05-02-2010, 07:05 AM   #4
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msmorto, the best way to determine proper tire pressures is to have all 4 corners of your motorhome weighed individually. Just because an axle weighs 7220 lbs doesn't mean each tire is carrying 3610 lbs. Many coaches are heavier on one side due to slides, position of water tanks, load, etc.

If you can't get individual corner weights, then if it were me, I would assume one side is at least 10% heavier than the other. I would inflate BOTH tires on that axle for the heavier weight. Your tire manufacturer should have an inflation table on their web site.

As far as safe inflation pressure, the maximum inflation pressure will be stamped onto the sidewall of the tire. Although it is generally safe to inflate the tire to this maximum pressure, overinflating the tires may result in rough ride, handling problems and/or uneven tire wear. That's why its best to have all 4 corners weighed.
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Old 05-02-2010, 12:11 PM   #5
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The short answer is yes, you can increase to 85. Or even 90. Excess pressure mostly just gives a harsher ride. I always keep mine at least 5 psi over the minimum recommended pressure for my actual weight.

But what makes you think they are under-inflated at 80? You can't really know that unless you weigh the coach and get the actuals.
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Old 05-02-2010, 08:35 PM   #6
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To Bob King
These are the GAWR and according to Michelin Inflation Tables for
a 245/70R19.5 xrv they call for 80PSI

To Gary
I did weight the MH as stated on my post, They just look under-inflated at 80 PSI maybe it just me
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Old 05-03-2010, 08:04 AM   #7
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I wasn't sure if the weights you stated were the axle GAWR (max loads) or actual weights.

Do not assume each end of the axle is exactly half the total. In fact, that is rarely the case. Always allow for one side weighting 5-10% more than half. That means you should estimate the tire load at about 300 lbs more than what you suggested. Find that weight in the inflation table and then go up one tier, which will typically be 5 psi higher.
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Old 05-03-2010, 10:23 PM   #8
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msmorto
You state the numbers are GAWR and you say you have weighed your unit. Are you saying that your unit is perfectly balanced side to side and exactly at the GAWR? If you are at GAWR you need to unload some stuff.
RE judging the amount of air by looking. I suggest you look at the Photos section and in members galleries go to Tireman9 and see if you can tell how much air is in the tire Dual-1 and how much in dual-21?

RV Roamer
On what enginering experience do you base your sugestion to exceed the MAX inflation specified by the manufacturer? In addition to harder ride, increased inflation also increases the possibility of an impact break.
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Old 05-04-2010, 07:53 PM   #9
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To All
I need to make a clarification on my original post as follow

This is for (245/70R19.5 LRF XRV) tires
The weight of my motor home fully loaded (Steering axle = 7220 lbactual weight
Drive axle = 12520 lb actual weight) not GAWR
The GAWR from Michelin is Single - 4080 lbs at 95 PSI
dual - 3860 lbs at 95 PSI
Steering axle = 7220 lb
Acording to my calculation ea. Front wheel can carry 3610 lb plus/minus
Drive axle = 12520 lb
Ea.Rear wheel can carry 3130 lb plus/minus
Acording Michelin chart I shoul run 80 PSI
Would 85 PSI be OK for all tires ?
Thank you.
Mario
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Old 05-04-2010, 08:13 PM   #10
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Actually you could run them all the way to 95 lbs which would allow for more weight than you have and not hurt a thing. A while back there were a lot of people subscribing to the theory that you should always inflate your tires to the max pressure. The ride will just gets rougher with the increased pressure.
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Old 05-04-2010, 08:30 PM   #11
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Is there a standard temperature that the tire inflation tables are based on? I've looked at a couple of different manufacturers but cannot find it. When I air up my tires in Yuma it could be 105 degrees outside, when I air them up in Michigan it's more like 40 degrees. It seems to me that has to affect the weight calculations.

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Old 05-05-2010, 12:27 AM   #12
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msmorto:
I still don't see what the GAWR as printed on a sheet that should be on the wall in back of the closet is. The ratings for the tires have nothing to do with the chassis GAWR. That only tells you what the tires will carry at different pressures and may still be overloading the chassis. Which chassis is under you rig and what does the manufacturer list for front and rear GAWR? Sorry if I'm being dense.
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Old 05-05-2010, 09:16 AM   #13
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Quote:
Originally Posted by msmorto View Post
Would 85 PSI be OK for all tires ?
The answer to your question is yes. It never hurts to add a little to the chart recommendations.
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Old 05-05-2010, 12:15 PM   #14
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Quote:
Originally Posted by DandS View Post
The answer to your question is yes. It never hurts to add a little to the chart recommendations.
If a person is willing to *maybe* sacrifice a small bit of ride comfort, a REASONABLE degree of added PSI also reduces rolling resistance, and delivers better MPG - a trade-off *I* am sure willing to accept!
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