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InTheDogHouse

Tire Pressure vs. Axle Weights

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Greetings All...

I recently had six new 245/R7019.5 Michilen XZE (Load Range H) tires mounted on my coach.  (The total installed price of $2,862.51 through the FMCA Tire Program seems a bit shocking compared to some other prices I've seen in this forum, but I suppose that's the result of the cost of Michelin's compared to some other brands...)

My old tires were Goodyear G670 (Load Range F).  The front were inflated to 85 psi and the rear to 100 psi by the previous owner who said this was based on the axle loads.

I noticed when I picked my coach up from the tire dealer after getting the new tires mounted, they were all inflated to 85 psi...  Even though the ride is VASTLY improved with the new tires, it got me to thinking about the pressure vs. weight vs. load range factors I've read about on this and other forums and sites.  Below are the placard numbers for my coach:

GVWR: 20,500-lbs

GAWR: 7,000-lbs (front)

GAWR: 13,500 (rear)

We loaded our coach of all our goodies, plus I filled the grey, black and water tanks and my wife and I headed over to a local truck stop to weight on our way out of town for our first quick weekender of the year.  The weights shown are the maximum we ever expect to have as we don't always carry all the gear included and we rarely travel with much in either grey or black tanks and almost certainly never with everything full.  We do routinely carry water to our destination, but not full holding tanks at the same time. (Results of the weight-in are as follows:

Steer Axle: 6,280-lbs

Drive Axle: 12,840-lbs

Gross Weight: 19,120-lbs

For the front axle, that works out to an average of 3,140 per tire (I understand weighing each corner would be better, but nobody in the area can do this...) According to a pdf downloaded from Michelin's website, the closest air pressure shown for each of these weights works out to 75 psi, which is shown as supporting a load of 3,390 -lbs and is the lowest pressure shown on the charts provided.  According to another chart on Michelin's website, the weights shown are half this value (1,695-lbs)  Should I assume the larger is per axle and the smaller per axle end?

For the rear axle, it works out to an average of 6,420-lbs per dual-wheel-set.  The charts show 6,420-lbs load capacity at 75 psi, again this being the lowest pressure shown on the chart. (Same issue re the two charts showing half/double values and being uncertain if they're indicating per tire...)

I've read that it's not advisable to run pressures that result in being right at or too close to the maximum loads listed and also that you don't want to be over or under inflated.  With all that being said... What would our resident tire experts recommend for pressures on these new tires?

 

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Little difficult to say, never heard of anyone carrying full black and gray thanks.  What is the max air on each cold tire? 

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With coach loaded as YOU go down the road (including tanks, people and gear) use the heavier wheel position on each axle to go to the Michelin inflation table to determine the MINIUM PSI for a given weight.  All tires on that axle inflated based on the heavier wheel position.  Most add 5-10 PSI to the MINIMUM.

If you only have axle weights, add a little more for left/right imbalance.  If you know left/right is way out of balance (example galley slide on one side with only a recliner on the other side) add even more to the minimum.

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All of my heavy stuff is on the driver side of coach.  Sofa, full residential galley, bathroom, bed, fresh water and black/grey!  I carry 15 PSI margin...120 instead of 105 !  Placard say's 95....Where these Engineers get their degrees, is beyond me!  But hey, it's a great floor plan.

I have 315/80/22.5 for steer axle and the other 6 are 295/80/22.5 !

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3 hours ago, manholt said:

Where these Engineers get their degrees, is beyond me!  But hey, it's a great floor plan.

 

Actually, that second sentence says it all. Marketing trumps engineering.  Got to believe there are a lot of times that the engineers just bite their tongue as some of the things the sales guys/customers want to do on their chassis.

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Yes, Michelin doesn't do us any favors with their Load & Infl tables.  They know that individual tire loads are important and that you should inflate all tires on any one axle to at least the pressure specified for the tires with the heavier end but then they go and make stuff complicated by publishing axle loads.

I suspect the reason for this is that their tables are aimed at truckers who only do axle weights because their loads are more balanced side to side than RVs.  You can do yourself a favor by writing down your own version of Michelin numbers but show the load per tire for fronts and per set of duals for the drive position as the rest of the tire industry does.

You can confirm you did the math correctly by looking at a Goodyear or Bridgestone or other truck tire chart. The numbers should be the same or almost the same as in the Michelin table.

(The reason for the minor differences can be confusing so just accept that the French have their way of thinking)

 

Minimum inflation.  You are correct to not run lower than the lowest number on the table. Just be sure you compare to a different company chart to confirm your math.

 

RE "Engineers"  I sometimes wonder if the RV companies have Automotive or similar engineers involved in the design of the coach part of a motorhome.  The chassis does, I'm just not sure about the coach based on some of the stuff I have seen in RV coaches and trailers.

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Thanks Tireman9.  I've been hoping you'd chime in on this.  I've read your comments in other threads on this topic and you seem to be the resident expert on the subject.  Not to diminish what manholt and wolfe10 stated.  I've read many of their comments in other threads and both also seem to bring a wealth of knowledge and experience to the table and I appreciate each of you sharing!

Thanks to each of you!

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On 5/12/2018 at 1:11 PM, tireman9 said:

....RE "Engineers"  I sometimes wonder if the RV companies have Automotive or similar engineers involved in the design of the coach part of a motorhome.  The chassis does, I'm just not sure about the coach based on some of the stuff I have seen in RV coaches and trailers....

The problem is the engineers have never spent a day in the RV they are "engineering" and designing.  So they design something that is incredibly stupid, and would realize that....if they used the product they are designing.  A past president of American Coach had a program started where the designing engineer had to spend a week in the coach he just designed.  However, that gentleman either retired, resigned or was fired before his program was implemented.

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22 minutes ago, FIVE said:

The problem is the engineers have never spent a day in the RV they are "engineering" and designing.  So they design something that is incredibly stupid, and would realize that....if they used the product they are designing.  A past president of American Coach had a program started where the designing engineer had to spend a week in the coach he just designed.  However, that gentleman either retired, resigned or was fired before his program was implemented.

And, the problem is even worse in the off-shore sailing business. 

Unlikely 5% of those assembling them have ever been off-shore in one of the products they build.  Lines not fairlead, winch self tailers that are installed such that they strip line overboard. Hawse pipe is not lined up with the anchor locker-- easily discerned if one fits chain over the windlass gypsy......

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3 hours ago, wolfe10 said:

And, the problem is even worse in the off-shore sailing business. 

Unlikely 5% of those assembling them have ever been off-shore in one of the products they build.  Lines not fairlead, winch self tailers that are installed such that they strip line overboard. Hawse pipe is not lined up with the anchor locker-- easily discerned if one fits chain over the windlass gypsy......

Gesundheit wolfe10...  ;)  

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Don't get me started on French, reverse, engineering!

That said, yes the tables should be the same.  Brett, different terminology, but the meaning is the same also!

Bill.  Your welcome!  Good Luck and safe travels, come back anytime! :) 

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