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

 

Tire Loads & Inflation - A refresher

 Just read another post on a FB page where more incorrect info was published....Soooo lets go over this topic again

 Today’s key points: Know the minimum tire inflation based on manufacturer estimates. At a MINIMUM Check your inflation with a good gauge at least monthly and every morning before travel. Better yet get, program and use a Tire Pressure Monitor system  TPMS to the knowledgeable.

Tire inflation seems to be a topic that confuses some and has others believing in misleading or just plain incorrect information. Tire inflation is one item that directly affects the safety of your RV, truck or car as you travel down the highway. Many of my posts have been background information that you really don’t have to know or fully understand. The intent of these topics is to give you a better foundation of understanding more about tires but, if you only pay attention to one series of posts this is it.

Tires do not carry the load. They are just a container of air. It is the inflation air that does the work. Think for a moment of an impact wrench. It can’t do the work of loosening or tightening nuts on your wheels without the air, and a tire can’t do the work of carrying the load or provide the traction needed to turn, start or stop if it doesn’t have air. The load a tire can carry is basically based on the air volume of the tire and pressure of that air inside the tire.

If the load was carried by the tire construction we would have Tire Load and Construction tables. But we don't. We have Load and Inflation tables because, for a given type and size tire the load is just a function of air pressure. More pressure gives more load carrying capacity.

If you want to get a feel for how little load your tire can carry without air in it, you can test this yourself. Simply take an unmounted tire and stand on the beads of the tire. I think you will find that the tire can support less than 5% of the rated load before it deflects more than an inch or two. Some people believe that tires with higher Load Range can carry more load at the same inflation. This is just not correct. You would be hard pressed to measure the uninflated load capacity difference between a Load Range D and E tire or between a G or H Load Range tire because almost any significant load would compress the tire to be flat.

The term Load Range replaced to old "Ply Rating back in the early 70's when tire construction materials became stronger and with the introduction of Radial construction.

If you are not sure, the "Load Range" is marked on the sidewall of your tires. If you don't see the words "Load Range" look for "LR" followed by a letter usually between C and G. "Load Range" may not be in large letters but you should be able to find them on your tires.

TireRack has a good information page on "Load Range" HERE.

The simple answer to how much air you need is on your Certification Label AKA Tire Placard that I wrote about last week. Depending on the year your RV was made, this information is on a sticker on the side of your trailer or near the driver’s seat in your Class-A or on older RVs it might be glued to the wall inside a cabinet or on the inside of your entry door. Yoyr car or Pick=up has the sticker on the driver door jam. Where ever it is, you should also have the information in the stack of owners manuals you received when you bought the new RV. Find this data sheet and write down the information for loads, minimum inflation, tire size and load range and place this info where you can easily find it. This inflation is the minimum based on what the RV manufacturer thought you would load into your vehicle. They may have guessed correctly or you may have gone way over that estimate depending on how much “stuff” you carry.

##RVT1038