By Roger Marble
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.
The post on the forum: “I have a 2003 Fleetwood Pace Arrow 37A about 20,000 lb loaded. Tires are Hercules 255/70R22.5. Sidewall states 120 PSI. Camping World—who I do not trust and will not go back to (whole other story)—set tire pressure at 80 PSI. 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?”
Well, the RV owner gets points for getting some of the information needed and he provided that data in his first post. I started by doing an Internet search to learn if this RV has slides. It does have a large slide on the driver side and a small one at the rear of the passenger side. 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 one 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’s being out of balance side-to-side by more than 1,000#. We should probably estimate a greater margin, just to be safe.
2. A 6% unbalance could mean 3,528# on the heavy end of the front. That would indicate 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 of an 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 let’s round it 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 until they have the opportunity to get their actual 4 corner weights.
Have a tire question? Ask Roger on his new RV Tires Forum here. It’s hosted by RVtravel.com and moderated by Roger. He’ll be happy to help you.