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