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Friday, April 4, 2014

Another post on "Calculating" inflation to avoid a blowout or tread separation

It seems this will be a never ending task of explaining the "Why and How" of setting tire pressure on RVs.

Quick answer:
A. Trailers should follow the placard which is probably the tire sidewall pressure unless the size or type has been changed. BUT they should also confirm that no individuakl tire is overloaded due to un-balance.
B. Motorhomes should run the tire placard inflation unless they know the actual load on each tire.
C If a Motorhome owner knows the actual corner load he should consult the load inflation charts from the tire manufacturer for his brand in RV use. Select the inflation needed to exceed the actual load of the heaviest side of each axle and then inflate all tires on that axle to the minimum inflation plus 10%
D, If the tires on the trailer have been changes then the individual loads for each tire need to be known. Using load inflation tables select a tire with sufficient Load Range to carry that load and then set the cold inflation to the inflation shown on the tire sidewall.

Following the above should decrease the damage done by running a tire in overload and under-inflated condition. Hopefully this will also prevent Tread separations and Blowouts.

 I try and follow a few RV forums. Some are general and others are focused on a single brand or manufacturer. I would prefer it if I could simply post a few links to this blog so I don't have to spend time saying the same stuff over and over but some sites will not allow a link to this blog. (If you are reading this you should know that you can post a link to this blog but as the Blog writer I am not allowed to.)

Here is a recent post on a thread titled "Calculating the correct tire pressure". This forum is 95% trailer oriented.

 "A while back, we weighed our travel trailer on the CAT scales and got a total weight of 7,300 pounds, rigged for towing. Rigged for towing means with a full water tank, all of the misc. junk that we normally carry and weight distribution bars tensioned, but no clothing or food on board. On average, this translates to 1,825 pounds per tire.
Today, we were able to obtain individual wheel weights using portable scales. The results are (rigged for towing)

Left Front - 2,000 pounds Left Rear - 1,900 pounds

Right Front - 1,900 pounds Right Rear - 1,700 pounds
"

We can immediately see the unbalance in this trailer with the heavy tire having 27% of the total. We also learn that the TT was not fully loaded with food or cloths which would add more weight. We also see that when the owner simply used the original CAT scale weight he assumed equal weight distribution of 1825# per tire.

In this case the owner was not aware of the special loading seen by trailer tires.

In this same thread we found one owner that switched from ST235/75R15 LR-D to P23575R15/XL ! giving up about 23% of the load capacity. This thread is at 85 posts and counting.

Clearly the need for more knowledge on care and maintenance of tires in RV application is needed. I hope I have provided helpful information here and in my seminars. Please spread the word. If you have a specific question you can send a question to   Tireman9   at  Gmail  dot com

2 comments:

  1. i read all this info on tire air pressure. my rv has a placard at the drivers side that read as follows: 2948 front, 4990 rear, 245-70/19.5G, 133/132L, 82PSI. if i am reading the tire chart correctly, it seems with this weight i should use 95PSI?? when i have inflated to 95PSI and have had my dealer check to insure no leakage, the dealer reduces the tires to 82PSI stating due to the placard that is all that can air to. PLEASE explain the charts. my tires are goodyear G670.

    ReplyDelete
  2. The loads on the placard are the MAX you can put on the axle. The 82 is the inflation recommended by the RV mfg based on the estimate for what stuff you will actually be carrying. The only way to know the proper cold inflation is to check the actual load on each tire, look up the required minimum inflation needed and I suggest you run at least 10% more psi but not exceed the infl on the tires on the motorhome. I have a number of posts on load and inflation calculations.

    ReplyDelete

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