Age old question tire pressure?

  Please forgive me if I sound like a broken record.        I found another post on an RV forum on inflation. It's almost as if no one knows that the answer to most questions on RV life has already been asked and answered. That is certainly the case when it comes to tire inflation. But I just can't ignore these questions as I hope that if some learn how to properly inflate their tires they may avoid a costly failure.

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"I have a 2003 Fleetwood Pace Arrow 37A about 20,000 lb loaded. Tires are Hercules 255/70R22.5. Sidewall states 120PSI. Camping World who I do not trust and will not go back to ( whole other story) set tire pressure at 80psi. I think they are way low. Am I right or wrong? Have not had each wheel weighed. Recent CAT weigh in: Front axle 6300 and Rear axle 14640. Any recommendations? "

 

My reply: 

Well the RV owner gets points for getting some of the information needed and he provided that data in his first post. I started off doing an Internet search to learn if this RV has slides and it does have a large slide on the driver side and a small one at the rear of the passenger size. This suggests the potential for some significant weight unbalance side to side.

Since we do not have the "4 corner weights" I am going to suggest we assume a 4% side to side unbalance because of the weight of the large slide.

With a Front axle scale reading of 6,300# that would suggest that on end could be supporting 3,402# on the heavy end.  The heavy end of the rear could be supporting 7,906#.

There was an answer posted that the owner could use a Firestone load/inflation chart since he could not find one for Hercules brand. This information is correct in that almost all brands other than Michelin have the same numbers in their tables. I had a Bridgestone chart that shows a 255/70R22.5 LR-G tire can support 4,190# @ 80 psi in single (front) position. his size in a dual position (rear) is rated to support 3,970# per tire at 80 psi. or 7,940# per dual pair.

So what would I recommend for this specific application?

1. Since we do not have the actual "4 corner weights" and are estimating a 4% side to side unbalance, we need to be careful as we could easily be wrong with that estimate. I have heard of some large Class-A being out of balance side to side by over 1,000#. We should probably estimate a greater margin jsut to be safe.

2. A 6% unbalance could mean 3,528# on the heavy end of the front which would suggest that our suggested 80 psi is probably enough for the front position. The rear position calculates to 8.198# on the heavy end or 4,100#.   The table suggests that inflation of 85 psi would be sufficient to support 4,110# per tire in dual application.  This gives us a goal minimum inflation of 80/85 F/R.

 

3. If you have been reading my posts you know I like to have a bit extra margin on the inflation number so the RV owner doesn't have to chase inflation as the Ambient temperature changes so an extra 10% on inflation is good.

 

4 We end with 88F/ 94R  but lets round to a nice 90/95 psi.

 

I trust that if people follow the concepts I covered in this post they should be able to arrive at a reasonable inflation guide till they have the opportunity to get their actual 4 corner weights.

 

##RVT1039

 

Another example of how I arrive at tire inflation recommendation for Motorhomes.

Got this request and supporting information:

Can you help me find the proper tire pressure for the following?

Winnebago Class C motorhome on 2018 MB Sprinter chassis
Dual rear wheels, Tires are LT215/85R16 115/112Q  Load range E  (Continentals)

GVWR = 11030
GAWRF = 4410
GAWRR = 7721
Door placard says 61psi, front and rear.

Actual weight as loaded, including driver & passenger

Front = 4200
Rear  = 6600
Total = 10800

According to the Michelin load chart for this size tire, the front about matches my placard at 61psi

However, if my rear with duals is 6600, should I set the rear pressure closer to 50? 
At 61psi the rear impact harshness seems high over road cracks and expansion joints.

Would love to hear your comments as I’m thinking about lowering my rear pressure some.  Trying to improve ride, but not hurt tire performance.
 

My Reply:

Looking at your GAWR and scale readings tells me you are not overloaded. You are relatively close on the front and only have a 5% margin at 210#. The Rears are better off with a 14% margin. Given that I have seen some Class-C RV have 3% to 4% side to side unbalance, I would be more comfortable if you could reduce the actual load on the front tires until you can confirm actual "4-Corner" weights at a building supply scale or gravel pit, or some other location where you can get the individual tire loading.

Your weights in the rear are a bit lighter than I normally see as on my 24' Class-C which has scale tire loading of 1,900# & 2,100 on F  (4,000 axle)
and  3,550 &3,850 on R  (7,400 axle)

But back to your situation

I confirmed that the weight chart from Michelin is same as general industry standard. Sometimes Michelin has different numbers than the rest of the industry so unless we are running Michelin tires I tend to not even look at the Michelin charts. I also prefer to deal with individual tire loading and some Michelin charts are axle loads.

4,200 x 51% = 2142# and consulting the chart I find 60 psi as the minimum. I Also always suggest +10% inflation margin to learn the cold set pressure which gives 66psi

6,600 x 50% gives 3,300# 51% gives 1683 and the chart suggests 45 psi as the minimum.  Adding my 10% yields 50 psi.

I add this inflation margin to avoid the need to adjust my tire pressure whenever the Ambient Temperature changes. I don't adjust tire pressure till the 10% drops to 5% inflation margin.


BUT the 51%/49% side to side split is conservative and I am not comfortable suggesting lower than Certification Label inflation without knowing actual tire loading. So I suggest:
66 Front and no less than 61 psi on the rears.

Ride and harshness are really a function of chassis design, springs and shocks. Using the tires to "improve ride" by lowering pressure without all the numbers, may result in shorter tire life.

I trust that this is clear enough such that others can do the calculations on their own. 

 

##RVT1018